Fermi and dark matter

[Reality Check]

What you're suggesting here Ben includes all the following "assumptions" that are all part of your package 'hypothesis".

A new form of matter exists beyond the "standard" particle physics theory.
This new particle is "long lived" and doesn't deteriorate in say a millisecond.
It only interacts with other matter in the specific way that you claim at the "weak" level of particle physics. It "decay's" in some way that releases both an electron and a positron at whatever rate you evidently are allowed to "postdict" from whatever observation you're claiming they relate to now and into the future.

The observations of dark matter mean that we know a lot of its properties. This means that we can make hypotheses about what would have those properties. The hypotheses can then be tested by further observations. This is a basic part of the scientific method.

As Ben said dark matter has a a set of observed properties. The list below is quoted from Ethan Siegel's blog article What is Dark Matter?

1. It isn't in dense clumps. Not faint, dark stars, not planets, not basketballs, and not gas and dust. We've already gone over that it can't be normal matter, but it also can't form structures the same way that normal matter does.
2. It doesn't interact with itself. Yes, it interacts gravitationally, but that's not what I mean. It doesn't feel the strong nuclear force, it doesn't feel the electromagnetic force, and if it feels the weak force, it feels it even more weakly than neutrinos do! (And you need about a light year of lead to stop your typical neutrino.) As a result of this, dark matter is incredibly fluffy and diffuse, which means even in our Solar System, by the heart of our galaxy, dark matter's effects are negligibly unobservable.
3. These dark halos form very early. This is very important. Looking back as far as we can see, we find that these diffuse collapsed structures have been around for pretty much the entire history of the Universe. This tells us that the dark matter is cold, or that its kinetic energy has pretty much always been much, much less than its mass. This rules out every single standard model particle that we know of from being dark matter, since the stable ones are relatively light and are all produced hot.

I will add point 3 to the list of problems with your "lumpy stuff" idea (even though "lumpy stuff" is ruled out by point 1).

Ethan Siegel goes on to list three candidates that meet the crteria for dark matter:

1. WIMPs
2. Cold massive particles like axions
3. Something even more exotic.

An additional hypothesis is that the WIMPs either

• decay with a long lifetime or
• annihilate.

IMO: This is a reasonable hypothesis beacuse we know of particles that decay with various lifetimes and there is no reason to suppose that WIMPs do not have anti-particles or are their own anti-particles, e.g. the neutralino found in SUSY theories.

If this additional hypothesis is correct then dark matter will emit gamma rays that can be detected. The detection of these gamma rays shows that if we eliminate all known sources of gamma rays then the gamma rays are from an unknown source of gamma rays. That unknown source may be dark matter.

 

[DazzaD]

Was Newtonian mechanics a 'bodge job' too once GR became popular?

No...

but then I am not seriously suggesting that because Newtonian mechanics has served us so well for hundreds of years we shouldnt ever look for an alternative explanation to that experimental stuff we see, that we should be seeking ways, forever and a day, to try and explain that stuff using just Newtonian mechanics.

This is exactly my point.

When and where is it appropriate to apply Occum's razor if not to this specific scenario? If empirical physics can explain what we observe, why wouldn't it be considered a "superior" theory to one that is based on hypothetical entities a half dozen hypothetical "properties", none of which enjoy empirical support?

Can I just point out that Occam's razor is not a "rule" in science.

In any case, it should only be used to make a judgement when "all other things are equal".

In this case plasma cosmology and the mainstream explanation (say a variant of lambda-cdm) are NOT equal. Not by a long shot.



Please note, that in pretty much all my posts I am talking generally, I am NOT specifically referring to the Fermi results on gamma rays and their possible explanation as possibly being evidence of a possible dark matter distribution.

 

[Michael Mozina]

Can I just point out that Occam's razor is not a "rule" in science.

Well, let me ask you a "philosophical" question then about "science". When is it appropriate to choose/favor a solution that is based upon pure empirical physics vs a pure act of faith?

