Some questions regarding WIMPs as a DM candidate

[frazzle]

WIMPs as a DM candidate - Some questions about detection.

Hello everyone, I am new here, so sorry if this has already been covered. I ran a search on 'WIMP' for the forum, and it seems I'm in the clear here..

So, I am writing a report on dark matter at the moment, but there are a few things that have me confused. At the moment, I am trying to figure out some aspects of detection of the WIMP.

1) I have been reading about indirect detection of WIMPs via their annihilation products. It suggests that WIMPs may cluster at the centre of bodies such as galaxies (due to their gravitational pull) and annihilate, as particles such as the Neutralino are their own anti-particle. However, I was previously under the impression that the majority of the dark matter present in galaxies was supposed to be located within the outer regions? i.e. a dark matter 'halo'? I am also confused as to why the WIMPs would be located in the halo at all. Would gravity not pull them into the centre, in much the same way the luminous matter is largely concentrated at the centre of the galaxy?

2) Another detection method involves the annual modulation of the WIMP flux signal in ground based detectors. As I understand it, this is due to the addition and subtraction of the Earth velocity to that of the WIMPs as the Earth orbits the sun, which is in turn orbiting the galaxy centre. My confusion here, is why would the WIMPs all move in one direction, like a 'wind'? Is this due to the fact that they too are in orbit around the galaxy centre?

3) Finally, I have been reading about possible indirect WIMP detection in the form of high energy gamma-rays. They suggest that the WIMPs that have annihilated to cause this signal might be TeV in magnitude. I was under the impression that this sort of mass is well outside the 'allowed' mass boundary of such a particle?


Sorry if I have rambled a bit here. Hope I'm not imposing at all. If you've read far enough to have reached this point, thanks very much

 

[SpaceTiger]

However, I was previously under the impression that the majority of the dark matter present in galaxies was supposed to be located within the outer regions? i.e. a dark matter 'halo'?

The vast majority of the mass of dark matter is located in the halo, but it's mostly at a very low density. Only in the regions of the highest density could there be enough dark matter annihilation to detect a signal.

I am also confused as to why the WIMPs would be located in the halo at all. Would gravity not pull them into the centre, in much the same way the luminous matter is largely concentrated at the centre of the galaxy?

Unlike luminous matter, dark matter doesn't interact (at least, not very much), so it has no way of dissipating energy. As such, the dark matter particles will oscillate around the galaxy relatively unhindered, while the luminous matter will interact with its surroundings and radiate energy away.

2) Another detection method involves the annual modulation of the WIMP flux signal in ground based detectors. As I understand it, this is due to the addition and subtraction of the Earth velocity to that of the WIMPs as the Earth orbits the sun, which is in turn orbiting the galaxy centre. My confusion here, is why would the WIMPs all move in one direction, like a 'wind'? Is this due to the fact that they too are in orbit around the galaxy centre?

On average, WIMPs shouldn't be rotating much with respect to the galactic center, so in that frame there would be no wind. However, the sun is in orbit about the GC, so there should be a net flow of WIMPs relative to the sun.

3) Finally, I have been reading about possible indirect WIMP detection in the form of high energy gamma-rays. They suggest that the WIMPs that have annihilated to cause this signal might be TeV in magnitude. I was under the impression that this sort of mass is well outside the 'allowed' mass boundary of such a particle?

You can see in the figure you posted in the High-Energy physics section -- the mass scale goes up to 10^4 GeV (10 TeV).

 

[Chaos' lil bro Order]

Are WIMPs neutrinos? Or are they other stuff.

 

[EL]

Are WIMPs neutrinos? Or are they other stuff.

WIMP stands for Weakly Interacting Massive Particles. Although neutrinos are weakly interacting, they are certainly not very massive, and hence does not belong to the class of particles we call WIMPs.
A typical WIMP is the (hypothetical) Neutralino, which exists in supersymmetric extensions of the Standard Model.
See also: http://en.wikipedia.org/wiki/WIMP

 

[frazzle]

The vast majority of the mass of dark matter is located in the halo, but it's mostly at a very low density. Only in the regions of the highest density could there be enough dark matter annihilation to detect a signal.

