Exploring the Impact of Expansion on Galaxy Clustering

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In summary, cosmological expansion does not affect gravitationally bound systems, like galaxy clusters. However, the dynamics of expansion do play a role in the formation and behavior of clusters. On a smaller scale, gravity dominates and pulls clusters closer together while on a larger scale, expansion dominates and moves clusters farther apart. Dark matter also plays a role in the gravitational attraction of clusters, and its existence is supported by various observations. Overall, while expansion does not directly affect clustering, it does have an impact on the dynamics and evolution of clusters.
  • #1
PoPpAScience
Question: When Galaxies are being drawn into clusters, does expansion of the universe enter into the equation? In other words, does expansion effect clustering? :shy:
 
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  • #2
Cosmological expansion does not affect gravitationally bound systems, like galaxy clusters.

- Warren
 
  • #3
This is the hardest thing for me to understand in cosmology. From what I understand the expanding Universe moves galaxies apart. But mapping shows them moving towards each other. i sure want to find out where my thinking is wrong.
 
  • #4
It might help to think about just the nearby ~100 Mpc of the universe ("Mpc" = megaparsec, a unit of distance which is approx 320 million light years). Near to us is the http://www.seds.org/messier/more/virgo.html [Broken] of galaxies, a relatively modest cluster. The galaxies in this cluster attract each other, as they each have a mass of several million to several billion (and more) Msol (this is a unit of mass, equal to the mass of the Sun). This mutual attraction, together with whatever relative motions each proto-galaxy had when it was formed (either in the first ~few hundred million years after the Big Bang, or from the collision in which it was created), makes for a very complicated picture - galaxies whizzing this way and that, passing close by each other and changing their 'orbits', etc. However, the total mass of the cluster is large enough that no (or very few) galaxy can 'escape' the combined gravitational grip of the whole cluster.

What about expansion of the universe? The Virgo galaxies, if there were no gravity, would expand away from each other, as they are part of the universe. However, there is gravity, and for the Virgo galaxies it is far 'stronger' than the universal expansion, so the net result is the Virgo cluster isn't 'pulled apart'.

Note that I've presented a very simplified word picture - there are many other things going on which make it all quite complicated. However, if you set up a model - using equations from Relativity etc - it all becomes a lot clearer. In fact, there are simulations which show what happens to 'point masses' as the universe expands (sorry that I don't have a link to one for you) - you see that most quite quickly clump together (as clusters and super-clusters) and the clumps move apart as the universe continues to expand.
 
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  • #5
Dark matter would increase the gravitational attraction. As I understand it, the strongest evidence for dark matter comes from cosmologists studying the behaviour of galaxies. although do they take into account the possible existence of black holes at the centre of each galaxy?
 
  • #6
Indeed expansion does not affect clustering in a way that it is visible today, let's say, galaxies inside clusters moving apart from each other.

But the dynamics of expansion affects the dynamics of clustering. As far as I know the cosmological models reveal that clusters tend to form faster or slower depending on the strengh of expansion.

Specifically, clusters form easier in models with no acceleration of the expansion (no dark energy). One should be able to find more or less of clusters at high redshifts (number count) depending on whether expansion decellerated or accelerated during the formation of galaxy clusters.

Deeper elaborations are behind my level, but as fas as I remenber there is a formula called Press-Schechter which may be used to describe this phenomenon.

Regards.
 
  • #7
The Local Groupe of gallaxies, in which we reside, is also held together by gravity.

But the Virgo Cluster is moving away from the Local Groupe. Cosmic expansion is sufficient to move the Local Groupe away from Virgo, but will probably be overcome some day, and the Milky Way and all the rest of us will begin moving towards Virgo.

So on the "small" scale, like superclusters and gallactic groupes, gravity prevails, pulling things closer together. And on the very large scale, like the distance between supurclusters, expansion dominates gravity, and moves things farther apart.

And the distance between the Local Groupe and the Virgo Supercluster is kinda on the boarderline between those two.

Hmmmm... Virgo Supercluster; does that have nougat and caramell?
 
  • #8
Blueplanetbob said:
Dark matter would increase the gravitational attraction. As I understand it, the strongest evidence for dark matter comes from cosmologists studying the behaviour of galaxies. although do they take into account the possible existence of black holes at the centre of each galaxy?
Well, it's probably more astronomers studying galaxies :wink: (cosmologists only run computer models and make theories :tongue2:)

On the BH in galactic nuclei: these are, in fact, the easiest to 'see' - the inferred motions of stars, gas clouds, etc in the inner parts of a galaxy are fairly easy to measure (lots of light), and the physics is very straight-forward (pre-dates Newton, in a way; Kepler's Laws). The dark matter which is 'observed' is via gravitational lensing (or other gravitational distortions in images of more distant objects), via galaxy rotation curves (the outer parts indicate far more mass than is inferred from the light), via cluster X-ray (assumed to come from inter-galactic gas in equilibrium), and via galaxy motions in a cluster (mass estimates via the Virial Theorem). The good news for cosmologists is that the amounts of dark matter so 'observed' is approx the same as they think they need to have in order for their models to match other observations. :smile:
 
  • #9
PoPpAScience said:
Question: When Galaxies are being drawn into clusters, does expansion of the universe enter into the equation? In other words, does expansion effect clustering? :shy:
Yes expansion effects clustering
 
  • #10
Thanks for the replies, I sure would like to find the link to the simulations Neried mentioned. I'm away from home right now and look forward to having time at home to converse on this subject more.
 

1. What is galaxy clustering?

Galaxy clustering refers to the tendency of galaxies to group together in certain regions of space. This phenomenon is caused by the gravitational pull between galaxies, as well as the distribution of dark matter in the universe.

2. How does expansion affect galaxy clustering?

Expansion of the universe causes galaxies to move away from each other, which can impact their clustering. As the distance between galaxies increases, the strength of their gravitational pull decreases, leading to a decrease in clustering.

3. What methods are used to explore the impact of expansion on galaxy clustering?

Scientists use a variety of methods to study the impact of expansion on galaxy clustering, including computer simulations, observational data from telescopes, and statistical analyses. These methods help us understand how galaxies are distributed in the universe and how they are affected by expansion.

4. Can galaxy clustering provide insights into the expansion of the universe?

Yes, galaxy clustering can provide valuable insights into the expansion of the universe. By studying the distribution and clustering of galaxies, scientists can measure the rate of expansion and track its changes over time. This information can help us better understand the evolution of the universe.

5. What are the potential applications of studying the impact of expansion on galaxy clustering?

Studying galaxy clustering can have various applications, including helping us understand the nature of dark matter and dark energy, testing theories of gravity, and improving our understanding of the large-scale structure of the universe. This knowledge can also have practical applications, such as improving cosmological models and informing future space missions.

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