APOD: Gamma Rays from Galactic Center Dark... (2014 Mar 10)

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APOD: Gamma Rays from Galactic Center Dark... (2014 Mar 10)

Post by APOD Robot » Mon Mar 10, 2014 4:06 am

Image Gamma Rays from Galactic Center Dark Matter

Explanation: What is creating the gamma rays at the center of our Galaxy? Excitement is building that one answer is elusive dark matter. Over the past few years the orbiting Fermi Gamma-ray Space Telescope has been imaging our Galaxy's center in gamma-rays. Repeated detailed analyses indicate that the region surrounding the Galactic center seems too bright to be accounted by known gamma-ray sources. A raw image of the Galactic Center region in gamma-rays is shown above on the left, while the image on the right has all known sources subtracted -- leaving an unexpected excess. An exciting hypothetical model that seems to fit the excess involves a type of dark matter known as WIMPs, which may be colliding with themselves to create the detected gamma-rays. This hypothesis is controversial, however, and debate and more detailed investigations are ongoing. Finding the nature of dark matter is one of the great quests of modern science, as previously this unusual type of cosmologically pervasive matter has shown itself only through gravitation.

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Boomer12k » Mon Mar 10, 2014 5:27 am

Would these be around the CMBH of the Milky Way?

Would they be drawn to it, and thus start hitting or interacting with other WIMPS?

Are they observed in other galactic centers?

If they are so pervasive, why do we not see them in other places, interacting and giving off Gamma Rays?

Can our heroes solve this puzzling riddle???
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Markus Schwarz » Mon Mar 10, 2014 9:53 am

Boomer12k wrote:Would these be around the CMBH of the Milky Way?

Would they be drawn to it, and thus start hitting or interacting with other WIMPS?

Are they observed in other galactic centers?

If they are so pervasive, why do we not see them in other places, interacting and giving off Gamma Rays?
A common density assumed for WIMPs is about three WIMPS per glass of beer on average. Since they are Weakly Interacting Massive Particles, having a very low density, they are extremely difficult to detect. That's why some experiments looked at centers of large gravity, such as the sun. The idea being that the density is higher there and, thus, WIMP annihilation into gamma rays occurs more often.

Direct searches, at the LHC, have found no trace of them so far. Other, indirect, experiments have found to signal either.

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by BDanielMayfield » Mon Mar 10, 2014 12:40 pm

So what their looking for is the particle-antiparticle annihilation signature for these proposed particles, but if this occurs INSIDE a black hole’s event horizon it could never be detected anyway. Therefore if events are being detected they would be from the accretion area in the vicinity of, but not inside the black hole.

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by NGC3314 » Mon Mar 10, 2014 12:49 pm

These searches all key not on the supermassive black hole, but the much larger and more massive region in the galactic center where the density of dark-matter particles would be highest. This gives a strong peak in the annihilation signal (always assuming that the particles are self-annihilating, known in the jargon as Majorana particles); collisional processes have a rate scaling as density squared. The SMBH has a small enough mass that its effect essentially doesn't matter for the detection of such an annihilation signal. In fact, some X-ray searches use dwarf galaxies, because they have the minimal amount of other astrophysical excitement that could mimic an annihilation signal.

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by neufer » Mon Mar 10, 2014 2:14 pm

Markus Schwarz wrote:
Boomer12k wrote:
Would these be around the CMBH of the Milky Way?
Would they be drawn to it, and thus start hitting or interacting with other WIMPS?
Are they observed in other galactic centers?
A common density assumed for WIMPs is about three WIMPS per glass of beer on average.
  • Three WIMPS for Colonel Blimp!
    Sure he has not got much fairy shrimp.
    And surely any he has are quite limp.
Markus Schwarz wrote:
Since they are Weakly Interacting Massive Particles, having a very low density, they are extremely difficult to detect. That's why some experiments looked at centers of large gravity, such as the sun. The idea being that the density is higher there and, thus, WIMP annihilation into gamma rays occurs more often.
But why would the density be higher in "gravitational wells" :?:

Non relativistic massive particles would be moving their fastest
(and hence spending the least time) deep within "gravitational wells."

