# Crashing into Andromeda

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53 minutes ago, Col Not Colin said:

Hi again.

You could start another thread for what is another topic - but it's your call.

Greatest contribution to cosmology?    The Hubble Law.  Obviously,  just one opinion.

Einstein and General Relativity showing how mass/energy density warp, curve, twist spacetime.

Person? Carl Sagan who personally sparked my interest in cosmology but sadly too late to really do much about it except learn. As he said Cosmology/astronomy is a humbling and character building experience...from that unforgetable description entitled "Pale Blue Dot"

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1 hour ago, Col Not Colin said:

Hi again.

You could start another thread for what is another topic - but it's your call.

Greatest contribution to cosmology?    The Hubble Law.  Obviously,  just one opinion.

I would put into consideration a different contribution made by Hubble prior to his Law.  In 1924 he discovered Cepheid variable stars in Andromeda, which he was able to use to show that Andromeda was far outside our own galaxy, and not a part of it.   While there had been debate about this., his observations were the first to provide exidence that the cosmos was much more vast than previously suspected and extended well beyond the Milky way.

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Hi @beecee,

Yes GR is definitely a significant contribution and I nearly said that.   Not sure about Carl Sagan but that's just a geography thing.  In my part of the world we had other science presenters.

Hi @Janus,

Also a good suggestion.

Thanks for quoting me but I think it was Wildie9 who started the new discussion - so I'll wave his flag.  @wildie9  You may also want to read some of the above.

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6 hours ago, beecee said:

Einstein and General Relativity showing how mass/energy density warp, curve, twist spacetime.

It “warps” and “curves” (in the sense of geodesic deviation), but it doesn’t “twist” - in GR there is (by definition) no torsion.

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4 hours ago, Markus Hanke said:

It “warps” and “curves” (in the sense of geodesic deviation), but it doesn’t “twist” - in GR there is (by definition) no torsion.

By twist, I meant the Lense Thirring Effect. Poor choice of word.

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On 2/10/2021 at 4:02 AM, Bufofrog said:

Because the gravitational attraction between Andromeda and the Milky Way is greater than the expansion.  All the galaxies in our local group are moving towards each other.

All of the galaxies in our supercluster, not only the local group, are ORBITING each other.  Some are even moving towards each other, such as Andromeda and the Milky Way.

One of the biggest misunderstanding in cosmology is: due to dark energy, galaxies are ALL moving apart.  ONLY superclusters move apart.  All galaxies inside the supercluster are gravitationally bound forever, and will eventually merge together into one giant galaxy, then one giant supermassive black hole, right?

"Larger black holes of up to 1014 (100 trillion) M may form during the collapse of superclusters of galaxies."

Edited by Airbrush
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Hope you are well.

6 hours ago, Airbrush said:

The biggest misunderstanding in cosmology is saying, that due to dark energy, galaxies are ALL moving apart.

Yes, although the emphasis probably needs to be put on "dark energy" and we might just as well replace it with Neil deGrass Tyson's favourite description of it as "badly behaved gravity".  We don't have a good understanding of what it is or where it is.  Perhaps it's not even a physical thing but just a requirement that the metric is found from an equation with a suitable cosmological constant.  Finally, perhaps it's a sign that our model of gravity is breaking down.  It's badly behaved gravity that is yet to be explained.

We are reasonably certain that, on the largest scales, space is expanding.  We are considerably less certain about whether space is expanding everywhere in the same way.  On the smallest scales, is the space between two atoms in a molecule expanding?  I can find references on both sides.

Whatever the case, we can just point out that the dark energy problem isn't important on small scales, or if you prefer, that gravity isn't always that badly behaved - our local group is gravitationally bound exactly as we would expect.

If dark energy was some kind of particle, then maybe it's just a case of checking to see if there's a particle of dark energy in-between atoms of a molecule, or thousands of particles in-between galaxies   (but that would be too easy, so we can be fairly sure it won't be anything like that).

Also, an earlier version of your post asked a bit more about what happens when galaxies collide -  Joigus put a nice simulation up on this thread about 1 page back.

Edited by Col Not Colin
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9 hours ago, Col Not Colin said:

Perhaps it's not even a physical thing but just a requirement that the metric is found from an equation with a suitable cosmological constant.