IMO this Fermi thread is pretty much a "classic" example of this specific bias playing out at the microscopic level of cosmology:

In this case plasma cosmology and the mainstream explanation (say a variant of lambda-cdm) are NOT equal. Not by a long shot.

When you choose "equal", how exactly do we "compare" theories? IMO even if we put "qualification" on an equal footing with "quantification', there's no logical way to claim that exotic forms of "dark matter" had anything to do with gamma rays that have been seen in Fermi images, or cosmology in general.

In fact the mainstream seems to be going out of their collective way to *NOT* mention the fact that "electrical discharges" are the likely source of the bulk of these emissions. Instead we're seeing a "leap of faith" into a pure theoretical construct related to 'dark matter', and no mention of the fact that planets and suns are the most likely 'source' of these emissions. It's not even logical IMO.

In any case, it should only be used to make a judgement when "all other things are equal".

Well, they aren't even "equal" yet. I've provided a mathematically supported paper that links these gamma ray excesses that have been attributed to "dark matter" in some papers to "binary stars". Which is a more "logical' solution here?

Even if quantification only counts as say a single point, I've handed you a 'slam dunk' that includes mathematical explanations as well visual (in Fermi images) verification of the fact that planets and suns are "known sources" of gamma rays. It's therefore no "leap of faith' for me to believe that binary stars are 'brighter' and "scattering happens" in a dusty core filled with binary stars.

"Dark matter" isn't even an option because it's purely a mathematical construct. There's no empirical physics whatsoever behind it. It only gets one point for quantification, and no point for qualification. They aren't even "equal" theories yet, it's 2 to 1 in favor of 'discharges".

If you ask me then is "dark matter" a more viable "explanation" than "electrical discharge', my answer is clearly no, certainly no in *this particularly* instance. In this specific instance "electricity in the atmospheres of bodies in space did it" enjoys both mathematical quantification and empirical qualification.

"Dark matter did it" theory will forever remain a "second rate" theory IMO until and unless it ever gains some sort of empirical support. Until that time, it will only ever be able to receive one point of a possible two points on the "qualification/quantification" scales of science.

IMO that is not unlike your monopole example. If and when they can be shown to exist, then it's worth considering them. In the mean time I'm not beholden to explain 'why' they don't exist in nature.

 

[Michael Mozina]

The observations of dark matter mean that we know a lot of its properties.

No you don't. You're making up those properties as you go, just like the "dark matter annihilates to produce gamma rays" claim.

 

[Reality Check]

No you don't. You're making up those properties as you go, just like the "dark matter annihilates to produce gamma rays" claim.

It is not like "dark matter annihilates to produce gamma rays" claim. That is a hypothesis that may explain a couple of observations (the Fermi haze and the spectrum of gamma radiation).


The observations of dark matter give us its properties:

1. It isn't in dense clumps.
2. It doesn't interact with itself..
3. These dark halos form very early.

These observations of the properties of dark matter allow use to construct hypotheses about what is it. Note the order: Observations -> properties -> hypotheses.

 

[ben m]

By "catching on", you mean I'm supposed to simply agree with everyone as one "complete package", inseparable, with exactly all the various "properties" you claim

If you want to discuss Fermi, then YES---you need to accept that this, this package I describe, is what we call the WIMP hypothesis. You must accept that definition of the hypothesis before you can even know what Fermi is on about. Moreover, to avoid more retreads, I think you should for the sake of discussion allow that this hypothesis is not otherwise ruled out.

If you want to say "that hypothesis is nonsense from the start", then there's no point discussing Fermi at all---in which case I think you need another thread.

including distribution which "just so happens" to coincide with the binary star populations near the core?

What? Who said anything about that? The Milky Way core is the place where we have the least detailed knowledge of the DM density. Halos and cluster cores are the places where we have the most detailed knowledge. None of this has anything to do with binaries.

