Dark energy is continuously expanding space. I was listening to one of my space podcast the other day. They said that dark energy has been speeding up since the big bang. Also we are at the peak of how many stars the universe can create. Is there a correlation between the population of stars and the expansion of the universe? Could photons of light be driving the expansion of the universe? The reason why I made the connection is because they were also talking about the balance between light pressure and gravity at the core of a star. The light pressure prevents gravity from collapsing the star. There is so much light pressure from the whole universe it’s expanding space itself. And after this peak of star formation. When all the stars begin to burn out. There will be no more “light pressure” to keep up the expansion of the universe. After all the stars have gone the balance of power shifts to gravity and the universe begins to collapse in on itself. Collapses into one point and creates another big bang.
The main discovery that led to the theorizing of dark energy was that the expansion of the universe is accelerating, and has been for some time. However, it wasn’t always accelerating. It started out very, very fast, and slowed for a decent amount of time (I can’t remember what, but it’s at least a couple billion years) before it expanded to the point that dark energy dominated over the contractive pull of gravity. Most theories for dark energy is that it is the vacuum energy density, which exerts a repelling force, and exists as long as there is empty space. The more space there is, the less matter there is per space, and thus the less net gravitational force, while dark energy density is constant. Cosmologists have some pretty decent theories about all this, but it’s all stuff that’s well beyond me.
In the very early Universe, the light content of the Universe was relevant to its evolution. But the more the Universe expands, the less relevant light becomes, and so the light-dominated era gave way to the matter-dominated era. Matter also becomes less relevant with expansion, though not quite as quickly, so we’ve now moved past the matter-dominated era as well, and are now in the dark energy-dominated era. Dark energy stays exactly as relevant no matter how much the Universe expands (or at least, so far as we can tell), so we don’t anticipate the dark energy-dominated era to ever give way to domination by anything else.
And the pressure of dark energy is extremely relevant, and in fact is what’s responsible for its peculiar properties. But get this: The pressure of dark energy is actually negative. I know this is counterintuitive: One would expect a negative pressure to cause contraction, not expansion. About all I can say to that is that gravity is quite often counterintuitive.