Early philosophers believed the Universe was made up of five "elements": Earth, Air, Fire, Water, and the mysterious Quintessence (also known as aether). Our understanding of physics has come a long way since then as we now know that the Universe consists of a multiplicity of subatomic particles held together by four fundamental forces, which are the Strong force, the Weak force, the Electromagnetic force, and the force of Gravitation.

One of the first cosmological models was the geocentric model developed by the Greek astronomer Ptolemy. Ptolemy's model of the Universe placed the Earth at the center with the sun and planets located in concentric crystal spheres surrounding Earth. These spheres rotated, causing the sun and planets to appear to rise and set. The stars were fixed in a stationary outer sphere. During the Middle Ages, this model became widely accepted in Europe, because the central location of Earth reaffirmed the importance of man.

Nicolaus Copernicus (1473-1543)

By the 1400s, scientists were beginning to question Ptolemy's model. In his book On the Revolutions of the Heavenly Spheres, the church canon and astronomer Nicolaus Copernicus proposed a heliocentric model which placed the sun, instead of the earth, at the center of the solar system. Copernicus's model would later be championed by the famed scientist Galileo Galilei.

Today, cosmologists have a much grander view of the Universe, extending far beyond our solar system. Far from being the center, our solar system is but a single system swirling around in the arm of our galaxy, the Milky Way, a spiral galaxy containing billions of stars. In fact one of the cornerstones of modern cosmology, the Cosmological Principle, asserts that the Universe has no center at all!

So what else is out there? Move forward to see what is in our Cosmic Backyard...

Like astronomers throughout history, modern cosmologists are interested in making an accurate model of the Universe. Starting with the laws of physics which explain how fundamental particles and forces interact, physicists derive general equations describing the evolution of the Universe's structure. Cosmologists use experimental evidence to select a set of initial conditions enabling them to solve the general equations, and calculate the state of the Universe at times in the past, present, or future. This generates a possible model, which can be tested by comparing the phenomena it predicts with observational data. In this manner, following the rigorous scientific method, cosmologists work to build a successful Universal model.

In the next section we will examine evidence for the current Big Bang model, one which arises from a set of initial conditions describing a hot dense universe. But first, in order to understand what cosmologists do, we'll have to look at the fundamentals of light, matter, and time.

Looking Back in Time

The first concept we must discuss involves how cosmologists observe the Universe. Scientists use a very special tool; one that enables them to see the Universe both in the present and in the past...

Move forward to learn about the stellar time machine...

Roll-over the spectrum to see images of the Crab Nebula taken using filters sensitive to different wavelengths.

Light allows us to interact with our world. We can see our surroundings because light bounces off objects into our eyes. We are all familiar with visible light, but this is merely a small portion of the electromagnetic spectrum. In fact, there are many different types of light ranging from short wavelengths, like x-rays, to longer wavelengths, like radio waves. These types of light blend together to make a continuous spectrum of electromagnetic radiation. By filtering out various regions of the electromagnetic spectrum, one can receive different types of information.

Light is composed of tiny wave-particles called photons which all travel at the same speed through space. We call this speed c , and it is approximately equal to 300 million meters per second. Instinctively, we might think that when we turn on a light switch the room lights up instantly, but this is not true. Light takes time to travel.

Video 1Travelling Photons(sıra1)

Bu videoyu oynatmak için Flash Player'i yükleyiniz.

Virtual Time Travel

So what can we see from our vantage point in the Universe? We can see every object whose light has had time to reach us. Light does not travel instantaneously; it takes time to leave an object and get to us. The farther away an object, the longer the light takes. When we see the light, we see the object as it was very long ago: looking farther out in space is looking back in time!

In addition to enabling us to see back in time, light can tell us much more about the Universe. One of the most important aspects of light is that it serves as an astronomical ruler. Scientists can use the relationship between how bright a star appears to be as observed from earth and a measure of its intrinsic brightness, or luminosity, to calculate the distance to the star.

Before telescopes, people looked at the sky and classified the objects they saw by their brightness. Hipparchus, a Greek mathematician, classified over 850 cosmic objects into six categories of brightness. Scientists later adopted the word magnitude, keeping and extending the scale developed by Hipparchus. The brightest stars were called first magnitude stars, the next brightest being second magnitude stars, etc. Today, we measure the brightness of an object using this same scale, but with much more precision and using a much larger scale. The scale is formatted so that the lower the magnitude the brighter the object, which means a star with a magnitude of -1 is brighter than a star with magnitude 2.

Luminosity, Distance, and Brightness are interrelated. Observed Brightness is what we see here on Earth, while Luminosity is the actual light energy produced by a star.

You probably know from direct experience that a light source seen from far away appears much dimmer than the same source viewed from up close. This is the basic idea that enables scientists to use light to measure astronomical distances. A measure of the energy emitted from a star (in the form of light) per unit time is called the star's luminosity. This value is independent of distance, hence astronomers treat it as a measure of the star's intrinsic brightness. Observed brightness, what we see, varies with the distance to the observer.

Apparent and Absolute Magnitudes

When the Greeks categorized celestial objects by their brightness, they could only see how bright they looked from Earth. Their scale only tells us a star's apparent brightness, or apparent magnitude. We know that as we get farther away from a source of light, the light looks dimmer. Some stars appear brighter than others because they are closer to us, not because they give off more light.

How do we know which stars really shine the brightest? In addition to an apparent magnitude, we can also determine a star's absolute magnitude, which tells us what we would see if we were 10 parsecs away (A parsec is the same as 3.26 light years, 31x10¹² km, or 19x10¹² miles).

Measuring Distance

Once we know both the apparent and the absolute magnitudes of an object, we can figure out how far away it is from the Earth. If we call the apparent magnitude m_app and the absolute magnitude m_abs, then we can find the distance (in parsecs) by using the following equation:

Proxima Centauri, that nice red star, is the star closest to our own sun (and a popular choice in sci-fi for future colonization). However, it's quite dim and very difficult to see at night. Its apparent magnitude is just 11.1, and it's absolute magnitude is a mere 15.5. How far away is it?

Solution:

We know m_app=11.1, and m_abs=15.5, so let's plug them into our distance equation:

and we see that d=1.32 parsecs, or about 4.3 light years away.

The brightness of a star is proportional to its luminosity divided by its distance from the observer squared. For this example at one unit of distance from the star the brightness is one, but at three units of distance from the star the brightness is nine times smaller.

As light from a star spreads out, its energy covers larger and larger areas causing the energy per unit area to decrease in an inverse square relationship. This means that doubling the distance actually cuts the energy by four. With more area to cover, the light from the star appears dimmer. We call "energy per unit area" brightness.

From our vantage point on Earth, it is relatively simple to measure the apparent magnitude of a distant star. Measuring luminosity on the other hand is incredibly difficult (one cannot simply send a probe out to a star many lightyears away). In fact measuring stellar distances is one of the most difficult ongoing problems in cosmology. Often scientists must utilize several different methods in combination to arrive at a reasonable estimate for the distance to a stellar object. Many of these methods make use of unique astronomical objects called standard candles

Luminosity

Just as we can measure the energy output of a power plant, so can we for a star. Every second, a certain amount of energy (mostly in the form of light) is flowing radially outward from a star.

If we sum the energy flowing out of a star per second, we get the star's luminosity, L. Essentially, we are adding up the "flux", or flow of energy through a unit area over the entire surface of the star. So, a star's luminosity is equivalent to the star's "total surface flux", which is equal to the flux at the surface of the star, multiplied by the surface area of the star.

Observed Brightness

We don't necessarily have to measure flux at the surface of a star. We can measure the flux from a star at any point in space.

Imagine a giant mathematical sphere (with a star at its center) whose radius (R) is the star's distance to an observer on Earth. The flux at the surface of the sphere is what we call the observed brightness of the star.

Observed brightness, b is defined as the energy passing through a unit area at the observer every second. To find b, we divide the star's net surface flux (luminosity) by the mathematical sphere's surface area. As one can see, the flux at the star's surface is much greater than the flux at distance R.