 

[Reality Check]

...snip...
In this particular case I have shown you evidence that the excess gamma rays we're looking for can be explained by an excess of binary stars in the core, and the shape of that core and gamma ray excess just so happens to align itself pretty well with where we believe such stars are located.

Can you give the citations to your source for these statements?
I cannot find them in the thread.

As far as I can see:

• All stars emit gamma rays.
• Fermi detects the gamma rays from the Sun because the Sun is close to us.
• Fermi does not detect the gamma rays from normal stars including normal binary stars because they are far from us, i.e. the intensity of gamma rays is too low to detect.
  For example I can see no mention of Alpha Centauri (a binary star) being a strong gamma ray source.
• If a binary star system involves strong magnetic fields, e.g. one of the pair is a neutron star then it could be a source of gamma rays.
  However I suspect that it will be a point source of gamma rays that can be distinguished from the gamma ray background.

The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze subtracts the 3-month Fermi point source catalog as well as the LMC, SMC, Orion-Barnard’s Loop, and NGC 5090.

 

[Reality Check]

This question seems to have been forgotten.

First asked 5 November 2009:

The answer to Can Micheal Mozina answer a simple question? post about colliding blobs is not how astronomers confirmed that the Bullet Cluster, MACS J0025.4-1222 and Abell 520 contain matter that does not interacting electromagnetically like normal matter. The distribution of the dark and visible matter in the clusters was a big clue. The scientific confirmation though is that computer simulations of colliding blobs containing a mixture of normal, gaseous matter and matter that only has gravitational friction show the same properties as in my post:

• The dark matter goes straight through the normal matter.
• The normal matter collides, slows down and heats up.
• As the collision progresses the dark matter emerges from the cluster (the outlying blobs seen in the Bullet Cluster).
• As more time passes the dark matter reverses direction and heads back into the cluster. MACS J0025.4-1222 is just entering this stage while Abell 520 is well into this stage.

See Dark Matter Part 3.5: When Clusters Collide!

Michael Mozina: What is wrong with the computer simulations?

P.S. One result of the computer simulations is the prediction that under the right conditions dark matter will from a ring around the cluster.
Here is a news article (since you like these) about a ring of dark matter:
NASA finds further proof of dark matter
(I really dislike that "proof" word - it should be "evidence").

Now we have four observations or dark matter that agree with it not interacting electromagnetically like normal matter!

 

[Michael Mozina]

Can you give the citations to your source for these statements?
I cannot find them in the thread.

http://www.internationalskeptics.com/forums/showpost.php?p=5294456&postcount=359
http://www.nature.com/nature/journal/v451/n7175/abs/nature06490.html

As far as I can see:

• All stars emit gamma rays.
• Fermi detects the gamma rays from the Sun because the Sun is close to us.
• Fermi does not detect the gamma rays from normal stars including normal binary stars because they are far from us, i.e. the intensity of gamma rays is too low to detect.

• 
  
  Er, you're now telling me that not just "some" of the gamma rays (the excess) comes from dark matter, but *all* of it? Could you point me to a paper that attributes "all" the gamma rays to "dark matter" rather than simply an "excess" of them?

 

[Reality Check]

Michael Mozina rocks = dark matter idea (Questions 4 and 5)

And now back to the galactic clusters!

First asked 16 November 2009
Astronomers have measured the distribution of matter in galactic clusters using gravitational lensing. This distribution has a massive, roughly spherical background (dark matter) with the spikes of galaxies poking out from the background.

IMO: "Lumpy stuff" (rocks and black holes) will not be moving fast. They will especially will not exceed the escape velocity of galaxies. If the "lumpy stuff" formed in intracluster space then it will have roughly the velocity of the cluster. If the "lumpy stuff" formed in the galaxies then there is the problem of extracting it from the galaxies. Collisions can explain the observed intracluster objects such as stars but not enough "lumpy stuff" to match the mass of dark matter.

