# How do galaxies form and move?

Just from general ideas gained over time I am under the impression that structure in our universe started with slight anisotropy in the CMBR. That slight fluctuation is gradually compounded and leads to structure, i.e. galaxy formation.

Building on that idea I am concluding that black holes form inside galaxies and a balance is reached in the distribution of mass in the galaxy so the central mass usually represents about 1% or 2%, perhaps regulated by the jets near the center and accretion at the outer reaches?

Having asked this question a year or so on a forum about whether galaxies form from the black hole or if the black hole forms inside the galaxy I have found that both views were supported.

Is there now any prevailing view on this matter?

## Answers and Replies

russ_watters
Mentor
Unless there is a miswording there, a galaxy cannot form "from" a black hole, since matter can't come out of the black hole to form a galaxy.

Unless there is a miswording there, a galaxy cannot form "from" a black hole, since matter can't come out of the black hole to form a galaxy.
I think that view included the mass of a black hole attracting the matter that formed the galaxy, not the galaxy coming out of the black hole.

russ_watters
Mentor
Since the total gravitational field strength of a giant cloud of "stuff" is the same for the same amount of mass regardless of whether there are black holes in the middle or not (and doesn't change before or after they form), I don't see how it could be said that the black holes pull in the matter to form the galaxy.

That sounds a lot like the common "cosmic vacuum cleaner" misconception about black holes.

Since the total gravitational field strength of a giant cloud of "stuff" is the same for the same amount of mass regardless of whether there are black holes in the middle or not (and doesn't change before or after they form), I don't see how it could be said that the black holes pull in the matter to form the galaxy.
Agreed. I'm trying to get to the current view of galaxy formation. Is my impression and conclusion in the OP anywhere near an acceptable description?

hellfire
Science Advisor
Galaxy formation is an extremely complex process of hierarchical structure formation with dissipative collapse of baryons, formation of dark matter halos, collisions and mergers, and feedback effects of star formation. A good pedagogical overview is given here:

http://abyss.uoregon.edu/~js/ast123/lectures/lec25.html

To my knowledge the role of supermassive black holes in that process is not fully understood.

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Galaxy formation is an extremely complex process of hierarchical structure formation with dissipative collapse of baryons, formation of dark matter halos, collisions and mergers, and feedback effects of star formation. A good pedagogical overview is given here:

http://abyss.uoregon.edu/~js/ast123/lectures/lec25.html

To my knowledge the role of supermassive black holes in that process is not fully understood.
Thank you for the link. That is a very good explanation. I see the graphic at the beginning showing the Big Bang, then the Dark Ages, then reionization.

Can you tell me where photon decoupling and the origin of the CMBR is on that time line? Is it at the start of the Dark Ages?

Is there a connection between the anisotropy of the CMBR and the formation of galactic structure?

Wallace
Science Advisor
Can you tell me where photon decoupling and the origin of the CMBR is on that time line? Is it at the start of the Dark Ages?

Yes, at the time indicated as ~300,000 years after the Big Bang

Is there a connection between the anisotropy of the CMBR and the formation of galactic structure?

Yes, tiny fluctuations seen in the CMBR are what develop into the structure we see in the universe. The precise mechanics of galaxy formation is not well understood as suggested above by others, but what that means is we can't perfectly model the way in which those fluctuations grew and turned into the structures we see. In the end, without those fluctuations having been there we would not have had galaxies form.

...

Yes, tiny fluctuations seen in the CMBR are what develop into the structure we see in the universe. The precise mechanics of galaxy formation is not well understood as suggested above by others, but what that means is we can't perfectly model the way in which those fluctuations grew and turned into the structures we see. In the end, without those fluctuations having been there we would not have had galaxies form.
Thank you.

OK, so given that we don't yet know precisely, we can make the connection between galaxy formation and the fluctuations in the CMBR.

So the formation of black holes in the center of galaxies is the next part as I mentioned in the OP. I often am wrong but logic would suggest that as galaxies form there is a natural process that enables them or requires them to form a black hole. Is my logic good or am I off on this?

It appears that there is a balance in the distribution of the mass as I mentioned, with 1% or 2% positioned in the black hole. Is this the case or am I working with wrong or out dated ideas? Do we know anything about this process or do we have ideas of how such a balance could be maintained?

Wallace
Science Advisor
Remember that black holes also form because of the fluctuations seen in the CMBR, we are just not completely sure of all the details leading to the kinds of arrangements of galaxies and black holes that we see.