Luminosity and Brightness

From Earth, we can only measure the brightness, b, of the Sun using telescopes since there is no way for us to get close to the surface of the Sun. From the previous slide, we know that we also have an equation which relates the distance (R), luminosity (L), and brightness (b). How can we use this information to find the luminosity, L, of the Sun?

It turns out that most stars are very close approximations to black bodies: black bodies have the special property that their surface temperature, T_eff, and the wavelength, λ_max at which they radiate their energy are related linearly. We can measure λ_max with a telescope so we can also find the star's T_eff.

What about L? It so happens that L is directly proportional to T_eff⁴. The exact equation is too involved to be derived here, but this makes sense because the hotter the star (the higher the value of T_eff) the larger the flux. And the larger the flux, the larger the luminosity. Essentially, this means that hotter stars are brighter, as we would expect.

Standard Candles

Standard Candles are used to calculate astronomical distances.Each of these candles have the same intrinsic luminosity, the only difference is the distance from the observer.

A standard candle is a general term for any class of objects that have the same intrinsic luminosity (produce the same amount of light energy per unit of time). Objects that astronomers have traditionally used as standard candles include the largest galaxies in clusters, type Ia supernovae, and particularly bright stars called cepheid variables. Since standard candles of the same type possess the same luminosity astronomers can calculate distance ratios simply by measuring the apparent magnitude of two candles, because distant candles will appear dimmer than closer candles according to the inverse square law.

These distant objects that we see give off light and consist of visible matter.

Blackbody

Click the button to see the stove in action.

All matter emits light, which is a type of electromagnetic radiation. The wavelength of this radiation depends on the object's temperature: higher temperature corresponds to a short wavelength while lower temperature means a longer wavelength. For example, think of the coils on a metal stove; When the stove is cold (off) the coils look black, but When you turn the stove on you begin to feel the coils giving off heat. If you turn the stove to its highest setting, the coils will begin to glow red. If you could set it any higher the coils would glow yellow, then white.

Temperature

The atoms in ice are more densely packed and move less than the atoms in lava.

From our stove example, we can see that temperature and electromagnetic radiation are related, but what exactly is temperature? If one considers a sample of matter on the molecular scale, temperature is a measurement of the sample's internal energy: the random motion or excitement of particles comprising the sample. The particles in a system with high internal energy will be moving faster than the particles in a system with lower internal energy. The less internal energy there is in a system the lower the temperature; the greater the internal energy the higher the temperature. A sample of water, for example, exists in a solid state below 0° C. The constituent H₂O molecules are densely packed and move very little. At higher temperatures the water sample transitions into liquid or gaseous forms in which the molecules are widely separated and rapidly moving.

Video2Movie: Temperature(sıra2)

Bu videoyu oynatmak için Flash Player'i yükleyiniz.

Matter and Atoms

"If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is that...all things are made of atoms."

-Richard P. Feynman, winner of the 1965 Nobel Prize in Physics

Matter is made of atoms, and atoms are comprised of protons, neutrons, and electrons.

Everything in the Universe is made of matter. Though matter exists in many different forms, each form is made out of the same basic constituents: small particles called atoms. Atoms themselves are made of smaller particles: protons, neutrons, and electrons. Protons and neutrons are composed of even smaller particles called quarks.

Scientists postulate that there are even smaller particles, but for the purpose of studying cosmology we will focus on the atom.

Anatomy of an Atom

The classical orbit representation of the ground state of a Helium atom on the left showing the nucleus being orbited by two electrons in a distinct shell. On the right the Quantum Mechanical Model of the Helium ground state shows the 1s orbital with the same two electrons existing in regions of greater probability.

In the center of the atom, the strong force binds protons and neutrons tightly together forming the atom's nucleus. Electrons are found in the surrounding region, rapidly orbiting the nucleus. Classical mechanics describes the electrons as being held in distinct orbits by the electromagnetic force, very similar to the way gravity holds the moon in orbit around Earth. A more accurate model is given by quantum mechanics which describes the electrons as occupying regions of greater probability density called orbitals. On much larger scales, matter is attracted to other matter through the gravitational force.

Gravity: The Main Attraction

Gravity's effect is apparent even at the largest scales: just as gravity keeps the Earth orbiting the sun, it holds these two irregular galaxies M32 and M110 in orbit around the larger Andromeda galaxy.

In the late 1600s, the English mathematician Sir Isaac Newton gave the first scientific description of gravitation. Gravity is an attractive force existing between any two objects that have mass, causing them to accelerate towards each other. It is the weakest of the four fundamental forces but can act over great distances and is responsible for the formation of planets, stars, galaxies, and even larger scale structures such as groups and superclusters. Gravity is also the force that governs the motion of celestial bodies.

Gravitation

Gravity and Mass

From the graph we can see that as the mass of the alien increases, the gravitational force (the alien's weight) also increases: mass varies directly with gravitational force. The more massive two objects are, the greater the gravitational attraction between them.

Force_g ∝ Mass

Click the "Increase Mass" button to see the effect of increased mass on gravitational attraction.

Gravity and Distance

The force of gravity is inversely related to the distance the alien is from Earth; this means that force of gravity decreases as the distance between the alien and Earth increases.

As the distance between the alien and the surface of the planet increases, the force of gravity decreases. This relationship is true in all cases: as the distance increases between any two objects, the gravitational force gets much smaller very fast in an inverse square relationship.

Force_g ∝ 1/d²

As distance increases, gravitational attraction decreases.

Putting it Together: The Universal Law of Gravitation

Newton combined the inverse square relation between distance and gravitational attraction with the direct relation between mass and gravitational attraction as well as an additional constant of proportionality. His end result would be one of the most powerful laws in classical physics: the Universal Law of Gravitation.

Where:

F_g = force between objects 1 and 2
G (constant of proportionality) = 6.67 x 10^-11 Nm²/kg² (from experiments)
M = mass of first object
m = mass of second object

Early Models of the Universe

Space exploration missions contribute greatly to our understanding of the Universe.

With our new understanding of the fundamental workings of light, matter, and gravity, we can begin to examine the theories and discoveries that led to the development of the Big Bang model. Currently the most widely accepted scientific model of the Universe, the Big Bang model describes a dynamic evolving Universe that began in an extremely hot dense initial state nearly 14 billion years ago, and has since expanded into the complex structure that we see today.

The Newtonian Model

"Disrupt" to disrupt the gravitational equilibrium of the apples.

Scientists following Newton used his theory of gravity to successfully explain the arrangement and motion of the planets in our solar system. However when they considered the structure of the Universe on larger scales an apparent paradox arose: if everything in the universe is gravitationally attracted to everything else, the stars would accelerate towards each other causing the universe to collapse.

To avoid this problem, it was hypothesized that the universe is infinitely large and that the stars are positioned perfectly so as to hold each other in gravitational equilibrium. However even the slightest disruption of a single star could create a domino effect, resulting in a collapse. Something obviously wasn't right.

General Relativity

.

In 1915, Albert Einstein expanded Newtonian gravity into the theory of General Relativity. The theory describes space and time as a 4-dimensional entity called space-time. The presence of a massive object warps or bends the fabric of space-time such that the path of other objects (even light itself) is curved inwards towards the mass.

The more massive an object is, the greater the degree of warping it causes, and hence, the greater the tendency of other objects to "fall" toward it. 

When Einstein formulated General Relativity he believed, as did most of his contemporaries, that the Universe was static and unchanging. To his surprise, however the solutions to the relativistic equations indicated that the universe must either be expanding or contracting. To fix this troubling anomaly Einstein postulated an additional term in the equations called the cosmological constant, Λ, which counteracted the attraction caused by gravity at large distance scales and preserved the hypothesis of a static universe. He would later come to regret this move calling it his "greatest blunder", when observational data showed that the universe is expanding.

Video3General Relativity(sıra3)

Bu videoyu oynatmak için Flash Player'i yükleyiniz.

The Cosmological Principle

The distribution of matter across the Universe is approximately even, homogeneous, when considered at large scales.

