ask a physicist

[freyar]

Somewhat related to what you chaps have been discussing:

Even though dark matter is rather isotropic thoughout the visible universe, there must be regions where it is clumped. Also, it supposedly makes up the majority of the matter in the universe.

So, why don't we see a great deal of gravitational lensing due to it? Or, do we, and we just don't hear about it?

I can see how the lensing of objects within our own galaxy would be minimized if the dark matter is spread so uniformally within it but, if dark matter is so ubiquitous in the visible universe, shouldn't nearly all of the furthest galaxies be lensing in our scopes?

The thing to remember is that since dark matter interacts through gravity, clumps of dark matter will correspond to clumps of normal visible matter drawn to it - so, those clumps of dark matter are where galaxies are. So, when they talk about lensing around a galaxy, they are implicitly talking about lensing around dark matter, too.

As Umbran says, we think that big clumps of dark matter draw visible matter into them during the formation of structure, so you generically expect a big visible structure to be sitting in a dark matter counterpart. So each galaxy has a "halo" of dark matter around it. By looking at motions of stars in galaxies, we can get a reasonable idea of how dark matter is spread in these galactic halos. But it's not just galaxies. We believe that there are smaller lumps of dark matter (called subhalos) inside the big galaxy-sized halos, though it's not clear if they'd all be associated with clumps of stars due to the non-gravitational physics that
affects normal matter but not dark matter. Going the other direction in size, galaxies come in groups and clusters, and clusters of galaxies are sitting in really big halos of dark matter. When we see images of very far galaxies lensed by the gravity of closer stuff, it's typically clusters of galaxies and their dark matter halos that do the lensing. It's also possible to use the lensing of an image to map out the location of the total mass in a cluster (this is pretty common for people to do), and that doesn't always track where the visible matter is --- typically it extends out a bit.

 

[Landifarne]

Makes good sense but, with a 5-to-1 ratio of DM to Regular Matter (RM), it seems like there couldn't be a perfect correlation. In fact, all of the previous comments on the vastness of the universe (and corresponding variety of structures) would suggest that we should observe many cases of DM without RM. Seems like a statistical model should give some idea of how many non-correlating clumps we should observe.

If there really is such a corelation, wouldn't that imply that the DM formed first? If so, have they figured out how much more quickly the DM formed?

[EDIT: Re-reading your comment on sub-halos, I'd have to say that, if such exist, then we should see a tremendous amount of lensing on every scale. Is the amount of grav-lensing that high?]

 

[Landifarne]

I'm glad that I began reading this thread. I've learned more about cosmology over the last few weeks from looking here (and visiting Wikipedia) than I did as an undergrad. Thanks all around!

OK, here's another one:

If our universe is a false vacuum [a concept I had never heard of before], could the dark matter then be a manifestation of the true vacuum that it is "falling into"?

 

[AbdulAlhazred]

Makes good sense but, with a 5-to-1 ratio of DM to Regular Matter (RM), it seems like there couldn't be a perfect correlation. In fact, all of the previous comments on the vastness of the universe (and corresponding variety of structures) would suggest that we should observe many cases of DM without RM. Seems like a statistical model should give some idea of how many non-correlating clumps we should observe.

If there really is such a corelation, wouldn't that imply that the DM formed first? If so, have they figured out how much more quickly the DM formed?

[EDIT: Re-reading your comment on sub-halos, I'd have to say that, if such exist, then we should see a tremendous amount of lensing on every scale. Is the amount of grav-lensing that high?]

There ARE 'dark galaxies', halos that appear to have little or no baryonic matter associated with them. They're generally small, and like many dwarf galaxies they tend to orbit larger galaxies. Its hard to tell though, since they're practically invisible. There has been some work on mapping out nearby ones, but we still don't even have a really good count of all the VISIBLE dwarf galaxies associated with ours.

 

[freyar]

Makes good sense but, with a 5-to-1 ratio of DM to Regular Matter (RM), it seems like there couldn't be a perfect correlation. In fact, all of the previous comments on the vastness of the universe (and corresponding variety of structures) would suggest that we should observe many cases of DM without RM. Seems like a statistical model should give some idea of how many non-correlating clumps we should observe.

If there really is such a corelation, wouldn't that imply that the DM formed first? If so, have they figured out how much more quickly the DM formed?

[EDIT: Re-reading your comment on sub-halos, I'd have to say that, if such exist, then we should see a tremendous amount of lensing on every scale. Is the amount of grav-lensing that high?]

There ARE 'dark galaxies', halos that appear to have little or no baryonic matter associated with them. They're generally small, and like many dwarf galaxies they tend to orbit larger galaxies. Its hard to tell though, since they're practically invisible. There has been some work on mapping out nearby ones, but we still don't even have a really good count of all the VISIBLE dwarf galaxies associated with ours.

On lensing, AbdulAlhazred is correct. It's also true that, within a biggish galaxy like ours, we don't know if many/most of the subhalos have been smoothed out by the gravitational effects of normal matter (which are more important than dark matter in the inner parts of galaxies like ours). So we wouldn't necessarily see much (micro)lensing in our galaxy from subhalos. If we're talking about lensing of very far away galaxies, we're talking about strong gravitational lensing, which requires a lot of mass. Typically the only thing large enough to make a lens we'd see is a cluster of galaxies and associated dark matter halo, and we do find that that kind of lens is very common.

