The History of the Universe -
Summary of Cosmology and Dark Energy

To begin with, what is this "cosmology"?

The science of cosmology is the study of the origins of the universe.  It is an attempt to use observations of the present day universe to infer the mechanisms by which the universe has evolved from the beginning of time.  This page is an attempt to describe the observations used in cosmology, and to explain the major concepts of present day cosmology.  We hope to lead the reader to a basic understanding of the global properties of the universe that have been seen, and the main ideas and leading theories of today's cosmology.

The basic facts: a laundry list

We'll start by describing the basic observations.  These are well established facts about today's universe that are most important to the description of the early universe. First we'll rattle them off in a big laundry list, then describe them in a bit more detail.

  • The matter in the universe is made up primarily of the light elements hydrogen and helium in a ratio of about 3 to 1.
  • The amount of visible matter in the universe is a tiny fraction of the total matter content of the universe.  The larger, unseen, fraction of matter in the universe is called dark matter.
  • The universe is permeated with microwave photons, much like the ones in a microwave oven, called the cosmic microwave background (CMB).  These photons look just like they would if they were emitted by a room whose temperature was about 500 degrees below zero Fahrenheit.
  • The size of the universe is expanding.
  • On the largest distance scales galaxies tend to form large structures, rather than being spread out homogeneously throughout space.
  • The CMB has tiny inhomogeneities, which means that it looks slightly different across different parts of the sky.  Although it's barely noticeable, this "splotchiness" is incredibly important to the study of cosmology.
  • The rate of expansion of the universe is increasing!
Now a bit more detail on each item in the list:

What's the Matter in the Universe?

Light elemental abundances:

Measurements of the relative abundances of the light elements can be done directly, by trying to observe the particles themselves in the universe, or indirectly through astronomical observations of light associated with certain types of matter.  It has been found that the visible matter in the universe is mostly made of the light elements hydrogen and helium, with smaller amounts of heavier elements.

Invisible Forces: Dark Matter

Many measurements indicate that a large fraction of the mass of the universe is unseen.  The first time this was seen came from studies of the Coma and Virgo Clusters of galaxies.  Again, the simple study of the velocities of celestial objects proved to be very revealing.  In the 1930's, astronomers found that the individual galaxies within the clusters were moving much faster than anticipated.  Since the motion of the galaxies stems from gravitational forces, this observation indicates that there is more mass inside the cluster than can be seen.  The gravitational forces of this greater amount of matter lead to higher velocities.

Cosmic Microwave Background (CMB)

In the early 1960s, two radio astronomers named Penzias and Wilson discovered, quite by accident, that there is a strong background of microwave radiation coming at the earth from all directions.  The CMB had been anticipated by the leading cosmologists of the day, and in fact a group at Princeton led by Tom Dicke was building an experiment to search for this when it was discovered by Penzias and Wilson.  Many, more precise, measurements followed the discovery, and the CMB is now known to exactly resemble the thermal radiation emission of a body whose temperature is about 500 degrees below zero Fahrenheit.  The fine details of the CMB are extremely important to cosmology, and we'll see this topic again.  But first, an aside about the nature of light.

Light is one part of a larger physical phenomenon broadly called electromagnetic radiation.  One can think of light as collections of electric and magnetic fields that act in concert with each other in specific ways.  The fields behave as waves, rising and falling with each other; they are coupled together, which means that as the electric wave rises the magnetic wave falls and vice-versa.  The waves are self-propagating, which means that they do not require a different medium to allow them to travel, like sound does; they can travel through empty space.  The waves are characterized by a wavelength, which can be thought of as the distance between successive peaks in the waves.  In visible light, this is what is called color: the color of light is determined by its wavelength.   All light travels through empty space at the same velocity, which is given the name "c."  Quantum mechanics teaches us that light is also quantized, which means that these waves come in discrete bunches, called photons, that cannot be broken down into smaller bunches.