Galaxies in a galactic cluster move. They will act like vacuum cleaners and collect "lumpy stuff". This means that as time passes the balance between galaxies will shift - galaxies will get heavier and the amount of dark matter will reduce. If enough time passes galaxies will become heavier than the "lumpy stuff" in between them.

Michael Mozina:

Why have astronomers not found seen that galaxies become less dense with increasing distance from us?
Why is the "lumpy stuff" in intracluster space so much of the mass of the cluster when there has been ~13 billion years for the galaxies to vacuum up the "lumpy stuff"?

 

[Michael Mozina]

I started the other thread for you. Feel free to stop hijacking this thread now and please keep this thread "on topic". Got any evidence that "dark matter" emits gamma rays?

 

[Reality Check]

http://www.internationalskeptics.com/forums/showpost.php?p=5294456&postcount=359
http://www.nature.com/nature/journal/v451/n7175/abs/nature06490.html

Er, you're now telling me that not just "some" of the gamma rays (the excess) comes from dark matter, but *all* of it? Could you point me to a paper that attributes "all" the gamma rays to "dark matter" rather than simply an "excess" of them?

Er, No.
I am saying that the excess cannot come from "binary stars" you have been stating. You do not supply a qualifier so it looks like you are referring to any binary star system. I assumed you meant normal binary stars which is obviously wrong.

The paper states that there are X-ray binaries whose distribution fits the the distribution of gamma rays. The abstract concludes:

Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

So they are stating what has been explained to you many times:
Dark matter might or might not be an explanation for the Fermi data after all of the known sources of gamma rays (including this one) are removed (the Fermi haze).
IOW: Removing the known sources of gamma radiation from the Fermi data leaves the unknown sources, one of which could be dark matter.

No one is stating that dark matter is the explanation for the Fermi haze.

The other paper does what scientists do all the time - it takes an observation and sees if there is a hypothesis to fit it. In this case the observation is the shape of the spectrum of gamma rays from the Fermi data and the hypothesis is that it is caused by dark matter. This leads to a testable, falsifiable prediction that if dark matter is the cause of the spectrum then it must have certain properties. There are ongoing controlled, empirical experiments in labs here on Earth that may confirm or falsify this hypothesis.

 

[ben m]

I started the other thread for you. Feel free to stop hijacking this thread now and please keep this thread "on topic". Got any evidence that "dark matter" emits gamma rays?

OK, so for the purpose of discussion, the dark matter we are looking for is in particular some as-yet-undiscovered stable massive particle.

Various specific hypotheses for the identity of the particle include, by necessity, gamma ray emission in pair annihilation reactions. The details are 100% standard particle physics: just like the hypothesis of the top quark obviously required that it decay into bottom quarks, the hypothesis of the Higgs obviously requires it to decay to the heaviest available particle, and so on. Examples of gamma-predicting hypotheses include SUSY dark matter and Kaluza-Klein dark matter. Each such hypothesis makes a specific prediction about the gamma ray spectrum.

Other hypotheses predict NO gamma ray emission. Examples include axion dark matter and some forms of sterile neutrino dark matter.

Let's see, what do scientists do when they have a hypothesis that predicts something observable? They try to make the observation and see if it confirms or excludes the hypothesis. FERMI is currently making observations. It has ruled out several hypotheses so far. If it sees a convincing SUSY-like or KK-like signal it might help confirm one of those hypotheses, obviously after lots of followup work. It can't say anything about axions, because the axion hypothesis do not predict gamma rays.

In other words, MM, your question is a bizarre one to which you ought to have known the answer. Fermi is looking for (among other things) new things, things we haven't seen before. No, we haven't seen those things before---that's why we have to look before knowing if they are there.

 

[Reality Check]

I started the other thread for you. Feel free to stop hijacking this thread now and please keep this thread "on topic".