Based on various types of evidence, dark matter structures did form first, and normal matter then fell into the dense regions of dark matter. Here's a reasonable picture of how things look today: there are these really big clouds of dark matter that hold clusters of galaxies together. Inside these cluster halos are large amounts of intergalactic gas and also smaller halos of dark matter, which by-and-large contain galaxies. Some are reasonably big, like our Milky Way, some are dwarf galaxies (generally satellites of galaxies like ours) that may either have very little normal matter or may be separated from their original dark matter halos by tidal effects, and some are really big monster-sized galaxies formed by the collision of other galaxies. Inside the galaxies, there are probably subhalos and other structures, but we don't have as good of a picture of that. The cluster-sized halos of galaxies themselves form into large walls and seem to lie at the intersection of filaments of dark matter. On the flip side, the walls surround large voids with very little matter of any kind (dark or normal). On the average, on very very large distances, the universe is pretty smooth though, and in fact this is what we see in the cosmic microwave background light, which is uniform with fluctuations of only one part in 100,000 roughly.

 

[freyar]

I'm glad that I began reading this thread. I've learned more about cosmology over the last few weeks from looking here (and visiting Wikipedia) than I did as an undergrad. Thanks all around!

OK, here's another one:

If our universe is a false vacuum [a concept I had never heard of before], could the dark matter then be a manifestation of the true vacuum that it is "falling into"?

The false vacuum doesn't really "fall into" the true vacuum. It takes a quantum decay process to get to the true vacuum, and it's quite a traumatic (read: likely destroys the universe as we know it) event. So, fortunately, it shouldn't happen very often (like even once in our volume of the universe for many many ages of the universe). But anyway, that doesn't have much to do with dark matter; back to that in a moment. First I want to mention that it is possible in theories with lots of false vacua to look for evidence of the decay of another false vacuum into our false vacuum, and people are looking. I think it's a long shot --- I agree with arguments that say any such evidence is likely pushed farther away than our ability to see it --- but it is a possibility.

There are also models that combine dark energy (general models that can explain the accelerating expansion of the universe) and dark matter. Most of these I've seen are highly speculative, and I don't put a lot of credence in them. There are just a lot of ways they can be sick (have subtle but important mathematical inconsistencies).

 

[Landifarne]

"...within a biggish galaxy like ours, we don't know if many/most of the subhalos have been smoothed out by the gravitational effects of normal matter (which are more important than dark matter in the inner parts of galaxies like ours). So we wouldn't necessarily see much (micro)lensing in our galaxy from subhalos."

Do the gravitational attractions of DM-to-DM and DM-to-baryonic follow those of baryonic-to-baryonic?

Your statement begs the question: How could baryonic matter smooth out DM inside a large galaxy? The result of elecroweak interactions/pressure? Or, is the ratio of DM to baryonic much smaller than 5:1 inside such galaxies, with that large overall ratio attributable to much higher % of DM strewn throughout the universe?

 

[Landifarne]

The false vacuum doesn't really "fall into" the true vacuum.

I got that; understood the context.

It takes a quantum decay process to get to the true vacuum, and it's quite a traumatic (read: likely destroys the universe as we know it) event. So, fortunately, it shouldn't happen very often (like even once in our volume of the universe for many many ages of the universe).

[Probably in my genuine ignorance] this sounds incredibly speculative. Is this so intertwined with our understanding of quantum mechanics to say it's a given? "To the extent that the following is probable" is likely the answer...

First I want to mention that it is possible in theories with lots of false vacua to look for evidence of the decay of another false vacuum into our false vacuum, and people are looking. I think it's a long shot --- I agree with arguments that say any such evidence is likely pushed farther away than our ability to see it --- but it is a possibility.

There are also models that combine dark energy (general models that can explain the accelerating expansion of the universe) and dark matter. Most of these I've seen are highly speculative, and I don't put a lot of credence in them. There are just a lot of ways they can be sick (have subtle but important mathematical inconsistencies).

You're saying that the models that connect DM to DE are sick, not the existance of DE being sick? [The existance of DE is well established, I assume]

 

[Landifarne]

Oh, and again, props to all of you guys for putting up with this. "The patience of Job" comes to mind...

 

[freyar]

"...within a biggish galaxy like ours, we don't know if many/most of the subhalos have been smoothed out by the gravitational effects of normal matter (which are more important than dark matter in the inner parts of galaxies like ours). So we wouldn't necessarily see much (micro)lensing in our galaxy from subhalos."

Do the gravitational attractions of DM-to-DM and DM-to-baryonic follow those of baryonic-to-baryonic?

Your statement begs the question: How could baryonic matter smooth out DM inside a large galaxy? The result of elecroweak interactions/pressure? Or, is the ratio of DM to baryonic much smaller than 5:1 inside such galaxies, with that large overall ratio attributable to much higher % of DM strewn throughout the universe?

The answers to these questions are kind of related. While all matter, dark or baryonic/normal, follows the same law of gravity (in any usual type of theory), which is just Newtonian gravity (since relativistic effects are tiny). The issue is that baryonic matter, as you say, has electromagnetic interactions and therefore pressure. Essentially, dark matter interacts so rarely that it can just pass through everything, but normal matter acts like a fluid. That means it's possible for normal matter to collapse into small objects. Think of it this way: the same amount of matter in a smaller space has a lower energy (that's what you get from falling, after all), so you have to lose energy somehow to go from being spread out to being compact. Dark matter doesn't have an effective way to lose energy. In the end, what this means is that, even though there is much more dark matter over all, there's a lot more normal matter in the center of a galaxy like ours. Also, outside the center, the normal matter forms a disk structure (that's where we are), but dark matter mostly stays in a large spherical halo (at least that's what a typical model would say; it's interesting to think about alternatives). In any event, the overall behavior of normal matter is a lot more complex than that of dark matter, even though they obey the same law of gravitation. And those complex processes can feed back into the dark matter via gravity. It's not clear exactly what the feedback will do since there are competing contradictory effects possible, but one possibility is breaking up or smoothing out the subhalos.