Albert Einstein's theory of General Relativity permits many possible types of universes. In applying the theory to describe the dynamics of our Universe, Einstein made a central empirical assumption to limit the number of possible solutions to the equations. He assumed that on very large scales the distribution of matter in the Universe is constant, making the Universe appear smooth. This idea is a form of the modern cosmological principle.

This principle is not exact since much of the Universe's matter is found clustered together in planets, stars, and galaxies, but when considered at sufficient scales the distribution of galaxies and clusters is approximately even. A useful analogy is a pane of glass which appears smooth to the touch but which is in fact on the microscopic level a highly irregular surface, dotted with peaks and valleys.

The Modern Model of the Universe

Dr. Edwin Hubble

In 1929, astronomer Edwin Hubble made a truly startling discovery. By examining the light emitted from neighboring galaxies and making detailed observations of an electromagnetic property called redshift, Hubble showed that other galaxies appeared to be accelerating away from the Milky Way. Contrary to what Einstein had predicted, the Universe was actually expanding...

The Big Bang, Lance Akiyama

Hubble's discovery, coupled with the modern interpretation of the cosmological principle, led to the development and eventual acceptance of the Big Bang model. Based on the theoretical work of Alexander Freidmann and Georges Lemaitre, the model describes the fiery origins of the Universe as a "primordial atom" and its subsequent evolutionary history of expansion and cooling.

The Big Bang model is the most widely accepted scientific

theory for the origin and evolution of our Universe. Since its inception in the mid-twentieth century, scientists have continued to make important theoretical and experimental discoveries which test and support the model's predictions. The current body of evidence for the Big Bang falls into four broad categories:

• The expansion of the Universe
• The presence of Cosmic Microwave Background Radiation
• The various abundances of elements
• The evolution of stars and galaxies
• [ next ]
• [ close ]

How Hubble Made the Law

Before 1929, it was generally believed that the universe was static, or at least moving apart at a constant speed. Even Einstein was pushing for a steady state model of the universe in his theory of relativity. However, Edwin Hubble made a startling discovery -- the universe is actually accelerating apart. Every galaxy is accelerating away from us, and we're accelerating away from each galaxy.

Hubble also discovered a simple relationship between how far away a galaxy is and how quickly it's moving away from us:

v = H₀×D

We usually measure a galaxies speed v in km/s and its distance D in Megaparsecs (Mpc). H₀ is the expansion rate: Hubble Constant, usually measured in km/s/Mpc. This number does change over time, but it's currently measured around 70 - 75 km/s/Mpc.

Finding Velocity

Not every galaxy is moving away from us; some of our closer, more loving neighbors are being accelerated towards us due to gravity. Overall, though, we see most of the galaxies moving away. This ring galaxy, called Hoag's Object, is about 600 million light years away (183 Mpc), so we expect it to be receding very fast. Just how fast is it getting away from us?

Solution

Don't worry; they get harder than this. H₀=70, and D=183, so:

v = 70×183

and we see that this galaxy is running away at a breakneck 13,000 km/s

Expansion of the Universe

Redshift

The expansion of space redshifts light.

As space expands, light waves get stretched and their wavelengths shift. The more that light is stretched, the longer its wavelengths become, and the color of each wave shifts toward the red end of the light spectrum. We say that this light is redshifted.

A moving fire truck's siren changes pitch as it moves past you. This is known as the Doppler Effect.

To get a better idea of how this actually works, we'll look at a common phenomenon: the Doppler Effect. Imagine you hear a fire truck coming right toward you. As the truck approaches, the pitch of the siren gets higher and higher. As soon as the truck passes you however, the pitch drops lower as the sound fades away.

This common sonic experience, known as the Doppler effect, is analogous to the redshifting of light waves from a moving source. The soundwaves coming toward you were compressed because of the fire truck's velocity. Once it passed, those wavelengths had to stretch out in order to reach you, making the pitch go down. The sound waves shifted to a longer wavelength.

Light: Our Window into the Universe

Light emitted from stars or galaxies can be separated into a band of colors, called a spectrum. Each color has an associated wavelength.

This shifting of wavelengths can be observed on a graph. This figure shows spectra of a star and of galaxies with typical redshifted spectral lines given by the equation:

The Redshift Equation
Z=(λ - λ₀)/λ₀

The Apparent Movement of Galaxies

The expansion of space itself makes it appear as though galaxies are moving apart and causes the spectral lines to shift, changing their colors.

The expansion of space causes galaxies to appear to be moving apart from each other. Although it may seem that the galaxies themselves are moving through space, in reality it is the space between the galaxies that is growing.

Models of Expansion

The Universe as a Loaf of Bread

The bread has no center of expansion but expands at every point.

Another way to think about expansion is to imagine the Universe is a loaf of raisin bread. When baked in the oven, the bread expands, but the raisins do not. The bread represents the space in the Universe, and the raisins represent galaxies and other astronomical objects . While the bread itself undergoes a large change in structure, the raisins themselves stay the same.

How does Hubble's Law explain the receding galaxies?

Imagine space as being represented by a rubber band. If one stretches the rubber band the galaxies will move apart. From the perspective of each galaxy, it will appear as though all of the other galaxies are moving away from it, while it remains stationary. The closer two galaxies are to each other, the more slowly they appear to move apart. This is in accordance with Hubble's Law, which states that the redshift in light coming from distant galaxies is linearly roportional to its distance from EarthThe Univers
The bread has no center of expansion but expands at every point.

Another way to think about expansion is to imagine the Universe is a loaf of raisin bread. When baked in the oven, the bread expands, but the raisins do not. The bread represents the space in the Universe, and the raisins represent galaxies and other astronomical objects . While the bread itself undergoes a large change in structure, the raisins themselves stay the same.

The Universe as Latticework

Cubic Space Division by M.C. Escher

Another helpful way to visualize the Universe is as a huge latticework, like the one in M.C. Escher's sketch.

In this analogy, each cube is a galaxy (or cluster of galaxies), and the connecting rods represent the space between these galaxies. When the Universe expands, imagine that every rod connecting the cubes grows longer at an even pace. Every single cube gets further and further away from every other cube, but the size of the cubes themselves stays the same. The cubes are not moving along the latticework but are being carried by the expansion of the lattice itseThe Big Bang, then, is not an explosion in space, but rather, an explosion of space

While the space in between galaxies increases, the galaxies themselves remain approximately the same size.

We are Not at the Center

Does this mean that we are at the center of the Universe?

If you view the expansion of space from the Earth's perspective, it appears that everything is expanding away from us. However, no matter where you are in the universe, that location will appear to be the center of expansion, too. The universe expands from every point!

Our View of the Universe

All observations that have been made using the most powerful telescopes show that the universe looks the same in all directions.

The average density of galaxies is the same throughout the universe and does not change with distance or direction. This is called the Cosmological Principle.

On average and at large scales, the distribution of galaxies is the same throughout the universe.

Since the expansion of space occurs evenly at every point in the universe, galaxies are separating from each other at about the same pace, giving the universe a nearly uniform density and structure. As a result, the universe appears smooth at large distance scales. In scientific terms, it is said to be homogeneous and isotropic.

In the image on the left, the universe is isotropic. This means that if you stand at the center and look in every direction, the universe will look the same. In the image on the right, the universe is homogeneous. This means that if you stand in any place and look around, the universe will be the same.

Homogeneous and Isotropic: What's the Difference?

·         Homogeneous (usually pronounced homo-GEE-nee-us) literally means "to be the same throughout," no matter where you are in the universe. If you look at the universe from Earth or from a galaxy a million light-years away, it will look the same.

·         Isotropic (pronounced eye-so-TRO-pic) means to appear the same in every direction or viewing angle. This approximation breaks down when viewing the night sky from Earth since our planet is located inside of the Milky Way, but if you were able to stand at any point outside of a galaxy the universe would look the same in all directions.

Scientists are careful to distinguish between these separate concepts: uniform with respect to position (homogeneous), and uniform with respect to viewing angle (isotropic). While most intuitive examples will be both homogeneous and isotropic like our universe, in theory, there can be universes which exhibit one property and not the other.