Unlike luminous matter, dark matter doesn't interact (at least, not very much), so it has no way of dissipating energy. As such, the dark matter particles will oscillate around the galaxy relatively unhindered, while the luminous matter will interact with its surroundings and radiate energy away.

Thankyou so much for your reply Spacetiger, that's really cleared some things up for me.

Something that has further confused me however, is that I have read that dark matter is located largely in halos around galaxies, but largely in the centre of galaxy clusters. I don't understand why/how this could be so? i.e. why would the particles cluster differently in these two scenarios?

 

[SpaceTiger]

Something that has further confused me however, is that I have read that dark matter is located largely in halos around galaxies, but largely in the centre of galaxy clusters. I don't understand why/how this could be so? i.e. why would the particles cluster differently in these two scenarios?

It's hard to say what the source meant by that, but dark matter halos all have similar shapes, whether they're in galaxy clusters or dwarf galaxies. In pretty much all such scenarios, the halo is the densest at the center and falls off as ~1/r^2 in the visible outskirts. The luminous matter in galaxies is more condensed than in clusters, so if that's what you're comparing to, one might say that the dark matter is more centrally concentrated in a cluster. I think the outer dark matter profiles are pretty poorly known, however.

 

[frazzle]

It's hard to say what the source meant by that, but dark matter halos all have similar shapes, whether they're in galaxy clusters or dwarf galaxies. In pretty much all such scenarios, the halo is the densest at the center and falls off as ~1/r^2 in the visible outskirts. The luminous matter in galaxies is more condensed than in clusters, so if that's what you're comparing to, one might say that the dark matter is more centrally concentrated in a cluster. I think the outer dark matter profiles are pretty poorly known, however.

thanks again for your reply!

in case you are curious, here is the quote:

An important difference between the distribution of dark matter in galaxies
and clusters needs to be emphasised: whereas dark matter appears to
increase with distance in galaxies, in clusters exactly the reverse is true, the dark matter distribution actually decreases with distance. Indeed, for certain dwarfs (such as DD0154) the rotation curve has been measured to almost 15 optical length scales indicating that the dark matter surrounding this object is extremely spread out (see also figure 1). A foreground cluster, on the other hand, acts as a gravitational lens which focuses the light from background objects such as galaxies and QSO’s thereby allowing us to determine the depth of the cluster potential well. Observations of strong lensing by clusters indicate that dark matter is strongly concentrated in central regions with a projected mass of 10^13 − 10^14[solar masses] being contained within 0.2 - 0.3 Mpc
from astro-ph/0403324, page 4.

i suppose it may be the case that the text is a series of statements without explanations because nobody really knows why the matter appears to be distributed this way?

 

[Chronos]

Inflation nicely explains why DM is unevenly distributed in the universe. The pre-inflation universe was uniformly saturated with DM. Inflation smeared it out in clumpy, fractal patterns across what is now the entire universe. Baryonic matter was naturally attracted [gravitationally] to these overdense regions. The rest is history.

 

[Garth]

The vast majority of the mass of dark matter is located in the halo, but it's mostly at a very low density. Only in the regions of the highest density could there be enough dark matter annihilation to detect a signal.

ST, what matter (DM & bayonic) is thought to lie in the Inter Galactic Medium between galactic clusters? Although of very low density there may be significant contribution to $\Omega_m$ because of the vast volume.

Garth

 

[frazzle]

Inflation nicely explains why DM is unevenly distributed in the universe. The pre-inflation universe was uniformly saturated with DM. Inflation smeared it out in clumpy, fractal patterns across what is now the entire universe. Baryonic matter was naturally attracted [gravitationally] to these overdense regions. The rest is history.

