Dark Mattter

[Badly Shaved Monkey]

"Dark Galaxy" found

Could someone please give me a brief rundown on Dark Matter? I am confused.

I understand that it is supposedly constrained to consist of matter that is basically different from "ordinary" matter and that particles acting as "WIMPs" are invoked. (Could someone briefly explain the arguments that imply this constraint not to be ordinary baryonic matter?). If WIMPs or other exotica are required, what does that do to the basic framework of the Standard Model? I don't see them on the list and I don't understand if the envisaged extensions of the Standard Model like supersymmetry with its selectrons and squarks etc simply subsumes the idea of WIMPs as being a description of one set of those additional particles. There is a p.s. to that, bearing in mind JAK's recent thread: is the Higgs boson a component of the Standard Model or is it already outside that construct?

But, and this is where I get confused, I thought there were also serious suggestions that Dark Matter may be relatively boring things like brown dwarfs and other normal matter objects that don't actively shine.

Basically, does Dark Matter have to be made of weird stuff, or do more prosaic explanations remain viable?

Thanks.

 

[SpaceFluffer]

I understand that it is supposedly constrained to consist of matter that is basically different from "ordinary" matter and that particles acting as "WIMPs" are invoked. (Could someone briefly explain the arguments that imply this constraint not to be ordinary baryonic matter?).

To be specific, what the article is referring to is Cold Dark Matter (CDM). Once it was realized that the total observed mass in the universe (stars, etc.) was not nearly enough to account for it's dynamics, it became clear that there had to be something else around providing this extra mass. This observation follows from the observation of the recession velocity of galaxies (Hubble's law), studies of galaxy cluster dynamics and the like.

Another important yardstick in examining the past history of the universe is the Cosmic Microwave Background (CMB) radiation. This represents the state of the universe at 300,000 years (this is when radiation decoupled from the matter - the CMB radiation is this leftover radiation). This places constraints on exactly how the universe could have evolved since this time (which is why WMAP results were so awesome).

If you're going to find something to account for this missing mass, you would prefer to use a particle that you know already exists. One of the first DM candidates were neutrinos.

Neutrinos have recently been discovered to have a small mass, but cannot account for more than ~0.1% of the missing mass. If they did, they would affect small-scale (galactic) structure in such a way that getting from the 300,000 year state (CMB) to now would not be possible. This kind of Dark Matter is usually termed Hot Dark Matter (HDM), since it affects the rest of the matter in the universe directly (non-gravitationally).

Various other particles were examined as DM candidates, but it currently appears that you do need non-baryonic matter in the current model. The reason for this is that non-baryonic matter would not have coupled to the radiation and therefore wouldn't have contributed to the CMB. In other words, the CMB is a snapshot of the baryonic matter distribution at 300k years.

If WIMPs or other exotica are required, what does that do to the basic framework of the Standard Model? I don't see them on the list and I don't understand if the envisaged extensions of the Standard Model like supersymmetry with its selectrons and squarks etc simply subsumes the idea of WIMPs as being a description of one set of those additional particles. There is a p.s. to that, bearing in mind JAK's recent thread: is the Higgs boson a component of the Standard Model or is it already outside that construct?

The Higgs is indeed part of the standard model; it is the only particle in the model that has not been observed directly. If the next generation of accelerators don't see the Higgs (which is in the 100-200GeV mass range), we will have to start to question our understanding of standard model physics.

If you accept that the CDM has to be non-standard model, there are many possible candidates for the WIMP (Weakly Interacting Massive Particle). But whatever it turns out to actually be in terms of theory, we know enough about what it's properties must be to detect it (which is what I work on, BTW). The mass is in question (although it must be >~50GeV), but the mass density is not. So up the mass, you bring down the number density. The exact mechanism of it's interaction with 'ordinary' matter is also unknown, but again doesn't affect your quest to directly detect it. The current most popular WIMP candidate is the neutralino, which is the most stable particle in Supersymmetry - a natural extension to the standard model. Other candidates are the axion, axino, WIMPzilla, and weird Kaluza-Klein constructs.

