The topic of this document deals with scientific theories pertaining to the ultimate fate of the universe, or otherwise, how it will end. The theories and beliefs expressed in this document are not accepted as the only explanation of the ultimate fate of the universe, and are also not related to religious beliefs. The viewer is invited to read the following information and decide for themselves, if any, which fate of the universe he or she might actually believe to be true.

The ultimate fate of our universe is a topic in cosmology, or the study of the universe and man's place in it. Rival scientific theories predict whether the universe will be of finite or infinite duration (weather or not it will end). Once it was accepted that the universe had a beginning a finite time ago, the ultimate fate of the universe becomes a valid cosmological question, one depending upon the universe's average density of matter and rate of expansion.

The fate of the universe strongly depends on its shape, and how much dark energy it contains. Dark energy is a hypothetical form of energy which permeates all of space and has strong negative pressure. According to the theory of relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe.

Closed Universe

If the end of the universe is geometrically greater than one, then the geometry of space is closed, like the surface of a sphere. The sum of the angles of a triangle exceeds 180 degrees and there are no parallel lines; all lines eventually meet. The geometry of the universe is, at least on a very large scale, elliptic.

In a closed universe lacking the repulsive effect of dark energy, gravity eventually stops the expansion of the universe, after which it starts to contract until all matter in the universe collapses to a point, a final singularity termed the Big Crunch, by analogy with the Big Bang. If, however, the universe seethes with dark energy, as recent findings suggest, then the expansion of the universe can continue forever even if the end is greater than one.

Open Universe

If the end of the universe is geometrically less than one, then the geometry of space is open, or negatively curved like the face of a saddle. The angles of a triangle sum to less than 180 degrees, and lines that do not meet are never equidistant; they have a point of least distance and otherwise grow apart. The geometry of the universe is hyperbolic. Hyperbolic refers to how two lines can both intersect a single point, but not intersect each other. Basically, this refers to time. Even without dark energy, a negatively curved universe expands forever, with gravity barely slowing the rate of expansion. With dark energy, the expansion not only continues but accelerates. The ultimate fate of an open universe is either universal heat death, better known as the big freeze, or a big rip, where the acceleration caused by dark energy eventually becomes so strong that it completely overwhelms the effects of gravitational and electromagnetic binding forces.

Flat Universe

If the average density of the universe exactly equals the critical density so that the end is equal to one, then the geometry of the universe is flat: as in Euclidian geometry, the sum of the angles of a triangle is 180 degrees and parallel lines never meet. Without dark energy, a flat universe expands forever but at a continually decelerating rate, the rate of expansion asymptotically approaching zero. With dark energy, the expansion rate of the universe initially slows down, due to the effect of gravity, but eventually increases. The ultimate fate of the universe is the same as an open universe; either a heat death, Big Freeze or a Big Rip. Most astrophysical data to date is consistent with a flat universe.

The fate of the universe is determined by the density of the universe. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will not collapse.

Big Freeze or Heat Death

The Big Freeze is a scenario under which continued expansion results in a universe that is too cold to sustain life. It could occur under a flat or hyperbolic geometry, because such geometries are a necessary condition for a universe that expands forever. A related scenario is Heat Death, which states that the universe goes to a state of maximum entropy in which everything is evenly distributed, and the temperature of the universe drops to absolute zero, or the temperature at which all movement ceases.

Big Rip: infinite time, finite lifespan

The Big Rip is a cosmological hypothesis about the ultimate fate of the Universe, in which the elements of the universe, from galaxies to atoms, are progressively torn apart by the expansion of the universe. First, the galaxies would be separated from each other. Arguably, this is what is happening right now, with galaxies that move outside the observable universe (approximately 13.7 billion light years away). About 60 million years before the end, gravity would be too weak to hold the Milky Way and other individual galaxies together. Approximately three months before the end, the Solar system will be gravitationally unbound. In the last minutes, stars and planets will be torn apart, and an instant before the end, atoms will be destroyed. However, time would still continue, although all matter would cease to exist.

Big Crunch: finite time and lifespan

The Big Crunch theory is a symmetric view of the ultimate fate of the universe. Just as the Big Bang started a cosmological expansion, this theory postulates that the average density of the universe is enough to stop its expansion and begin contracting. The end result is unknown; a simple extrapolation would have all the matter and space-time in the universe collapse into a dimensionless singularity, but at these scales unknown quantum effects need to be considered (See Quantum gravity).

This scenario allows the Big Bang to have been immediately preceded by the Big Crunch of a preceding universe. If this occurs repeatedly, we have an oscillatory universe. The universe could then consist of an infinite sequence of finite universes, each finite universe ending with a Big Crunch that is also the Big Bang of the next universe. Of course, it is meaningless to distinguish between a Big Crunch and a Big Bang; we should only speak of recurring singularities.

Multiverse: no complete end

The Multiverse (or parallel universe in the singular case) scenario states that while our universe may be of finite duration, it is but one universe among many. Moreover, the physics of the multiverse may permit it to exist indefinitely. In particular, other universes may be subject to physical laws differing from those that apply in our own universe.


False Vacuum

If the vacuum is not in its lowest energy state (a false vacuum), it could collapse into a lower energy state. This is called the Vacuum metastability disaster. This would fundamentally alter our universe; the various physical constants could have different values, severely affecting the foundations of matter.