Yes, this would be the best way to look at it. The form of the gravitational field equations is determined by a set of basic mathematical and physical requirements, and the most general form of equation that fulfils these requirements just happens to be the Einstein equations with cosmological constant. It’s essentially just a background curvature that is there even in the absence of all other sources; the presence of such a background curvature modifies all other solutions obtained from the equation - bearing in mind, of course, that this modifications isn’t just a linear combination of solutions, but something more complicated.

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Hi @Markus Hanke,   hope you are well.

I like GR as much as the next person and I'd love nothing more than Einstein's Field Equations to apply everywhere with a cosmological constant - but is that enough to overlook the level of ignorance we have about Dark Energy?

What makes you so sure that the Einstein Field Equations with cosmological constat applies to such small scales as a bond length within a molecule?

We don't have an exact value for $\Lambda$ but just an approximation based on Astronomical observations and this includes adjusting the proportion of dark matter to obtain a good fit to the observations.   To the best of my knowledge it isn't a full picture or a perfect fit to the data but just the best we can get.  Worse than this, Dark matter is much more likely to be some sort of particle we may identify and then if we obtain some measurements of the amount of it in the universe, the value of the cosmological constant would need some adjustment again.

If our estimated proportions of dark matter and dark energy tell us anything at all, then it's got to be that most of what's out there isn't what we thought it was.  Don't get me wrong, I'd hate it if it was true  - but the statement made earlier   "perhaps it's a sign that our model of gravity is breaking down" is something we have to consider.

Edited by Col Not Colin
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2 hours ago, Col Not Colin said:

I like GR as much as the next person and I'd love nothing more than Einstein's Field Equations to apply everywhere with a cosmological constant - but is that enough to overlook the level of ignorance we have about Dark Energy?

What makes you so sure that the Einstein Field Equations with cosmological constat applies to such small scales as a bond length within a molecule?

Is there supposed to be a connection between these statements? If so, what is it?

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1 minute ago, swansont said:

Is there supposed to be a connection between these statements? If so, what is it?

Ummm....  I'm not sure what you're asking.  I try to have an opening that just says "hi" or otherwise sets a more reasonable scene for discussion.  You could take it out if you wanted but the rest of the post may seem a bit too hard and direct then (and I really wouldn't want you to do that).

Statement 1 was intended to soften the impact of the rest of the post rather than making it appear to be an outright disagreement.  I do like General Relativity and would like to suggest that I am not the enemy or deliberately antagonistic to any view that GR should be applied to all situations.

Statement 2 was put there to indicate that, whatever we may like to do, that doesn't mean that we can or should do that thing.  In particular, I'm not sure that space expands everywhere in the same way as that indicated by the Hubble law.   In particular, it may not apply over these small scales (bond lengths).

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1 hour ago, Col Not Colin said:

Ummm....  I'm not sure what you're asking.  I try to have an opening that just says "hi" or otherwise sets a more reasonable scene for discussion.  You could take it out if you wanted but the rest of the post may seem a bit too hard and direct then (and I really wouldn't want you to do that).

Statement 1 was intended to soften the impact of the rest of the post rather than making it appear to be an outright disagreement.  I do like General Relativity and would like to suggest that I am not the enemy or deliberately antagonistic to any view that GR should be applied to all situations.

Statement 2 was put there to indicate that, whatever we may like to do, that doesn't mean that we can or should do that thing.  In particular, I'm not sure that space expands everywhere in the same way as that indicated by the Hubble law.   In particular, it may not apply over these small scales (bond lengths).

Apologies for not being clear: you mention dark energy, and then measuring the effect of GR on the level of a bond length. Are these connected? If so, how?

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Hi Swansont,

2 minutes ago, swansont said:

Apologies for not being clear: you mention dark energy, and then measuring the effect of GR on the level of a bond length. Are these connected? If so, how?

Thanks and best wishes to you.  I think the main thing I wanted to get across was that   the expansion of space   may not apply uniformly.   It applies on the largest scales of distances between distant galaxaies but not necessarily on the scale of lengths between atoms in a molecule.   However, yes, since we don't have an adequate understanding of dark energy the typical form of the Einstein Field Equations with a cosmological constant may not apply on these small scales, if you want to look at it that way.

