Does Space Expand?

Drakkith

What's up with your equation? Is something confusing or are you joking?

I am uncertain if you can say that the universe was brighter earlier. I think that depends on the amount of light being output by stars at various times in the universe. If there are more stars now than in the past, today may be "brighter". Either way it's a confusing issue that doesn't really say much.

GeorgeDishman

Some context might help:

OP: Me: A small black body at a "typical" location in the universe now (i.e. probably in intergalactic space) would be at equilibrium somewhere near 3K, at the time of last scattering what we see as the CMBR was at 2975K and our test body would also have had that temperature. If that applied today, the Earth could be no cooler. Before 48,000 years, the universe was the "radiation dominated" era so I think my statement is valid.

If the OP is curious what the sky would have looked like back then, he can put 2975 into the box at the bottom right of this applet:

http://webphysics.davidson.edu/alumn...ava/bb_mjl.htm

The colour is the circle marked "composite" in the left panel.

Drakkith

While all true, what I mean is that saying the universe is brighter in the past may only be correct when you look back to a certain point in time. My question is if there are more stars now than in the past, and if those stars are outputting more total light than they were in the past. That combined with the density of the universe could mean it is definately brighter in the past, or not. I have no idea if it was brighter, nor do I know how to find out.

GeorgeDishman

The OP was talking about the first instants of the universe and prior to the release of the CMBR, it was brighter earlier. After that is more complex ;-)

Once the plasma combined into neutral hydrogen, there was the period called the "dark ages" because there were no stars at all. Then the first Pop III stars formed perhaps after around 100 million years. They were probably very large and short lived and put a lot of "metals" (elements beyond helium) into the mix when they ended as supernovae. As the proportion of heavy elements increased, stars could be smaller and new star production peaked then fell. All that time the universe was expanding so the density was falling too. Overall, there would have been a peak in stellar brightness when the universe was perhaps 1 to 3 billion years old. The rate of new star production now is conventionally thought to be perhaps one tenth of the peak but that is a point of current debate.

Drakkith

Hmmm. I have a few ideas/questions but I'll save that for another thread.

Trenton

The idea of expanding space seems to me to be precicely as strange as in-falling space in the vacinity of black holes. One is simply the reverse of the other!

Since the big bang appears to be a singularity that for some reason became disrupted and exploded outwards, it would follow that space would expand in a collapsing or diminishing gravitational field. The further out the space is, the faster it expands - Just as with a black hole the further in the faster it shrinks.

This seems to me to be a better explanation than dark energy for the accelerating expansion - not least because we don't have to look for anything new!

And yes it can go faster than light because the speed of space itself is not limited in GR.

Orion1

Equation of State:
[tex]P(r) = -\rho(r) c^2[/tex]

Inflationary metric:
[tex]ds^2 = - (1 - \Lambda r^2) c^2 dt^2 + \frac{1}{1 - \Lambda r^2} dr^2 + r^2 d\Omega^2[/tex]

If space is still expanding, then would this metric also still be applicable in the Universe at present time?

Reference:
Cosmic inflation - space expands - Wikipedia