What's the Matter with the Universe? CMB: Smoothness and Splotchiness

The COBE satellite mission in the late 1980s and early 1990s measured the blackbody spectrum very precisely.  In addition to making a very precise measurement of the blackbody temperature of the CMB, the COBE team found that flux of CMB photons has very slight inhomogeneities.  This means that some patches of the sky emit slightly more or slightly fewer photons than the average.  The magnitude of these fluctuations is very slight (1 part in 100,000), and they are seen on many different scales.   This means that you can look at patches of any size you like and see the same sorts of behavior.  This means that if you divide the sky up into 10 pieces and compare the amount of CMB in each patch and then you divide the sky into 250 pieces and compare them, both sets of sky-pieces will exhibit the same types of fluctuations.  Studies of the CMB have become incredibly precise, prompting many scientists to call this the onset of the age of precision cosmology. Finer and finer measurements were made by experiments such as DASI and Boomerang, and have culminated with the WMAP satellite mission.

Figure 1. Comparison of CMB anisotropy sky maps from the COBE and WMAP satellite missions. COBE showed that the CMB does have real anisotropies, but the finer resolution of WMAP ushered in the era of precision observational cosmology. Taken from the WMAP website.

Large Scale Structure

The locations of galaxies as seen from our viewpoint show great clusters of galaxies and great voids.  The clusters are regions with many galaxies clumped together very closely, and the voids are regions with very few galaxies at all. The Sloan Digital Sky Survey is doing a comprehensive survey of the large scale structure---looking closely at the distribution of galaxies in space to understand the clumps and voids.

What's the Universe Doing?

Figure 2. The original Hubble diagram. The relative velocity of galaxies (in km/sec) is plotted against distance to that galaxy (in parsecs; a parsec is 3.26 light years). The slope of the line drawn through the points gives the rate of expansion of the universe. (In the original publication that the plot is taken from, it was labeled "Figure 1".)

The Universe is Growing

By studying the relative velocities of other galaxies, astronomers have found that other galaxies are moving away from ours, and that the relative velocity is greater for the more distant galaxies.  The rate of expansion is given by the slope of a curve plotting velocity of a galaxy against distance from us.

This was first discovered by Edwin Hubble in 1929.  Although the data and measurements in Hubble's time had very large uncertainties, he correctly saw the relationship between velocity and distance.  His original value of the expansion rate has been improved since then, but the parameter that describes the rate is nevertheless named in his honor.  It is called the Hubble constant.

The expansion of the universe gives rise to a new way to express distances between astrophysical objects called redshift.  To talk about the redshift, we first have to discuss the Doppler effect.  This is a property of waves, so it happens for both light and sound.  To understand it, consider a train that approaches and passes you.  The sound of the train's whistle will have a higher pitch as it moves toward you, and a lower pitch as it moves away from you.  In the same way a light bulb that is moving with respect to you is affected.  The light has a characteristic color when it is emitted, but that color is modified if the source is moving relative to you, the observer.  If the light bulb moves away from you, the color becomes more red, and if it moves toward you the color becomes more blue.  Since distant stars and galaxies are moving away from us, their light is modified by the Doppler effect, and this is called redshift.  Since more distant objects are receding away at a faster rate, the redshift is greater for these more distant objects.  Thus a measurement of the change in the color of emitted light, the redshift, can be used as a measure of distance.

Accelerating Universe

Two independent groups, The Supernova Cosmology Project and The High Z Supernova Search Team, began studying the Hubble diagram using supernovae in the early 1990s.  (We describe what a supernova is below.)  The idea is to simultaneously measure the redshift and the brightness of supernovae.  Both of these observations should give you a good measurement of the distance of each supernova.  The redshift shows how much the spectrum is modified by the motion of the supernova due to the expansion of the universe.  The brightness shows the actual distance traveled by the light form the supernova.  If the two measurements agree, then the expansion of the universe is flat, meaning that the universe has always expanded at the same rate.  However, the observations indicate that the expansion is accelerating!  This is because the brightness of the supernovae do not have a linear relationship with redshift; the brightness seems to fall off as redshift increases.  If we assume that the brightness measurement is the correct measure of distance, then this result means that the expansion rate is increasing with increasing distance.

A supernova signals the end of a star's life as a star.  There are several types of supernovae.  The cosmological projects use Type Ia supernovae, which are described here.  A star is a burning ball of hydrogen that is held together by a delicate balance of competing forces: gravity and nuclear fusion.  Gravity tries to compress the hydrogen down into as small a space as possible, and this compression increases the temperature, which makes the nuclear reactions fueling the star happen faster.  The increased nuclear reactions push the matter in the star out against gravity's pull, and a star burns like this for many millenia.  As time progresses, the hydrogen in the star is converted into heavier elements that do not burn as easily, and the temperature begins to decrease.  As this happens, gravity begins to win the battle and it shrinks the star.  If the mass of the star is not too large, the Eventually, if all the hydrogen and other light elements are used up, the star can fizzle and collapse on itself.  However, when it collapses, the gravitational energy gained will heat up the star again rapidly and begin to burn the heavy elements (such as carbon and oxygen) so fast that the star will explode.  The explosion is called a supernova.

