How do galaxies form and move?

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  • #1
bogie
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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?
 

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  • #2
russ_watters
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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.
 
  • #3
bogie
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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.
 
  • #4
russ_watters
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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.
 
  • #5
bogie
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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?
 
  • #6
hellfire
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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.
 
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  • #7
bogie
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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?
 
  • #8
Wallace
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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.
 
  • #9
bogie
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...

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?
 
  • #10
Wallace
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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.
 
  • #11
bogie
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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?
 
  • #12
Wallace
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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.
 
  • #13
bogie
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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?
 
  • #14
Wallace
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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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  • #15
bogie
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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.
 
  • #16
bogie
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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?
 
  • #17
Nereid
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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?
 
  • #18
bogie
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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).
 
  • #19
Nereid
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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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  • #20
bogie
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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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  • #21
bogie
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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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  • #22
cosmopot
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astro-ph/0510535, 0510536, 0605213

astro-ph/0510535, 0510536, 0605213
are the simple analytic models of galaxies and the universe.
 
  • #23
bogie
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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 canceled 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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  • #24
Nereid
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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 canceled 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)?
 
  • #25
Nereid
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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.
 
  • #26
bogie
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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.
I was going to take exception to the abstract because though I have ideas that go beyond the mainstream (who doesn’t) I also see a correlation between curved space and gravity even if we (mainstream and I) disagree about what causes curved space. GR does a fine job with gravity from what people say, and an “open” universe in GR trends toward flatness and that is what I think the consensus is moving toward, i.e. the universe is “open” and the expansion appears to even be accelerating.

You are right about your suspicion that I may not fully grasp the abstract I quoted but I am a pretty enthusiastic “buff” when it comes to having a grip on the big mainstream picture. The devil is in the details though so I will never question your suspicions about my level of understanding which is far below adequate by any professional standards.

I have no idea why this remark is addressed to me ... surely it's a matter for Janus or SpaceTiger (or chroot or Greg)?
Sorry about mentioning you in that remark.
 
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  • #27
bogie
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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.
Yes, and I sometimes get so impatient with how slowly the wheels turn.

I find myself sitting on the edge of my chair waiting for great new findings and conclusions, and my butt is getting sore . Can’t the professionals hurry things up a bit :)?

Remember that the idea of last scattering is still theory. Granted it fits nicely. Anyway, galaxy formation in the context that you point out, i.e. the top down and the bottom up approach still is linked to what mainstream calls the surface of last scattering in one way or another.

And there is still the issue of the hot beginning and where it all came from. Mainstream starts the instant after the “beginning” of the expansion. I can’t help it but to me the logic for considering the existence of a greater universe before and beyond the Big Bang is inescapable.

If that was the case, then almost any pre-conditions would include a greater universe surrounding our expanding universe. That would change the drivers of the expansion wouldn’t it?

This thread is about galaxy formation and movement but I should specify that I am talking about their movement relative to each other, i.e. expansion and accelerating expansion and the causes.
 
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  • #28
Wallace
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If I could summarise your the thrust of what you are asking Bogie, I would do it like this:

1) How do Galaxies form and is their formation related to the CMBR?

2) How and why do galaxies move apart?

3) Is there a link between the two?

Is this a fair summary? Assuming it is (correct me if I'm wrong...) I'll attempt to answer them.

1) If we take the question more broadly, i.e. 'How does structure form in the universe' then we see an very good link between structure and the CMBR. WE have analytically models supported by simulations that let us literally start with a universe that like the CMB tells us it looks at z~1100 and evolve it forwards to today and we find that the universe in our simulations looks very much like the universe we see around us.

Now galaxies are small scale structure in this picture and are harder to model due to complex gas physics and star formation. However these bits of physics tell us what particular form galaxies will take. The distribution of galaxies in the universe (how much they clump near each other, how big and frequent are 'void' regions containing no galaxies) follow very closely to what the simulations, that start with the information from the CMBR, tell us.

2) This one is deceptively simple. Galaxies move apart because they did so in the past. That's it. There is no mystical property of space or dark energy or anything else that we need to understand this. The material in the universe got an initial 'kick' in the beginning (i.e. the big bang) and had been moving away from each other ever since.

3) In short no, not in the way you seem to be asking. Considering the rate of expansion at different epochs changes the way structure forms but not in any way that is some new physics. As I said in answer to 1) the simulations we can perform have an expanding slightly lumpy universe that evolves to what we see today.
 