Non relativistic massive particles would be moving their slowest
(and hence spending the most time) in the outskirts
(e.g., Oort clouds, galactic edges, etc.) of "gravitational wells."
Last edited by neufer on Mon Mar 10, 2014 2:58 pm, edited 1 time in total.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by NGC3314 » Mon Mar 10, 2014 2:29 pm

Even if moving more slowly when farther out, they're spread over a lot more volume (actually, for the same reasons stars in galaxies concentrate toward the center). The situation in many galaxies gives less bias than this, in fact - flat rotation velocity with radius, as often measured, reduces the velocity difference between inner and outer parts of the orbit compared to our intuition honed on systems where the mass is practically a point source - effectively, the particle sees more mass when farther from the center. Also, gravitational interactions will gradually segregate orbiting particles by energy, so some particles do end up trapped deep in the potential well (which is due more to dark matter than baryons outside the innermost parts of most galaxies).

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Ron-Astro Pharmacist » Mon Mar 10, 2014 2:39 pm

The one on the left looks like a Jalapeño. It must be hot dark matter. The Wiki link says "it neither emits nor absorbs light or other electromagnetic radiation at any significant level" Wonder how its existence can be indirectly inferred from radiation? Probably a hot topic amongst the PhD's.

The evidence is clear. The one on the left must be a Habanero? Even hotter dark matter.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by neufer » Mon Mar 10, 2014 3:14 pm

NGC3314 wrote:
Even if moving more slowly when farther out, they're spread over a lot more volume (actually, for the same reasons stars in galaxies concentrate toward the center). The situation in many galaxies gives less bias than this, in fact - flat rotation velocity with radius, as often measured, reduces the velocity difference between inner and outer parts of the orbit compared to our intuition honed on systems where the mass is practically a point source - effectively, the particle sees more mass when farther from the center. Also, gravitational interactions will gradually segregate orbiting particles by energy, so some particles do end up trapped deep in the potential well (which is due more to dark matter than baryons outside the innermost parts of most galaxies).
It would seem that the most logical place to look (with gamma rays) for such
gravitational condensations of WIMPS would be the centers of globular clusters.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Psnarf » Mon Mar 10, 2014 3:22 pm

Why not just call it a mystery? FMTM - Mystery To Me. How did they count every single star in a nearby galaxy, including the black holes, supernovae remnants and all? Without a clue as to what is inside a galactic center, how do we know there is insufficient force to keep a galaxy from flying apart? Nobody knows exactly what goes on at the quantum level, just experiments that match predicted equations. No one has yet to come up with a satisfactory explanation of the expansion of space-time. We think the expansion is accelerating because photons that put flowers in their hair and split for the coast 12 billion years ago show things were still flying apart pretty fast during the early billions of years after the big badaboom. We have no way to tell what's the deal out there today.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Chris Peterson » Mon Mar 10, 2014 3:39 pm

Psnarf wrote:Why not just call it a mystery? FMTM - Mystery To Me. How did they count every single star in a nearby galaxy, including the black holes, supernovae remnants and all?
There are solid methods for doing this, looking at luminosity and mass.