Aha! Is this to say that the large amount of matter at the centre of the galaxy cluster is was there 'first', and the individual galaxies were attracted to it (and then to each other) which is what caused the cluster to form?

 

[SpaceTiger]

ST, what matter (DM & bayonic) is thought to lie in the Inter Galactic Medium between galactic clusters?

Outside of bound objects, dark matter is very difficult to observe. Gravitational lensing surveys (most of which are still in preparation) ought to give us a relatively direct measure of the overall distribution of matter in the universe on large scales. The theoretical prediction from $\Lambda CDM$ is that it will be a gaussian random field, with a scale-invariant power spectrum at the largest scales.

Baryonic matter in the IGM is a bit more complicated because it depends on the detailed history of the universe -- that is, how much matter was expelled or stripped from galaxies and galaxy clusters. This matter is generally observed in absorption along sightlines to distant quasars.

 

[SpaceTiger]

Aha! Is this to say that the large amount of matter at the centre of the galaxy cluster is was there 'first', and the individual galaxies were attracted to it (and then to each other) which is what caused the cluster to form?

Galaxy clusters often have a great deal of mass at the center in the form of a giant elliptical galaxy, but the formation of the cluster wasn't due to an individual galaxy or small-scale concentration of matter. It was due to the combined gravitational effect of all galaxies within an "overdense" region of the universe.

 

[Garth]

Outside of bound objects, dark matter is very difficult to observe. Gravitational lensing surveys (most of which are still in preparation) ought to give us a relatively direct measure of the overall distribution of matter in the universe on large scales. The theoretical prediction from $\Lambda CDM$ is that it will be a gaussian random field, with a scale-invariant power spectrum at the largest scales.

Baryonic matter in the IGM is a bit more complicated because it depends on the detailed history of the universe -- that is, how much matter was expelled or stripped from galaxies and galaxy clusters. This matter is generally observed in absorption along sightlines to distant quasars.

Thank you ST. Are there any upper limits? We have already looked elsewhere at WHIM, but what about cold hydrogen and the material causing the Lyman $\alpha$ forest?

Garth

 

[SpaceTiger]

Thank you ST. Are there any upper limits? We have already looked elsewhere at WHIM, but what about cold hydrogen and the material causing the Lyman $\alpha$ forest?

We can estimate the amount of mass in the Ly$\alpha$ forest and I think it comes well short of what's needed for $\Omega_b$ from nucleosynthesis and the CMB. I don't know the numbers, but a search of arxiv might turn something up.

 

[Garth]

Yes thank you - I have two recent papers Rao's Damped Lyman alpha Surveys and Statistics: A Review and Wolfe et al's DAMPED Ly alpha SYSTEMS.

The latter paper gives the neutral gas $\Omega_g \sim 0.001$ and stellar $\Omega_* \sim 0.002$. However, the first paper makes the point:

DLAs are the highest column density neutral hydrogen absorption lines seen in quasar spectra (N(HI) > 2 × 10²⁰ cm⁻²), and consequently, have been described as harboring the bulk of the neutral gas in the universe.

but later makes the point

Selection effects that preclude us from observing the highest column density gas might be affecting our interpretation of what DLA statistics really mean. Evidence for this comes from a comparison of DLA and luminous mass densities, SFRs, and metal mass densities as a function of redshift.

So there may be more out there than is thought.

Garth

 

[SpaceTiger]

Yes thank you - I have two recent papers Rao's Damped Lyman alpha Surveys and Statistics: A Review and Wolfe et al's DAMPED Ly alpha SYSTEMS.

Damped systems are only a small fraction of the full Ly$\alpha$ forest. Specifically, it's only those absorbers with hydrogen column densities greater than about 10²⁰ cm^-2. Even though they may carry most of the neutral gas, the Ly$\alpha$ lines in the forest will often be from warmer, more heavily ionized gas with large hydrogen column densities.