If we expanded at the same rate as space, we would not perceive any expansion.

Planets, stars, and galaxies are bound together by gravity. On the short distance scales present in these systems the force of gravity is great enough to resist and stop the expansion of space. Gravity opposes the expansion of the universe on larger scales as well, but since the strength of gravity falls with the square of the distance according to the inverse square law, the force is not enough to halt the expansion. If every object expanded with space, including ourselves, we would not perceive any expansion at all.

What is the Universe expanding into?

The expansion of the universe is consistent with the Big Bang theory, but, what is the universe expanding into?

In short, nothing. Most cosmologists believe that the universe is infinite; there is nothing outside of it. Space and time only exist within the universe. So, what happens when infinity expands? It is still infinity, just a bit bigger.

Rewinding the Universe

Think of the universe today as a movie playing. We see expansion: everything moving apart from everything else.

But what happens when we rewind the movie? Everything becomes closer and closer to everything else (and the universe becomes hotter and denser) until we reach the edge of scientific understanding: the Big Bang.

Cosmic Microwave Background Radiation

"Once you eliminate the impossible, whatever is left, no matter how improbable, it must be the truth." -Sir Arthur Conan Doyle, Sherlock Holmes

Accidental Discovery

In 1964, Bell Laboratory scientists Arno Penzias and Robert Wilson were trying to detect sources of radiation that might potentially harm satellites. Their data, however, showed background noise from a microwave signal corresponding to a temperature of approximately 2.7 K that appeared to be emitted from every direction. This apparent aberration was recognized by scientists at Princeton as remnant radiation from the earliest observable moment in the evolution of the universe, now called the Cosmic Microwave Background.

Arno Penzias and Robert Wilson with the Horn Antenna used to discover the Cosmic Microwave Background.

Their discovery was a tremendous one for astrophysics, providing a glimpse of the earliest observable moment in the evolution of the Universe.

What is the CMB?

The Cosmic Microwave Background, or CMB, is ancient radiation leftover from a time roughly 380,000 years after the Big Bang when the hot, dense plasma that permeated the Universe cooled with the expansion of space. At a nearly uniform temperature of 2.7 Kelvin, the CMB fills the entire Universe and can be detected everywhere we look. If the human eye could see microwaves, the entire sky would glow with nearly equal brightness in every direction.

A map of the Cosmic Microwave Background. The different spots of color correspond to different temperatures and in turn, different densities.

Origins of the CMB

The first light radiated after decoupling is now known as the CMB.

During the first 380,000 years after the Big Bang, the universe was so hot that all matter existed as plasma. During this time, photons could not travel undisturbed through the plasma because they interacted constantly with the charged electrons and baryons, in a phenomenon known as Thompson Scattering. As a result, the universe was opaque.

As the universe expanded and cooled, electrons began to bind to nuclei, forming atoms. The introduction of neutral matter allowed light to pass freely without scattering. This separation of light and matter is known as decoupling. The light first radiated from this process is what we now see as the Cosmic Microwave Background. Similarly, in the video below, the precipitate in a solution of magnesium hydroxide scatters light from a flashlight, making it opaque to radiation.

Video4 The Last Scattering (Sıra4)

Bu videoyu oynatmak için Flash Player'i yükleyiniz.

Why is the CMB so Cold?

Light from the CMB is redshifted as the universe expands, cooling it over time.

The CMB is a perfect example of redshift. Originally, CMB photons had much shorter wavelengths with high associated energy, corresponding to a temperature of about 3,000 K (nearly 5,000° F). As the universe expanded, the light was stretched into longer and less energetic wavelengths.

By the time the light reaches us, 14 billion years later, we observe it as low-energy microwaves at a frigid 2.7 K (-450° F). This is why CMB is so cold now.

The expansion of space cools down the CMB.

What do the Colors on the CMB Map Represent?

Although the temperature of the CMB is almost completely uniform at 2.7 K, there are very tiny variations, or anisotropies, in the temperature on the order of 10^-5 K. The anisotropies appear on the map as cooler blue and warmer red patches. But what do these minute fluctuations mean?

Map of the CMB created from data gathered by the Wilkinson Microwave Anisotropy Probe (WMAP).

These anisotropies in the temperature map correspond to areas of varying density fluctuations in the early universe. Eventually, gravity would draw the high-density fluctuations into even denser and more pronounced ones. After billions of years, these little ripples in the early universe evolved, through gravitational attraction, into the planets, stars, galaxies, and clusters of galaxies that we see today.

Why are Maps of the CMB Shaped like Ovals?

The spherical map of the CMB translates to an oval in the same way a globe translates to a familiar oval map when flattened.

The CMB is shaped like an oval for the same reason that many maps of the world are ovals. You can't take a sphere and make it flat without tearing it, because a sphere is fatter in the middle than at the top and bottom.

To see why this is true, peel an orange and try to flatten it. The only way you can accomplish this is by tearing the peel, or distorting it. Instead of "tearing" the map of the CMB, it is depicted as an oval, which is the shape with the least angular distortion of the original sphere.

The Predictive Power of the CMB

In 1992, physicists used the orbiting COBE satellite to make the first detailed measurements of the CMB anisotropy.

The CMB is one of the strongest pieces of evidence for the Big Bang model. The theory makes highly accurate predictions about the size and types of anisotropies in the CMB as well as its nearly perfect blackbody spectrum, all of which have been verified by experiment and observation. The discovery of the CMB in the 1960s marked the end for several competing cosmological models including the Steady State Theory.

With the information attained from the CMB, we can begin to understand the formation of the structure and matter of the universe.

Elemental Abundances

Understanding Our History

An accretion disk forms during the birth of a star.

There are precise physical and chemical processes that govern the evolution of planets, stars, and galaxies. By analyzing the structure and chemical content of astronomical objects, scientists can garner valuable information about what the universe's conditions must have been like long ago in order to account for currently observed elemental ratios.

The Composition of the Earth

We can begin in our own backyard. Geologists have determined that the Earth is composed primarily of heavy elements (those containing many protons and neutrons). The crust and mantle are made up of compounds containing large traces of oxygen, nickel, aluminum, magnesium, iron, silicon, and sulfur. Beneath these layers lies a molten iron core. The atmosphere is made of heavier gases, principally nitrogen and oxygen.

The Composition of the Sun

Scientists use spectroscopy to determine the composition of the Sun.

Unlike the Earth, the Sun is made primarily of light elements. It is a fairly typical main sequence star composed of 74% hydrogen, 25% helium. In the core, nuclear fusion converts hydrogen to helium, releasing large amounts of energy in the form of solar radiation and neutrino showers, while producing small traces of heavier elements in the process.

Using a technique called spectroscopy scientists analyze the absorption spectrum of the Sun in order to determine its chemical structure.

Hydrogen and Helium

Relative elemental abundances in the universe

Using powerful telescopes, scientists have made extensive spectroscopic surveys of distant stars and galaxies. The data indicates that hydrogen and helium make up nearly all of the nuclear matter in the universe. The most abundant element, hydrogen, accounts for 74% of the mass while helium contributes 25%. Heavier elements comprise less than 1% of the total.

The observed 3:1 ratio of hydrogen to helium along with the relative scarcity of heavier elements yield critical clues about the density, temperature, and expansion rate of the early universe. The correlations between these observations and the predictions of the Big Bang model are striking pieces of evidence in support for the theory.

Nucleosynthesis

The Big Bang model predicts that nucleosynthesis, the process by which the elements formed, began approximately 100 seconds after the Big Bang. Driven by the immense temperature and pressure, nuclear fusion reactions converted hydrogen into helium.

As space expanded, temperatures dropped below those required to sustain fusion, and as a result nucleosynthesis only lasted for about three minutes. A third of the atomic hydrogen was converted into helium and no elements heavier than lithium could synthesize.

Formation of Light and Heavy Elements

see how hydrogen and helium nuclei are formed by fusion.