Explaining the Observations: A Short List

So, we have now seen a few of the major observations used.  Below are short descriptions of the cosmological theories that have been developed to describe the observations. We'll describe the big bang model and inflation, and then discuss briefly the ideas of the cosmological constant and dark energy.

The Big Bang

The hot big bang model stipulates that the universe began about 13.7 billion years ago as a tiny point of infinite density and zero size.  This spot exploded in a tiny, hot, dense explosion of all matter in the universe.  It was tiny in the sense that all matter, and even space itself, was condensed into a single point.  It was very hot and dense, and as the universe exploded, all the particles of matter and anti-matter rushed outward, away from each other.  Initially, the universe was comprised of the fundamental constituents of matter and lots of radiation rushing around and interacting with each other freely.  It was too hot for them to combine into stable systems, such as atoms or even protons, but too dense for them not to interact at all.  It was so hot and dense, that even the types of interactions that existed at that time were different than they are today.  As time progressed, the universe expanded and cooled.  As this happened, different types of interactions became dominant in the universe, defining the different cosmological epochs.

Within a ridiculously small fraction of a second, the universe expanded enough, and thereby cooled sufficiently, for the fundamental interactions to evolve into the ones we know about today.  Very soon after that, but still only a fraction of a second old, the fundamental particles in the universe were cool enough to condense into protons and neutrons.  After about a 100 seconds, the protons and neutrons were cool enough to begin to form nuclei.  The energy density of the universe at this point was still dominated by radiation, however.  This intense and hot radiation continuously bombarded the protons, neutrons and electrons in the universe.  After about 50,000 years, the universe was cool enough that the matter overcame the radiation domination in the total energy density.  At this point, called the epoch of matter domination, gravitational effects began to become important.  After about 400,000 years, the universe was sufficiently cool and the density sufficiently low for electrons and protons to form hydrogen atoms.  This epoch is called recombination.

At that point, the radiation stopped interacting constantly with the matter in the universe.  This is called decoupling.  The temperature of the universe at that point is thought to have been about 5000 F.  Think of the radiation as having been emitted from every point (in space) in the universe, with an intensity proportional to the matter density at that point in space.  It is expected that this radiation should be observed today, but red-shifted by an amount due to the expansion of the universe since the time the radiation was released.  This radiation from decoupling is the CMB.

The hot big bang model explains very well many of the above observations.  The expansion of the universe is a natural consequence of the big bang.  The CMB was a huge triumph for the big bang model when it was discovered.  A blackbody spectrum of temperature ~5000 F red-shifted by the expansion of a 13 billion year old universe should give a blackbody spectrum with temperature -500 F, which is just what is observed.  The big bang model successfully predicts the relative abundance of the light elements: 75% hydrogen, 25% helium, and traces of lithium and beryllium.

But the big bang model alone cannot explain all of the observations.  In particular, it cannot account for the inhomogeneities seen in the CMB, or the large scale structure of galaxies.  Also, the big bang does not predict the fate of the universe.  Will it expand like this forever?  Will it collapse back on itself?  For this, an additional theory is needed, which brings us to inflation.

Inflation: it's not just for economists anymore

Inflationary models of cosmology stipulate that the earliest epoch of the universe was a period of rapid expansion called inflation.  Thus, immediately after the hot big bang that we know from the previous section, the universe grew very rapidly.  Space itself grew at an astonishing rate during the inflationary period.  After that, the universe proceeded in roughly the same way as was described in in the Big Bang section.  The rapid expansion, and then usual big bang evolution, led to the CMB anisotropies, and to large scale structure.  Since it's not obvious, we'll spend some more time describing how this happens.

Figure 3. A schematic diagram showing the evolution of the universe. (Taken from the WMAP web site.)