 

[Garth]

Damped systems are only a small fraction of the full Ly$\alpha$ forest. Specifically, it's only those absorbers with hydrogen column densities greater than about 10²⁰ cm^-2. Even though they may carry most of the neutral gas, the Ly$\alpha$ lines in the forest will often be from warmer, more heavily ionized gas with large hydrogen column densities.

Is it thought that this latter ionized gas is WHIM at 10⁵ - 10^7oK or just warm gas?

Garth

 

[SpaceTiger]

Is it thought that this latter ionized gas is WHIM at 10⁵ - 10^7oK or just warm gas?

The WHIM is very highly ionized, so it produces virtually no signature in the Ly$\alpha$ forest. This is a part of the reason that we have such difficulty constraining the mass of the WHIM. The Ly$\alpha$ forest is largely diffuse, ionized gas, presumably at temperatures nearer 10⁴ K.

 

[SpaceTiger]

A web search turned up this:

http://ursa.as.arizona.edu/~rad/research.html"

It gives a nice, brief summary of the distribution of baryons in the universe.

 

[Garth]

A web search turned up this:

http://ursa.as.arizona.edu/~rad/research.html"

It gives a nice, brief summary of the distribution of baryons in the universe.

Yes thank you, I had found that already.

Just to include a paper today on WHIM Warm-hot intergalactic medium contribution to baryonic matter which states

The average density of the WHIM in the local universe amounts to 7 - 11 x 10^-32 g cm^-3 (Omega_WHIM = 0.7 - 1.2 %).

, which is even lower (by a factor of ~ 3) than the lower limit of Nicastro, Elvis, Fiore & Mathur's paper: Measured Cosmological Mass Density in the WHIM: the Solution to the 'Missing Baryons' Problem, which as I have posted before gives an upper limit of:
$\Omega_b$^WHIM > 4.3 × 10^0.47 % = 12.6%
and a lower limit of:
$\Omega_b$^WHIM > 1.3 × 10^0.32 % = 2.7%.


Garth

 

[frazzle]

apologies if you've already seen this one, but i found this paper on the distribution of baryons to be quite useful

Cosmic Matter Distribution: Cosmic Baryon Budget Revisited

http://www.arxiv.org/PS_cache/astro-ph/pdf/0312/0312517.pdf

 

[Garth]

apologies if you've already seen this one, but i found this paper on the distribution of baryons to be quite useful

Cosmic Matter Distribution: Cosmic Baryon Budget Revisited

http://www.arxiv.org/PS_cache/astro-ph/pdf/0312/0312517.pdf

Thank you that is a useful paper, I had his 'Cosmic energy inventory', but not this one.

One comment: the paper uses the total baryonic content parameter obtained from the WMAP results, $\Omega_b = 0.04$. This is consistent with mainstream BBN, however both WMAP and BBN estimations are theory dependent on GR, and a degeneracies could exist with modifications of GR.

Just a point to be kept in mind...

Garth

 

[Chaos' lil bro Order]

Awesome conversation in this thread, ST and Garth are really firing up a great Q & A session. Keep it up fellas, I'm learning so much.

Question for Garth, how is GR used to interpret WMAP data that says Baryonic matter = 4% of the critical Mass? And what wiggle room in GR is there that could lead to inaccuracy in WMAP's interpretation? This is probably a question that requires a very long answer, so if you could just summarize some key points, I'd really appreciate it.

Thanks

Keep it up fellas!

 

[Garth]

Question for Garth, how is GR used to interpret WMAP data that says Baryonic matter = 4% of the critical Mass? And what wiggle room in GR is there that could lead to inaccuracy in WMAP's interpretation? This is probably a question that requires a very long answer, so if you could just summarize some key points, I'd really appreciate it.

There has been a tremendous amount of work analysing the WMAP data and I do not pretend to be able to, or even intend to, contradict it all overnight so to speak.