The graph below represents the abundances of the lightest elements during the first three hours after the Big Bang. Notice that at higher temperatures, only free protons and neutrons existed. As the universe cooled, deuterium (₁H²) was formed, and then helium (₂He⁴), resulting in a decrease in the number of free protons and neutrons. Tiny amounts of beryllium and lithium were produced at cooler temperatures. Nucleosynthesis was finished by t=10³ seconds and since then the elemental ratios of the universe have remained constant.

Relative abundance of various elements following the Big Bang.

We just saw how most of the universe's hydrogen and helium (and trace amounts of lithium and beryllium) was synthesized after the Big Bang. But why were these four extremely light nuclei the only ones to form? The answer is that fusion products involving 5 to 8 nucleons are very unstable, as shown in this animation with beryllium-8 (which falls apart almost immediately after it forms).

In the early universe, the right conditions were present only long enough to synthesize two elements: hydrogen and helium. The heavy elements were created later by extended fusion processes inside of stars, and scattered across the universe when the massive stars exploded, a phenomenon known as a supernova.This stellar debris formed the seeds of comets, planets, and even you! The oxygen and carbon in our bodies could only have been formed inside of stars.

Evolution of Stars and Galaxies

Just as ring structure gives us clues about the history of this tree, galaxy structure provides evidence about the evolution of the Universe.

A consequence of the Big Bang theory is the evolution of the universe over time. By examining galaxies with different redshifts, astronomers are able to "look back in time" and obtain data regarding the shape, size, and distribution of galaxies throughout the history of the Universe. The observations made indicate that the trends followed by stars, galaxies and clusters are much different from those we currently see, providing further evidential support for the Big Bang.

Evolution of Galaxies

The shape of galaxies 14 billion years, the Big Bang are irregular and blob-like, while those from 9 billion years and onward are more regular spirals.

Occasionally the gravitational attraction between galaxies can cause two galaxies to collide. While individual stars rarely slam into each other during such collisions, the tidal forces caused by dramatic shifts in the gravitational field will often reshape and deform the galactic structure. Observations show that primordial galaxies were much more likely to undergo such interactions. Some collisions would result in the merging of small irregular galaxies to form larger spiral or elliptical structures. This cosmic dance is an ongoing process by which galaxies continue to grow and evolve. In fact, in roughly 3 billion years, the Milky Way will collide with its neighbor Andromeda.

Evolution of Stars

Roll-over each generation of star to see its chemical composition.

Changes in stellar structure also provide evidence of an evolving Universe. The first heavy elements were produced by nuclear fusion reactions inside the earliest stars. When these stars reached the end of their life spans some of them exploded as supernovae, scattering traces of heavy elements across the Universe. Subsequent stars formed from this debris and as the cycle of birth and death continued the average ratios of heavy elements inside stars rose with each new generation. Scientists have confirmed these predictions by broad spectrographic surveys.

Compiling the Evidence

The Big Bang Theory continues to be verified through experimental and observational evidence.

To date, the hot Big Bang model is the most successful scientific theory explaining the origins and evolution of the Universe. No alternate theory has been able to satisfactorily account for the range of observed natural phenomena that are intrinsic elements of Big Bang cosmology.

Still, there are lingering questions that must be resolved and theoretical assumptions which will be empirically tested for the first time in the coming decades. Cosmology remains an open field, yet given the immense body of evidence in its support, scientists are confident that the Big Bang model will continue to provide the framework upon which new discoveries will build.

Consequences of the Big Bang

Using the wealth of empirical information from redshift surveys, particle accelerator experiments, and detailed studies of galactic evolution and CMB anisotropy, scientists can predict what types of conditions must have been present in the early Universe. Combining these parameters with the theoretical framework provided by General Relativity, cosmologists have generated a powerful model capable of describing both the history and fate of the Universe.

Satellites, such as WMAP, provide scientists with important information about the history and fate of the Universe.

We can estimate the age of the Universe by uncovering the ages of some of the cosmic bodies in the Universe.

• Earth: Using radioactive dating, we have discovered that the approximate age of Earth is 4.2 billion years. So, the Universe must be older than that.
• Stars: We can observe many stars at different ages. We can deduce from this that the oldest stars formed 10 to 12 billion years ago. So, the Universe must be older than that.

Cosmologists estimate the Universe to be 13.7 billion years old. How did they arrive at that?

Accurately Determining the Age of the Universe

Analyzing data from supernovae is one technique scientists use to estimate the age of the Universe.

Cosmologists get a more accurate estimate for the age of the Universe by analyzing the Universe's expansion rate. By studying the history of the expansion rate using redshift data from distant galaxies and supernovae, we can project the expansion of space back to the beginning of time: the Big Bang. Running the expansion model backwards in this way tells us that the Universe is roughly 13.7 billion years old, by our most accurate estimates.

The Visible Universe

The Universe is infinitely big. Even with the best imaginable telescopes, we can only see a small fraction of it. Why? Because it takes time for light to travel. So if the Universe is now 13.7 billion years old, light can only have traveled a distance of 13.7 billion light-years since the beginning of time. Thus, the part of the Universe we can observe (the Visible Universe) lies within a sphere with a radius of 13.7 billion light years centered around the Earth.

We can only observe 13.7 billion light years of the Universe.

The Visible Universe vs. The Whole Universe

The distinction between the "Visible Universe" and the Universe in its entirety is an important one. Current models assume that the Universe is infinite and has always been so since it first came into existence. Spheres or circles labeled "Our Visible Universe" show different stages in the evolution of what we can see in the Universe today (a sphere with a radius of 13.7 billion light years across and the earth at its center).

We are only able to see the parts of the Universe whose light has had time to reach us.

The important idea is that outside any shape labeled "Our Visible Universe", the rest of the Universe exists and stretches on infinitely. Light, or any other information, cannot reach Earth from this region.

Expansion and Our Growing Visible Universe

The parameter that describes the Universe's rate of expansion is called the Hubble Constant. Far from being a constant however, the Hubble Constant has been continuously changing over the course of the Universe's 14 billion year history. We know from both observational and theoretical data that there have been at least two epochs during which the rate of expansion was accelerating or speeding up. The first and most dramatic era was inflation, which occurred briefly during the first second after the Big Bang. The second (believe it or not) is happening right now!

Video5Velocity versus Acceleration(sıra5)

Bu videoyu oynatmak için Flash Player'i yükleyiniz.

As time goes on, expansion reveals more of the Universe, increasing the size of our Visible Sphere.

This is an animation showing how the Big Bang altered the sphere of space we now see as our Visible Universe, in terms of space expanding. In other words, for this demonstration, think of the visible Universe as a "geometrical sphere of space," as opposed to what it really is, which is "what we can see of the Universe at any point in time." The circle in the center of the sphere is where the Earth would be if it had existed at the beginning of the Universe.

The Planck Epoch

In the beginning...

"...we have a viable theory of the universe back to about 10^-30 seconds. At that time, the currently observable universe was smaller than the smallest dot on your TV screen, and less time had passed than it takes for light to cross that dot."
-George F. Smoot, Winner of the 2006 Nobel Prize in Physics

In the time before the first 10^-44 seconds of the Universe, or the Planck Epoch, the laws of physics as we know them break down; the predictions of General Relativity become meaningless as distance scales approach the Planck length at which random quantum mechanical fluctuations dominate. Most particle physics models predict that during this epoch the four fundamental forces were combined into one unified force. Very little else is known about the early part of this era, and the mystery it poses is perhaps the central question in modern physics.

We will come back to the Plank Epoch in the last section when we examine the frontiers of scientific knowledge.

Big Bang Theory predicts that sometime during the first second of Era 1, an unusual energy drove the Universe through a rapid, accelerating expansion. During this inflationary period, the Universe increased in size on the order of 10²⁷.

Era 1: Inflation

Isotropy of the CMB

Inflation was not originally part of the Big Bang Model, but was proposed as an addition to the theory in the early 1980's as a way to solve several problems with earlier versions of the model. One problem inflation successfully explains is the almost perfect isotropy of the CMB. Inflation specifies that a very small region of space could rapidly expand to be very large without changing the scale factor. This would have allowed thermal equilibrium to be established on a scale as large as the surface of last scattering.

