Expanding Now But Later What?
Solutions of Einstein’s general relativity equations allowed three possible destinies for the universe, depending on the average density of matter in it. Given the initial expansion at an average velocity and the continuous deceleration that gravity causes, the universe may grow to a certain size and then fall back in on itself, like a ball thrown upward from the surface of the Earth. However, if the initial velocity is large enough and if the deceleration of gravity is weak enough because the average density of matter is low enough, the universe may expand forever and never lose all of its initial velocity. The third possibility is the limiting case between the first two. The universe may have just enough initial velocity to overcome the deceleration of gravity after expanding for a very long time. In this case, as the universe approaches infinite size its remaining initial velocity will approach zero. Cosmologists say that such a universe expands at the “critical rate.”
The rockets we launch now use most of their fuel in the first few minutes of flight to accelerate to high speed. If a rocket’s final speed after leaving the Earth’s atmosphere is greater than “escape velocity” then the rocket will move forever away from the Earth, but if the rocket’s final speed is too slow it will fall back to the Earth. The universe may act in a similar way, eventually collapsing back inwards or expanding forever.
None of the destinies is appealing. If the universe keeps on expanding then after a very long time all the fuel for all the stars will be exhausted. If our own galaxy burns out first, any other galaxy with a remnant of fuel left would be too far away for our posterity to reach it, no matter how advanced their space flight technology may become. Such a universe will end with all remaining energy and material spread out too thinly to support life of any kind. That will be the “heat death” of the universe described by Sir James Jeans.[i]
If the universe falls in on itself everything will be crushed together, burned up, and destroyed. Such a universe will finally end up as one huge black hole, and nothing will ever escape from it.
Of the three possibilities, the least unappealing is expansion at the critical rate, because that postpones disaster for the longest time. By 1981 astronomers knew that the expansion rate is at present within a factor of 100 of the critical rate.
If the universe expands more slowly than the critical rate then gravity will eventually slow the expansion to a halt and then produce contraction and a Big Crunch at the end of time. If the expansion had been substantially less rapid than the critical rate then the Big Crunch would come far too soon. The expanding material would have lost its outward impulse very quickly. Providing the full number of atomic elements, the basis for life, takes thousands of millions of years. In a universe that expanded too slowly hardly anything could have happened before everything collapsed back into a black hole. Not even light could have escaped from it.
But if the expansion had been substantially more rapid than the critical rate, the particles would have dispersed outward into empty space. They would have moved quickly so far from each other that their mutual gravity could never bring them together again in dense regions. All the material would be blown away as a fine gas of atoms. There could be no gravitational regathering of material to produce the galaxies, stars and planets. Without planets there could be no life in the universe.
If the expansion rate were either too much more or too much less than the critical rate, life would not be possible anywhere in the universe.
At the beginning the expansion rate must have been adjusted very precisely for the present expansion rate to be as close as it is to the critical rate. See Appendix A for an estimate of the precision required. Such fine tuning requires a fine-tuning mechanism.
We do not know what fine-tuning mechanism operated or how it caused the physical change that enables the universe to support life. Guth showed that the right phase change could have produced the fine adjustment required for the expansion rate. This would help the universe achieve the present rate of expansion, the rate that makes life possible. However, neither Guth nor anyone else has yet found a phase change that works. Very many physical changes are possible. Even if physicists ever find the right phase change, they will still have to explain why that change occurred instead of any other. The one phase change that would make life possible happened. Was that outrageously good luck, or did a benevolent Fine Tuner choose the right mechanism and cause it to operate in the early universe to make life possible?
Today we know that the rate of expansion is equal to the critical rate plus or minus one percent. It is not reasonable to believe that “we were just lucky.” The chances are so small that mathematicians generally agree to say it is impossible. If something impossible happens, it is a miracle, something that shows the effective working of a beneficent intelligence. Only God does miracles.
[i] Jeans, James, The Universe around Us (Cambridge, 1960), Fourth Edition, p. 280.
The rockets we launch now use most of their fuel in the first few minutes of flight to accelerate to high speed. If a rocket’s final speed after leaving the Earth’s atmosphere is greater than “escape velocity” then the rocket will move forever away from the Earth, but if the rocket’s final speed is too slow it will fall back to the Earth. The universe may act in a similar way, eventually collapsing back inwards or expanding forever.
None of the destinies is appealing. If the universe keeps on expanding then after a very long time all the fuel for all the stars will be exhausted. If our own galaxy burns out first, any other galaxy with a remnant of fuel left would be too far away for our posterity to reach it, no matter how advanced their space flight technology may become. Such a universe will end with all remaining energy and material spread out too thinly to support life of any kind. That will be the “heat death” of the universe described by Sir James Jeans.[i]
If the universe falls in on itself everything will be crushed together, burned up, and destroyed. Such a universe will finally end up as one huge black hole, and nothing will ever escape from it.
Of the three possibilities, the least unappealing is expansion at the critical rate, because that postpones disaster for the longest time. By 1981 astronomers knew that the expansion rate is at present within a factor of 100 of the critical rate.
If the universe expands more slowly than the critical rate then gravity will eventually slow the expansion to a halt and then produce contraction and a Big Crunch at the end of time. If the expansion had been substantially less rapid than the critical rate then the Big Crunch would come far too soon. The expanding material would have lost its outward impulse very quickly. Providing the full number of atomic elements, the basis for life, takes thousands of millions of years. In a universe that expanded too slowly hardly anything could have happened before everything collapsed back into a black hole. Not even light could have escaped from it.
But if the expansion had been substantially more rapid than the critical rate, the particles would have dispersed outward into empty space. They would have moved quickly so far from each other that their mutual gravity could never bring them together again in dense regions. All the material would be blown away as a fine gas of atoms. There could be no gravitational regathering of material to produce the galaxies, stars and planets. Without planets there could be no life in the universe.
If the expansion rate were either too much more or too much less than the critical rate, life would not be possible anywhere in the universe.
At the beginning the expansion rate must have been adjusted very precisely for the present expansion rate to be as close as it is to the critical rate. See Appendix A for an estimate of the precision required. Such fine tuning requires a fine-tuning mechanism.
We do not know what fine-tuning mechanism operated or how it caused the physical change that enables the universe to support life. Guth showed that the right phase change could have produced the fine adjustment required for the expansion rate. This would help the universe achieve the present rate of expansion, the rate that makes life possible. However, neither Guth nor anyone else has yet found a phase change that works. Very many physical changes are possible. Even if physicists ever find the right phase change, they will still have to explain why that change occurred instead of any other. The one phase change that would make life possible happened. Was that outrageously good luck, or did a benevolent Fine Tuner choose the right mechanism and cause it to operate in the early universe to make life possible?
Today we know that the rate of expansion is equal to the critical rate plus or minus one percent. It is not reasonable to believe that “we were just lucky.” The chances are so small that mathematicians generally agree to say it is impossible. If something impossible happens, it is a miracle, something that shows the effective working of a beneficent intelligence. Only God does miracles.
[i] Jeans, James, The Universe around Us (Cambridge, 1960), Fourth Edition, p. 280.
