History of Time - End of time
Related quote goes here. Said Who
Some say the world will end in fire;
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
-Robert Frost “Fire and Ice”
Though not the same theories that Robert Frost offered in the poem above, there are two theories that leading cosmologists subscribe to concerning the probable future of the world. One of these is the possibility of “endless expansion,” and the other is the idea of a “Big Crunch.” The two of these together, expansion and gravity, form a tug-of-war in which the evolution of the universe hangs in the balance.
We can tell that the universe is expanding because all galaxies appear to be moving away from us. That does not mean that we are the center of the universe, but rather that if the entire universe is expanding, the perspective of anyone from any star or planet would be such that everything else seems to be moving away from where they are. The Hubble Constant, whose value is hotly debated among the leading experts, is a measure of the rate of this expansion. The fact that the Hubble Constant is speeding up seems to indicate that the universe will simply always continue to expand. This theory parallels the second law of thermodynamics, where entropy and thus disorder are always increasing. If the expansion were slowing down, it would make sense to assume that it will eventually stop, but since the expansion is still accelerating, no stoppage is in sight.
Gravity is dependent primarily on the density of the universe. There is a certain Critical Density that is especially important in determining the fate of the world. It is calculated that if the actual density of the universe falls below this Critical Density point, the universe will continue to expand forever. If the actual density of the universe surpasses this point, the universe will stop expanding and begin contracting upon itself. We appear to have nothing to worry about however since the most recent measurements reveal that the two densities are nearly equal. This means that the universe will stop expanding, but will not collapse upon itself either. There are some that believe that even if the Big Crunch did occur, the universe would remain eternal anyway by returning to the state that it was in prior to the Big Bang and then starting over again. That is, the universe would exist indefinitely in a cycle of birth, death, and rebirth.
To put the beginning and end of the universe on a timeline, scientists have attempted to divide the time between the two events into the following 4 eras: Star-Filled Era, Degenerate Era, Black Hole Era, and Dark Era.
Star-Filled Era (10^6 - 10^14 years after the Big Bang)
We are in the middle of this era right now. Here, energy is generated by the creation and destruction of stars such as our sun.
Degenerate Era (10^15 - 10^37 years after the Big Bang)
Stars will no longer be created, the mass and energy level of the universe will continue to decline, and all that will be left is dead planets and black holes. According to physicist Greg Laughlin, there will occasionally be remaining energy when dead stars collide with each other, but “on average this will happen only once every quadrillion years in a galaxy the size of the Milky Way” (Pobojewski).
Black Hole Era (10^38 - 10^99 years after the Big Bang)
Black holes will grow larger for awhile but will begin dissipating into radiation along with the remaining stars.
Dark Era (10^100 years after the Big Bang)
All black holes will evaporate. As the universe continues to expand, all free energy will be spread so thin throughout the universe that the temperature will reach absolute zero and all movement will cease.
If all this seems bleak, fear not. Even if all this theory is true, we are not scheduled to enter into the next era for another million years, so we have plenty of time to develop a solution!