Imagine the universe during the first tiny fraction of the first second, before the inflationary period.  Random chance, arising from quantum uncertainties, allows for some areas of space to have slightly more matter and energy than other areas of space.  This isn't much different from the observation that the density of air has slight variations across a room.  But then if space itself grows quickly, those slight density fluctuations will be turned into big density fluctuations.  To illustrate this, suppose we have a quilt with patches one inch across of different colors.  Now suppose the quilt grows a million times in size.  The color patches now are 15 miles across instead of one inch.  In the same way the areas of density fluctuations grow rapidly in size.  At the end of the inflation period, they are much bigger than they were originally, and also they are much bigger than they could ever have gotten through the slow expansion predicted by the big bang model.

The density fluctuations continued to exist on these larger scales throughout the evolution of the universe, and so after the epoch of matter domination these density fluctuations acted as the seeds of large scale structure.  This happened because the areas with greater density had stronger gravitational forces, and the particles in these regions attracted each other more rapidly than those in the areas of lower density.  Over many billions of years, this led to the structure we see: clusters formed from the high density regions and this left behind the empty spaces of the voids.
Figure 4. Evidence for the acceleration of the universe. The brightness ("magnitude") of supernovae is plotted against the redshift of the same supernovae.

At the epoch of recombination, when the 5000 F radiation was decoupled from the matter in the universe, the density fluctuations led to a splotchy pattern in the radiation across the universe.  This splotchiness became the CMB inhomogeneities seen by COBE and WMAP.

Inflation is also interesting as a physical theory because it is a theory of cosmology that originally grew out of particle physics calculations.  That is to say that a theory of the universe as a whole came from a theory of the behavior of matter on the smallest scales imaginable.
But inflation does not contain all the answers either.  Why is most of the matter in the universe dark?  Why is the growth rate of the universe accelerating?  We need more answers, so we must look for more theories.

The Cosmological Constant, or "Out like a Lambda"

The cosmological constant was first introduced by Einstein as a way to incorporate the idea that empty space has its own density and pressure into his general theory of relativity.  The name "cosmological constant" is really just a mathematical description of this idea.  It is usually represented by the Greek letter lambda.  It was abandoned by Einstein when the universe was discovered to be expanding, but the idea keeps popping back up.  Today, it is used to explain why the rate of expansion of the universe is growing. As the universe expands and the matter and energy densities decrease, the inherent energy of space itself becomes more important, and this now begins to drive the expansion of the universe.

The idea that empty space has its own energy is not new; in fact it is an advanced consequence of quantum physics.  But calculations of the value of it based purely on quantum physics give much larger values than observations suggest.  Thus the cosmological constant presents a problem for cosmologists.  Another problem presented by the cosmological constant is its very nature, which is as yet unknown.  Its behavior is understood, it drives the expansion of empty space, but its origin ands its makeup and anything else about it is pretty much unknown.

The Ultimate Conspiracy Theories: Dark Matter and Dark Energy

Whatever it is that drives the expansion of empty space is invisible.  And it has no gravitational effect; in fact it has an ANTI-gravitational effect, so it can't be ordinary matter.  For these reasons, and for a few more subtle reasons, cosmologists have given this mysterious stuff the name dark energy.

Recent results now indicate that 70% of the energy density of the universe is dark energy. The remaining 30% is matter, but only a tiny fraction of this, about 0.1%, is visible matter. The vast majority of the matter in the universe is dark matter, and the majority of the universe is dark energy.

But what is dark energy, really? That's a good question, but it does not have a good answer. It could be some kind of energy field that was unknown until now. Some cosmologists favor the quintessence model, which says that a fifth fundamental force is acting to push the expansion of the universe. It could be new new gravitational physics that drives the expansion.

Or it could be some as yet unknown explanation waiting to be discovered by a future experiment. We hope that future experiments such as SNAP will make the measurements that will answer these questions.

We also don't have a good explanation for all the dark matter. Until its composition is known, it is difficult to know exactly how it fits into the evolution of the universe. Many experiments are busy looking for evidence of dark matter. From direct searches for exotic particles with strange names, like axions or WIMPS (weakly interacting massive particles), that could be the mysterious dark matter, to indirect searches which just look for evidence of the gravitational effects of dark matter, like SDSS, dark matter research will remain hot for at least the next decade.

Morgan Wascko
Lousina State University, MiniBooNE Experiment