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
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.
Now a bit more detail on each item in the list:
- 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
- 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!
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
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
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.
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
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
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.
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
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
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.
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
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.
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
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.
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
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.
Lousina State University, MiniBooNE Experiment