Flatness of the Universe

The Universe appears flat as a result of inflation.

Another problem that inflation accounts for is the observed flatness of the Universe. To better understand what this means, imagine the Universe is the surface of a ball. As inflation causes the ball to expand, the curvature of the ball lessens as well. Thus, as the Universe expands, it starts to appear flatter.

Galactic Seeds

see the role of inflation in the formation of galaxies.

Due to inflation, space was almost perfectly homogeneous. However, quantum mechanics predicts that on small scales there must be fluctuations in the energy density of space. These fluctuations were on the order of 1 part in 100,000. As they occurred, the rapid expansion of inflation stretched the fluctuations to astronomical scales.

These fluctuations are the seeds that later formed stars, galaxies, and clusters. Our own galaxy is the result of one such quantum fluctuation.

Video6Structure Formation(sıra6)

Bu videoyu oynatmak için Flash Player'i yükleyiniz.

What Happened Before 10^-44 Seconds After the Big Bang?

We have no idea. Era 1 has provided cosmologists with several as-of-yet unanswered questions:

• What were the initial conditions for the Big Bang?
• How did the Big Bang start?
• What physical laws applied before the Big Bang?
• What is time?

Unfortunately, inflation appears to wipe out the clues that might help answer these questions. Inflation spreads out any initial conditions so that they are so diluted that the chance of finding anything from before inflation would be like finding a needle in a hay stack.

Era 2: From Quarks to Nucleons to Nuclei

As the Universe expands, it also cools. With this cooling, the intense concentration of energy from the beginning of the Universe spreads out and cools. Although energy can take the form of matter, and matter can convert back into another form of energy, over a cosmically short period of time, the Universe had cooled enough that a significant amount of energy stayed in the form of matter.

E = mc²

Matter is just one of the many forms of energy. The equivalency of these two seemingly unrelated things is reflected quantitatively in Einstein's famous equation E = mc². In other words, it takes a lot of energy to create a little bit of matter. Conversely, it only takes a little matter to get a lot of energy. A one gram paperclip could be converted to enough energy to run a 100 W light bulb for 28479 years!

E stands for "energy," m for "matter," and c for the speed of light, which is
approximately 3 x 10⁸ m/sec.

Fundamental Particles

All matter in the universe is comprised of fundamental particles.

So what exactly makes up this matter? All matter is made of fundamental particles that came into being at the birth of the Universe. Quarks experience the strong force which is carried by massless particles called gluons. They bond together in specific combinations to form protons, neutrons, and other hadrons. Leptons do not experience the strong force but may interact via the electromagnetic force, the weak force, or both. Anti-quarks and anti-leptons are exactly the same as their quark and lepton counterparts, but have an opposite charge. All massive particles are influenced by the force of gravity.

Quark-Gluon Plasma: 10^-12 Seconds After the Big Bang

A Quark-Gluon Plasma

At around 10^-12 seconds after the Big Bang, the Universe is a hot mess of "plasma soup" with essentially equal parts matter and antimatter.

see how matter is turned into energy.

When a particle and its antiparticle collide, their mass is converted into pure energy during an explosive annihilation. This process can also be reversed to produce a particle-antiparticle pair from pure energy.

It takes more energy to produce a particle-antiparticle pair than it does to annihilate one. When the Universe was young, there was energy everywhere, as evidenced by the higher temperature. As the Universe expanded and cooled to its present state, the amount of free energy dropped below the threshold necessary to produce pairs.



More energy is required to produce a particle-antiparticle pair now than in the past due to a decrease in available free-energy. Now, more particle-antiparticle pairs are being annihilated than created.

Quarks Come Together: 10^-4 Seconds After the Big Bang

Quarks combine to form neutrons and protons.

Due to circumstances still unknown, the amount of matter and the amount of antimatter in the Universe was no longer equal. The excess matter in quarks came together in a specific combination of three quarks to make either a proton or a neutron. The quarks that did not combine were annihilated. Thus, by 10² seconds (a little over one and a half minutes after the Big Bang), all protons and neutrons had formed.

Atomic Nuclei Form: 10² Seconds After the Big Bang

see how protons and neutrons come together to form Atomic nuclei

By 10² seconds after the Big Bang, all protons and neutrons had formed, and these nucleons began to fuse together to form atomic nuclei in a process known as nucleosynthesis.

The Surface of Last Scattering

.

The formation of atoms at the division between Era 2 and Era 3 corresponds directly with the decoupling of light and matter. Once atoms formed, light and matter stopped constantly interacting with one another, and photons were suddenly able to travel freely. As a result, the Universe became transparent. Scientists refer to this epoch as the "Surface of Last Scattering;" light from this period is observed today as the CMB!

Era 3: From Atoms to Stars to Galaxies

Atoms Begin to Form: 3 x 10⁵ Years after the Big Bang

"Expand the Universe" to see how atoms formed in the early Universe!

After many thousands of years, expansion has cooled the Universe significantly. As a result, nuclei and electrons were able to come together to form atoms.

Celestial Bodies Form: 3 x 10⁸ Years after the Big Bang

The first stars and galaxies were the result of gravitational attraction.

By 3 x 10⁸ years after the Big Bang, gravitational attraction between denser regions of visible matter and halos of dark matter cause atoms to come together to form the first stars and galaxies.

The Formation of Galaxies

An artist's conception of a black hole and forming galaxy.

The study of galactic formation is an open area of research in modern astrophysics. Scientists suspect that as luminous matter is drawn inwards towards a halo of dark matter, a massive star-like object takes shape. Because of its huge mass this object soon collapses into a black hole. As more matter is accumulated, it begins to rotate around the halo, forming a flat disk which spins faster and faster as the mass increases. This gaseous disk begins to resemble an irregular galaxy as hydrogen gas and dust collapse to form stars. Over billions of years the newborn galaxy will continue to grow and evolve as it interacts gravitationally with other primordial galaxies.

Dark Matter and the Formation of Galaxies: 3 x 10⁸ Years after the Big Bang

Although its makeup remains a mystery, scientists have strong evidence indicating that most of the matter in the Universe is dark. Unlike luminous matter, dark matter does not emit any electromagnetic radiation and thus cannot be seen directly.

It is predicted that dark matter has an especially important role in the formation of galaxial structure. In the early Universe, the concentration of radiation was much higher and the gravitational attraction between pockets of luminous matter was opposed by the force of radiation pressure. Since dark matter does not absorb or emit photons, it was not subject to the opposing force of radiation pressure and was able to collect long before luminous matter could. Without dark matter, the formation of galaxies would have occurred billions of years later than it actually did.

Era 4: Today's Accelerating Universe

The Universe Today

Dark matter has aided in forming the universe we see today; however, many questions regarding the cosmos remain.

• What is the status of the Universe today?
• We know the Universe is expanding...
• But what do we know about the expansion?

Surveys of supernova provide scientists with information about the history of the Universe

In 1997 advances in telescope technology allowed astronomers to conduct redshift surveys of very distant type Ia supernovae. This enabled them to look further back into the Universe's history than previously possible. Their stunning results rivaled Hubble's original discovery and turned cosmology on its head. While most theoretical models predicted that the expansion of the Universe would continue to slow, in fact today the expansion rate is faster then it was a few billion years ago. The expansion of the Universe is accelerating.

Expansion: Chunk-by-Chunk

A very small portion of the Universe.

In order to better understand the significance of expansion, let's look at a cubic sample of space. By considering a finite volume we can follow changes in the size of the Universe as we move forwards and backwards in time. Remember, only the size of the cube will change. The galaxies inside the cube stay the same size.

If the Universe followed the simplest expansionary models, its size would increase linearly with time. The Universe would continue to expand at a constant rate forever. If you look at only a narrow time-slice of the Universe's history, it does, in fact, appear that this is how the Universe expands.