I'm not quite sure what you mean by asking how 'such a balance could be maintained'. Are you asking how a black hole could sit in the middle of a galaxy and not gobble it up? If so then you may have been mislead by the "cosmic vacuum cleaner" misconception as suggested by Russ previously. There is nothing particularly special about black holes in the macroscopic sense, they are just a gravitating body so things can happy orbit them as easily as anything else. We don't worry about why the Sun dosn't gobble up the solar system and the same reasoning applies for galaxies and black holes. Apologies if this is not what you were thinking, I wasn't exactly sure what your question was asking.

Remember that black holes also form because of the fluctuations seen in the CMBR, we are just not completely sure of all the details leading to the kinds of arrangements of galaxies and black holes that we see.
I understand. I'll check again in a year or so :).
I'm not quite sure what you mean by asking how 'such a balance could be maintained'.
I am basing that question on my impression that there is a black hole in most galaxies and that it makes up about 1.5% of the mass of the galaxy. This could be completely wrong so let's clear that up first. Are you familiar with this 1.5% relationship?

Wallace
Science Advisor
Sorry I'm not familiar with any estimate of the standard black hole to galaxy mass ratio.

Perhaps I and others have been too negative about what we know about galaxy formation. We know a hell of a lot, we just don't yet claim to have the full picture, so 'checking again in a year or so' might not be the best option. You could spend a year learning what we do know so far about galaxies and you still wouldn't have learned it all!

When (if?) we do work out exactly how galaxies form it won't be in the form of a several sentence answer that you could post on a forum! In basic terms we do have a handle of galaxy formation, the unknown is in the details.

Sorry I'm not familiar with any estimate of the standard black hole to galaxy mass ratio.

Perhaps I and others have been too negative about what we know about galaxy formation. We know a hell of a lot, we just don't yet claim to have the full picture, so 'checking again in a year or so' might not be the best option. You could spend a year learning what we do know so far about galaxies and you still wouldn't have learned it all!

When (if?) we do work out exactly how galaxies form it won't be in the form of a several sentence answer that you could post on a forum! In basic terms we do have a handle of galaxy formation, the unknown is in the details.
Thank you. In this case just knowing that there is a relationship between anisotropy of the CMBR and galaxy formation, and that we know a lot but we don't know the exact details is good enough.

One more thing while I have a thread going about galaxies. I think it is safe to say that galaxies are in motion relative to each other. I think it is safe to make a relationship between the concept of expansion and the relative motion of galaxies; in general they are all moving away from each other.

I think it safe to say the if accelerating expansion is the case, that galaxies would appear to be moving away from each other at an accelerating rate.

My question is, if accelerating expansion is the case, can we conclude that in terms of closed, flat or open, the universe would be open? In other words, there is no indication in theory that deceleration would ever occur given the case of acceleration?

Wallace
Science Advisor
I think it safe to say the if accelerating expansion is the case, that galaxies would appear to be moving away from each other at an accelerating rate.

This is a reasonable statement, with some caveats about the 'moving away' part, since thinking entirely kinematically about receding galaxies implies that they can move apart at greater than the speed of light, which can't be explained kinematically. This question however is a can-o-worms that has been opened in many other threads, so it's probably best not to confuse this present discussion with it! So yes, in basic terms your statement that the galaxies appear to be moving away from each other at an increasing rate is reasonable.

My question is, if accelerating expansion is the case, can we conclude that in terms of closed, flat or open, the universe would be open? In other words, there is no indication in theory that deceleration would ever occur given the case of acceleration?

The simple closed (universe collapses in big crunch), open (expansion rate goes to a linear relationship with t) and flat (the balance line between the two) picture you may have seen is only valid for matter only universes. In this case the universe model depends only on the total amount of mass relativity to a critical value.

You cannot get acceleration in this picture at all, since matter will not accelerate the expansion, you need dark energy to do this (in the standard picture anyway) Once you add dark energy into the picture the universe models become more complex, since it depends not only on the amount of dark energy but also its properties. So you could have a flat universe that has a big crunch or a big rip or a closed universe that never re-collapses etc etc. It all depends on the properties of the dark energy.

This attached image may help to guide your understanding of our current best guess model of the universe. What it shows is that initially the universe was in fact deccelerating as matter was the dominant energy component. At a certain time (roughly 5-6 Billion years ago I think, somewhere around there) dark energy became the dominant component in the universe, and began to cause the expansion to accelerate. So the theory does in fact have a deccelerating and accelerating phase. Once you add dark energy the flat, closed and open models are not a very useful tool to understanding the dynamics of the expansion.

The reason that matter dominates early on and dark energy later is that the density of dark energy stays roughly the same for all time (if dark energy is a cosmological constant it stays exactly the same for all time). By contrast the density of matter drops with the inverse cube of the scale factor of the universe, since if you double the volume of the universe, but still only have the same amount of matter then the density will halve. Therefore the greater the total expansion since t=0 the more dominant dark energy will be over matter. Whichever component has the greater density at a given time will dominate the dynamics of the universe, early on matter, later dark energy.