This thread has not been hijacked.
Your argument is that dark matter cannot emit gamma rays. Your evidence for this is an idea that it is "lumpy stuff". Refuting this idea is easy and should be (has been) done in this thread.

If your argument is that dark matter may emit gamma rays then this thread is over. That is what everyone has been saying. In that case we can take the "lumpy stuff" rebuttal to the other thread.

Which is it MM?

Got any evidence that "dark matter" emits gamma rays?

Yes

• The unknown sources for the Fermi haze which include dark matter.
• The fit to the Fermi gamma ray spectrum from the hypothesis that dark matter emits gamma rays.

Is it strong evidence? - no.
The first observation might be explained by something other than dark matter.
The second hypothesis needs independent confirmation.

 

[ben m]

RC, MM, you aren't even talking about the same gamma rays.

The gamma ray excess seen by EGRET at the galactic center was (specifically) 511 keV gammas, the energy that you expect from positron annihilations. This specific energy line---and no other---appeared to come 50% from the galactic center and 50% from a cluster of LMXBs. This rules out those specific dark matter models which predicted very large 511 keV fluxes.

The Fermi telescope is searching for gamma rays between 30,000 keV and 300,000,000 keV. This has nothing to do with the LMXB positrons. This range is sensitive to totally different dark matter models, and contaminated by totally different backgrounds. In fact, the places you'll see attempts to use dark matter to explain Fermi data are (AFAIK) always focusing on the highest-energy end of this range.

 

[Michael Mozina]

OK, so for the purpose of discussion, the dark matter we are looking for is in particular some as-yet-undiscovered stable massive particle.

So let us agree that the "base" hypothesis "assumes":

1. A new massive particle exists.
2. It only weakly interacts with other forms of matter.
3. It is "long lived"

Are these all the assumptions of the "base hypothesis', or does it also include a given decay rate?

Various specific hypotheses for the identity of the particle include, by necessity, gamma ray emission in pair annihilation reactions.

Um, I don't quite accept that "necessity" concept. Is it a "necessity" that a WIMP decay at some specified rate under some specific condition? Is the gamma ray emission process also part of the "basket of goodies" that I have to accept or is that part open to challenge?

The details are 100% standard particle physics: just like the hypothesis of the top quark obviously required that it decay into bottom quarks, the hypothesis of the Higgs obviously requires it to decay to the heaviest available particle, and so on. Examples of gamma-predicting hypotheses include SUSY dark matter and Kaluza-Klein dark matter. Each such hypothesis makes a specific prediction about the gamma ray spectrum.

So you're taking this number from some part of SUSY theory rather than "winging it"? How can I "verify" or falsify that number?

Other hypotheses predict NO gamma ray emission. Examples include axion dark matter and some forms of sterile neutrino dark matter.

Ok. "Note to self: This isn't the only possible "exotic particle" that might explain "missing mass". " Got it.

Let's see, what do scientists do when they have a hypothesis that predicts something observable? They try to make the observation and see if it confirms or excludes the hypothesis. FERMI is currently making observations. It has ruled out several hypotheses so far.

Did it rule out that binary star study somehow?

If it sees a convincing SUSY-like or KK-like signal it might help confirm one of those hypotheses, obviously after lots of followup work. It can't say anything about axions, because the axion hypothesis do not predict gamma rays.

In other words, MM, your question is a bizarre one to which you ought to have known the answer. Fermi is looking for (among other things) new things, things we haven't seen before. No, we haven't seen those things before---that's why we have to look before knowing if they are there.

The problem from my skeptical perspective Ben is that you're "looking" to stuff the gaps of ignorance related to any gamma radiation with "dark matter" with unverified "properties" galore. Even if I give you one or two of them to start with, the whole thing requires the existence of something new whereas the solution I provided you earlier does not. Same question to you then: When is it appropriate to choose/favor a "simpler" theory over one that requires multiple unverified things to be true?