Dark Energy can mean several things.  Here's a few ideas (although I'm not suggesting you didn't already know them, just trying to open the discussion to anyone to follow):

1.     In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales....    [Wikipedia - see below]

2.    When we talk about dark energy, it might turn out to be a cosmological constant. Certainly, when we take all of the observations we have so far, it appears that dark energy is consistent with being a cosmological constant....      [https://www.forbes.com/sites/startswithabang/2020/12/25/ask-ethan-is-einsteins-cosmological-constant-the-same-as-dark-energy/]

13 hours ago, Markus Hanke said:
23 hours ago, Col Not Colin said:

(Referring to dark energy)....Perhaps it's not even a physical thing but just a requirement that the metric is found from an equation with a suitable cosmological constant.

Yes, this would be the best way to look at it. The form of the gravitational field equations is determined by a set of basic mathematical and physical requirements, and the most general form of equation that fulfils these requirements just happens to be the Einstein equations with cosmological constant. It’s essentially just a background curvature that is there even in the absence of all other sources; the presence of such a background curvature modifies all other solutions obtained from the equation.

3.   Two proposed forms of dark energy are the cosmological constant,[11][12] representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities having energy densities that can vary in time and space.              [https://en.wikipedia.org/wiki/Dark_energy]

4.   In an earlier post, I suggested more generally that we don't know much about dark energy at all and it could be described as "badly behaved gravity".  I must admit I'm struggling to find an exact reference to where Neil DeGrasse Tyson called it "badly behaved gravity", maybe that's a term I only half-remembered.  I can find references to Tyson calling it "Dark Gravity", "Unexplained Gravity", suggesting we don't call it anything, or that we call it "Fred" if we must.  The essence of the idea remains - it is potentially misleading to give it a name which suggests it's a thing like a particle;  or to suggest it's just a constant required in an equation.  It is gravity behaving badly, something yet to be fully explained.

------

Now to connect with the later statement concerning bond lengths:   If dark energy is adequately explained as a cosmological constant in the Einstein Field Equations then it is (fairly obviously) a constant that appears in the equations and it should apply wherever and whenever we apply those field equations to some region of space.  In particular space would generally be seen to be expanding and this applies fairly uniformly from the scale of the lengths between distant galaxies right down to the lengths between atoms in a molecule.  (There is an exception*).

However, if dark energy is a particle or some field with values that vary in space and time, then the effects of this dark energy are not uniform.  In particular, it is quite possible that the spaces between atoms in a molecule don't show the same kind of expansion that the universe as whole exhibits.  We have astronomical observations that distant galaxies are receeding which leads to our requirment for the expansion of space but these observations can be satisfied by a space where most of that expansion is happening between the galaxies rather than within them.

*We can even accept the cosmological constant and still attempt to exhibit small-scale situations where the expansion of space doesn't appear.  It's already been mentioned that the field equations are NOT linear and so the cosmological constant doesn't always force a solution to the field equations which appears to have anything like a time-dependant scale factor multiplying the spatial components of the metric.

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34 minutes ago, Col Not Colin said:

Hi Swansont,

Thanks and best wishes to you.  I think the main thing I wanted to get across was that   the expansion of space   may not apply uniformly.   It applies on the largest scales of distances between distant galaxaies but not necessarily on the scale of lengths between atoms in a molecule.   However, yes, since we don't have an adequate understanding of dark energy the typical form of the Einstein Field Equations with a cosmological constant may not apply on these small scales, if you want to look at it that way.

The facts are that over smaller scales, the strong nuclear, weak nuclear, and gravity, overcome the expansion rate.

Is the expansion uniform? Best observational evidence so far tells us yes, it is, and that the universe is isotropic and homogenous  in all directions. However there maybe reason to believe this may not be the case.....

extracts:

"Based on our cluster observations we may have found differences in how fast the universe is expanding depending on which way we looked,” said co-author Gerrit Schellenberger of the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Massachusetts. “This would contradict one of the most basic underlying assumptions we use in cosmology today.”

