Future of the Universe and Possible Death

Overview

There are basically two possible futures of the universe. The first is that the universe will eventually collapse in on itself in the reverse of the Big Bang - a process called the "Big Crunch." Everything in the universe pulls on everything else. Because of this, the Big Crunch would happen because there is enough matter in the universe for the collective gravity to overcome the expansion.

The second possible end is where the universe would continue to expand forever: Everything will eventually disappear, and the temperature of the universe will be absolute zero (0 K, -459.688 °F). This second process has many names, but the most common is the "Big Freeze;" it would happen if the universe does not have enough matter for the collective gravity to counteract the expansion. It would result from the reverse situation of the Big Crunch - there is not enough matter in the universe to halt the expansion.

A Geometry Lesson

The future of the universe ultimately depends upon its overall geometry: Flat, Spherical, or Hyperbolic (diagrams of these geometries lie throughout this page). The universe's geometry is determined by the average density of everything in it. There is one magic number, called the "critical density," that determines which fate the universe will take.

Big Crunch

The Big Crunch is one scenario for the end of the universe, and it will result if the universe has a spherical geometry. This "spherical geometry" is not an abstract idea: It actually relates to what the shape of the universe would be if one could observe it from the "outside."

In this case, the universe contains enough mass - it is above the critical density - to actually stop its expansion. Once it stops expanding, it will start to contract. Slowly at first, and then faster and faster, the universe will contract and galaxies will come closer to each other. Eventually, everything will merge, for the universe will no longer be large enough for separate galaxies or stars. As it continues to shrink, the universe will heat to huge temperatures, and everything will be compacted into a black hole. Finally, at the end, the universe will be as it began - an infinitely small, infinitely dense, and infinitely hot point. No one knows what, if anything, would happen after that *.

An easy way to think of this is by throwing a ball; you throw a ball up into the air. Your release is like the Big Bang, and starts the ball's acceleration. As the ball climbs skyward, it slows its ascent because the Earth has enough gravity to slow it down and pull it back to it. This is like the mass of the universe being enough to overcome its expansion. As the ball reaches its maximum height, it stops, which is what the universe will do if it is over the critical density. Then, ever so slowly, the ball begins to fall back down, faster and faster, until it reaches your hand again (unless you miss). This is the end of the ball's throw, and is like the end of the universe.

*A popular idea is that the universe will then be re-born and it would continue to oscillate between Big Bangs and Big Crunches forever.

The Big Freeze

This scenario for the universe's future will result from either hyperbolic or flat geometry. As with spherical geometry discussed in the above section on the Big Crunch, these geometries are not abstract terms that only weird astrophysicists with thick glasses and poofy white hair use, but rather they are real shapes. A flat geometry is like a sheet of paper: It is flat; there is no curvature. Hyperbolic geometry is usually pictured with a saddle, and is depicted below to the right.

Either one of these geometries will result in a universe that effectively expands forever. If the universe is hyperbolic - the density is lower than the critical density - then it will eventually reach a fixed rate of expansion, and continue to expand at that rate forever. If the universe is flat - the density is exactly the critical density - then it will "asymptotically" (get closer and closer to but will only reach at an infinite time) reach an expansion rate of 0.

Both of these pose the future of a never-ending universe. After enough time, all galaxies beyond our Local Group will have disappeared beyond the edge of the observable universe**. After a longer time, all the stars in all the galaxies will have died, and there will be nothing left to make new ones. The universe will be a dark and cold place. Eventually, there will be nothing left but a vast, frozen emptiness.

**We can only know a small bit of what the universe contains due to the finite speed of light (300,000 kmps; 186,000 miles per second). Because the universe is a certain age, we can only see that many light-years out; for any part of the universe beyond that, the light has not had enough time to reach us.

Current Research

At least three main independent projects are trying to determine the universe's overall geometry. The initial results all point to that critical balance -- it appears as if the density of the universe is exactly that critical density that would result in a flat geometry, to within a very small error tolerance. This means that present research shows that the universe will have no end, that it will continue to expand forever but in a decelerating nature. The future of the universe is the Big Freeze.

For a more thorough discussion of current research in this area, please see the Advanced version of this site.