Dark Matter?

[Ian]

On space.com, they said that Neutrenos might be dark matter and now this nasa website claims that a cloud around a nebula is made of dark matter. Here is the website. http://www.nasa.gov/missions/deepspace/chandra_dark_matter_halo.html

Scientists keep saying that different kinds of matter is dark matter and it is confusing when they say that different things may by the missing mass of the universe.

 

[wollery]

Neutrinos are a dark matter candidate, but it all depends on whether or not they have mass, and the jury is still out on that one.

The other main candidates are;

MACHOs - MAssive Compact Halo Objects - which include clumps of cold inert gas, ancient brown dwarfs, ancient white dwarfs and miniature black holes.

WIMPs - Weakly interacting Massive Particles - which include any number of exotic particles that have mass, and therefore gravitational attraction, but do not interact in any other way with normal matter.

I suspect that, as in many cases in the past, the answer is some sort of combination of some or all of the candidates.



ps There's also a small but determined band who are pushing the idea that Newtonian mechanics is not constant, but changes in low acceleration (ie low gravity) environments. The theory is called MOND - MOdified Newtonian Dynamics - and it can reproduce many of the effects that are seen, but not all of them, and it is a bit of an ad hoc fix.

 

[Zombified]

The evidence that neutrinos have mass is pretty strong at this point - there are at least three different experiments in SuperK (fully-contained muons and upward-going muons examine different energy and path-length ranges, plus K2K if it's back on track) as well as other experiments like SNO, which IIRC had preliminary evidence of electron neutrino oscillation a couple years ago.

Evidence from SuperK is that muon neutrinos have about a 10^-3eV mass difference from some other neutrino flavor. The simplest explanation is that the other neutrino is the electron neutrino but other hypotheses have been considered (such as sterile neutrinos flavors and funky higher-dimensional neutrino states that I don't understand).

The last I heard was that given this low a mass neutrinos weren't expected to contribute sufficient mass, but I guess that would depend a lot on the electron neutrino mass.

 

[SpaceFluffer]

Neutrinos are a type of 'hot dark matter', and due to evidence from the WMAP satellite, the Sloan Digital Sky Survey, and the like, we know that hot dark matter cannot make up much of the total dark matter.

When I say 'dark matter', I'm referring to matter that is non-luminous, but that makes it's presence known via gravitational effects. Hypothesizing the existence of dark matter explains several observed effects:

1) The rotation curves of galaxies suggest a halo of matter surrounding each galaxy,
2) There doesn't appear to have been enough time since the Big Bang for matter to accumulate into clumps (i.e form galaxies)
3) The dynamics of clusters of galaxies suggest a large amount non-luminous matter is present
4) The universe appears to have an energy density that far exceeds the mass that we observe.

One can suggest that MACHOs and neutrinos make up the missing matter, but current evidence suggests that neutrinos can, at most, make up around 0.1% of the matter. MACHOs are thought to make up <20% of the galactic halos, but this number has been decreasing of late.

You can then posit the existance of lots more 'stuff' that we simply can't see. The problem here is that the Cosmic Microwave Background (CMB) gives a snapshot of the universe when it was 300,000 years old, and places a limit on the amount of baryonic matter in the universe. Baryonic matter is protons, neutrons, electrons - basically all the stuff we know about. The only way that we can see to solve this problem is to hypothesize 'cold' dark matter (CDM), that would not contribute to the CMB. It would have to be much more massive that 'ordinary' matter (otherwise collider experiments would have observed it already), which leads it to form clumps under gravity quicker than the hydrogen and helium in the early universe. In this way, the dark matter 'seeds' galaxy formation. Such a scenario would also give the observed rotation curves of galaxies and explain galactic cluster dynamics.

The problem, of course, is that we have no idea what this stuff is! The term WIMP is given to the possible candidates for the CDM, but there are many possible sources for this particle. The theory does place limits on how it would interact with regular matter though, and it's through this that we can try to detect it. It's still very hard to do, but so much evidence points to it's existence that it's a good bet that the dark matter is out there.

The WMAP satellite measured the CMB and placed strong limits on the amount of baryonic mass in the universe. The numbers, as a fraction of the total energy density of the universe, are:

Baryonic Matter ~4%
Cold Dark Matter ~21%
Dark Energy ~75%

Dark Energy is the name given to the energy associated with the accelerating expansion of the universe, also known as the cosmological constant.

 

[Zombified]

What temperature do you mean by "hot"? 3°K? Higher?

How long would it take for the neutrinos in the universe to reach thermal equilibrium with visible matter if they can only interact through the weak force?

Is the neutrino background presumed to be primordial, or is it largely produced by meson decay? (the latter case would make it very hot, I guess.)

How exactly does Sloan rule out hot neutrinos as the missing matter?

I ought to know the answers to these questions, but unfortunately, I don't.

 

[Drooper]

Well, from a complete non physicist it seems to me that through history whenever a theory rests on the existance of some undetectable substance or energy the theory never stays the distance.

I am thinking "flux" and "ether" here. Do any of you physics boffins think there is much chance that dark matter will join these illustrious concepts somewhere down the line?

 

[Zombified]

Well, dark matter isn't really one theory, it's a group of theories, and there are alternatives (like modifications to general relativity, or odder quantum gravity theories). But there is definitely observations of gravitational effects that can't be explained by visible matter alone, and they need an explanation of some sort.