However there are many parameters (Spergel et al's paper http://lambda.gsfc.nasa.gov/product/map/dr2/pub_papers/threeyear/parameters/wmap_3yr_param.pdf Table 1 page 7 lists 23), which are made to fit the data refining the mainstream $\Lambda$CDM model, and the value of one parameter often depends on the value of more basic ones, the "priors".

The first peak of the power spectrum is interpreted as being consistent with a flat, or nearly flat, spatial geometry. (This yields the total density $\Omega_t = 1.010 +0.009/-0.016$.)

However, as the data is angular in nature and conformal transformations are angle preserving, the data is consistent with a conformally flat spatial geometry as well.

A finite and closed conformally flat geometry would also be consistent with a low-l power deficiency. The interpretations of the other peaks, such as the baryon density determined by the third peak, are also degenerate with respect to conformal transformations.

Whereas such conformally flat geometries are not predicted by GR, the mainstream model is so predicted. Hence such verification of the mainstream GR model (from Spergel et al's paper: "The standard model of cosmology has survived another rigorous set of tests.") is a circular argument, for GR is needed to interpret the data that is used to confirm GR. By fitting the many parameters and invoking Inflation, non-baryonic DM and DE, all undiscovered in laboratory physics, the process and data have been made self-consistent. However, the possibility exists that the data is also consistent with a non GR gravitational theory conformally related to it.

Just showing that "wriggle room in GR" does exist, the same "wriggle room" that shows up as the Pioneer anomaly perhaps?

Garth

 

[Chaos' lil bro Order]

Excellent post Garth, thanks.

I'm puzzled by your last comment 'Just showing that "wriggle room in GR" does exist, the same "wriggle room" that shows up as the Pioneer anomaly perhaps?'

What did you mean by Pioneer anomaly?

 

[Chronos]

Objection. WMAP Y3 insists upon DM. Arguing the amount is fair game, but not its existence. I must draw the line here. No amount of MOND can make that go away.

 

[SpaceTiger]

Objection. WMAP Y3 insists upon DM. Arguing the amount is fair game, but not its existence. I must draw the line here. No amount of MOND can make that go away.

To be fair, there's only so much of the parameter space of "alternative gravity" models that they can explore. It's true that simple modifications like those made by MOND will not produce the third peak, but there may be other theories that can. I strongly suspect Garth's theory would give similar results to MOND, but he hasn't modeled it in detail, so it's hard to say.

 

[Garth]

Objection. WMAP Y3 insists upon DM. Arguing the amount is fair game, but not its existence. I must draw the line here. No amount of MOND can make that go away.

Hi Chronos! To which post are you referring? I personally am not advocating a specific theory here, just showing that although the WMAP results are consistent with the mainstream model the interpretation of that data is dependent on GR, so the possibility exists that they could be also self consistent under a modified gravitational theory.

Actually the TeVeS version of MOND is undergoing a revival see an article in the latest New Scientist Gravity: Were Newton and Einstein wrong?

It's a long-standing question whether his law of gravity, supposed to explain everything from falling apples to spinning galaxies, might actually be flawed. That is the claim of a growing number of physicists who support a controversial alternative theory called modified Newtonian dynamics or MOND..."From being really out in the cold from a theoretical point of view, MOND is now being taken very seriously," said University of Oxford physicist James Binney

The TeVeS underpining of MOND apparently now explains galactic rotation curves, cluster lensing and is a good fit to the WMAP data.


Garth

 

[SpaceTiger]

The TeVeS underpining of MOND apparently ...is a good fit to the WMAP data.

Do you have an academic reference for this? Last I checked, it couldn't fit the CMB.

 

[Garth]

I'm puzzled by your last comment 'Just showing that "wriggle room in GR" does exist, the same "wriggle room" that shows up as the Pioneer anomaly perhaps?'

What did you mean by Pioneer anomaly?

