I have just finished summarizing this lecture about the nature of the universe, where it came from, and where it is headed (here is another presentation of the same lecture, on youtube, which is shorter but also less complete).
The lecture is from last year, with very recent data and theories.
Here it is:
Thanks to Einstein we know that matter and energy cause space to curve. According to his equations, the curvature of space is relative to the energy-momentum inside it (curvature = energy-momentum)
This, however, posed a problem: gravity (i.e. energy) should cause galaxies to contract, which is contrary to observation. This led Einstein to add the cosmological term to the equation (curvature+cosmological term = energy-momentum), which was a very small force that would not be noticeable to us, but that would build up over distance so it’s strong enough to hold galaxies apart, resulting in the universe that was then thought to be static.
Later, Hubble confirmed beyond doubt that the universe is in fact expanding, and not static. He also discovered that other galaxies are moving away from us, relative to their distance to us. The further a galaxy is, the faster it moves away. This means that the universe is expanding uniformally in all directions and enables us to calculate the size of the universe, how fast it is expanding, and how old it is.
Now we are posed with a new question: is there enough gravity to slow down and reverse expansion?
After these discoveries, Einstein dropped the cosmological constant, calling it his biggest blunder. However, it turns out that there is in fact a cosmological constant, but in a different way and for different reasons: curvature = energy-moment + cosmological constant.
The constant is the energy needed for the universe to expand, and apparently it comes from empty space. That may sound like nonsense, but it’s actually completely reasonable in quantum mechanics.
Virtual particles pop in and out of existence on a timescale so short that they can’t be measured directly, but their effects can be measured indirectly. Most of the mass or energy of a proton comes from these virtual particles.
It was then concluded that the energy of empty space is 10^120 times the energy of all matter in the universe, which is an incredibly bad prediction, because in order for us to exist they must be roughly equal.
The number is much too high to cancel out somehow, so it must be zero, which is only possible if there is a fundamental symmetry in nature. Theorists knew this, but there was no evidence for it and observation didn’t agree with it.
In order to solve this problem, scientists had to measure the energy of the universe, using gravity. That way we could know what kind of universe we live in:
-A closed universe would eventually stop expanding and then start contracting.
-An open universe would expand forever.
-In a flat universe expansion would slow down, but never quite stop. It’s also the boundary between an open and a closed universe.
After ‘weighing’ the universe, it was concluded that the weight between galaxies was fifty times greater than the weight of galaxies themselves, which means that most of the mass in the universe is dark, and therefore called dark matter, a new elementary particle that’s present everywhere, but has not yet been observed.
So now we know the weight of the universe. In order to determine what kind of universe we live in, we have to compare the density of matter to the density of matter in a flat universe (matter density/matter density for a flat universe), expressed as Ω. If Ω > 1, our universe is closed. If Ω < 1, we live in an open universe. If Ω = 1, we live in a flat universe.
According to observation, Ω = ~.30, which would mean we live in an open universe.
Another way to find out what kind of universe we live in is to measure the curvature of the universe:
-In an open universe, light would be curved by .5°, and objects would appear smaller.
-In a flat universe, light would be curved by 1°, and objects would appear at their real size.
-In a closed universe, light would be curved by 2°, and objects would appear bigger.
Observation tells us that light coming from distant galaxies is curved by 1°. We know this with 1% accuracy, which is incredibly good. In other words, we know for a fact that our universe is flat.
This begs the question of where the remaining 70% of the energy needed for a flat universe is (the Ω ratio): it must be in empty space, which means there is a cosmological constant, which is a repulsive force. All of this fits perfectly with observations.
To sum up, we know today that our universe is flat. That means that its expansion will gradually slow down, but never stop, and that it came about out of nothing, through quantum mechanics. We also know that the matter in the observable universe is only a fraction of the matter in the entire universe, which only 30% of everything in the entire universe.
I think that pretty much answers the question of whether or not the universe is infinite.