But, and this is where I get confused, I thought there were also serious suggestions that Dark Matter may be relatively boring things like brown dwarfs and other normal matter objects that don't actively shine.

You're referring to MACHOs (MAssive Compact Halo Objects), which studies have shown cannot account for more than 10-15% of the missing mass (i.e. the rest must be dark matter). But again, this is baryonic so the constraints from the CMB put an upper limit on how much of this stuff can exist. Of the quantity of baryonic matter that we believe exists (~4% of the total energy density, ~15% of the total mass), only about half has been directly observed. The rest is assumed to exist as dust, etc. and there are papers on this from time to time (a recent Wired article covered one such investigation)

It's worth pointing out that we are looking for something for which several lines of investigation strongly suggest it's existance, but we could of course be wrong. It's just a fairly good bet that this stuff is out there.

 

[Badly Shaved Monkey]

Thanks for that.

So, in summary, it's the fact that Dark Matter cannot interact with CMB that means that the bulk of it must be something non-baryonic.


And the galaxy found in the link is CDM and therefore, made of weird stuff. Is that right?

Is it a surprise to find a galaxy-sized lump of it instead of haviong it all loosely distributed amongst ordinary matter?

 

[SpaceFluffer]

Is it a surprise to find a galaxy-sized lump of it instead of haviong it all loosely distributed amongst ordinary matter?

DM halos extend out of the galaxies to many galactic radii.

The DM halo is a distribution of DM with a density proportional to 1/r - this is what we'd expect from simple dynamics, and also accounts for the strange rotation curves that we see in almost all galaxies. The dark matter distributions would have formed first in the universe, and then 'seeded' galaxy formation by way of gravitational interaction.

Recent numerical simulations have resulted in a number of satellite distributions of dark matter around each galaxy. For example, our galaxy is known to have a few dwarf galaxies (the Magellanic Cloud is one example), but we might expect more judging from the simulation. The most popular explanation for this is that not all clumps of dark matter would necessarily attract luminous matter (or matter capable of becoming luminous). i.e. our galaxy may have more dwarf galaxies, there just aren't any stars in them. The alternative is that our dynamics or something else is wrong.

This new observation seems to support the idea of clumps of dark matter existing without being associated with luminous matter. So this just makes this possibility a bit more favored.

 

[Badly Shaved Monkey]

Thanks

 

[Badly Shaved Monkey]

Small fly in ointment;

http://en.wikipedia.org/wiki/Cold_dark_matter

This says that MACHOs are CDM, whereas I thought you were saying that CDM is necessarily non-baryonic. Is your point that, CDM may include baryonic MACHOs, but most CDM must be non-baryonic?

 

[SpaceFluffer]

There are varying definitions, but it doesn't really matter. As you say, a MACHO (be it brown dwarf, neutron star or white dwarf) is baryonic and therefore would contribute to the CMB.

Technically hot and cold differentiate between relativistic and non-relativistic particles respectively (i.e. velocity comparable, or not, to speed of light). These definitions almost amount to the same thing, but since MACHOs are non-relativistic they should really be defined as CDM.

WIMPs are non-relativistic and are expected to have a galactic velocity ~200km/s, which although fast, is non-relativistic.

 

[Badly Shaved Monkey]

Thank you again.

 

[Badly Shaved Monkey]

Uh, oh. The monkey's been thinking again!

While attempts were being made to discover the geometry of space, I think I'm right in saying that although efforts to find enough matter to flatten space and just close the Universe kept failing, there ws an implicit assumption that space really would be flat and the Universe would asymptotically approach closure with an almost infintely prolonged deceleration in its expansion.

Am I right I recalling that the curvature of space and whether there is the critical mass density are not necessarily the same thing, though just enough matter density could have produced a flat, just-closed Universe?