  • #29
bogie
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If I could summarize the thrust of what you are asking Bogie, I would do it like this:

1) How do Galaxies form and is their formation related to the CMBR?

2) How and why do galaxies move apart?

3) Is there a link between the two?

Is this a fair summary? Assuming it is (correct me if I'm wrong...) I'll attempt to answer them.

1) If we take the question more broadly, i.e. 'How does structure form in the universe' then we see a very good link between structure and the CMBR. WE have analytically models supported by simulations that let us literally start with a universe that like the CMB tells us it looks at z~1100 and evolve it forwards to today and we find that the universe in our simulations looks very much like the universe we see around us.

Now galaxies are small scale structure in this picture and are harder to model due to complex gas physics and star formation. However these bits of physics tell us what particular form galaxies will take. The distribution of galaxies in the universe (how much they clump near each other, how big and frequent are 'void' regions containing no galaxies) follow very closely to what the simulations, that start with the information from the CMBR, tell us.

2) This one is deceptively simple. Galaxies move apart because they did so in the past. That's it. There is no mystical property of space or dark energy or anything else that we need to understand this. The material in the universe got an initial 'kick' in the beginning (i.e. the big bang) and had been moving away from each other ever since.

3) In short no, not in the way you seem to be asking. Considering the rate of expansion at different epochs changes the way structure forms but not in any way that is some new physics. As I said in answer to 1) the simulations we can perform have an expanding slightly lumpy universe that evolves to what we see today.
I appreciate your analysis and you have expressed my questions nicely and you have answered pretty nicely :) … but:
I guess it is safe to say that those simulations start with the WMAP data and using what we observe and the best theory, are brought forward and as you say, our simulations look very much like the universe we see around us.

That was what I expected and of course that is what a causally connected universe in BBT would look like. The causal connection to the BB was intact when the CMBR was emitted and the evolution into galactic structure and even larger scale structure was under way from the beginning of the expansion, whether we know what initiated the expansion or what drives the expansion.

When I consider the expansion, and your answer #2 boiling it down nicely to the fact we get expansion from the initial kick, then we are toying with the right question but we are still missing the mark.

My reference to the logic for considering the existence of a greater universe before and beyond the Big Bang was mentioned because I think that such a consideration would result in simple explanations for what we observe.

Let’s consider the question: Has mainstream thinking addressed the issue of expansion and accelerating expansion from the perspective of the initial “kick” occurring within a pre-existing greater universe?
 
  • #30
Wallace
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Let’s consider the question: Has mainstream thinking addressed the issue of expansion and accelerating expansion from the perspective of the initial “kick” occurring within a pre-existing greater universe?

Well kind of, but it's shear speculation really. We can only observe the universe directly back until t~300,000 years (when the CMB photons were last scattered). We can trace back further by using theory to match to observables such as Big Bang Nucleosynthesis matching the observed H/He ratio. This gets us to t~some fraction of a second. Early than this our theories break down.

If you want to philsophies about 'before' the big bang (which in terms of science is a sentence devoid of meaning) and a 'greater' universe beyond our own, but it is pure speculation, it cannot be tested and it dosn't make any of our current theories more or less simple.

Cosmology is a science that deals with t~smalll fraction of a second until today. Anything outside of this cannot be science since it can not be based on observations or experiment.
 
  • #31
bogie
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Well kind of, but it's shear speculation really. We can only observe the universe directly back until t~300,000 years (when the CMB photons were last scattered). We can trace back further by using theory to match to observables such as Big Bang Nucleosynthesis matching the observed H/He ratio. This gets us to t~some fraction of a second. Early than this our theories break down.

If you want to philsophies about 'before' the big bang (which in terms of science is a sentence devoid of meaning) and a 'greater' universe beyond our own, but it is pure speculation, it cannot be tested and it doesn't make any of our current theories more or less simple.

Cosmology is a science that deals with t~smalll fraction of a second until today. Anything outside of this cannot be science since it can not be based on observations or experiment.
That is the generally accepted position and if science was limited to what we think we can prove then we have to wait until our thinking changes. Proofs can be subtle and unexpected.

It has been said that, "The discovery of dark energy was one of the biggest surprises in astronomy." But the apparent repulsive force of dark energy must be considered carefully before any monumental change is made to mainstream thinking.

Is the feeling best described as uncomfortable? Aren’t we uncomfortable that such a repulsive force could reside within the parameters of our known, visible, familiar expanding universe? Certainly it will take years of hard work to put the math together that can be accepted as mainstream.

