# Dark Galaxy found?

Gold Member
Dark Galaxy found!?

You know, I've always been secretly hoping that dark matter was an aberration of our observations or calculations, some value to be tweaked or some normalizing factor to be applied thing that was making galaxies turn faster than formulae predicted. I just couldn't believe that we are only observing 4% of the universe.
But it seems that idea has been blown apart.
They've discovered a Dark Galaxy.
http://www.space.com/scienceastronomy/050223_dark_galaxy.html" [Broken]
http://www.discover.com/issues/dec-05/cover/?page=2" [Broken]

It is completely invisible to light, it is only visible by its radio emissions. And even at that, only a tiny portion of it is visible even there. They can tell by its gravity that there's many, many times more mass in it than can be seen. Whatever the mass, it is completely undetectable directly. This is a galaxy that contains no (zero) stars.
Incredible. Stranger than fiction.

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Garth
Gold Member
Well they discovered ordinary hydrogen, from its 21cm radiation emission, so that is hardly exotic DM. But they also state: "From the speed it is spinning, we realized that VIRGOHI21 was a thousand times more massive than could be accounted for by the observed hydrogen atoms alone," Minchin said."

Now in an ordinary galaxy/cluster the ordinary baryonic matter is 0.002 closure density whereas the total baryonic matter is ~ 0.04 closure, so about 95% of ordinary baryonic stuff is invisible even in the Mainstream LCDM model. In this dark galaxy it seems this 20:1 invisible/visible baryonic matter ratio is some 50 times larger. Now this could require an exotic form of DM to be invoked, but we could be more certain if we knew where that 95% invisible baryonic matter was!

It could be that here is a resevoir of exotic (totally new physics) non-baryonic DM, or that in this galaxy the ordinary baryonic stuff never got round to forming stars and becoming visible, which do you think it is?

Garth

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SpaceTiger
Staff Emeritus
Gold Member
Unfortunately, this isn't the first claim of a "dark galaxy" to hit the presses. Even more unfortunately, these claims usually turn out to be bogus or easily explained some other way. In this particular case, it looks like the object could just as easily be tidal debris from a recent interaction. See here:

http://xxx.lanl.gov/abs/astro-ph/0505580"

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Chronos
Gold Member
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SpaceTiger,

That was an interesting link. I wonder if by applying Occam's razor we might be more inclined to believe it to simply be debri, rather than jumping to the conclusion that it is a dark matter galaxy?

Also, if there really is all this dark matter floating about, what does that say for our observations regarding the expanding universe? Wouldn't "dark energy" be required to overcome it and cause the perceived acceleration in the universe, and wouldn't this dark energy nullify the effect of dark matter so we must have more dark matter to overcome dark energy? Then, wouldn't we need more dark energy to overcome the effects of the of dark matter? And then wouldn't we need more dark matter... well, I think you get the picture.

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I would be extremely skeptical of any "dark galaxy" found in a cluster environment, since we know dynamical interaction and ram pressure stripping are both effective gas removalists.

If one of these things were to be found in isolation, then that would be a different matter.

Also, in collapsed systems, the effect of the dark energy is negligible as gravity dominates.

matt.o said:
Also, in collapsed systems, the effect of the dark energy is negligible as gravity dominates.
Sure, but everyone seems to have forgotten the basic rule about action and reaction. Applying this simple rule seems to indicate that you cannot have two distinct entities (dark matter and dark energy).

Where is the dark matter's proposed position in galaxies? It is mostly confined to a halo around the visible mass. Therefore, its edges are most exposed to the ant-gravity (I hate that word) effect of dark energy, effectively meaning that dark energy should nibble away at dark matter's placement in galaxies.

Think about it. Dark energy is strong enough to push on entire galaxies. Dark matter is a loose conglomeration of mass on the edges of these galaxies. If dark energy is a force, it must extend into the gravitational field of these galaxies, thereby nullifying (to some extent) the gravity on the edges of galaxies (this effect should be strongest at the edges if gravity rules as you say). Mass on the edges should simply sling away!

Since dark energy's purported task is to separate mass from mass, it should obviously work well at separating small mass and particles from galaxy edges. I.e. nibble away at galaxies. That is it should if it's an acceleration like gravity.

In short, it seems to me that the combination of these two opposing hypothetical forces in our universe acting in this way, would cause galaxies to evaporate away into space. Therefore, there should be a more homogenous dispersal of matter in the universe.

Supposing it is only a confining or "pushing" force that doesn't disturb the gravitational attraction of dark and ordinary matter (essentially creating a demarcation line at the edges of galaxies) we should then note that galaxies are denser at the edges. In other words, galaxies would need to exhibit a ring or sphere of denser matter at the edges, due to the compression effects of dark energy. Additionally, this would tend to make galaxies collapse upon themselves!

In conclusion, if dark energy deeply penetrates galaxies in radians, galaxies should evaporate. If it only pushes on the edges of galaxies, then galaxies should show signs of compression and perhaps collapse.