It's dangerous to call anything a mystery, since to many people that means a question that has to be answered with a silly explanation, and to others it means a question that can't be answered. When a scientist says something is a mystery, it means a problem that needs attention.
Without a clue as to what is inside a galactic center, how do we know there is insufficient force to keep a galaxy from flying apart?
We have a very good idea what is inside a galactic center, with a rich set of observations supporting theory (including very high resolution imagery of our own galactic center).
Nobody knows exactly what goes on at the quantum level, just experiments that match predicted equations.
How is that different from "knowing exactly what goes on"?
No one has yet to come up with a satisfactory explanation of the expansion of space-time. We think the expansion is accelerating because photons that put flowers in their hair and split for the coast 12 billion years ago show things were still flying apart pretty fast during the early billions of years after the big badaboom. We have no way to tell what's the deal out there today.
We have superb explanations, supported by observation. How is it unreasonable to infer what's going on "now" (whatever that means) from observing a progression of past events?
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by neufer » Mon Mar 10, 2014 4:03 pm

Chris Peterson wrote:
Psnarf wrote:
Nobody knows exactly what goes on at the quantum level, just experiments that match predicted equations.
How is that different from "knowing exactly what goes on"?
When we know exactly what is going on:
  • 1) We'll be able to explain it all to a 3rd grader
    2) There will be no unsolved "mysteries."
Chris Peterson wrote:
Psnarf wrote:
No one has yet to come up with a satisfactory explanation of the expansion of space-time. We think the expansion is accelerating because photons that put flowers in their hair and split for the coast 12 billion years ago show things were still flying apart pretty fast during the early billions of years after the big badaboom. We have no way to tell what's the deal out there today.
We have superb explanations, supported by observation. How is it unreasonable to infer what's going on "now" (whatever that means) from observing a progression of past events?
We have a few explanations, supported by observation.

We probably need better (or fuller) explanations, and more observations.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by JuanAustin » Mon Mar 10, 2014 4:18 pm

Is there an artist depiction of what the night sky would look like if the galactic center turned on and was actively feeding again and what percentage of the jet's content emminating from active galactic centers are gamma rays? if the majority of the stuff being expelled in jets is indeed all or close to mostly gamma, what can be deduced about dark matter for all active galactic cores?
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by neufer » Mon Mar 10, 2014 5:06 pm

JuanAustin wrote:
What percentage of the jet's content emanating from active galactic centers are gamma rays?
In the extreme case of NGC 6251 the peak (jet/lobe) energy output lies at around 1 GeV (or ~1023 Hz)

(Visible light output at around 1015 Hz of NGC 6251's jet/lobe is at a minimum.)
http://iopscience.iop.org/0004-637X/749/1/66/article wrote:
The Astrophysical Journal, Volume 749, Number 1

SUZAKU X-RAY IMAGING OF THE EXTENDED LOBE IN THE GIANT RADIO GALAXY
NGC 6251 ASSOCIATED WITH THE FERMI-LAT SOURCE 2FGL J1629.4+8236

Y. Takeuchi1, J. Kataoka1, Ł. Stawarz2,3, Y. Takahashi1, K. Maeda1,
T. Nakamori1, C. C. Cheung4,6, A. Celotti5, Y. Tanaka2, and T. Takahashi2

Broadband spectral energy distributions of the outer jet and the NW lobe in NGC 6251, including the Fermi-LAT source 2FGL J1629.4+8326. The radio and X-ray data for the outer jet are from Sambruna et al. (2004b), and Evans et al. (2005), respectively. The radio fluxes of the NW lobe (red squares and red arrow) are described in Section 4.2. The X-ray fluxes for the lobe, as found in this paper, are given in the 0.5–1.0 keV, 1.0–2.0 keV, 2.0–4.0 keV, and 4.0–8.0 keV bins (open red circles). Finally, the LAT fluxes of 2FGL J1629.4+8326 (magenta stars) are taken from the 2FGL catalog (Abdo et al. 2012). The thin solid line represents the beamed IC/CMB model for the outer jet, assuming an association with 2FGL J1629.4+8326. The thick gray line represents the IC/(CMB+EBL) model for the NW lobe, again assuming an association with 2FGL J1629.4+8326.>>
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Anthony Barreiro » Mon Mar 10, 2014 6:25 pm

These gamma ray results are interesting and need further study. I would imagine that weakly interacting massive particles are only one possible explanation for the extra gamma rays. Weakly interacting massive particles are the currently favored mechanism to explain the excess gravity seen at galactic and intergalactic scales. However huge underground detectors that should have found a few WIMP's have failed to make any statistically significant detections. So maybe we need to build bigger detectors and let them observe longer. Or maybe WIMP's don't exist.