All of these theories are pretty speculative, really. And many of the experiments so far are about ruling things out rather than settling the answer definitively.

 

[TillEulenspiegel]

SpaceFluffer


"Dark Energy is the name given to the energy associated with the accelerating expansion of the universe, also known as the cosmological constant."

Hmmm

Einstein's Lambda seems to exist but to use that to account for the missing mass , seems duplicitous on it's face as DE is a repulsive , not attractive force. That in and of itself begs the question of galactic cohesion. Altho a background scaler field would ( due to Einsteins equivalentcy statement) make up the mass that is absent. The path to make that a reality births a strange beast with disparate qualities.

 

[phildonnia]

Well, from a complete non physicist it seems to me that through history whenever a theory rests on the existance of some undetectable substance or energy the theory never stays the distance...

And yet, the neutrino itself was originally an "undetectable" entity proposed as an ad-hoc fix to the theory of weak interactions.

 

[SpaceFluffer]

What temperature do you mean by "hot"? 3°K? Higher?

When I refer to 'hot' dark matter, that's just an adopted term that references anything that contributes directly to the CMB as 'hot' and anything that's decoupled (frozen out) as 'cold'. Not really to do with temperature per se.

How long would it take for the neutrinos in the universe to reach thermal equilibrium with visible matter if they can only interact through the weak force?

Not sure of the timescale...what are you thinking of in particular?

Is the neutrino background presumed to be primordial, or is it largely produced by meson decay? (the latter case would make it very hot, I guess.)

The neutrinos were around during the radiation dominated period (along with photons and anti-neutrinos), before matter could become stable in large quantities.

How exactly does Sloan rule out hot neutrinos as the missing matter?

An excess amount of relativistic particles during the epoch of galaxy formation would wash out small scale structure, preventing agreement with the observed matter density distribution today. One uses a combination of galaxy clustering measurements (such as SDSS), CMB and observations of the Lyman-alpha forest to obtain an upper limit on the neutrino contribution.

 

[SpaceFluffer]

Well, from a complete non physicist it seems to me that through history whenever a theory rests on the existance of some undetectable substance or energy the theory never stays the distance.

I am thinking "flux" and "ether" here. Do any of you physics boffins think there is much chance that dark matter will join these illustrious concepts somewhere down the line?

Well firstly, dark matter is not undetectable. Secondly, flux is a perfectly well-accepted part of physics - just look at an elementary E&M textbook.

Ether is really the only substance that was thought to exist and was eventually proven not to. However, bear in mind that there was no evidence that it existed, directly or indirectly; it was a postulation because at the time nobody could come up with a way for light to propagate through space.

Things are compared to ether (and epicycles, for that matter), so damn often that it gets on my nips.

We strongly believe dark matter to exist because everything we know up to this point gives strong indication of some new, unseen 'stuff' existing. Granted that the only evidence of dark matter is indirect - if we don't detect it in the next few years, most physicists will begin to doubt the theory and look to alternatives. Not to say there aren't alternatives now; they just don't fit the current data anywhere near as well.


Edited to add that the top quark was part of the standard model of physics since the late 60's, yet it wasn't actually seen until 1990 or so.

 

[SpaceFluffer]

SpaceFluffer
"Dark Energy is the name given to the energy associated with the accelerating expansion of the universe, also known as the cosmological constant."

Hmmm

Einstein's Lambda seems to exist but to use that to account for the missing mass , seems duplicitous on it's face as DE is a repulsive , not attractive force. That in and of itself begs the question of galactic cohesion. Altho a background scaler field would ( due to Einsteins equivalentcy statement) make up the mass that is absent. The path to make that a reality births a strange beast with disparate qualities.

I'm not entirely sure what you're saying, but I wasn't suggesting that lambda contributes to the missing mass, just that it contributes to the overall energy 'budget' of the universe.

According to WMAP,

Omega = 1.02 +- 0.02

which is

CDM ~ 0.22, Dark Energy ~ 0.76, Baryonic ~ 0.04 (not sure of the errors here. I could look it up, but hey - I'm not going to).

Omega=1 corresponds to a universe that has an overall flat geometry.


I should have made a distinction between

a) the study of the evolution of the universe as a whole, which tells us about omega and it's parts, and
b) study of the evolution of smaller scales (galaxies and galactic clusters), which tell us more about the ratio of baryonic matter to CDM.

 

[Zombified]

When I asked about timescale, I mean how long would it take the background neutrinos to reach thermal equilibrium with visible matter (more or less). This could only happen via the weak force so I'm assuming it would take a while. Of course, that's assuming the neutrinos aren't the same "temperature" in the first place.

My (limited) experience with neutrino physics is atmospheric neutrinos from cosmic rays, I don't know much about cosmology.

I guess I'm probably talking out my @ss since in the early universe when typical interactions are of sufficiently high energy neutrinos and visible matter would naturally be in thermal equilibrium to begin with.

I guess the "washing out" you're referring to would happen at about the same time, while neutrinos are still interacting with visible matter.

Sounds like you're the new resident cosmology expert. Handy person to have around.

(Aside on Drooper's comment, caloric/phlogiston was another one, as it were, but the nuts don't go on about that one.)