The expansion of the universe implies that the expansion began when the universe was very dense and hot. This early state of expansion is called the Big Bang. The time since the Big Bang can be estimated by the Hubble time, which assumes that the expansion has always gone on at the present rate.
The space in the universe may be either positively curved, negatively curved, or flat. If it is positively curved, the universe is finite in volume. Otherwise it is infinite. For all three cases, the universe has no center and is unbounded. In principle, geometrical tests can be used to determine the curvature of the universe. In practice, however, these are unworkable because the universe has changed with time.
The curvature of the universe can be tested by measuring the density of the universe. If the density is equal to the critical density (ρc), space is flat. If the density is less than ρc, space is negatively curved. If it is greater than ρc, then space is positively curved.
If the universe is negatively curved or flat, it will continue to expand forever. If the universe has positive curvature, contraction will eventually occur. The accuracy of the Hubble time as an estimate of the age of the universe also depends on curvature. Only for a low density and negative curvature is the actual age as great as the Hubble time. If dark energy exists, expansion may be accelerating.
Studies of ancient supernovae suggest that the expansion of the universe originally slowed but later began to accelerate. Dark energy may be responsible for the acceleration.
The ages of the oldest objects in the universe suggest that the Big Bang occurred more than 11 billion years ago. Estimates of how long the universe has been expanding range from 9.5 to 14 billion years.
During the first second of expansion, the universe was so hot that photons had sufficient energy to make matter and antimatter. At the same time, matter and antimatter continually destroyed each other through annihilation. After the first second, photons no longer had enough energy to make matter and antimatter, but annihilation continued to reduce the amount of matter in the universe. When this happened neutrinos were free to travel throughout the universe without interacting with matter.
From about 100 to 300 seconds after expansion began, conditions were right for nuclear reactions to produce helium from hydrogen. The abundances of isotopes made at that time show that the density of protons, neutrons, and helium was too low for the universe to have positive curvature unless the universe is mostly composed of dark matter.
After a few hundred thousand years, the universe cooled enough to permit atoms to form from nuclei and electrons. When this occurred, the universe became transparent to radiation, so radiation could begin to travel freely through the universe. The moment when light broke free from matter is known as the decoupling epoch or the recombination epoch. The radiation emitted before recombination is all gone, but most of the radiation emitted since that time is still present in the universe.
The cosmic background radiation is the light emitted at the time the universe became transparent. It is almost equally bright in all directions but must be observed at infrared and radio wavelengths because it comes from very remote parts of the universe and is strongly redshifted. Fluctuations in the cosmic background radiation suggest that space is flat and that normal matter contributes little to the density of the universe.
The standard model of the Big Bang cannot account for the isotropy of the cosmic background radiation, the near flatness of the universe, and the origin of structure in the universe.
The theory of inflation proposes that the early universe increased in size by an enormous factor. This solves the problems of isotropy of the cosmic background radiation and the flatness of the universe because the part of the universe that we can see was once extremely compact. The structure in the universe arose from random fluctuations in that compact region of space.
Inflation is believed to have been produced by scalar fields that caused density to remain high as the universe grew.
If space is positively curved and there is little or no dark energy, contraction will eventually occur. Contraction will lead to a dense state in which the early Big Bang is played out in reverse. At some point, conditions will become so extreme that we can't predict what will happen, although one possibility is another cycle of expansion and contraction. If space is negatively curved or dark energy dominates the universe, expansion will continue forever. Matter will decay, and the radiation in the universe will become redder and dimmer forever.