How exactly are you getting these positrons out of the "dark matter" in a way that is visible to us here on Earth? In other words, an "annihilation' at the core of a sun isn't going to do us much good. These decay/ e+ e- process have to occur somewhere "visible" to us here on Earth.

 

[Michael Mozina]

RC, MM, you aren't even talking about the same gamma rays.

The gamma ray excess seen by EGRET at the galactic center was (specifically) 511 keV gammas, the energy that you expect from positron annihilations. This specific energy line---and no other---appeared to come 50% from the galactic center and 50% from a cluster of LMXBs. This rules out those specific dark matter models which predicted very large 511 keV fluxes.

Such signature have a known source.

http://www.sciencenews.org/view/generic/id/49288/title/Signature_of_antimatter_detected_in_lightning

The Fermi telescope is searching for gamma rays between 30,000 keV and 300,000,000 keV. This has nothing to do with the LMXB positrons. This range is sensitive to totally different dark matter models, and contaminated by totally different backgrounds. In fact, the places you'll see attempts to use dark matter to explain Fermi data are (AFAIK) always focusing on the highest-energy end of this range.

So how exactly are you proposing that these wavelengths are in any way related to "dark matter"???

 

[ben m]

Such signature have a known source.

http://www.sciencenews.org/view/generic/id/49288/title/Signature_of_antimatter_detected_in_lightning

Yes, this is called a "background". We know that the Sun, LMXBs, etc., produce these low energy gamma rays. If the dark matter emits low-energy gamma rays, it will be difficult to conclusively observe because we'll have to convince ourselves that Observation X cannot be just background. This may be difficult or impossible. We are trying.

This process is called "science".

So how exactly are you proposing that these wavelengths are in any way related to "dark matter"???

Listen: the hypothesis that dark matter is made of SUSY LSPs includes everything we know about the SUSY LSP hypothesis---including things we learn about it from, e.g., collider data and electroweak theory. One aspect of this hypothesis is that the LSP annihilation products include > 50 GeV gamma rays. Understand? That's a prediction of the SUSY LSP hypothesis for what the dark matter is made of.

The data does not yet support or refute this hypothesis.

 

[ben m]

Um, I don't quite accept that "necessity" concept. Is it a "necessity" that a WIMP decay at some specified rate under some specific condition? Is the gamma ray emission process also part of the "basket of goodies" that I have to accept or is that part open to challenge?

Please reread the post. The hypothesis tells you the particle properties. In the SUSY hypothesis, one of the properties is "coannhihilation to gamma rays" (NOT decay!), like it or not. This comes from the same sort of particle theory calculations that tells Large Hadron Collider physicists that the "Higgs hypothesis" automatically includes Higgs decay to heavy pairs, or that tells neutrino physicists that the "Majorana neutrino hypothesis" automatically includes neutrinoless double-beta decay of the 76Ge nucleus.

So you're taking this number from some part of SUSY theory rather than "winging it"? How can I "verify" or falsify that number?

Yes, we're taking these claims from particular SUSY theories; there's a wide range of theories, which Jungman, Kamionkowski and Griest http://arxiv.org/abs/arXiv:hep-ph/9506380 surveyed appropriately. For a particular experiment, you have to ask what particular theories they were or were not sensitive to.

Ok. "Note to self: This isn't the only possible "exotic particle" that might explain "missing mass". " Got it.

Good, now you understand. A given observation can only test the theories that predict something related to that observable. Fermi can only possibly test SUSY and Kaluza-Klein dark matter. ADMX can only possibly test axion dark matter. EROS could only possibly have tested MACHO-type dark matter.

 

[ben m]

Did it rule out that binary star study somehow?

What, the 511 keV/LMXB association? To first order this is a totally different question than anything Fermi deals with. On the other hand: state a specific hypothesis for the origin of the 511 keV line and I'll tell you whether Fermi can say anything about that hypothesis.