The big bang starts off with a state of extremely high density and pressure for the
Universe. Under those conditions, the Universe is dominated by radiation. This
means that the majority of the energy is in the form of photons and other
massless or nearly massless particles (like neutrinos) that move at near the speed
of light. As the big bang evolves in time, the temperature drops rapidly as the
Universe expands and the average velocity of particles decreases.
reaches a state where the energy of the Universe is primarily contained in
non-relativistic matter (matter sufficiently massive that its average velocity is
very much less than the speed of light). This is called a matter dominated
universe. The early Universe was radiation dominated, but the present Universe is
Let us now give a brief description of the most important events in the big bang.
The Cast of Characters for the Big Bang
The primary cast of characters includes:
Because of the equivalence of mass and energy in the Special Theory of Relativity,
in a radiation dominated era the particles and their antiparticles are continuously
undergoing reactions in which they annihilate each other, and photons can collide
and create particle and antiparticle pairs. One says that under these conditions
the radiation and the matter are in thermal equilibrium because they can
freely convert back and forth.
- Photons ("particles" of light)
- Protons and neutrons
- Electrons and their antiparticles the positrons
- Neutrinos and their antiparticles the antineutrinos
Let us now follow the approximate sequence of events that took place in the big
bang in terms of the time since the expansion begins.
Time ~ 1/100 Second
At this stage the temperature is about 100 billion Kelvin and the density is
a billion times that of water. The Universe is expanding rapidly and is
very hot; it consists of an
undifferentiated soup of matter and radiation in thermal equilibrium. This
temperature corresponds to an average energy of the particles of about 8.6 MeV
(million electron-Volts). The electrons and positrons are in equilibrium with the
photons, the neutrinos and antineutrinos are in equilibrium with the photons,
antineutrinos are combining with protons to form positrons and neutrons, and
neutrinos are combining with neutrons to form electrons and protons. At this stage
the number of protons is about equal to the number of neutrons.
Time ~ 1/10 Second
Now the temperature has dropped to several times
10 billion Kelvin and the density is a
little over 10 million times that of water as the Universe continues to expand.
Because a free neutron is slightly less stable than a free proton, neutrons
decay to protons plus electrons plus neutrinos with a half-life of approximately 17
minutes. Thus, the initial approximately
equal balance between neutrons and protons begins to be tipped in favor of protons.
By this time about 62% of the nucleons are protons and 38% are neutrons.
The free neutron is unstable, but neutrons in composite nuclei can be stable, so
the decay of neutrons will continue until the simplest nucleus (deuterium, the
mass-2 isotope of hydrogen) can form. But no
composite nuclei can form yet because the temperature implies an average
energy for particles in the gas of about 2.6 MeV, and
deuterium has a binding energy of only 2.2 MeV and
so cannot hold together at these temperatures. This barrier to production of
composite nuclei, which allows the free neutrons to be steadily converted to
protons, is called the deuterium bottleneck.
Time ~ 1 Second
The temperature has dropped to about 10 billion K as the Universe continues to
expand, and the density is now down to about 400,000 times that of water. At this
temperature the neutrinos cease to play a role in the continuing evolution, but the
deuterium bottleneck still exists so there are no composite nuclei and the
neutrons continue to beta decay to protons. At this stage the protons abundance
is up to 76% and the neutron abundance has fallen to 24%.
Time ~ 13.8 Seconds
The temperature has now fallen to about 3 billion K. The average energy of the
particles in the gas has fallen to about 0.25 MeV. This is too low for photons to
produce electron-positron pairs so they fall out of thermal equilibrium and the
free electrons begin to annihilate all the positrons to form photons. The
deuterium bottleneck still keeps appreciable deuterium from forming and the
neutrons continue to decay to protons. At this stage the abundance of neutrons has
fallen to about 13% and the abundance of protons has risen to about 87%.
Time ~ 3 Min 45 Sec
Finally the temperature drops sufficiently low (about 1 billion K) that deuterium
nuclei can hold together. The deuterium bottleneck is thus broken and a rapid
sequence of nuclear reactions combines neutrons and protons to form deuterium, and
the resulting deuterium with neutrons and protons to form the mass-4 isotope of
helium (alpha particles). Thus, all remaining free neutrons are rapidly "cooked"
into helium. Elements beyond helium-4 cannot be formed because of the peculiarity
that there are no stable mass-5 or mass-8 isotopes in our Universe and the next
steps in the most likely reactions to form heavier elements would form mass-5 or
Time ~ 35 Minutes
The temperature is now about 300 million K and the Universe consists of protons,
the excess electrons that did not annihilate with the positrons, helium-4 (26%
abundance by mass), photons, neutrinos, and antineutrinos. There are no atoms yet
because the temperature is still too high for the protons and electrons to bind
Time ~ 700,000 years
The temperature has fallen to several thousand K, which is sufficiently low that
electrons and protons can hold together to begin forming hydrogen atoms. Until
this point, matter and radiation have been in thermal equilibrium, but now they
decouple. As the free electrons are bound up in atoms the primary cross section
leading to the scattering of photons (interaction with the free electrons) is
removed and the Universe (which has been very opaque until this point) becomes
transparent: light can now travel large distances before being absorbed.
Production of the Light Elements in the Big Bang
One important success of the big bang model has been in describing the abundance of
light elements such as hydrogen, helium, and lithium in the Universe. These
elements are produced in the big bang, and to some degree in stars.
Analysis of the oldest stars, which
contain material that is the least altered from that produced originally in the big
bang, indicate abundances that are in very good agreement with the predictions of
the hot big bang.
One particularly sensitive test involves the abundance of deuterium.
Because deuterium has a nucleus that is very weakly bound compared with
most nuclei, it is very sensitive to the conditions in which it is formed
(as we have just seen): if
the temperatures are too high, deuterium breaks apart, and it can only be formed
when there are free neutrons to combine with protons. Detailed analysis of the
deuterium abundance gives very strong support to the hot big bang picture.
The Steady State Model
The big bang model had an early challenger that was called the steady state
model. The steady state model did the cosmological principle one better by
invoking what has been termed the perfect cosmological principle:
Not only is the Universe the same at all places and in all directions when averaged
over a large enough volume; it is the same for all time too.
Since the Universe was known to be expanding, the steady state model had to
postulate continuous creation of matter in the space between the stars and galaxies
to maintain the same density over time and thus satisfy the perfect cosmological
principle of a universe unchanging in time on large scales.
This violates the law of mass-energy
conservation, but the rate of mass creation that is required is far too small to be
detectable by any conceivable experiment, so it cannot be ruled out experimentally
(the rate that is
required is to create approximately 1 hydrogen atom per cubic centimeter every
The Triumph of the Big Bang
For a time, the steady state theory and the big bang theory competed with each
other, but eventually observations all but ruled out the steady state theory while
providing strong support for the big bang. Probably the two most important
As a consequence of these and other findings, the steady state theory is no longer
considered viable by most astronomers.
- Deep space radio telescope observations (which therefore peered far
back in time
because of the finite speed of light) indicating that the early Universe
looked very different from the present Universe. For example, there appear to be
more quasars at great distances, implying that there were more quasars in the early
Universe than the present one. This contradicted the steady state
hypothesis that the Universe was unchanging over time on large scales.
- The discovery of the cosmic microwave background to be
discussed shortly, that appeared to permeate all of space. This was an expected
consquence of the big bang model, but was very difficult to explain in any simple
way in the steady state theory.