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I've explained that dark energy cannot be a distinct force acting between the galaxies, but I haven't explained what it could be.

Many think it could be an expansion of space itself, where the galaxies only appear to be accelerating because the space inbetween them is getting larger. However, I have issues with this because an apparent acceleration caused by expansion should exhibit the same local "anti-gravity" effects I listed in the prior post. That is that it should nibble away loose matter at the edges of galaxies. This is because this hypothesized expansion is occurring everywhere, only local gravity trumps it. However, weakly contained mass orbiting on the fringes should be spun away by even the slightest imbalance between their centripetal force connections (gravity) and a weakening of centripetal force caused by the effect of expanding space between the masses, thus stretching and diminishing the centripetal connections. In short, galaxies should radiate mass and energy from the edges (evaporate).

I think it must be a more pervassive force acting not on the galaxies, but on all of space at once. I propose that this force is gravity and that the galaxies are simply falling toward the universe's cosmological event horizon. I explain this hypothesis here.

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matt.o,

Those are good articles (I particularly liked the physicsweb article and will cite it in the future), but they only demonstrate my point. They treat galaxies like single entities reacting to dark energy and provide no mention of possible consequences to the galaxies themselves when considering the various hypothetical versions of dark energy.

Galaxies are not single entities. They are loose conglomerations of matter held in a thin balance between dispersal and collapse. You can't simply treat them as individual entities reacting to the vagrancies of dark energy.

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Chronos
Gold Member
Hi ubavontuba! I side with matt.o. on this one. There is a huge amount of observational and mathematical evidence he is sitting on that fits much better than the dark galaxy thing.

Chronos,

Chronos said:
Hi ubavontuba! I side with matt.o. on this one. There is a huge amount of observational and mathematical evidence he is sitting on that fits much better than the dark galaxy thing.
Actually I agree. Where matt.o and I seemed to disagree was on the local effects of dark energy on dark matter and galaxies. I then foolishly ran off on a tangent about dark energy in a dark matter thread (someone oughtta' come up with more distinctive names). Sorry.

Anyway, I think Occam's razor should lead us first to look at simpler possibilities, rather than jump to the initial conclusion that it's a dark matter galaxy. It could be a dark matter galaxy, but it could be a bunch of loose gases too. I think more study is required for a definitive solution.

I like your signature. My current favorite quote is:
"Beer is proof that God loves us and he wants us to be happy." -Ben Franklin

Actually, it does mention that one kind of dark energy called quintessence may lead to a 'big rip'. Since in this model dark energy may get stronger with time it will eventually tear apart clusters and galaxies and it is even poostulated that it will overcome all other forces in the universe, tearing apart atoms on the way. This is a rather large extrapolation of the current data, which I believe (I may be wrong) is not good enough to distinguish different models for dark energy.

Did you notice that they only notice the effects of dark energy at about a redshhift of 1? Does that not tell you the local effects at this present time are negligible and the things you describe are not going to be observed?

Garth
Gold Member
matt.o said:
Did you notice that they only notice the effects of dark energy at about a redshhift of 1? Does that not tell you the local effects at this present time are negligible and the things you describe are not going to be observed?
Or it might tell you that the GR-with-Inflation-and-DM model begins to break down when extrapolated out to z > 1.

Garth

matt.o said:
Actually, it does mention that one kind of dark energy called quintessence may lead to a 'big rip'. Since in this model dark energy may get stronger with time it will eventually tear apart clusters and galaxies and it is even poostulated that it will overcome all other forces in the universe, tearing apart atoms on the way. This is a rather large extrapolation of the current data, which I believe (I may be wrong) is not good enough to distinguish different models for dark energy.
Yeah. I kind of like this. It's about as wild as some of my own musings. It seems positively poetic that the universe which started from nothing might someday disolve back into nothing...

Did you notice that they only notice the effects of dark energy at about a redshhift of 1? Does that not tell you the local effects at this present time are negligible and the things you describe are not going to be observed?
Yes, but even if it is very weak but pervasive force throughout the universe, we should see more fog in intergalactic space. It wouldn't take much of a nudge to strip mass off of the edges of galaxies.

It is possible that dark matter is somehow viscuos and acts as a kind of glue-barrier between dark energy and normal matter, but then we'd have to come up with a bunch more new physics to explain this efffect. Perhaps it might have something to do with it's purported weak interaction with normal matter? Even so, it should still fall in gravity just the same though, right? Therefore shouldn't it also fall away the same under the influence of a separating force?

Garth said:
Or it might tell you that the GR-with-Inflation-and-DM model begins to break down when extrapolated out to z > 1.
Yeah, or the extrenum of distance might have effects not yet anticipated in relativity.

In either case, I think dark matter is interesting. Did anyone ever get anywhere with the hypothesis that dark matter might be nuetrinos?