See a brief account in Wikipedia article "Pioneer Anomaly" or for a full account see The Study of the Pioneer Anomaly: New Data and Objectives for New Investigation.

My favourite explanation is that it is caused by a clock drift between ephemeris and atomic clock time.

The connection with the subject of this thread is simply that if the PA is showing us that GR requires some modification then that modification may also explain the nature of DM. That is: whether it exists in the first place and explained by a theory such as TeVeS-MOND, or whether it may be baryonic after all.

Garth

 

[Garth]

Do you have an academic reference for this? Last I checked, it couldn't fit the CMB.

I only have this week's New Scientist article to go on. They quote Constantinos Skordis of the Perimeter Institute in Waterloo Ontario Physical Review Letters vol 96 011301, but I have not seen that paper.

The article also has a WMAP power spectrum diagram with the mainstream and MOND predicted curves, which you may want to examine (NS 29 April 06 page 54).

Garth

 

[Garth]

I have now found that Skordis et al. paper on the arXiv: Large Scale Structure in Bekenstein's theory of relativistic Modified Newtonian Dynamics

Since the baryon content is set by the abundance of light elements, we must compensate with a high value of the cosmological constant, i.e. with $\Omega_{\Lambda} \sim 0.95$. An obvious consequence of this is that the angular-distance relation will be modified as compared to the standard adiabatic $\Lambda$CDM universe [11]. Indeed the position of the peaks in the angular power spectrum of the CMB will be shifted to higher ls which would lead to a severe mismatch with the current available data from the Wilkinson Microwave Anisotropy Probe and other experiments. A natural solution to this is to include a small component of massive neutrinos, $\Omega_{\nu} \sim 0.15$. As we can see in the top panel of Fig. 4, with this modification we can reproduce the temperature anisotropy data.

So they get rid of DM in galaxies and clusters by introducing an enormous DE component and a substantial cosmological density component of massive neutrinos...

There you go, another DM candidate! - But I thought the neutrino mass has been established to only allow $\Omega_{\nu} \sim 0.01$?

Garth

 

[Garth]

Actually the TeVeS version of MOND is undergoing a revival see an article in the latest New Scientist Gravity: Were Newton and Einstein wrong?

It's a long-standing question whether his law of gravity, supposed to explain everything from falling apples to spinning galaxies, might actually be flawed. That is the claim of a growing number of physicists who support a controversial alternative theory called modified Newtonian dynamics or MOND..."From being really out in the cold from a theoretical point of view, MOND is now being taken very seriously," said University of Oxford physicist James Binney

Just some further information from James Binney's homepage

James Binney graduated with a BA from Cambridge University in 1971, and with a doctorate from Oxford University in 1975. From 1975{1979 he was a Fellow by Examination of Magdalen College, Oxford. During 1976 he was a Lindemann Fellow at Princeton University, whither he returned in 1979 as a Visiting Assistant Professor in Astrophysical Sciences. In 1981 he became University Lecturer, and in March 1990 Ad Hominem Reader in Theoretical Physics at Oxford University. In July 1996 he became Professor of Physics at Oxford University. From 1981 he has been a Fellow and Tutor in Physics of Merton College, Oxford.

Garth

 

[SpaceTiger]

But I thought the neutrino mass has been established to only allow $\Omega_{\nu}<0.01$

I think that limit is assuming $\Lambda CDM$.

Their fit to the power spectrum looks very strange. They seem to be fitting to a combination of first- and third-year WMAP data (strange, since the third-year includes the first-year) and ignoring results from other experiments. Even with neutrinos, their model produces a small third peak that would probably be inconsistent with other CMB experiments.

 

[Garth]

I concur; a wicked thought: perhaps they need to add another field, say a spinor field? With enough fields and tunable parameters I am sure they will eventually be able to make it fit. The ad hoc nature of the theory and the specific required values of those parameters could always be explained as an anthropic coincidence!

Of course this might be seen as 'epicycle fitting'...

Garth