Kynan said:
That is really well explained Chalnoth.
I appreciate it when someone goes to such detail to explain things well.
So am I right in that what you described below is what they are calling "dark energy"? and this energy is believed to permeate all of space before the big bang such that meaning it is a constant and not changing with with the expansion of the universe or with any respect to time.
This is one potential sort of dark energy, yes. Any dark energy model ends up being very similar, however, in that it must permeate all of space and vary at most extremely slowly with time.
Kynan said:
I'm still curious about this particular statement.
"We do not yet have a good theoretical understanding of the observations that the expansion of the universe, is accelerating again, after a long period of slowing down."
What I gather from this is that the expansion of the universe was slowing down at some point.. so like a deceleration? and then randomly begun accelerating again? have I misunderstood this statement? If the dark energy is a constant then what would cause these fluctuations?
Well, yes, in the distant past, our universe was most definitely decelerating. This is what happens when you have a universe that is dominated by either radiation or normal matter. You can see this by examining the first Friedmann equation again:
H^2 = \rho
(note: I didn't mention this before, but I've dropped the constants in the equation for maximal clarity)
Where we also define:
H = \frac{1}{a}\frac{da}{dt}
If we are to have a deceleration, we want da/dt to get smaller as the scale factor a increases. If da/dt gets smaller as a increases, then this means that H must get smaller faster than 1/a. This means that H^2 must get smaller faster than 1/a^2, and therefore the energy density of the universe must get smaller faster than 1/a^2.
In other words, as long as you have an energy density that dilutes, and in particular dilutes faster than 1/a^2, you get a universe that decelerates. In fact, this is exactly what happens as long as you have a universe that is made of nothing but matter and radiation. Normal matter, as the universe expands, just gets further apart. You have the same amount of mass in a larger volume. So normal matter dilutes as 1/a^3: with normal matter you have deceleration.
With radiation, you get even more deceleration, because radiation not only dilutes in number, it also gets redshifted, so that radiation dilutes as 1/a^4.
So basically the very early universe, when most of the energy was in radiation, was decelerating. But the radiation's energy dropped more rapidly than the normal matter, and so later on the normal matter ended up dominating the energy in the universe. The universe continued to decelerate, but more slowly. As time moved on, the normal matter continued to get more and more dilute, its energy dropping more and more, until the originally much smaller (but not decreasing!) energy density in dark energy came to dominate.
Now, today, the energy density of the universe is still decreasing, because the matter is still getting more and more dilute, but with matter already at only about 25% of the energy density and falling, the constant (or nearly so) energy density of dark energy has caused the expansion to accelerate.
