
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." This ending would result if there is enough matter in the universe to counteract the force of 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 in the reverse situation of the Big Crunch  if the universe does not have enough matter for the collective gravity to counteract 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  a function of the mass. There is one magic number, called the "critical density" which is represented by ρ_{c}, 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 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 reborn 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 reach an expansion rate of 0. Both of these pose the future of a neverending 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 lightyears out; for any part of the universe beyond that, the light has not had enough time to reach us. Recent and Current Research Since 1992, there have been many different projects to determine the overall geometry of the universe. The only successful way to determine this so far has been to study the cosmic microwave background radiation*** (CMB). The first such was COBE, which stands for COsmic Background Explorer. It presented the first allsky picture of the CMB, but its resolution was too poor to accurately determine the geometry (temperature resolution was about 0.002 K; angular resolution was 7°  14 times the size of the full moon). It did show that the actual density of the universe is very close to the critical density. The most recent and complete research is from the Wilkinson Microwave Anisotropy Probe (WMAP for short), sponsored mainly by NASA. It has made the highestresolution image of the CMB: The angular resolution of WMAP was 0.3° and the temperature resolution is 20 µK. The WMAP results show that the universe is flat, meaning that the the universe will expand forever at an ever decelerating rate. Other results from the WMAP mission are:
***This is radiation that is left over from the early universe  from the time when atomic nuclei first combined with electrons which made the universe transparent. This happened at the ripe ol' age of 380,000 years, based upon WMAP results. To learn more about the cosmic microwave background radiation, see the History page of this section.