Now we know that baryonic matter is only 4% of the total energy density, does that mean that we were out by a factor of 25 when the low estimates of density kept coming out, but there was still a feeling that if we looked hard enough and found enough little Black Holes or brown dwarfs then we would get to the magic 100%?

What does the discovery of expansion mean for flatness and critical energy density? Has this result separated the answer for these parameters i.e. is the Universe still geometrically flat even though it's expansion rate is increasing, or does the expansion mean that the geometry is hyperbolic?

The monkey really should return to peeling those bananas with his feet and not think too hard, but the problem is that I'm bored being stuck with this bloody typewriter and a whole load of other monkeys and we've been told that we can't leave until one of us has written Hamlet. Bo-Bo, over there, wrote a Monty Python sketch last week, but that just made the researcher cry. I don't think he understood the jokes.

 

[Bodhi Dharma Zen]

Re: Re: Dark Mattter

It's worth pointing out that we are looking for something for which several lines of investigation strongly suggest it's existance, but we could of course be wrong. It's just a fairly good bet that this stuff is out there.

Correct me if Im wrong, but "dark matter" is just a theoretical construct necessary to keep the current explanation about the relative movement between galaxies.

There are several clusters and macro clusters of galaxies, yet, their movement is not what it should be regarding their mass.

Can you explain this a bit more?

 

[SpaceFluffer]

Re: Re: Re: Dark Mattter

Correct me if Im wrong, but "dark matter" is a theoretical construct necesary to mantain the current theoretical framework in order to explain the relative movement about galaxies.

There are several clusters and macro clusters of galaxies, yet, their movement is not what it should be regarding their mass.

Can you explain this a bit more?

Yes, dark matter is a theoretical construct in the sense that we believe it's effects have been observed but it has not been observed directly in the laboratory. This is no different from black holes - the evidence is just stronger for them.

I also object to the statement that they're necessary "to maintain the current theoretical framework". We have no interest in maintaining the theory if it's wrong. But dark matter as a hypothesis can account for several mysteries:

1) The rotation curves of galaxies suggest that extra mass is located in an around all galaxies
2) The universe seems to be approximately flat, yet the visible matter cannot account for anywhere near the required amount for this.
3) The universe is not old enough for galaxies to have had time to form without some other mechanism assisting this process.
4) This missing mass cannot be baryonic; neutrinos would wash out small-scale structure, MACHOs are a small component, etc. The CMB also places a strong limit on the amount of baryonic matter at 300,000 years.

All this is a convincing case, given what we know at present. Again though, it could turn out that we're wrong about something fundamental. But this is how science works - if you have multiple lines of inquiry from many independent anaylses that all point to a single solution, it's a reasonable bet.

 

[SpaceFluffer]

What does the discovery of expansion mean for flatness and critical energy density? Has this result separated the answer for these parameters i.e. is the Universe still geometrically flat even though it's expansion rate is increasing, or does the expansion mean that the geometry is hyperbolic?

There was a mid of a problem in the mid-1990s, since it was believed at the time that the universe was approximately flat, and that the energy density due to (all) matter was therefore about 1. However, this implies that the universe is about 9 billion years old, which is contradicted by some globular clusters that are observed to be much older.

Subsequently, the possibility of a non-zero cosmological constant (aka. dark energy) was examined and found that the energy density breakdown of matter~0.3, dark energy~0.7 gives excellent agreement with both stellar evolution and radioactive decay measurements.

The attached image below shows how, by varying the dark energy contribution, we can still have a universe that is currently approximately flat, but that has a range of past and future outcomes.

The monkey really should return to peeling those bananas with his feet and not think too hard, but the problem is that I'm bored being stuck with this bloody typewriter and a whole load of other monkeys and we've been told that we can't leave until one of us has written Hamlet.

I'll be sure to check back with you in 10^40 years when you've typed a single sentence correctly...

 

[SpaceFluffer]

gah...sorry

 

[SpaceFluffer]

try again...

