1

Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe?If the universe is expanding, the galaxies should have been much closer in the past so should we see a larger distribution of images of galaxies from the past than these young closer galaxies and clusters of galaxies? If it were no space expansion should the density of galaxies on the picture be just proportional with the square of the distance from our position which should not be if the universe is expanding...

Qmechanic
  • 201,751
Janko Bradvica
  • 1,825
  • Note though that the observed past far universe should look magnified where smaller distances should actually look larger, because the expanding space should act as a giant telescope: https://physics.stackexchange.com/questions/422644/is-the-universe-a-giant-telescope – safesphere Sep 15 '21 at 19:08

2 Answers2

1

Clusters of galaxies generally become denser over time, because they accrete more nearby galaxies as time goes by. In addition, galaxies orbiting within a cluster will tend to lose energy to dynamical friction caused by their interaction with the dark matter in the cluster, so they will gradually sink towards the center of the cluster.

Galaxies are a bit more complicated. They can become denser by accreting gas, which can concentrate in the center and form new stars, making the galaxy denser. They can also merge with other galaxies; if there is little or no gas in the two galaxies, then the energy of their orbital motion is converted into the kinetic energy of the stellar motions within the merged galaxy; the result is that the outer parts of the galaxy are more extended, and so the galaxy is, on large scales, less dense.

It is important to understand that galaxies and galaxy clusters do not participate in the general expansion of the universe -- that applies to the space outside galaxy groups or clusters, or to the space outside isolated galaxies that are not in groups or clusters.

Peter Erwin
  • 1,213
0

Galaxies and clusters of galaxies are held together by gravity. They do not expand as the universe expands. Therefore they are not closer together and not denser as time passes.

Buzz
  • 462
  • You are correct that galaxies do not expand as the universe expands. But they can get denser (by accreting gas that forms new stars near their centers) and they can get closer together (otherwise we would't see merging galaxies). – Peter Erwin Oct 05 '21 at 15:05