Garth
Gold Member
ubavontuba said:
Yeah, or the extrenum of distance might have effects not yet anticipated in relativity.
In either case, I think dark matter is interesting. Did anyone ever get anywhere with the hypothesis that dark matter might be nuetrinos?
Yes, neutrinos have an upper limit mass that means they can only account for 1% max. of $\Omega_{total}$, not 23% as required for DM.

Garth

"Yes, neutrinos have an upper limit mass that means they can only account for 1% max. of , not 23% as required for DM"

Can you clarify what "upper limit mass" means?

Is it basically the calculation that, if every possible neutrino that could be produced was produced, they would still only account for 1% of the universe's total mass?

If they were all located in a certain place, say on the outskirts of galaxies, couldn't this have a fairly sizable impact?

Garth
Gold Member
jhe1984 said:
"Yes, neutrinos have an upper limit mass that means they can only account for 1% max. of , not 23% as required for DM"
Can you clarify what "upper limit mass" means?
Is it basically the calculation that, if every possible neutrino that could be produced was produced, they would still only account for 1% of the universe's total mass?
Yes
If they were all located in a certain place, say on the outskirts of galaxies, couldn't this have a fairly sizable impact?
Neutrinos are 'hot', that is they 'zoom' everywhere at relativistic speeds and would not congregate on the outskirts of galaxies. Others have suggested that neutrinos are the exotic DM but they don't fit, their mass is too small and they are too active to explain the large scale structure and galaxy formation required by the standard theory.

I hope this helps.

Chronos
Gold Member
Affirmed. I agree with Garth's explanation. Neutrinos are like photons, just more abundant. They cannot clump, or possibly make up more than a few percent of the 'missing' mass in current models of the universe.

ubavontuba said:
Yes, but even if it is very weak but pervasive force throughout the universe, we should see more fog in intergalactic space. It wouldn't take much of a nudge to strip mass off of the edges of galaxies.
What do you mean by "fog"?
I think you need to go and crunch some numbers to prove to yourself that this is not the case.
ubavontuba said:
It is possible that dark matter is somehow viscuos and acts as a kind of glue-barrier between dark energy and normal matter, but then we'd have to come up with a bunch more new physics to explain this efffect. Perhaps it might have something to do with it's purported weak interaction with normal matter? force?
I don't understand what you are trying to say here. Dark matter does not interact with baryonic matter apart from it's gravitational influences, which are more than enough to stop a galaxy being nibbled away at by dark energy (to use your terms).
ubavontuba said:
Even so, it should still fall in gravity just the same though, right? Therefore shouldn't it also fall away the same under the influence of a separating force?
Again, I don't understand what you are trying to say here.
It sounds as though you are drawing a lot of conclusions from your own unfounded hypothesis.

matt.o said:
What do you mean by "fog"?
I think you need to go and crunch some numbers to prove to yourself that this is not the case.
By "fog," I mean that galaxies might not have such definable boundaries. That is, might they appear to be more "fuzzy?"

I don't understand what you are trying to say here. Dark matter does not interact with baryonic matter apart from it's gravitational influences, which are more than enough to stop a galaxy being nibbled away at by dark energy (to use your terms).
Right, but my point is, what's to stop the dark matter from being nibbled away, by this same separating force?

Again, I don't understand what you are trying to say here.
It sounds as though you are drawing a lot of conclusions from your own unfounded hypothesis.
It's the same point I was making regarding dark matter's interaction with dark energy above.

I am not making any speculative conclusions based on any personal theories. I am merely asking questions regarding the interactions between gravitational and dark energy forces. If dark energy reacts with baryonic matter in an antigravity sort of way and dark matter reacts with baryonic matter in a gravitational way, shouldn't dark energy also react with dark matter in an antigravity sort of way?

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Garth
Gold Member
Both non-baryonic Dark Matter and Dark Energy are hypothetical entities that have not been discovered in a laboratory. Therefore we do not know what they are or how they behave. However they are invoked to explain some non-Newtonian behaviour of matter and the universe as a whole at galactic and larger scales.

The invoked DE is considered to be cosmological and spread out evenly throughout space whereas DM self gravitates and collapses into massive halos into which the galaxies fall and form.

Although your question is an interesting one, the behaviour of DE and DM is modelled in very large numerical calculations that do not demonstrate the sort of interactions you speculate on.

Garth

Garth said:
Although your question is an interesting one, the behaviour of DE and DM is modelled in very large numerical calculations that do not demonstrate the sort of interactions you speculate on.
Why not?

This has been my point all along. It seems that science may have missed the interplay between these two hypothetical forces that Newtonian Mechanics would demand. Perhaps the mathematical model is incomplete or too simple?

See? Galaxologists (?) need a force to hold galaxies together, so we get dark matter. Cosmologists need a force to explain cosmic acceleration, so we get dark energy. Both of these concepts (if real) must interact with each other in the universe (since everything is relative) and I don't know if the two camps have realized this.

Therefore I ask the questions: Could one preclude the other? Can both hypothetical forces live comfortably in the same universe?

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Garth