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
antibaryons. However, observations suggest that the Universe, including its most distant parts, is made almost
entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis
to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that
C-symmetry and CP-symmetry are violated and that the Universe depart from thermodynamic equilibrium. All these
conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon
asymmetry.

Horizon problem.

The horizon problem results from the premise that information cannot travel faster than light. In a Universe of
finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in
causal contact. The observed isotropy of the CMB is problematic in this regard: if the Universe had been
dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that
time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to
have the same temperature.

A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and
isotropic scalar energy field dominates the Universe at some very early period (before baryogenesis). During
inflation, the Universe undergoes exponential expansion, and the particle horizon expands much more rapidly than
previously assumed, so that regions presently on opposite sides of the observable Universe are well inside each
other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region
was in causal contact before the beginning of inflation.

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. Inflation predicts that the primordial fluctuations are nearly scale invariant and
Gaussian, which has been accurately confirmed by measurements of the CMB.

If inflation occurred, exponential expansion would push large regions of space well beyond our observable
horizon.

Flatness problem.

The overall geometry of the Universe is determined by whether the Omega cosmological parameter is less than,
equal to or greater than 1. Shown from top to bottom are a closed Universe with positive curvature, a hyperbolic
Universe with negative curvature and a flat Universe with zero curvature.





The flatness problem (also known as the oldness problem) is an observational problem associated with a
Friedmann–Lemaître–Robertson–Walker metric. The Universe may have positive, negative, or zero spatial curvature
depending on its total energy density. Curvature is negative if its density is less than the critical density,
positive if greater, and zero at the critical density, in which case space is said to be flat. The problem is
that any small departure from the critical density grows with time, and yet the Universe today remains very close
to flat.] Given that a natural timescale for departure from flatness might be the Planck time, 10-43 seconds,
the fact that the Universe has reached neither a heat death nor a Big Crunch after billions of years requires
some explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis)
the Universe density must have been within one part in 1014 of its critical value, or it would not exist as it
does today.

A resolution to this problem is offered by inflationary theory. During the inflationary period, spacetime
expanded to such an extent that its curvature would have been smoothed out. Thus, it is theorized that inflation
drove the Universe to a very nearly spatially flat state, with almost exactly the critical density.

Magnetic monopoles.

The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted topological
defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the
hot early Universe, resulting in a density much higher than is consistent with observations, given that searches
have never found any monopoles. This problem is also resolved by cosmic inflation, which removes all point
defects from the observable Universe in the same way that it drives the geometry to flatness.

The future according to the Big Bang theory.

Before observations of dark energy, cosmologists considered two scenarios for the future of the Universe. If
the mass density of the Universe were greater than the critical density, then the Universe would reach a maximum
size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in
which it started—a Big Crunch. Alternatively, if the density in the Universe were equal to or below the critical
density, the expansion would slow down but never stop. Star formation would cease with the consumption of
interstellar gas in each galaxy; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very
gradually, collisions between these would result in mass accumulating into larger and larger black holes. The
average temperature of the Universe would asymptotically approach absolute zero—a Big Freeze. Moreover, if the
proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually,
black holes would evaporate by emitting Hawking radiation. The entropy of the Universe would increase to the
point where no organized form of energy could be extracted from it, a scenario known as heat death.

Modern observations of accelerating expansion imply that more and more of the currently visible Universe will
pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of
the Universe contains dark energy in the form of a cosmological constant. This theory suggests that only
gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death
as the Universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest
that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the
ever-increasing expansion in a so-called Big Rip.

Speculative physics beyond Big Bang theory.

This is an artist's concept of the Universe expansion, where space (including hypothetical non-observable
portions of the Universe) is represented at each time by the circular sections. Note on the left the dramatic
expansion (not to scale) occurring in the inflationary epoch, and at the center the expansion acceleration. The
scheme is decorated with WMAP images on the left and with the representation of stars at the appropriate level
of development.





While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is
known about the earliest moments of the Universe's history. The equations of classical general relativity
indicate a singularity at the origin of cosmic time, although this conclusion depends on several assumptions.
Moreover, general relativity must break down before the Universe reaches the Planck temperature, and a correct
treatment of quantum gravity may avoid the would-be singularity.

Some proposals, each of which entails untested hypotheses, are:

       •	Models including the Hartle–Hawking no-boundary condition in which the whole of space-time is
finite; the Big Bang does represent the limit of time, but without the need for a singularity.

       •	Big Bang lattice model states that the Universe at the moment of the Big Bang consists of an
infinite lattice of fermions which is smeared over the fundamental domain so it has both rotational,
translational, and gauge symmetry. The symmetry is the largest symmetry possible and hence the lowest entropy of
any state.

       •	Brane cosmology models in which inflation is due to the movement of branes in string theory; the
pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and
the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model
the Big Bang was preceded by a Big Crunch and the Universe endlessly cycles from one process to the other.
 
       •	Eternal inflation, in which universal inflation ends locally here and there in a random fashion,
each end-point leading to a bubble universe expanding from its own big bang.
 
Proposals in the last two categories see the Big Bang as an event in either a much larger and older Universe, or
in a multiverse.












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