Observations on the current state of the universe such as the relative velocities of galaxies show that it is expanding. A range of models suggests different fates for the universe from this point. Some models suggest that the universe will continue to expand forever, some that it will slow down and eventually collapse back on itself, perhaps repeating expansion/contraction cycles many times. Still other models suggest that the universe will reach a steady state, with the expansion slowing until it just stops. The factor deciding which of these models will occur depends primarily on how much mass is present in the universe. With a sufficient quantity, gravity may take over and cause the universe to collapse eventually; if the universe contains too little mass, then the expansion can continue indefinitely. A thin critical mass in between would suggest a steady-state universe that would eventually slow its expansion, attaining a steady state. The amount of matter per volume of space it takes to determine the difference among these different possible scenarios depends on the rate of expansion and is known as the critical density.
The future of the universe is vitally dependent on this critical density. If the density of matter is above the critical value, then the gravitational attraction will be enough to halt the expansion, and gravity would start to pull all the matter of the universe back inward, with all points moving toward one another as space contracted just in the opposite way from when it was expanding. This would take precisely the same amount of time that it took for the universe to expand, and everything would happen in reverse. observers would see that nearby galaxies were blue-shifted—but distant galaxies could still show red-shifts, since the time it takes for the light to reach the observer could be greater than the time since the contraction started.
As the universe and space itself contract in this model, the frequency of collisions between galaxies increases, and the whole universe heats up and becomes denser. The process continues until the temperature is so high the whole universe is hotter than typical stars, and the contraction will continue to the point of a superhot, superdense, supersmall singularity, very similar or identical to that from which the universe first expanded. At this point the normal laws of physics break down again, and understanding the meaning of the universe that has no space and no time and infinite mass is beyond comprehension. Nonetheless, many cosmologists speculate that at this point the universe may suddenly go through another big bang and expansion phase, and that the universe may experience an infinite number of expansions and contractions, in an oscillating or bouncing universe model.
If the density of the universe is below the critical value, then its fate and future will be dramatically different. A universe with a density below the critical value will expand forever, and the radiation and light received from distant galaxies will eventually fade and disappear. Eventually even the light from the stars in the local Milky Way would disappear as they use up their fuel and go dark. The fate of this universe is a cold, dark death. At present it is difficult to tell which of these models may be closer to the real fate of the universe, but the critical measurement is being able to tell the true density of the universe.
Determining the density of the universe is not a simple task, since only about 4 percent of matter in the universe is visible and about 22 percent is dark matter, and about 74 percent of the rest of the universe is estimated to be dark energy. When astronomers calculate the density of the universe, using all the matter that they can see, the density of this luminous matter comes to about 10-28 kg/m3, or about 1 percent of the mass needed to be the critical mass of the universe. Even if we include the present estimates for the dark matter in the universe, however, most models for the density suggest that the universe has only 20-30 percent of the mass of the critical density, implying that the universe will expand forever, and not retract upon itself.
Large uncertainties about the amount of dark matter in the voids in space and on very large scales may cause the amount of mass in the universe to be grossly underestimated. Some estimates of the amount of dark matter in these areas place the density of the universe very close to the critical value, so the present limits of detection cannot tell the future of the universe, or whether the future is dark and cold, or bright and very hot.
See also astronomy; astrophysics; dark matter; galaxies; origin and evolution of the universe; universe.
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