If we look at the Universe over a much longer timespan, however, we can see that the rate of expansion was not always constant. During Era 1, the Universe underwent a brief period during which the expansion rate rapidly accelerated. During Eras 2 and 3, it decelerated as the force of gravity resisted the expansion.

More Information" button to learn more about the possible fates of the Universe.

Different models predict different fates of the Universe. In some, gravity continues to slow the expansion rate. Although the Universe would expand forever, as the expansion rate critically approaches zero, the Universe would increasingly resemble Einstein's Static Universe. In other models, the force of gravity is strong enough to overcome the expansion. After reaching its maximum size the Universe would contract and end in a Big Crunch. Still, other models predict that the expansion rate will accelerate and the Universe will continue to grow in size forever.

This graph shows the history of the Universe's expansion rate. The data points represent redshift measurements from distant supernovae and fall along a set of possible curves in the blue region of the graph. All of these curves are consistent with histories detailing a post-inflationary period of decelerating expansion followed by the current era of acceleration. The curves in the gold region that are ruled out by the data are consistent with histories in which the expansion rate has been constantly decelerating since inflation.

The Dark Side of Expansion

There is no explanation from classical physics to explain the current acceleration of the expansion rate. The acceleration indicates that gravity (which slows down the expansion) is counterbalanced by a mysterious repelling force. Physicists call this mysterious force dark energy. It takes the form of an outward pressure constant and seems to be pushing the Universe to expand faster and faster in every direction.

Dark Matter

"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
-Sir Isaac Newton, English mathematician and physicist (1642-1727)

What is Dark Matter?

The Composition of the Universe

We "see" visible matter because it is able to emit and reflect visible light. However, imagine matter that did not interact with light at all and was therefore, quite literally, invisible. This is what scientists today believe to be Dark Matter (DM). In fact, DM is most probably non-baryonic, meaning it does not interact with any electromagnetic radiation at all.
Even though we cannot visually observe it, scientists are convinced that 70-90% of matter in The Universe is non-baryonic DM and that ordinary luminous matter constitutes only a small fraction of The Universe's mass density.

Evidence of Dark Matter

Finding physical evidence of DM is difficult because it cannot be directly detected with any optical equipment. Even though it does not interact with light, DM still interacts gravitationally, and this quality helps verify its presence. If DM does exist, there must be more gravity present in The Universe than visible matter alone can produce.

Evidence of this "extra" gravity includes:

• The strange rotational velocities of stars in galactic disks can only be explained if much more mass is present in each galaxy.
• The relative bulk motions of galaxies combined with restrictions from nucleosynthesis show that the majority of the Universe's matter must be non-baryonic dark matter.
• The degree to which light is bent by galaxies and clusters indicates that 90% of the Matter in the Universe is missing.
• The density of the Universe is just right to make its geometry flat. Moreover, no viable theory of structure formation exists that does not contain a minimum amount of dark matter.

The Case for Dark Matter

Bizarre Rotational Velocities

What if our solar system behaved like a galaxy?

All of the planets would have the same tangential velocity. That is, they would cover the same distance in the same amount of time. However, the bodies in the outer orbits (such as Neptune) would have slower angular velocities; it would still take them longer to make a complete revolution around the sun.

In reality, our solar system does not behave like this. The outer planets have a much slower tangential speed than the inner ones.

The velocity of a planet is inversely proportional to its distance from the sun.

As discussed in the Fundamentals: Gravity section we know that the gravitational force between two objects has an inverse square relationship with the distance between them. As such, Mercury experiences a greater gravitational force from the Sun than any other planet. Therefore, it's reasonable to expect Mercury to have the fastest tangential velocity in its orbit. This idea does not, however, extend to calculations of velocity of objects in clusters and galaxies; the predictions simply do not fit the observations.

The tangential velocity of a star is generally independent of its radial distance from Galactic Center.

When astronomers observe the rotational velocity of galaxies and clusters, they collect data which show measurements of objects at various distances from the obital center. Incidentally, in galaxies, stars' radial distances do not effect their tangential velocity around the Galactic Center. The Graph above shows a plot of this observation.

Since the prediction (stars further out should have lower tangential velocities) is inconsistent with observations (velocity is largely independent of distance), astronomers have proposed a theoretical matter, so-called "Dark Matter," to account for the results.The theory is that the dark matter is uniformly distributed, like a halo around the perimeter of the galaxy. The gravitational force resulting from the presence of this DM is what disrupts the inverse square relationship between stars' tangential velocity and their distances from the galactic center.

The Case for Dark Matter

Bulk Motions of Galaxies

The Structure of a typical Spiral Galaxy.

Galaxies are most often found in clusters and are thus subject to gravitational forces from their neighbors. The relative motion that results from these interactions causes deviations from the cosmological principle called bulk flow. Measuring the peculiar velocities enables cosmologists to calculate the masses of interacting galaxies. Using this technique scientists can estimate the total density of matter in the Universe.

During the recombination epoch, the Universe had cooled sufficiently for nucleosynthesis to occur. However, this process produces only a few elements, and looking at the abundance of these elements today is indicative of the rate of nucleosynthesis and the amount of baryonic mass that exists. Now, when scientists today look at the bulk motions of galaxies, they find that their velocities are far too great to be generated solely by the baryonic matter inferred from nucleosynthesis. There must therefore be large amounts of non-baryonic DM in these galaxies, generating gravitational forces large enough to explain the velocities observed. Furthermore, it is found that during recombination, baryonic matter was not able to provide the density needed to form stars and galaxies because it was ripped apart by photons. Thus DM must have provided the required density, which resulted in the formation of stars and galaxies.

Gravitational Lensing

Light rays from the two stars are bent by the Sun's gravity. As a result, the stars appear to be farther apart than they actually are.

In his theory of General Relativity, Albert Einstein predicted that the path of light can be bent by the presence of a gravitational field in an effect known as gravitational lensing. This phenomena was verified by Arthur Eddington during the solar eclipse of 1919 and marked important experimental confirmation of Einstein's theory.

The light from a distant galaxy is bent by the intermediary field created by a nearby cluster.

Any large galaxy or galaxy cluster can act as a gravitational lens; the light emitted from objects behind the lens will have a characteristic angular distortion and, because of the lens's poor optical quality, pronounced spherical aberration. By measuring the degree of gravitational lensing, cosmologists can calculate the mass of the intervening body as well as its approximate density distribution. This method is a particularly powerful one for studying the effects of dark matter.

The Case for Dark Matter

The Geometry of the Universe and Structure Formation

CMB data collected by the balloon-based BOOMERanG and MAXIMA experiments provided crucial evidence in support for the existence of both dark matter and dark energy.

Detailed measurements of the CMB power spectrum reveal that the Universe has a nearly perfectly flat geometry. Estimates of the total luminous matter density are far too low to account for this geometry, one more reason that scientists believe there must be dark matter

Another piece of evidence that we have already seen is the crucial role dark matter plays in theories about structure formation. Currently, no alternative theories exist which can explain the timing of galactic formation without a minimum amount of dark matter.

The Unusual Suspects: Dark Matter's True Identity

What do scientists think the true identity of dark matter is?

Currently, there is a consensus among cosmologists that DM exists and is physically composed of some yet-to-be-discovered non-baryonic fundamental particle. It cannot be said with absolute certainty that DM is non-baryonic because, due to ionization and the Universe's redshift, we are not yet able to send out electromagnetic waves far enough into space to observe their interaction with DM. As of today, the proposed composition of dark matter falls under two categories of fundamental particles:

• Hot Dark Matter (HDM) candidates include low mass fundamental particles which can travel at relativistic velocities (near the speed of light). For years one such particle, the light neutrino, was a leading suspect. However, current theories about structure formation cast doubt on most HDM candidates.
• Cold Dark Matter (CDM) candidates include heavier fundamental particles as well as large compact objects such as brown dwarf stars and primordial black holes. WIMPS (Weakly Interacting Massive Particles) are high mass particles which only interact via gravity and the weak force. Extensions to the Standard Model incorporating supersymmetry predict the existence of WIMPs, requiring that each known fundamental particle have a heavier supersymmetric partner. If they exist, these partner particles are considered to be leading CDM candidates.

