Ultimate fate of the universe

Either the Universe will continue in its present form forever, or it will cease to exist in its present form at some time.

Many religions have postulated an end to the Universe, for example as part of an Apocalypse ordained by God; see the article on eschatology for more discussion of these issues. This article is about scientific theories of the end of the Universe in the absence of such an event.

Scientific ideas of the end of the Universe

Until fairly recent times, even the scientific view of the Universe was one of eternal and unchanging existence. After Edwin Hubble's discovery of an expanding Universe, suddenly the notion of a start and, possibly, an end, was the subject of scientific investigation.

Theories can be divided into three major groups:

• that, despite the observation, the Universe is eternal as believed before: steady-state Universe and oscillatory Universe.
• that the Universe had a beginning, but not a proper end: heat-death of the Universe and the Big Rip
• that the Universe had a beginning, and will end in some way: Big Crunch

The first group is not discussed in this article, since it negates the very end of the Universe. In these theories, some kind of meaningful activity can last forever.

All theories must come to terms with General Relativity, which provides a common background for cosmological speculation. Most of these theories are solutions of GR equations, only changing parameters like average density, cosmological constant, etc.

Infinite time, but finite lifespan

In an open Universe, General Relativity shows that the Universe can exist indefinitely in the future, but will settle down into a state where life as we know it will cease to exist. For example, the following is a possible timeline, based on current physical theories, of an open Universe suffering from a heat death:

• 10¹⁴ years -- the estimated time until low-mass stars cool off
• 10¹⁵ years -- the estimated time until planets detach from stars.
• 10¹⁹ years -- the estimated time until stars detach from galaxies
• 10²⁰ years -- the estimated time until orbits decay by gravitational radiation
• 10³¹ years -- proton decay, if GUT theories are right.
• 10⁶⁴ years -- the estimated time until stellar black holes decay by the Hawking process
• 10⁶⁵ years -- the estimated time until all matter is liquid at zero temperature
• 10¹⁰⁰ years -- the estimated time until supermassive black holes decay by the Hawking process
• 10¹⁵⁰⁰ years -- the estimated time until all matter decays to iron (if the proton does not decay)
• 10^{100,000,000,000,000,000,000,000,000} years (10¹⁰²⁶) -- low estimate for the time until all matter collapses into black holes
• 10^{10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000} years (10¹⁰⁷⁶) -- high estimate for the time until all matter collapses into neutron stars or black holes

In 2003, New Scientist magazine reported a preprint by Robert R. Caldwell, Marc Kamionkowski and Nevin N. Weinberg which puts forward the hypothesis that the end of the Universe may occur as a "Big Rip", which will shred the physical structure of the Universe.

In this model, a cosmological constant causes the Universe's rate of expansion to accelerate. Taken to the extreme, an ever-accelerating expansion means that all physical objects in the Universe, starting with galaxies and down to individual human beings, bacteria, and grains of sand, will eventually be torn to pieces and then to elementary particles. The Universe will be then reduced to single elementary particles forever accelerating away from one other.

Finite time and lifespan

The Big Crunch theory is a symmetrical view of the life 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 a cosm-wide contraction.

It is unclear what the end result would be: a simple extrapolation would have all the matter and space-time in the Universe collapse into a dimensionless singularity, but at these scales quantum effects, ignored by General Relativity, should be considered. Some people use this opportunity to postulate an oscillatory Universe, that starts again to expand.

Life in a mortal Universe

Some well-known physicists have speculated that an advanced civilization could use a finite amount of energy to survive for an effectively infinite amount of time. The strategy is to have brief periods of activity, alternated by longer and longer periods of hibernation (see Dyson's eternal intelligence for more information).

The reverse is true for a civilization finding itself in the middle of the Big Crunch. Here, an effectively infinite amount of subjective time can be extracted from the finite remaning time, using the enormous energy of the Crunch to "speed up" life faster than the limit is approaching. (see Frank J. Tipler's Omega point)

Even if possibile in theory, it is not clear if a practical way to use those possibilities can be developed by any civilization, as advanced as it may be.

See also

• the arrow of time

External links

• Robert R. Caldwell, Marc Kamionkowski, Nevin N. Weinberg. Phantom Energy and Cosmic Doomsday, preprint at http://arxiv.org/abs/astro-ph/0302506