Future of the Universe

Future of the Universe, fate of all matter and energy on a cosmological timescale of many billions of years. According to the consensus in present-day cosmology, the universe was born in a gigantic explosion called the big bang and is still expanding today. Until recently, it was thought that its ultimate fate depends on how much matter it contains. Gravitation—the pull of each piece of matter on every other—has slowed the expansion since the big bang. It had been thought that if there were enough matter in the universe (more than the so-called “critical density”), the expansion would eventually halt and then reverse. Everything in the universe would fall together and be crushed in a “big crunch”, the reverse of the big bang. In these circumstances, the universe is said to be closed.

If the universe is of less than the critical density, it is said to be open, and it will carry on expanding forever. About a million million years from now, all star-making material will have been used up, and from then on galaxies will start to fade as stars die and are not recycled. Some stars will end up as black holes, others as cold balls of matter, in which, over enormous periods of time—10³³ years or more—even the protons may decay into radiation and positrons (the positive counterparts to electrons). Neutrons, the other major component of ordinary matter, also decay, into electrons and protons, so that ultimately all of this matter will have been converted into radiation and electrons and positrons, which will annihilate one another to leave more radiation. Black holes also “evaporate” eventually, emitting radiation as they do so. Nothing would be left in an open universe but radiation.

One of the most dramatic discoveries in astronomy in recent years, however, concerns the speed of the expansion of the universe. Measurements of the red shift of type 1A supernovae in distant galaxies indicate that the galaxies are fainter, and therefore farther away, than standard models predicted, suggesting that the expansion of the universe, far from slowing down, is actually accelerating. The most natural explanation of this is that empty space is filled with a form of energy (sometimes called dark energy) that acts like anti-gravity and pushes the expansion. This dark energy is effectively the same as the cosmological constant discussed by Albert Einstein, and is related to the forces that operated during inflation to start the universe expanding. It has always had the same strength, but when the universe was more compact and more dense gravity overwhelmed the dark energy. As the universe has expanded, it has thinned out and the influence of gravity has weakened so that dark energy now dominates.

Like all forms of energy, dark energy has a mass given by Einstein’s famous equation E = mc² (or, m = E/c²). Observations show that overall there is about three times as much mass in the form of dark energy as in all the matter in the universe (baryons and non-baryonic dark matter) put together. This is just enough to give the universe the critical density. But because of the dark energy, the universe will indeed expand forever, even if it is just closed. The scenario for its ultimate fate will be as for an open universe, but as the expansion proceeds more rapidly every galaxy beyond the Local Group, which includes our Milky Way galaxy, will be carried out of sight within a couple of hundred billion years.

Although it is very difficult to measure the density of the universe directly, observations of the cosmic microwave background radiation left over from the big bang, made by satellites such as the Wilkinson Microwave Anisotropy Probe (WMAP), confirm that it is close to the critical density. So the discovery of dark energy filled in a missing piece of the cosmological puzzle. By the time it was discovered that the expansion of the universe is accelerating, astronomers already knew the universe had the critical density, and that most of it must be in the form of unobservable dark stuff. About 4 per cent of the density of the universe is in the form of baryons (essentially, atomic matter), but only a fifth of this is in the form of bright stars. About 22 per cent of the universe is in the form of non-baryonic dark matter, sometimes known as WIMPs, or weakly interacting massive particles. About 74 per cent of the universe is in the form of dark energy.

A concerted effort is under way to detect the non-baryonic dark matter that is believed to exist. Studies of motions of galaxies show that their movements are slowed by unseen matter, accounting for at least part of the suspected matter. Some dark matter undoubtedly exists in the form of large numbers of brown dwarfs, masses of gas of less than one tenth of the mass of the Sun, too small to shine as stars, which began to be discovered in the mid-1990s. But these relatively “conventional” objects do not account for all of the mass. Physicists are searching with particle accelerators for a whole range of conjectured kinds of elementary particle, which, if they exist, would form an “ocean” underlying the universe with which we are familiar.

More speculatively, some cosmologists have suggested that the acceleration of the universe will itself get faster as time passes, eventually tearing atoms and particles apart in a “big rip” and leaving a featureless void. Just possibly, these conditions might trigger a new burst of inflation and the birth of a new universe—it might even explain the origin of our universe.

See also Universe, Origin of the.