Debate The Ultimate Fate Of Our Universe

The fate of our planet is fire. In around 5 billion years the Sun will become a Red Giant and expand to a size that fits Earth's entire orbit inside of it. The oceans will boil, land will become molten, and Earth will be a small charcoal ember orbiting on the inside of the Sun. Hopefully, we've made it off this rock by then and have successful colonies throughout the galaxy.

The fate of our planet can be predicted with reasonable scientific certainty. This thread is about the fate of the universe itself. How will it all end? Will it ever end? Does it reset and start over again, or is it doomed to expand exponentially?

How will the Universe die?

From Nasa:

What is the Ultimate Fate of the Universe?
"SOME SAY THE WORLD WILL END IN FIRE, OTHERS SAY IN ICE"

Just as Robert Frost imagined two possible fates for the Earth in his poem, cosmologists envision two possible fates for the universe:

* Endless expansion
* The “Big Crunch”



A graph showing the relative size of the universe over billions of years under different content senarios The evolution of the universe is determined by a struggle between the momentum of expansion and the pull (or push!) of gravity. The current rate of expansion is measured by the Hubble Constant, while the strength of gravity depends on the density and pressure of the matter in the universe. If the pressure of the matter is low, as is the case with most forms of matter we know of, then the fate of the universe is governed by the density.

If the density of the universe is less than the critical density, then the universe will expand forever, like the green or blue curves in the graph above. Gravity might slow the expansion rate down over time, but for densities below the critical density, there isn’t enough gravitational pull from the material to ever stop or reverse the outward expansion. This is also known as the “Big Chill” or “Big Freeze” because the universe will slowly cool as it expands until eventually it is unable to sustain any life.

If the density of the universe is greater than the critical density, then gravity will eventually win and the universe will collapse back on itself, the so called “Big Crunch”, like the graph's orange curve. In this universe, there is sufficient mass in the universe to slow the expansion to a stop, and then eventually reverse it.

Recent observations of distant supernova have suggested that the expansion of the universe is actually accelerating or speeding up, like the graph's red curve, which implies the existence of a form of matter with a strong negative pressure, such as the cosmological constant. This strange form of matter is also sometimes referred to as the “dark energy”. Unlike gravity which works to slow the expansion down, dark energy works to speed the expansion up. If dark energy in fact plays a significant role in the evolution of the universe, then in all likelihood the universe will continue to expand forever.


Theories about the end of 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 continue to expand indefinitely, resulting in the "big freeze" scenario below.[1]

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, in the absence of dark energy, occur only under a flat or hyperbolic geometry. With a positive cosmological constant, it could also occur in a closed universe. 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 there are no gradients — which are needed to sustain information processing, one form of which is life. The Heat Death scenario is compatible with any of the three spatial models, but requires that the universe reach an eventual temperature minimum.

Big Rip: Finite Lifespan

In the special case of phantom dark energy, which has even more negative pressure than a simple cosmological constant, the density of dark energy increases with time, causing the rate of acceleration to increase, leading to a steady increase in the Hubble constant. As a result, all material objects in the universe, starting with galaxies and eventually (in a finite time) all life forms, no matter how small, will disintegrate into unbound elementary particles and radiation, ripped apart by the phantom energy force and shooting apart from each other. The end state of the universe is a singularity, as the dark energy density and expansion rate becomes infinite. For a possible timeline based on current physical theories, see 1 E19 s and more.

Big Crunch

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. Theoretically, the oscillating universe could not be reconciled with the second law of thermodynamics: entropy would build up from oscillation to oscillation and cause heat death. Other measurements suggested the universe is not closed. These arguments caused cosmologists to abandon the oscillating universe model. A somewhat similar idea is embraced by the cyclic model, but this idea evades heat death, because of an expansion of the branes that dilutes entropy accumulated in the previous cycle.

Big Bounce

The Big Bounce is a theorized scientific model related to the beginning of the known Universe. It derives from the oscillatory universe or cyclic repetition interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe.

According to one version of the Big Bang theory of cosmology, in the beginning the universe had infinite density. Such a description seems to be at odds with everything else in physics, and especially quantum mechanics and its uncertainty principle.[citation needed] It is not surprising, therefore, that quantum mechanics has given rise to an alternative version of the Big Bang theory. Also, if the universe is closed, this theory would predict that once this universe collapses it will spawn another universe in an event similar to the Big Bang after a universal singularity is reached or a repulsive quantum force causes re-expansion.

Multiverse: no complete end

The multiverse hypothesis states that our universe is but one universe among infinite parallel universes, possibly with different physical laws. Whatever the ultimate fate of our universe may be, almost all parallel universes will have different fates. And while many universes may be closed, many others may be open. The multiverse as a whole may never end completely.

False vacuum

If the vacuum is not in its lowest energy state (a false vacuum), it could tunnel into a lower energy state.[citation needed] This is called the vacuum metastability event. This has the potential to fundamentally alter our universe; in more audacious scenarios even the various physical constants could have different values, severely affecting the foundations of matter, energy, and spacetime. It is also possible that all structures will be destroyed instantaneously, without any forewarning.

Many-worlds interpretation of quantum mechanics

According to the many-worlds interpretation of quantum mechanics, the universe will not end this way. Instead, each time a quantum event happens that causes the universe to decay from a false vacuum to a true vacuum state, the universe splits into several new worlds. In some of the new worlds the universe decays; in some others the universe continues as before.