The simple answer will have to be rejected for now. It is too much against the mainstream to consider that our known, visible, familiar expanding universe is expanding into a greater universe of lower energy density. It might seem that the only clue to the existence of such a greater universe is the acceleration itself if you are strongly ensconced in the mainstream, but if you are not so firmly ensconce there is simple logic that says that there must be a before and beyond the big bang.

If science can put any comprehensible explanation and math together to keep dark energy a repulsive force from within our familiar playing field they are committed to do that. IMHO, only as a last resort and with the greatest evaluation, study, theorizing and serious debate will science ever be able to say, "Maybe we are expanding into a greater universe."

Until then a good discussion of such a possibility is still an appropriate part of a deliberate approach to understanding dark energy. Such a discussion is appropriate because we may end up having to accept that the big bang occurred in pre-existing space from pre-existing energy.
 
  • #32
Nereid
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Yes, and I sometimes get so impatient with how slowly the wheels turn.

I find myself sitting on the edge of my chair waiting for great new findings and conclusions, and my butt is getting sore . Can’t the professionals hurry things up a bit :)?
I guess it's somewhat a matter of perspective, don't you think?

Consider that it's less than 100 years since Zwicky's paper on dark matter, Hubble's on the distance-redshift relationship, and even on http://en.wikipedia.org/wiki/The_Great_Debate" [Broken]. And photographic plates were the primary tools for obtaining images (in the visual waveband) for a significant part of the working lives of many currently active professional astronomers.

The discovery of astronomical gravitational lenses (http://adsabs.harvard.edu/cgi-bin/n...mp;db_key=AST&data_type=HTML&format="?) is not yet 30 years' old!
Remember that the idea of last scattering is still theory.
A rather odd expression ... after all, isn't everything in astrophysics, beyond our solar system, "still theory"?
Granted it fits nicely. Anyway, galaxy formation in the context that you point out, i.e. the top down and the bottom up approach still is linked to what mainstream calls the surface of last scattering in one way or another.
Well, in that sense, you could also say (as Wallace pointed out) that it's also linked with current theories of gravity (GR), and of quantum mechanics ... IOW, astrophysicists bring the full range of modern physics to bear on this puzzle.
And there is still the issue of the hot beginning and where it all came from. Mainstream starts the instant after the “beginning” of the expansion. I can’t help it but to me the logic for considering the existence of a greater universe before and beyond the Big Bang is inescapable.
Indeed.

But we're doing science here, not engaging in idle speculation ...

If there is anything potentially observable that could give us a handle on "a greater universe before and beyond the Big Bang", don't you think thousands of scientists would be working on it, day and night?
If that was the case, then almost any pre-conditions would include a greater universe surrounding our expanding universe. That would change the drivers of the expansion wouldn’t it?
Who knows?

Without something quantitative, and potentially testable, your speculation is no better, or worse, than anyone else's ... or do you have a tool that no one else has thought to use yet?
This thread is about galaxy formation and movement but I should specify that I am talking about their movement relative to each other, i.e. expansion and accelerating expansion and the causes.
I think Wallace covered this well.
 
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  • #33
Nereid
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That is the generally accepted position and if science was limited to what we think we can prove then we have to wait until our thinking changes. Proofs can be subtle and unexpected.

It has been said that, "The discovery of dark energy was one of the biggest surprises in astronomy." But the apparent repulsive force of dark energy must be considered carefully before any monumental change is made to mainstream thinking.

Is the feeling best described as uncomfortable? Aren’t we uncomfortable that such a repulsive force could reside within the parameters of our known, visible, familiar expanding universe? Certainly it will take years of hard work to put the math together that can be accepted as mainstream.

The simple answer will have to be rejected for now. It is too much against the mainstream to consider that our known, visible, familiar expanding universe is expanding into a greater universe of lower energy density. It might seem that the only clue to the existence of such a greater universe is the acceleration itself if you are strongly ensconced in the mainstream, but if you are not so firmly ensconce there is simple logic that says that there must be a before and beyond the big bang.

If science can put any comprehensible explanation and math together to keep dark energy a repulsive force from within our familiar playing field they are committed to do that. IMHO, only as a last resort and with the greatest evaluation, study, theorizing and serious debate will science ever be able to say, "Maybe we are expanding into a greater universe."