Another way to think about this might be that if dark energy is vacuum energy, i.e. some inherent energy associated with space itself (which is a possible candidate for dark energy, and almost equivalent to a cosmological constant) then as the universe expands there is more space in the universe hence more dark energy in total. By contrast the total amount of matter stays the same, reducing the matter density as the expansion proceeds.

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This is a reasonable statement, with some caveats about the 'moving away' part, ...
Wallace, thank you. I am very impressed with your post.

I asked the moderator to change the name of the thread from "How galaxies form" to 'How galaxies form and move" in hopes I could carry on more of this discussion.

Before I venture to pose some additional questions though, I am waiting to hear back from the moderator on guidelines that describe the extent that I can bring up speculative ideas about the thread topic. I'm not promoting any over speculative ideas but in the normal course of thinking through what you have said and thinking about the thread topic, thoughts enter my mind. I'm am sure that others have had these same thoughts and thoughts lead to questions, some of which display various degrees of speculation.

http://www.sciencedaily.com/releases/2007/04/070403134630.htm

“The greatest concentrations of star formation are found in the so-called starburst regions near the ends of the galaxy’s strong galactic bar.”

This article and the NGC 1672 galaxy seem to lead to the conclusion that the galaxy may be growing by accumulating “dust”.

That idea supports my thinking about galaxy formation and the sequence of events starting from the slight anisotropy that existed in the CMBR and the future structure of our universe, i.e. galaxy formation.

Any thoughts on this observation?

Nereid
Staff Emeritus
Science Advisor
Gold Member
http://www.sciencedaily.com/releases/2007/04/070403134630.htm

“The greatest concentrations of star formation are found in the so-called starburst regions near the ends of the galaxy’s strong galactic bar.”

This article and the NGC 1672 galaxy seem to lead to the conclusion that the galaxy may be growing by accumulating “dust”.
How did you so conclude?
That idea supports my thinking about galaxy formation and the sequence of events starting from the slight anisotropy that existed in the CMBR and the future structure of our universe, i.e. galaxy formation.

Any thoughts on this observation?
Sounds rather speculative - have you written a paper on this yet? Or worked - or sketched - out the mechanisms by which this might happen?

Thanks for the warning

How did you so conclude?Sounds rather speculative - have you written a paper on this yet? Or worked - or sketched - out the mechanisms by which this might happen?
Hello Nereid.

Did you read the article? Paragraphs two and three lead to the conclusion I made about the article. Maybe you have an different opinion? Please contribute (or did you just want to let me know that you are watching).

Nereid
Staff Emeritus
Science Advisor
Gold Member
Hello Nereid.

Did you read the article? Paragraphs two and three lead to the conclusion I made about the article. Maybe you have an different opinion? Please contribute (or did you just want to let me know that you are watching).
I read it twice the first time, and once again just now ... I still can't see how you came to that conclusion!

That barred spirals somehow funnel gas into the nucleus is hardly news - I recall an ESO PR on this several years ago, for example, and you'll find lots of papers on the topic if you do some searching on http://adswww.harvard.edu/" [Broken].

This PR, and the topic in general, is about how (spiral) galaxies are evolving "today"; there's little, if anything, from this research that points directly to mechanisms of galaxy formation.

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I read it twice the first time, and once again just now ... I still can't see how you came to that conclusion!

That barred spirals somehow funnel gas into the nucleus is hardly news - I recall an ESO PR on this several years ago, for example, and you'll find lots of papers on the topic if you do some searching on http://adswww.harvard.edu/" [Broken].
You're right. I like that ADS site. I've added it to my favorites.
This PR, and the topic in general, is about how (spiral) galaxies are evolving "today"; there's little, if anything, from this research that points directly to mechanisms of galaxy formation.
All we have to go on is what we see happening today, right? What research do you look at for mechanisms of galaxy formation. What scenario is there to link it to anisotropy of the CMBR?

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I read it twice the first time, and once again just now ... I still can't see how you came to that conclusion!

That barred spirals somehow funnel gas into the nucleus is hardly news - I recall an ESO PR on this several years ago, for example, and you'll find lots of papers on the topic if you do some searching on http://adswww.harvard.edu/" [Broken].

This PR, and the topic in general, is about how (spiral) galaxies are evolving "today"; there's little, if anything, from this research that points directly to mechanisms of galaxy formation.
There should be more interest in this topic IMHO.

Does it occur to anyone else that the reason that we aren't doing very good at describing how galactic structure comes about is that we don't yet have a consensus on what is causing expansion itself?

If that is the case, then the several proposed causes of expansion should each have a description of how galactic structure comes about. Where does theory stand on this subject, given that there are no solid facts and no "standard" theory that seems worthy of mention.

