Over the course of the past several
decades, cosmologists have used countless observations to come up with
a sort of biography of the universe, a model of the history and
structure of the universe often called the "Standard Cosmology".
A brief outline of the Standard Cosmology is given below - for more
detail, send a question to our FAQ page
The Big Bang Model
- The universe began (or, at least, became interesting)
approximately 13 billion years ago when it began expanding from an
almost inconceivably hot, dense state. Ever since then, the
universe has more or less continued its long process of expansion and cooling,
eventually reaching the cold, sparse state we see today.
- In the first 10-34 seconds or so of the universe's history, it underwent a brief period of extremely fast expansion, known as inflation. This period smoothed out the universe's original lumpiness and left it with the homogeneity and isotropy we see today. Quantum mechanical fluctuations during this process were imprinted on the universe as density fluctuations, which later seeded the formation of structure.
- The early universe was a soup of matter and energy, in which
particle/antiparticle pairs were constantly being born and
annihilating. As the universe cooled, it eventually became too
cold to produce certain kinds of particles - for example,
proton/antiproton pair creation stopped below a few trillion Kelvin,
while electron/positron pair creation continued until temperatures of a
few billion Kelvin. After this point, the remaining
particle/antiparticle pairs quickly annihilate, leaving little
behind. When this process happens for a particular species of
particle, that particle is said to have frozen out. The only reason
we have any matter in the universe at all
is because of a poorly-understood process called baryogenesis, caused by an
asymmetry in the physics of matter and antimatter.
- During the first 10 minutes or so, various light elements such as
deuterium (a heavy isotope of hydrogen), helium-3, helium-4, and
lithium-7 are created by the combination of free protons and neutrons
created in baryogenesis. This process of light-element formation
is called Big Bang nucleosynthesis.
- After about 100,000 years, the universe finally cools to a few
thousand Kelvin, cold enough for free nuclei and electrons to begin to
combine into atoms. This process occurs during a time period
called the era of recombination.
Before recombination, the universe was opaque to light and other
electromagnetic radiation - the large number of free electrons are just
too good at scattering light. After the formation of atoms, the
universe becomes transparent - it becomes possible for light to travel
large distances (for example, across the visible universe) without
getting knocked off course too badly. The light released at this
time is perceived today (after redshifting by the universe's expansion)
as the cosmic microwave background,
the afterglow of the Big Bang's heat. By this time, dark matter
(unaffected by the behavior of the baryonic matter) had already begun
to collapse into halos.
- Galaxies and stars began to form after a few hundred million
years, when the baryonic gas and dust collapsed to the center of the
pre-existing dark matter halos.
The Present Composition of the Universe
- Baryonic Matter: ~5% of the mass in the universe
This is ordinary matter composed of protons, neutrons, and
electrons. It comprises gas, dust, stars, planets, people, etc.
- Cold Dark Matter: ~25%
This is the so-called "missing mass" of the universe. It
comprises the dark matter halos that surround galaxies and galaxy
clusters, and aids in the formation of structure in the universe.
The dark matter is said to be "cold" because it is nonrelativistic
(slow-moving) during the era of structure formation. Dark matter
is currently believed to be composed of some kind of new elementary
particle, usually referred to as a weakly
interacting massive particle (WIMP).
- Dark Energy: ~70%
Through observations of distant supernovae, two research groups have
independently discovered that the expansion of the universe appears to
be getting faster with time. This seems to require some kind of
"antigravity" effect which we do not understand. Cosmologists
believe that the acceleration may be caused by some kind of new energy
field that permeates the universe, perhaps even the cosmological constant that Einstein
imagined almost a century ago. Whatever the source of this
phenomenon turns out to be, cosmologists refer to it generically as dark energy.
Jeff Filippini, UC Berkeley Cosmology Group (August 2005)