 

[Bodhi Dharma Zen]

Re: Re: Re: Re: Dark Mattter

Yes, dark matter is a theoretical construct in the sense that we believe it's effects have been observed but it has not been observed directly in the laboratory. This is no different from black holes - the evidence is just stronger for them.

What else can it be? Unseen mass is one explanation, but Im unaware of any other. Maybe another force (I know it would be a lot more difficult to "fit" that dark matter) that pulls galaxies with "more power" than gravity? What else can distort the space/time continuum?

But this is how science works - if you have multiple lines of inquiry from many independent anaylses that all point to a single solution, it's a reasonable bet.

I agree, btw, astronomy has been one of my favorite subjects since I have memory.

Now that we are on this, can you comment about the "acceleration" that the universe is, apparently, experimenting?

Oh, and a last question! There are, presumable, some very far galaxies that are moving so fast away from us that their relative speed is almost C... does that means that we know what it feels to travel at almost the speed of light?

Thanks

 

[Bodhi Dharma Zen]

just a reminder (of my last questions)

 

[SpaceFluffer]

What else can it be? Unseen mass is one explanation, but Im unaware of any other. Maybe another force (I know it would be a lot more difficult to "fit" that dark matter) that pulls galaxies with "more power" than gravity? What else can distort the space/time continuum?

Well, that's kind of the point. It's the simplest explanation we can come up with that fits the data, even though on the surface, it's a slightly weird idea. Modifying gravity (MOdified Newtonian Dynamics, or MOND) doesn't solve all the problems, though there are a few proponents of the idea.

Now that we are on this, can you comment about the "acceleration" that the universe is, apparently, experimenting?

See the plot I posted above. The cosmological constant is responsible for an extra 'push' in the universe's expansion, and Dark Energy is the name given to the energy associated with this dynamic. You can see from the plot that if you accept a flat universe with no cosmological constant, the universe cannot be as old as we suspect it is from many other observations. We know that the 'energy budget' of the universe (i.e. omega) is very close to 1, corresponding to a flat universe. So either the other measurements of the universe's age are wrong, or the cosmological constant is nonzero.

Oh, and a last question! There are, presumable, some very far galaxies that are moving so fast away from us that their relative speed is almost C... does that means that we know what it feels to travel at almost the speed of light?

What it feels like? Not really...there are galaxies receding from us very fast, but only galaxies formed at the moment of the big band would be receding at c, and they were none until many millions of years after the big bang. There are many galaxies receding so fast that their visible light becomes red-shifted away, but the further you look, the harder they are to observe due to the fact that they appear younger and dimmer.

 

[Carn]

Modifying gravity (MOdified Newtonian Dynamics, or MOND) doesn't solve all the problems, though there are a few proponents of the idea.

Why modifying newton?
Wouldn't genral relativity have to be modified to account for the difference?

And how is the 50 GeV limit supported?


Carn

 

[Bodhi Dharma Zen]

What it feels like? Not really...there are galaxies receding from us very fast, but only galaxies formed at the moment of the big band would be receding at c, and they were none until many millions of years after the big bang. There are many galaxies receding so fast that their visible light becomes red-shifted away, but the further you look, the harder they are to observe due to the fact that they appear younger and dimmer.

Movement is relative, so, if those galaxies are receding away from us at that speed, it is logical tu assume that we are receding from them at that speed... do I have a point? if not, why?

 

[Badly Shaved Monkey]

We know that the 'energy budget' of the universe (i.e. omega) is very close to 1, corresponding to a flat universe. So either the other measurements of the universe's age are wrong, or the cosmological constant is nonzero.

Sorry to add to the barrage of questions, but I think you are confirming that the cosmological constant- dicating the fate of the Universe, and omega- dictating its geometry are independent parameters. Is that right? Or do you just mean that it is geomterically flat in our epoch, but that its geometry may vary with time?

Once again, thanks.