MODERN COSMOLOGY V/S YESTERDAY’S COSMOLOGY
25 years ago, there were two versions of cosmologies. It was believed one of them was right. They were called Big Bang cosmologies and there were two versions – open and closed. In an open cosmology the universe was infinite but more than that it continued to be infinite, it continue to expand. A closed cosmology had the property that it expanded and contracted and crunched.
The left-hand side is the special starting point. Universe starts very small, it begins to expand. Interesting things happen during the expansion (all the good things we are looking for). Then it dies and crunches to a point and it’s over. That’s clearly a one-shot universe, you only get chance.
The radius of the universe expands and expands. Now because it expands all the particles fly apart, if they fly apart it’s still true that you get only one chance. They fly apart, once they are flown apart too far, they cannot interact with each other anymore. They don’t form galaxies, they get finished. They may still be there but they are not even useful for making a Boltzmann brain.
It was thought to be the solution until about 15 years ago when Dark Energy was discovered. To a physicist dark energy is called lambda (Λ), cosmological constant. It’s a condition in space whose effect are one or two for our purposes. Either it accelerates the expansion and makes it even faster. That’s if Λ is positive. If Λ is negative it does exactly the opposite. It accelerates the crunch.
In our universe, lambda is positive. It sounds great for a one-shot theory, but it’s not. The universe is expanding according to Hubble’s Law which says that the velocity of recession is proportional with a constant called the Hubble’s Constant times the distance. The further away you are, the faster you are moving.
Now, this constant is not a constant in most cosmologies. It decreases with time. But, in an accelerated universe, the kind of we live in, the Hubble constant is a constant. Now, that in turn means If you are far away, there is a place where things are receding away from us at the speed of light. If they are receding away from us at the speed of light, then anything that’s further away from that cannot be seen.
The circle where it happens is called the cosmic horizon. It’s very much like the black hole horizon. Except that it is a black hole inside out. You don’t lose things into the black hole (in this case), you lose things to the outside because they pass out to the cosmic horizon.
Everything out behind the cosmic horizon, we can never see so I painted it black. Inside, there’s our galaxy, there’s Andromeda (Messier 31), there’s all the distant galaxies. But they are rushing away from each other. Now, if you wait a while, all those galaxies will pass out to the horizon and we won’t see them. There would nothing be on our astronomical sky other than just plain emptiness or almost emptiness (our galaxy is still out there). Still sounds pretty good for one-shot.
But if you wait even longer, the galaxy itself will evaporate, the protons will probably disintegrate. It will form radiations. These radiations will pass out through horizon and there will be nothing left.
But quantum theorists know better. Stephen Hawking understood that the horizon of a black hole or the horizon of the cosmic horizon of the universe is not cold dead place. It has quantum energy and that quantum energy gives it a temperature. It behaves as though there were actually particles in the box.
Now, most of the particles are concentrated near the boundaries of the box, near the cosmic horizon. But they are constantly fluctuating. They are constantly in thermal motion. Every so often those particles will assemble themselves into a structure of some sort. If you wait long this newly formed Boltzmann brain too would evaporate; disintegrate. But wait a little longer, and another will appear.
There’s something wrong with this theory that says that universe simply forms with thermal particles in it. By virtue of the Dark Energy, which makes the universe expand in an accelerated way, it has led us literally right back to Boltzmann’s Box.