Replies

Thanks for taking the time to type up that reply. Happily I knew quite a bit of the basics of distance measurement.

They are the main reason we now think the Hubble "Constant" is not really constant, but drops off with distance (or, conversely, increases as you get closer in distance and time). That accelerating expansion is not explained by gravity - in fact, it contradicts the expected effect of gravity. Since the velocity (and hence energy) of observed objects seems to be increasing, they call the mechanism we can't otherwise detect "dark energy."

This makes me scratch my head. If the Hubble constant drops off then that implies a slower expansion at the outer edge. Would they expect gravitational slowdown to be equal at all distances? Therefore over a billion years the observed Hubble Constant would reduce evenly. I guess only if they expect even distribution of matter.

I remember my old astronomy books suggesting that galaxies were evenly distributed. How surprised they were to find clusters, filaments, voids and strange motions among them.

If the Hubble constant drops off then that implies a slower expansion at the outer edge.

The 'outer edge' is just the edge of the observable universe. There is no 'edge' to the expanding universe that shows a measurable dropoff in density of material. I've seen some estimates, but they all acknowledge they are just estimates. They tend to run that the "true" size of the universe is two or three times what we can observe. The time back to the beginning is known (reasonably well) but all we can see in all directions is out to the observable limits with no indication of an 'edge' in any direction.

So, instead of thinking of it as an 'edge' it's better to think of it as a distance over which light has travelled, and therefore as a time. The expectation was that the universe was originally expanding faster (hence a higher Hubble Parameter), and that it then slowed down due to gravity. If it was slowing fast enough, eventually it would stop expanding and collapse back in on itself. Instead, they found that it was "originally" (again, meaning as far back as we can see) expanding more slowly. So, what has been making it speed up since the Big Bang?

I remember my old astronomy books suggesting that galaxies were evenly distributed.

And so they are, when the question is whether there is any direction we might look where they are significantly more or less dense. They 'web' of the universe does not show any particular direction that is different than any other, except at the level of small 'ripples' in the overall distribution analogous to raindrops in a storm. If you're in the middle of it, it's raining in all directions. That doesn't mean there aren't individual drops (or galaxies, in this analogy) coupled with gusts and eddys.