Another possible explanation for the so-called "missing mass problem" is modified Newtonian dynamics. "Rather than invoking some invisible form of dark matter, [MOND] hypothesizes a subtle change to the effective force law at extremely low accelerations (< 10-10 m/s/s)."

MOND is a distinctly minority hypothesis that tends to make the WIMP majority faction sputter and fume. But just as we found curved light paths that were better explained by general relativity than by classical Newtonian dynamics, perhaps there are further theoretical tweaks needed to explain the extra gravity we're observing on large scales. And theoretical elaboration is a lot cheaper than filling abandoned mines with distilled water and liquid xenon and hoping to see ultraviolet flashes.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Spif » Mon Mar 10, 2014 7:15 pm

Anthony Barreiro wrote:Another possible explanation for the so-called "missing mass problem" is modified Newtonian dynamics. "Rather than invoking some invisible form of dark matter, [MOND] hypothesizes a subtle change to the effective force law at extremely low accelerations (< 10-10 m/s/s)."

...
And theoretical elaboration is a lot cheaper than filling abandoned mines with distilled water and liquid xenon and hoping to see ultraviolet flashes.
Theories don't get very far unless you can back them up with experimental results based on their predictions. Moreover, developing a theory that appears to have some credibility and then finding a null result is still very useful... It sets boundaries on the actual numbers or wipes out your hypotheses entirely and allows people to spend their brain cycles on other ideas. Sure it costs money, but real data is how science really gets done. Since as a society we value knowledge, even "failures" are worth investing in, since the failure still advances the boundaries of our knowledge.

I'm not familiar with the specific physical law theory that you link to, but I'll check it out if I get time. I've heard of people pursuing all kinds of angles on Dark Matter and Dark Energy and every once in a while I hear of "modified" models of gravity and such. A couple of those seem to have been convincingly ruled out by subsequent data findings a few years ago. I get the impression that the scientific community is more open to out of the box thinking today than they used to be. But with the rise of the internet that enables self-published "papers", there's a lot of clearly insane crack-pottery flying around. Not to mention recent controversies like all the journals that have accepted computer-generated gibberish and recent findings of unethical data tampering.

So the weird ideas that attempt to upturn the foundations of current knowledge still require some credible rigor and reproducible experimental findings to gain real attention. That's not necessarily oppression of the creative thinkers ... it's just filtering the signal out from the noise.

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Anthony Barreiro » Mon Mar 10, 2014 7:43 pm

MOND is not "self-published ... clearly insane crack-pottery." And yes, empirical validation is necessary to confirm a theory. But at some point if all you're getting is null results, maybe it's time to reconsider your hypothesis, rather than adding another epicycle that might preserve the possibility of your hypothesized particle. I don't know enough physics to have an opinion about whether MOND is true or not. But the consensus assumption, in the absence of empirical observation, that WIMP's must exist, strikes me as rather curious. And the principle of parsimony would seem to favor a reworking of equations (so long as the new equations conform to observations) over a fruitless snark-hunting expedition.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by chuckster » Mon Mar 10, 2014 7:59 pm

Just asking you people who seem to have a better overall knowledge of the flow of current investigations: Is the idea of multiple universes still too far-out to be incorporated into serious theoretical work ? I have thought that it's possible that all the hoopla about the search for dark matter may turn out to be a snark hunt, and the dark gravity out there is originating from mass that is not of this universe. The search for dark matter is necessary, but it may turn out to be an elimination of it as an actual feature of nature and relegate it back to place-holder status . It has been suggested that gravity acting over multiple universes explains its supposed weakness relative to the other basic forces in physics, which would mean that gravity from 'elsewhere' could affect this universe, as well as the reverse situation.

