Hubble's Law Expansion
Observations of distant galaxies and quasars show that these objects
are redshifted, meaning that the light emitted from them has
been shifted to longer wavelengths. This is seen by taking a
frequency spectrum of the objects and then matching the spectroscopic
pattern of emission lines or absorption lines corresponding to
atoms of the elements interacting with the light. From this analysis,
a redshift corresponding to a Doppler shift for the radiation
can be measured which is explained by a recessional velocity.
When the recessional velocities are plotted against the distances
to the objects, a linear relationship, known as Hubble's law,
v = H0 D
where v is the recessional velocity, D is the distance
to the object and H0 is Hubble's constant, measured to be 71 ± 4
km/s/Mpc by the WMAP probe.
The Hubble's Law observation has two possible explanations.
One is that we are at the center of an explosion of galaxies,
which is untenable given the Copernican principle. The second
explanation is that the universe is uniformly expanding everywhere
as a unique
property of spacetime. This type of universal expansion was developed
mathematically in the context of general relativity well before
Hubble made his analysis and observations, and it remains the
cornerstone of the Big Bang theory as developed by Friedmann-Lemaître-Robertson-Walker.
Cosmic Microwave Background Radiation
The Big Bang theory predicted the existence of the cosmic microwave
background radiation or CMB which is composed of photons emitted
during baryogenesis. Because the early universe was in thermal
equilibrium, the temperature of the radiation and the plasma
were equal until the plasma recombined. Before atoms formed,
radiation was constantly absorbed and reemitted in a process
called Compton scattering: the early universe was opaque to light.
However, cooling due to the expansion of the universe allowed
the temperature to eventually fall below 3000 K at which point
electrons and nuclei combined to form atoms and the primordial
plasma turned into a neutral gas. This is known as photon decoupling.
A universe with only neutral atoms allows radiation to travel
Because the early universe was in thermal equilibrium, the radiation
from this time had a blackbody spectrum and freely streamed through
space until today, becoming redshifted because of the Hubble expansion.
This reduces the high temperature of the blackbody spectrum. The
radiation should be observable at every point in the universe to
come from all directions of space.
In 1964, Arno Penzias and Robert Wilson, while
conducting a series of diagnostic observations using a new microwave
by Bell Laboratories, discovered the cosmic background radiation.
Their discovery provided substantial confirmation of the general
CMB predictions—the radiation was found to be isotropic and
consistent with a blackbody spectrum of about 3 K —and it
pitched the balance of opinion in favor of the Big Bang hypothesis.
Penzias and Wilson were awarded the Nobel Prize for their discovery.
In 1989, NASA launched the Cosmic Background Explorer satellite
(COBE), and the initial findings, released in 1990, were consistent
with the Big Bang's predictions regarding the CMB. COBE found a
residual temperature of 2.726 K and determined that the CMB was
isotropic to about one part in 105. During the 1990s, CMB anisotropies
were further investigated by a large number of ground-based experiments
and the universe was shown to be geometrically flat by measuring
the typical angular size (the size on the sky) of the anisotropies.
In early 2003 the results of the Wilkinson Microwave Anisotropy
satellite (WMAP) were released, yielding what were at the time
the most accurate values for some of the cosmological parameters.
This satellite also disproved several specific cosmic inflation
models, but the results were consistent with the inflation theory
Abundance of Primordial Elements
For a more detailed treatment of this topic, see the subarticle
Big Bang nucleosynthesis.
Using the Big Bang model it is possible
to calculate the concentration of helium-4, helium-3, deuterium
and lithium-7 in the universe
as ratios to the amount of ordinary hydrogen, H. All the abundances
depend on a single parameter, the ratio of photons to baryons.
The ratios predicted are about 0.25 for 4He/H, about 10-3 for
2H/H, about 10-4 for 3He/H and about 10-9 for 7Li/H.
The measured abundances all agree with those predicted from a
single value of the baryon-to-photon ratio. This is considered
strong evidence for the Big Bang, as the theory is the only known
explanation for the relative abundances of light elements. Indeed
there is no obvious reason outside of the Big Bang that, for example,
the universe should have more helium than deuterium or more deuterium
Galactic Evolution and Distribution
Detail observations of the morphology and distribution of galaxies
and quasars provide strong evidence for the Big Bang. A combination
of observations and theory suggest that the first quasars and
galaxies formed about a billion years after the big bang, and
since then larger structures have been forming, such as galaxy
clusters and superclusters. Populations of stars have been aging
and evolving, so that distant galaxies (which are observed as
they were in the early universe) appear very different from nearby
galaxies (observed in a more recent state). Moreover, galaxies
that formed relatively recently appear markedly different from
galaxies formed at similar distances but shortly after the Big
Bang. These observations are strong arguments against the steady-state
model. Observations of star formation, galaxy and quasar distributions,
and larger structures agree well with Big Bang simulations of
the formation of structure in the universe and are helping to
complete details of the theory.