Recent content by Chalnoth

It's a relative statement rather than a statement about the overall size, since the overall size is not observed. What it means specifically is that objects in our universe that we observe today were much closer together.

I don't think eternal inflation says anything one way or another about a multiverse, at least not an interesting one. All that you get from eternal inflation is a huge universe. What you need to also get a multiverse that is more interesting than just being really big is some mechanism to have...

What are you talking about?

Not quite. From what we can tell, the rate of expansion has done nothing but decrease. Early-on it decreased very rapidly, and less so more recently. The rate has been slowing down slowly enough over the last few billion years that objects have started accelerating away from one another. If...

If you want to see the derivation, see here, for example: http://www.astronomy.ohio-state.edu/~dhw/A5682/notes4.pdf The short version is that doing the calculation the way you did was incorrectly taking account of the symmetry of the system. When you use the symmetry of the system to make use...

One way you can do the Newtonian derivation, by the way, is to use Gauss's law. Since the universe is spherically-symmetric about every point, it's possible to simply reduce the problem to a single sphere: pick an origin and a radius, and you only need to consider the mass inside that sphere...

That's not true. Newton's laws recreate the first Friedmann equation exactly for the case of a uniform, matter-only fluid. If you modify Newton's laws to account for the cosmological constant, then that is included correctly as well. Radiation isn't included correctly, however.

I don't see why the underdense regions should cause the overdense regions to recede from one another more rapidly.

Probably a better way to tackle this problem would be to imagine an infinite, uniform grid of point masses as compared to the uniform fluid.

I really don't see how this can work. It's certainly very much contrary to the way gravity works in most every other situation. With a spherically-symmetric mass distribution, for example, the gravitational attraction at some specific distance depends only upon the mass contained within a...

Because the expansion doesn't involve creating matter. It's just existing matter getting further apart. That said, matter/mass can certainly be created or destroyed. Mass is a form of energy, and other forms of energy can be converted to mass under the right conditions. When heavy particles...

Anti-matter has all of the exact same properties of normal matter, except that its electric charge is opposite*. Dark matter has no electric charge at all, hence why it is dark. There's no reason to believe that normal matter and anti-matter are related in any sort of simple sense. There is...

Yes. CMB observations put the nail in that coffin, as the CMB was emitted before any compact objects would have formed, and the signature of dark matter in the CMB itself is very clear.

Gravitational waves from the early universe? The short answer is yes. It should, to some extent, be able to measure the overall background of gravitational waves in the universe. Provided it reaches its design sensitivity and the background signals can be effectively separated from the...

It's not the number of planets times the number of stars. There aren't ##10^{24}## planets per star, but close to ##10^{24}## planets in the entire universe. So a (somewhat) more reasonable calculation would be: Atoms on Earth (##10^{50}##) * (number of planets (##10^{24}##) + number of stars...