The Search for Dark Matter

Particle accelerators, such as the newly constructed LHC (Large Hadron Collider) at CERN in Geneva, Switzerland, use powerful magnets to accelerate particles to velocities near that of light and collide them into target beams. Physicists analyze the spray of particles created by the collisions which may contain clues about the properties of elusive dark matter particles.

Today the search for dark matter is carried out in labs, observatories, and particle accelerators around the world. Scientists hope that the next generation of experiments will finally uncover the identity of dark matter.

Alternatives to Dark Matter

Some cosmologists are looking for alternative theories that explain these phenomena without relying on unobservable dark matter. Most of these theories involve substantial theoretical modifications to General Relativity and Newtonian gravitation. However, none of these explanations have been experimentally verified yet, and dark matter remains the leading candidate theory in most physics circles.

Dark Energy

An artist's conception of Dark Energy.

Imagine you toss an apple straight up into the air. Due to gravity, one would expect the apple to come right back down to earth. But what if it doesn't? What if, due to some unseen force your apple continues going up, at an accelerated rate, no matter how much gravity pleads and begs for the apple to come back down. Could this really happen? Could there really be "anti-gravity?" On the scale of the Universe, there is; say "hello" to Dark Energy.

In the most basic sense, Dark Energy is akin to negative gravity. Where gravity is attractive, Dark Energy is repulsive. Dark Energy causes the Universe to expand at an increasing rate. For example, to a viewer on earth, gravity would attract a distant galaxy towards Earth, but Dark Energy would cause the galaxy to move away from the Earth. Similarly, neither force can be directly seen. We detect gravity by observing the effect between two masses. We detect Dark Energy by measuring the expansion of the Universe through the comparison of standard candles.

Where is Dark Energy?

Dark Energy is everywhere.

If Dark Energy is causing the Universe to expand, where is it?

Dark energy is everywhere. Dark energy is thought to be an inherent property of space itself. However we don't notice dark energy mostly because it is an incredibly small amount of energy per volume. The effect is only seen acting on the universe as a whole, much like how we can feel a gust of wind, but cannot feel the individual particles in air.

String Theory

What was the Universe like before 10^-44 seconds? This question is one of the most important and vexing problems facing modern physics. Big Bang cosmology coupled with the Standard Model of particle physics provides an excellent description of the Universe regarding that question. But in order to go on will most likely require a radical theoretical shift.

The Quest for Grand Unification

During the 1960s and 1970s physicists developed a theory to describe the interactions between particles in the standard model. The theory proposes that there is symmetry underlying the strong, weak, and electromagnetic forces. At sufficiently high energies, like those present in the early Universe, these three forces have the same strength and behave as a single unified force. At lower energies this symmetry is spontaneously broken and the three forces begin to take on their characteristic properties.

For years, scientists hoped that a similar path would combine all four fundamental forces (strong, weak, electromagnetic, and gravitational), revealing a single "Theory of Everything" to govern the natural world. However, deep inconsistencies between General Relativity and Quantum Mechanics stood in the way, and it became increasingly clear that a totally new theory was needed.

What is String Theory?

Vibrational modes of strings.

String theory, one of the most promising candidates, describes a physical model of the Universe in which the fundamental building blocks are not point particles but rather one-dimensional "strings". According to this model, every fundamental particle exists as a different vibration mode of the string where each vibration has different associated properties.

String theory arose, in part, as an attempt to bypass problems caused by the traditional model of point-like, zero-dimensional particles. Physicists had observed that the angular momentum of elementary particles was exactly proportional to the square of their energies. These results could not be explained using simple models that pictured these elementary particles as a set of smaller particles glued together, but could be explained under string theory.

One of the most intriguing implications of string theory involves the existence of multiple dimensions beyond the three spatial dimensions (length, width, height) and the fourth temporal dimension (time).

Applying the principles of string theory, physicists can expect a certain number of dimensions depending on the theory they choose. Superstring theory predicts 10 dimensions, while bosonic string theory predicts as many as 26 extra dimensions.

There are two competing models that try to explain the extra dimensions:

• Compacted Dimensions
• Braneworld Theory

Compacted Dimensions

From a distance, a cylinder looks like a line, while a torus appears to be a point.

The compacted dimension theory suggests that all the dimensions are present in our everyday world. However, these dimensions are so small that they are undetectable in our everyday experience. For example, imagine looking at a garden hose from a far away distance. From a distance, it seems to be a one-dimensional line, but upon closer inspection, we see it has thickness and a circumference.

In this way, our daily macroscopic experience of physical phenomena tells us there are four dimensions (three spatial and one temporal), but theorists propose that inspection at the subatomic level would reveal the presence of extra dimensions.

Braneworld Theory

The Braneworld Theory suggests we are confined to 4 dimensions and cannot experience the other dimensions postulated by string theory.

The braneworld theory suggests that extra dimensions exist all around us, but we are confined to a 3+1 dimensional subspace, and cannot experience these other dimensions. This is analogous to living on a sheet of paper as a two-dimensional figure. You would have no concept of depth - it is simply not a part of your physical world. This is the concept behind braneworld theory, which says that our four dimensional spacetime is like the sheet of paper, simply a subspace of some bigger, multi-dimensional space that we cannot perceive because all matter and forces (except possibly gravity) we experience are constrained to this subspace, or brane (as in, membrane).

Current Research

When launched, the Supernova Acceleration Probe (SNAP) will study the effects of dark energy by surveying distant type Ia supernovae and making detailed measurements of weak gravitational lensing.

With the new Large Hadron Collider (LHC) at CERN nearing full completion, experimentalists will soon be able to test certain elements of String Theory. While not definitive, these tests will cast some light upon the theory's parameters and may even provide clues into the identity of dark matter. Meanwhile, theorists continue to investigate the implications of String Theory for Big Bang cosmology, particularly the effects of strings on cosmic inflation.

As particle physicists eagerly await the results from the new LHC, observational cosmologists are busy developing astronomical experiments, such as the Supernova Acceleration Probe, to explore the fundamental questions posed by the existence of dark matter and dark energy.

Famous Observatories

More to Explore: Unanswered Questions

So that's the end? We've covered everything there is to know about the Universe? Wrong! There's still a lot we don't know or can't explain about our Universe:

• Are there undiscovered laws of nature?
• Is there a Grand Unified Theory (GUT)?
• What is Dark Energy?
• What is Dark Matter?
• What happened before 10^-44 seconds?
• Why are there so many different kinds of particles?
• Why did antimatter vanish?

And in the end, who knows, maybe you will be the one to solve the mysteries of the Universe!

Congratulations!

You have completed the Universe Adventure. You may be wondering, "Now that I've put so much work into learning the content of this website, what will I get out of it?"

You could start by rewarding yourself here (and while you're visiting, why not get to know your fundamental particles?). Don't forget to amuse yourself or to visit the universe every day.

In case you didn't know already, our website team is led by the Nobel Laureate, George Smoot, who won the Prize in 2006 for cracking the cosmos. We also make these awesome posters.

Of course, there is still lots of the Universe for you to explore, either with the Sidequests on this very site, or by going outside to experience the Universe first hand. There are also plenty of other great sites.

This Web site is the propiety of The Berkeley Center for Cosmological Physics (BCCP)

The Berkeley Center for Cosmological Physics is an integrated research and education enterprise directed by Nobel Laureate George Smoot. BCCP will develop research, education, and outreach to create a vision and direction for the 21st century study of cosmology for more  please clikc here BCCP)

Yukarıda SİZE  sınduğumuz web sitesi  Berkeley Universitesi Kozmoloji merkezi ve fizik bölümü tarafından  Nobel fizik ödülü sahibi  George Smoot idaresinde yaplımıştır.

Sizlere evrenin ilk oluşumundan bugüne kadar ceğirdiği evreleri  80 adet resimle ve bilimsel bilgilerle izah edilmektedir.