Until then a good discussion of such a possibility is still an appropriate part of a deliberate approach to understanding dark energy. Such a discussion is appropriate because we may end up having to accept that the big bang occurred in pre-existing space from pre-existing energy.
It may be worth your while, bogie, to invest some time in studying GR and how it applies to modern cosmology.

For example, "our known, visible, familiar expanding universe is expanding into a greater universe of lower energy density" is, I think you'll find, just a zero calorie word salad, devoid of anything useful that anyone could work on, from a scientific perspective.

Why? Well, to make sense of the various sets of good astronomical observational results, you need to use some theories. If you choose to reject those theories (GR, for example), and do not have any to put in their place, then the good astronomical observational results are essentially meaningless. And if you keep those theories (GR, for example), then the words after "is" are meaningless (or nonsense).

Although many folk no doubt wish it were otherwise, I for one can't see how cosmology can progress other than through the creation and testing of theories.
 
  • #34
bogie
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It may be worth your while, bogie, to invest some time in studying GR and how it applies to modern cosmology.

For example, "our known, visible, familiar expanding universe is expanding into a greater universe of lower energy density" is, I think you'll find, just a zero calorie word salad, devoid of anything useful that anyone could work on, from a scientific perspective.

Why? Well, to make sense of the various sets of good astronomical observational results, you need to use some theories. If you choose to reject those theories (GR, for example), and do not have any to put in their place, then the good astronomical observational results are essentially meaningless. And if you keep those theories (GR, for example), then the words after "is" are meaningless (or nonsense).

Although many folk no doubt wish it were otherwise, I for one can't see how cosmology can progress other than through the creation and testing of theories.
You do express the position of science vs. the realm of speculation. However, you did take a portion of my sentence out of context, label it, "zero calorie word salad, devoid of anything ..."

The full sentence was, "It is too much against the mainstream to consider that our known, visible, familiar expanding universe is expanding into a greater universe of lower energy density." With that you do agree from what you have said.

You did not address my comment about how subtle and unexpected proofs can be. I said that because to say that "our current understanding is that we can't know and much less prove anything beyond our event horizon" can change if the understanding of our universe changes.

You did not address the comments on dark energy, or of a discussion of dark energy including a discussion of how the existence of a greater universe might simplify the explanation of dark energy.

If you would rather that such a discussion be abandoned because it requires speculation, are you comfortable with abandoning all discussion in the scientific community that includes speculative ideas to start with?
 
  • #35
Nereid
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You do express the position of science vs. the realm of speculation. However, you did take a portion of my sentence out of context, label it, "zero calorie word salad, devoid of anything ..."

The full sentence was, "It is too much against the mainstream to consider that our known, visible, familiar expanding universe is expanding into a greater universe of lower energy density." With that you do agree from what you have said.
I've read this twice now, and must say that I don't know what it means - could you clarify please?
You did not address my comment about how subtle and unexpected proofs can be. I said that because to say that "our current understanding is that we can't know and much less prove anything beyond our event horizon" can change if the understanding of our universe changes.
Well yes, of course.

But that's not saying anything, is it?

I mean, "beyond our event horizon" implies GR (or something similar), so if a new theory replaces GR, then a new window opens up (or not; details matter).

And at any time, a new result - observational or experimental - may come along that leads to a change in our understanding of the universe.

And that change may be quite radical, or subtle, or totally boring ... but until that change, all we have is rank speculation, don't we?
You did not address the comments on dark energy, or of a discussion of dark energy including a discussion of how the existence of a greater universe might simplify the explanation of dark energy.
I thought I did ... unless you have something to replace, or extend, GR (or unless you have a much better handle on "DE"), all you've got is empty rhetoric, haven't you?

I mean, of course "the existence of a greater universe might simplify the explanation of dark energy" ... but then it might not ...
If you would rather that such a discussion be abandoned because it requires speculation, are you comfortable with abandoning all discussion in the scientific community that includes speculative ideas to start with?
Not me ... unless they've changed, discussion of overly speculative ideas is explicitly ruled out in PF's rules ...

OK, https://www.physicsforums.com/showthread.php?t=5374" (my bold):
Overly Speculative Posts:
One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in most of the PF forums, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional scientific discussion. Posts deleted under this rule will be accompanied by a private message from a Staff member, with an invitation to resubmit the post in accordance with our Independent Research Guidelines. Poorly formulated personal theories, unfounded challenges of mainstream science, and overt crackpottery will not be tolerated anywhere on the site.
 
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