The Cellar - 3/9/2004: New Hubble deep field

Quote:

Originally Posted by SteveBsjb

Einstein's equations take the amount of matter and energy in the universe (assumed, again by the consideration of symmetry, to be distributed uniformly) and as output, they give the curvature of space.

The tough part is... how much matter and space is there really out there?

The way I understand it, if all matter and energy in the universe were to be smeared uniformly throughout space, and if, after this was done, there turned out to be more than the so called critical density of of 10 to the -23 grams in every cubic meeter (about five hydrogen atoms per cubic meter) Einstein's equations would yield a positive curvature. If there were less than the critical density, there would be a negative curvature, if there were EXACTLY the critical density, the equations would tell us that space has no overall curvature.

It's impossible to prove this without knowing how much matter and energy is out there, and how it's spread out. Don't you agree?

The way you figure out how much stuff is out there is to look at it with at telescope and multiply it out, based on how much you believe each thing (galaxies, mainly) weighs. The problem is that you come up with a density which is lower than what you would determine based on the redshifts of distant objects. Thus, to reconcile accounts, astronomers theorize that there is other stuff out there that does not show up in telescopes for some reason. It could be particles or wandering planets or something more exotic, but each candidate leads to different side-effects and that's how one tries to choose among them.

Disclaimer: I used to be a physicist, but that was a decade ago.