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astro-ph/0510535, 0510536, 0605213

astro-ph/0510535, 0510536, 0605213
are the simple analytic models of galaxies and the universe.

astro-ph/0510535, 0510536, 0605213
are the simple analytic models of galaxies and the universe.
I love that third link.

http://arxiv.org/abs/astro-ph/0605213

It is perfect for this thread: "The standard model of expanding universe is based on the theory of general relativity (GR) which assumes that spacetime is curved. The reason of curved spacetime was given by Einstein that locally there is common acceleration for all test particles so that gravity is canceled. This is called the equivalence principle. The present paper shows that it is not true for Schwarzschild solution (static gravity of pure spatial inhomogeneity). The paper also presents isotropic but temporally inhomogeneous gravity. Freely falling particles locally have accelerations of any magnitude and any direction, which also indicates that the gravity can not be locally cancelled too. Realistic gravity is non-static which is the case in between. This indicates that the assumption of curved spacetime is a fundamental mistake. Therefore, a correct gravitational theory or a model of the universe must be based on the absolute flat background spacetime. The existence of such absolute spacetime is shown to be true from the following three basic principles about the universe: (1) the density of large-scale mass distribution of the universe varies with time (corresponding to an isotropic but temporally inhomogeneous gravitational field); (2) the gravity is described by a Lagrangian which is the generalization to the proper distance of special relativity (the metric form of GR); (3) Hubble law is approximately true. These lead to varying light speed and give account of accelerating expansion. Therefore, the assumption of big bang and expansion is incorrect."

Neried, this statement sounds terribly speculative and since you flinched at my earlier statement I would assume that this statement may be considered too speculative to discusss here. Am I correct, or can I make some comments about it without getting into trouble?

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Nereid
Staff Emeritus
Science Advisor
Gold Member
I love that third link.

http://arxiv.org/abs/astro-ph/0605213

It is perfect for this thread: "The standard model of expanding universe is based on the theory of general relativity (GR) which assumes that spacetime is curved. The reason of curved spacetime was given by Einstein that locally there is common acceleration for all test particles so that gravity is canceled. This is called the equivalence principle. The present paper shows that it is not true for Schwarzschild solution (static gravity of pure spatial inhomogeneity). The paper also presents isotropic but temporally inhomogeneous gravity. Freely falling particles locally have accelerations of any magnitude and any direction, which also indicates that the gravity can not be locally cancelled too. Realistic gravity is non-static which is the case in between. This indicates that the assumption of curved spacetime is a fundamental mistake. Therefore, a correct gravitational theory or a model of the universe must be based on the absolute flat background spacetime. The existence of such absolute spacetime is shown to be true from the following three basic principles about the universe: (1) the density of large-scale mass distribution of the universe varies with time (corresponding to an isotropic but temporally inhomogeneous gravitational field); (2) the gravity is described by a Lagrangian which is the generalization to the proper distance of special relativity (the metric form of GR); (3) Hubble law is approximately true. These lead to varying light speed and give account of accelerating expansion. Therefore, the assumption of big bang and expansion is incorrect."
I'm not sure how much of this preprint you've understood Bogie, but I don't think this has anything to do with what you want to discuss.

In any case, a) it's a preprint (v5 too!), and clearly is not yet in acceptable form (or it would already have been published); b) if you read the GPB prediction, which didn't make it into the GPB thread that Garth started, I think you'll see that it's likely this idea is falsified by the preliminary GPB results.
Neried, this statement sounds terribly speculative and since you flinched at my earlier statement I would assume that this statement may be considered too speculative to discusss here. Am I correct, or can I make some comments about it without getting into trouble?
I have no idea why this remark is addressed to me ... surely it's a matter for Janus or SpaceTiger (or chroot or Greg)?

Nereid
Staff Emeritus
Science Advisor
Gold Member
There should be more interest in this topic IMHO.

Does it occur to anyone else that the reason that we aren't doing very good at describing how galactic structure comes about is that we don't yet have a consensus on what is causing expansion itself?
The main reasons why our understanding of how (big, old) galaxies form is that there's too little in the way of observational constraints yet.

After all, galaxy formation occurred sometime between the surface of last scattering (~300k years after the BB) and z ~6 (which is about as far as we can see, so far).

AFAIK, there are two principal classes of models for galaxy formation - top down, and bottom up (hierarchical formation). The latter seems more consistent with the meagre data available to date, but it's a whole class of models, so no surprise that it's a vigorous field of research.
If that is the case, then the several proposed causes of expansion should each have a description of how galactic structure comes about. Where does theory stand on this subject, given that there are no solid facts and no "standard" theory that seems worthy of mention.
See above.