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Spif » Mon Mar 10, 2014 8:23 pm

Anthony Barreiro wrote:MOND is not "self-published ... clearly insane crack-pottery."
To be clear, I did not make that assertion. I started out conceding that I haven't read the paper ... that's my way of pointing out that I have no specific opinion about the work involved. 8-)

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Spif » Mon Mar 10, 2014 8:38 pm

chuckster wrote:It has been suggested that gravity acting over multiple universes explains its supposed weakness relative to the other basic forces in physics, which would mean that gravity from 'elsewhere' could affect this universe, as well as the reverse situation.
There was a Science Friday show last year where the physicist in the hot seat tossed this idea out there as one of the vague possibilities. But I think the general idea is that IS "way out there" beyond the frameworks of knowledge that we have developed to date. Generally, I think that people prefer pursuing more "low hanging fruit" ideas before re-inventing the entire field of cosmology on a grander scale than we can even see today.

But, yeah, fun idea.

So that's at least a one professional who has devoted a few brain cycles to it...

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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Chris Peterson » Mon Mar 10, 2014 8:56 pm

Anthony Barreiro wrote:MOND is a distinctly minority hypothesis that tends to make the WIMP majority faction sputter and fume.
I'm not sure I'd go that far. But the reason that so few physicists consider MOND to be a likely theory is that not only does it fail to predict some important features of the Universe, but some observational evidence appears to actually contradict it.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Psnarf » Mon Mar 10, 2014 8:58 pm

http://science.nasa.gov/astrophysics/fo ... rk-energy/

Boy, when I'm wrong, I really get it wrong, something that is self-evident to the casual observer. I did not know that in 1998 Hubble observations showed that things were accelerating a lot slower a long, long time ago in a land far, far away.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Chris Peterson » Mon Mar 10, 2014 9:01 pm

chuckster wrote:Just asking you people who seem to have a better overall knowledge of the flow of current investigations: Is the idea of multiple universes still too far-out to be incorporated into serious theoretical work ? I have thought that it's possible that all the hoopla about the search for dark matter may turn out to be a snark hunt, and the dark gravity out there is originating from mass that is not of this universe. The search for dark matter is necessary, but it may turn out to be an elimination of it as an actual feature of nature and relegate it back to place-holder status . It has been suggested that gravity acting over multiple universes explains its supposed weakness relative to the other basic forces in physics, which would mean that gravity from 'elsewhere' could affect this universe, as well as the reverse situation.
I think you're confusing dark matter with dark energy. Dark matter doesn't demonstrate any unusual forces. It has mass, and therefore is subject to gravitational interaction. It is only EM that it interacts with very weakly (or not at all), but that describes other particles, as well. In short, there's nothing necessarily exotic about dark matter particles, and I've never heard any suggestion that the quite strong effects that we directly observe could possibly relate to multiple universes.
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Chris Peterson » Mon Mar 10, 2014 9:03 pm

Spif wrote:Theories don't get very far unless you can back them up with experimental results based on their predictions.
That's a rather obsolete view of how science works. It would be much more accurate to say that theories are either bolstered or disproved by observational results (which may or may not be from experiments).
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Re: APOD: Gamma Rays from Galactic Center Dark... (2014 Mar

Post by Chris Peterson » Mon Mar 10, 2014 9:08 pm

neufer wrote:When we know exactly what is going on:
  • 1) We'll be able to explain it all to a 3rd grader
    2) There will be no unsolved "mysteries."
Knowing exactly what's going on doesn't imply it can be easily understood by everybody. If the explanation requires tensor calculus, it's going to be a different 3rd grade than I remember! And the response was only understanding "it all" with respect to one particular aspect of nature. That doesn't mean we'll know everything.
We probably need better (or fuller) explanations, and more observations.
Always true for non-trivial theories.
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