Great thread, I enjoy reading your posts. Here's something that never stops puzzling me. Tried to keep it short, but I failed.
Tidbits on Dark Energy
At the heart of Einstein's theory of general relativity, there is a direct connection between energy and spacetime. Basically, all energy warps spacetime, leading to what we know as gravity.
Most of this warping happens not in space, but in the 'time' part of spacetime. So on the surface of Earth, time passes faster than it does in empty space. This is because the Earth has a lot of mass, and mass equals energy (E = mc^2). Energy warps spacetime, and so time passes differently. That is the real-life result of relativity.
In other words, spacetime around the Earth is not straight. In fact, according to Einstein, the Moon is traveling in a straight line! So, there's no reason for spacetime to be straight anywhere, right?
Scientists have actually tested this question by measuring the angles between very distant galaxies. What they found is that on the largest scales, space is actually straight! This makes a lot of sense, but it's actually not obvious. Just like the Earth is curved, so could space be curved in a higher dimension. It seems this is not the case.
So spacetime is straight. But since spacetime is connected to energy, this means we can calculate the energy of the universe!
That is where it starts to get interesting. We already know how much mass the universe contains. This is quite certain, because only a certain amount of mass would make it possible for the galaxies and stars we see today to exist. This mass has a lot of energy, but it's not enough! In itself, it could never make spacetime straight. So there is a lot more energy out there, straightening spacetime. We call it dark energy, and there is more of it than all other energy combined!
Dark energy is very strange. If it was energy like we are used to, then it would make the universe contain a lot of strong gravity, and all galaxies would be on the way to a giant group hug. This is not the case. Our universe is expanding faster every day, like a balloon inflating at a great speed. This is happening despite all of the gravity in the universe. So something is counteracting the gravity of everything. Again, this is the work of dark energy.
Dark energy has a negative pressure. Compare it to an elastic string that, when you stretch it, actually starts pushing your hands further apart by its own! This is weird. But actually, it is only relativity that makes it weird. It comes from a very simple thing. That is: dark energy has a constant density. If space expands to twice its original size, it will still contain the same mass, but it will now contain twice the amount of dark energy.
Personally, I do not understand how, but I trust those that do. This simple characteristic gives dark energy an anti-gravitational effect.
Also, it means that the dark energy content of the universe used to be lower. It's only so dominating now because space has been expanding for a while. There was a time when the dark energy of the universe was insignificant! And back then, space didn't expand as fast as today.
Here's a mystery. Dark energy is said to be a property of space itself. But then, why doesn't it dilute as space expands? This must mean that more space is generated constantly. Either case, energy is entering the universe! How, from where, and how come it ends up being the exact right amount to keep spacetime straight?
If it keeps on like this, then after many billions of years, we wouldn't be able to see anything beyond our local neighbourhood of galaxies, which will have merged into one. Everything else would have moved away so far and so fast that we would never again receive light from it. A very strange thought.