The authors of this new study came up with two possible explanations for their results that involve cosmology. One of these explanations is that large groups of galaxy clusters might be moving together, but not because of cosmic expansion. For example, it is possible some nearby clusters are being pulled in the same direction by the gravity of groups of other galaxy clusters. If the motion is rapid enough it could lead to errors in estimating the luminosities of the clusters.

These sorts of correlated motions would give the appearance of different expansion rates in different directions. Astronomers have seen similar effects with relatively nearby galaxies, at distances typically less than 850 million light years, where mutual gravitational attraction is known to control the motion of objects. However, scientists expected the expansion of the universe to dominate the motion of clusters across larger distances, up to the 5 billion light years probed in this new study.

A second possible explanation is that the universe is not actually the same in all directions. One intriguing reason could be that dark energy – the mysterious force that seems to be driving acceleration of the expansion of the universe – is itself not uniform. In other words, the X-rays may reveal that dark energy is stronger in some parts of the universe than others, causing different expansion rates.

“This would be like if the yeast in the bread isn’t evenly mixed, causing it to expand faster in some places than in others,” said co-author Thomas Reiprich, also of the University of Bonn. "It would be remarkable if dark energy were found to have different strengths in different parts of the universe. However, much more evidence would be needed to rule out other explanations and make a convincing case."

Either of these two cosmological explanations would have significant consequences. Many studies in cosmology, including X-ray studies of galaxy clusters, assume that the universe is isotropic and that correlated motions are negligible compared to the cosmic expansion at the distances probed here.

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At present I would say the principle of isotropy and homogeneity still appear to hold overall...but obviously the science of cosmology is an ever changing discipline, based on new and further observational and experimental data.

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Hi Beecee and thanks for your article, I enjoyed reading that.  Some evidence that dark energy is not uniform, thank you very much +1.

2 hours ago, beecee said:

The facts are that over smaller scales, the strong nuclear, weak nuclear, and gravity, overcome the expansion rate.

Yes, total agreement on this.  Even if there was some expansion of space, strong short range forces will dominate so that atoms and molecules don't tend to fly apart.  The same applies upto scales of the local group where we can say the constituents are gravitationally bound together.  It doesn't matter if space is expanding there, this can be overcome as you stated.  I think this was discussed earlier on in this thread (Bufofrog, Joigus and Janus should get a mention for that).

Sadly (for me, at any rate), it doesn't show that space IS expanding there.  Worse than this it makes it hard to test.  Bizarrely, it implies that any sensible co-ordinate system you set up in this region wouldn't give us any reason to believe that space was expanding (until you look outside the local group).  Maybe the whole problem is caused just because we try and use co-ordinates that emerge from the usual cosmological models with the FLRW metric - but let's put that to one side for the moment (it's far too late for me to discuss anything like that coherently).

You also mentioned isotropy and homogeneity of space.  Yes, that's important and assumed on cosmological scales.  However, space is not at all isotropic or homogenous at small scales.  We can have some non-uniformity in dark energy, or some regions of space that aren't expanding the same way.

Tired now but enjoyed the discussion, thanks to everyone, bye for now.

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13 hours ago, Col Not Colin said:

I like GR as much as the next person and I'd love nothing more than Einstein's Field Equations to apply everywhere with a cosmological constant - but is that enough to overlook the level of ignorance we have about Dark Energy?

Not necessarily. Identifying DE with the cosmological constant is only one possible option among several. It is also conceivable that DE is the effective result of the interplay between more than one factor, such as the presence of a cosmological constant in conjunction with some background scalar field. There is no consensus about this as of now.

13 hours ago, Col Not Colin said:

What makes you so sure that the Einstein Field Equations with cosmological constat applies to such small scales as a bond length within a molecule?

The cosmological constant has orders of magnitude of ~10^-52 per meter squared; for localised solutions on small local patches it is thus irrelevant.

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• 2 months later...

The collision of Andromeda and the Milky Way has already begun. The reason the collision occurs several billion years ahead of schedule is because the Andromeda galaxy is much larger than it appears. The bright stellar disk of this galaxy is about 120,000 light-years across, slightly larger than the Milky Way.

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