There are a small number of proponents of non-standard cosmologies
who believe that there was no Big Bang at all. They claim that
solutions to standard problems in the Big Bang involve ad hoc modifications
and addenda to the theory. Most often attacked are the parts of
standard cosmology that include dark matter, dark energy, and cosmic
inflation. However, while explanations for these features remain
at the frontiers of inquiry in physics, each one is strongly suggested
to exist in some form by observations of the cosmic microwave background,
large scale structure and type IA supernovae. The gravitational
effects of these features are understood observationally and theoretically
even as models for them have not yet been successfully incorporated
into the Standard Model of particle physics. Though such aspects
of standard cosmology remain inadequately explained, the vast majority
of astronomers and physicists accept that the close agreement between
Big Bang theory and observation have firmly established all the
basic parts of the theory.
What follows is a short list of standard Big Bang "problems" and
puzzles:
The Horizon Problem
The horizon problem results from the premise that information cannot
travel faster than light, and hence two regions of space which
are separated by a greater distance than the speed of light multiplied
by the age of the universe cannot be in causal contact. The observed
isotropy of the cosmic microwave background (CMB) is problematic
in this regard, because the horizon size at that time corresponds
to a size that is about 2 degrees on the sky. If the universe
has had the same expansion history since the Planck epoch, there
is no mechanism to cause these regions to have the same temperature.
This apparent inconsistency is resolved by inflationary theory
in which a homogeneous and isotropic scalar energy field dominates
the universe at a time 10-35 seconds after the Planck epoch. During
inflation, the universe undergoes exponential expansion, and regions
in causal contact expand so as to be beyond each other's horizons.
Heisenberg's uncertainty principle predicts that during the inflationary
phase there would be quantum thermal fluctuations, which would
be magnified to cosmic scale. These fluctuations serve as the seeds
of all current structure in the universe. After inflation, the
universe expands according to a Hubble Law, and regions that were
out of causal contact come back into the horizon. This explains
the observed isotropy of the CMB. Inflation predicts that the primordial
fluctuations are nearly scale invariant and Gaussian which has
been accurately confirmed by measurements of the CMB.
Flatness
The flatness problem is an observational problem that results
from considerations of the geometry associated with Friedmann-Lemaître-Robertson-Walker
metric. In general, the universe can have three different kinds
of geometries: hyperbolic geometry, Euclidean geometry, or elliptic
geometry. The geometry is determined by the total energy density
of the universe (as measured by means of the stress-energy tensor):
the hyperbolic results from a density less than the critical
density, elliptic from a density greater than the critical density,
and Euclidean from exactly the critical density. The universe
is measured to be required to be within one part in 1015 of the
critical density in its earliest stages. Any greater deviation
would have caused either a Heat Death or a Big Crunch, and the
universe would not exist as it does today.
The resolution to this problem is again offered by inflationary
theory. During the inflationary period, spacetime expanded to such
an extent that any residual curvature associated with it would
have been smoothed out to a high degree of precision. Thus, inflation
drove the universe to be flat.
Magnetic Monopoles
The magnetic monopole objection was raised in the late 1970s. Grand
unification theories predicted point defects in space that would
manifest as magnetic monopoles with a density much higher than
was consistent with observations, given that searches have never
found any monopoles. This problem is also resolvable by cosmic
inflation, which removes all point defects from the observable
universe in the same way that it drives the geometry to flatness.
Baryon Asymmetry
It is not yet understood why the universe has more matter than
antimatter. It is generally assumed that when the universe was
young and very hot, it was in statistical equilibrium and contained
equal numbers of baryons and anti-baryons. However, observations
suggest that the universe, including its most distant parts,
is made almost entirely of matter. An unknown process called
baryogenesis created the asymmetry. For baryogenesis to occur,
the Sakharov conditions, which were laid out by Andrei Sakharov,
must be satisfied. They require that baryon number not be conserved,
that C-symmetry and CP-symmetry be violated, and that the universe
depart from thermodynamic equilibrium. All these conditions occur
in the big bang, but the effect is not strong enough to explain
the present baryon asymmetry. New developments in high energy
particle physics are necessary to explain the baryon asymmetry.
Globular Cluster Age
In the mid-1990s, observations of globular clusters appeared to
be inconsistent with the Big Bang. Computer simulations that
matched the observations of the stellar populations of globular
clusters suggested that they were about 15 billion years old,
which conflicted with the 13.7-billion-year age of the universe.
This issue was generally resolved in the late 1990s when new
computer simulations, which included the effects of mass loss
due to stellar winds, indicated a much younger age for globular
clusters. There still remain some questions as to how accurately
the ages of the clusters are measured, but it is clear that these
objects are some of the oldest in the universe.
Dark Matter
During the 1970s and 1980s various observations (notably of galactic
rotation curves) showed that there was not sufficient visible
matter in the universe to account for the apparent strength of
gravitational forces within and between galaxies. This led to
the idea that up to 90% of the matter in the universe is not
normal or baryonic matter but rather dark matter. In addition,
assuming that the universe was mostly normal matter led to predictions
that were strongly inconsistent with observations. In particular,
the universe is far less lumpy and contains far less deuterium
than can be accounted for without dark matter. While dark matter
was initially controversial, it is now a widely accepted part
of standard cosmology due to observations of the anisotropies
in the CMB, galaxy cluster velocity dispersions, large-scale
structure distributions, gravitational lensing studies, and x-ray
measurements from galaxy clusters. Dark matter has only been
detected through its gravitational signature; no particles that
might make it up have yet been observed in laboratories. However,
there are many particle physics candidates for dark matter, and
several projects to detect them are underway.
Dark Energy
In the 1990s, detailed measurements of the mass density of the
universe revealed a value that was 30% that of the critical density.
Since the universe is flat, as is indicated by measurements of
the cosmic microwave background, fully 70% of the energy density
of the universe was left unaccounted for. This mystery now appears
to be connected to another one: Independent measurements of Type
Ia supernovae have revealed that the expansion of the universe
is undergoing a non-linear acceleration rather than following
a strict Hubble Law. To explain this acceleration, general relativity
requires that much of the universe consist of an energy component
with large negative pressure. This dark energy is now thought
to make up the missing 70%. Its nature remains one of the great
mysteries of the Big Bang. Possible candidates include a scalar
cosmological constant and quintessence. Observations to help
understand this are ongoing.