Expansion of the Universe regarding Time and lightspeed

Question

According to our current understanding, the universe is expanding... it is often said that every galaxy is moving away from us (I'm assuming that isn't true for galaxies in our local group), and it's also often said that the farther a galaxy is away from us -- the faster it moves away from us.

We also know that light has a finite speed, so when we look at those far away galaxies, we see them as they were a long, long time ago.

But (1) if the oldest galaxies are moving away faster, and the younger galaxies are moving away slower, then wouldn't that mean that the expansion of the universe is slowing down?

But (2) when I'm thinking of an expanding 2d sheet of paper with dots (scaled up in both directions), then it makes sense that the farther galaxies would move faster away from our perspective.. (although if the universe were expanding at a constant rate, we would still see the farther galaxies moving faster from our perspective, but I'm assuming that it has been accounted for, and that the galaxies move even faster than that).

So... why is my first "but" incorrect? Something tells me that I'm looking at it at the wrong way, but I can't figure it out xD

Thanks!

Answer 1

Your first "but" is incorrect, but second one is not. Here's how should you think about your first "but".

Here I'll assume that you meant more distant galaxies (and thus their emitted photons took more time to reach our eyes) by "oldest galaxies" and conversely the nearer galaxies by "younger galaxies".

Now when we observe distant galaxies, they seem to recede faster than those nearby. Your explanation involved in the second "but" is correct as the analogy of an expanding sheet can be drawn here. However, there is a cosmological principle stating that there is no special place and direction in the universe. What it implies is that the observation of galaxy recession is not a special observable fact for the residents of earth only. If there is some observer in a very far away galaxy trying to observe other galaxies, that observer will also observe that the farther a galaxy from him/her is the faster it recedes. The fact that every galaxy is receding from each other doesn't depend on the acceleration or rate of expansion. Even if the rate of expansion is constant throughout the cosmic history, at any time any observer from any point in the universe will observe that the far a galaxy is from that observer the faster it recedes away.

It is important to note that, as the universe is expanding, the galaxies were nearer than present when they emitted the photons which are reaching our eyes now. But in the Hubble's law, which states the more distant the galaxy the more the recession speed, we mean the distance at the current time, not in the past. Although the photons were emitted when the galaxies were nearer to us, we can calculate the present distance of the galaxies from different measurements. For example, we can measure luminosity distance $d_L$ of a standard candle from the flux and redshit $z$ from the spectra. Then for a flat universe (which our universe very nearly is), the current proper distance $d_p(t_0)$ of the host galaxy of the candle is $$d_p(t_0) = \frac{d_L}{1+z}$$

Lastly, the expansion of the universe is actually accelerating as strongly indicated from various observations and this recent acceleration took over nearly 4 billion years ago.

Answer 2

The expansion of the universe was slowing down until a few billion years ago when it started speeding up again due to dark energy. However this isn't the answer to your question.

You say:

if the oldest galaxies are moving away faster, and the younger galaxies are moving away slower, then wouldn't that mean that the expansion of the universe is slowing down?

but all the galaxies are roughly (i.e. to within a factor of two or so) the same age. For example the nearest big galaxy, Andromeda, is thought to be about 9 billion years old which is two thirds the age of the universe. Even though Andromeda is very near it isn't young. The difference isn't which galaxies are young and which are old, but which are near to us and which are far away. The recession velocity is determined solely by distance (as you suggest in your point 2) and not by galaxy age.

Incidentally the oldest star known, SMO313 with an age of 13.6 billion years, is very nearby indeed as it's part of the Milky Way.

It's certainly true that if we look a long way away we see only old galaxies, but that's inevitable. It's like looking at a census form from 1914 and being surprised everyone in it is a hundred years old.

Answer 3

Your second thought is correct. That is why the farthest galaxies move so much faster away than the younger ones. That and the universe was expanding faster when the older galaxies were accreted. The radiation and matter domination phases made the expansion slow and it's only now speeding up again.