NASA pictures of dark matter collisions

[PWiz]

You may be right here, I was thinking of free neutron decay which can produce photons (https://en.wikipedia.org/wiki/Neutron#Free_neutron_decay). Maybe it's incorrect to interpret that as interaction with EM ?

All that I'm saying is that a neutron star consists of neutrons in its core and ionized gas in its exterior. The outside of the star should interact with photons, but the inside must be transparent. If a non-rotating neutron star with a particularly thin (i.e. negligible) layer of ionized gas is present somewhere, it should look pretty much invisible to the eye (remember that only rotating neutron stars emit EM radiation). Therefore, I thought that such invisible, yet massive objects would qualify as dark matter.
EDIT: Sorry for being so fussy, but I couldn't tell if you're joking or not:

Or a soup of quarks or something

Individual quarks do not exist in nature because of color confinement.

 

[wabbit]

All that I'm saying is that a neutron star consists of neutrons in its core and ionized gas in its exterior. The outside of the star should interact with photons, but the inside must be transparent. If a non-rotating neutron star with a particularly thin (i.e. negligible) layer of ionized gas is present somewhere, it should look pretty much invisible to the eye (remember that only rotating neutron stars emit EM radiation). Therefore, I thought that such invisible, yet massive objects would qualify as dark matter.

Interesting, thanks.

The argument I've seen simply states that neutron stars are hot so they emit blackbody radiation in the x ray range, but maybe this wouldn't apply to such a "bare" neutron star if it can exist - might it emit mostly hard to detect neutrinos ?

So I guess the one sound argument against that neutron star hypothesis is the one Orodruin gave, that MACHOs are currently ruled out as a significant component of dark matter.

 

[PWiz]

Interesting, thanks.

I would still suspect some EM from such a "bare neutron star" but it could very well be below detectability. And an undetectable neutron star would presumably be counted as a MACHO.

So I guess the only sound argument against that neutron star hypothesis is the one Orodruin gave, that MACHOs are currently ruled out as a significant component of dark matter.

I wouldn't call them ruled out, but it's definitely difficult to reconcile the observations of the current universe with that model, since most of the neutron stars would eventually form black holes after colliding with each other if the Tolman-Oppenhiemer-Volkoff limit is exceeded (it is a mass limit which when exceeded causes the gravitational force to overcome the neutron degeneracy pressure), and the black holes would eventually evaporate because they emit Hawking radiation. So no baryonic matter would be formed in that scenario (come to think of it, we would never have baryonic matter in the universe in the first place as it would eventually all be converted to EM radiation if 80% of the universe consisted of these things[I'm excluding primordial black holes for simplicity]).

 

[PWiz]

The argument I've seen simply states that neutron stars are hot so they emit blackbody radiation in the x ray range

Rotating neutron stars induce magnetic dipole moments in the outer ionized gas layers. Photons (any EM radiation) will then be emitted. The outer layer consists of degenerate matter and high energy neutrinos and photons are released because the pressure is less on the outside, permitting charged particles to come into picture. If you take a static neutron star with a thin outer layer (as I've previously speculated), you should have a very-hard-to-detect neutron star. (Remember that no matter how hot a group of neutrons in thermal equilibrium with its environment is, it will never emit any EM/black body radiation if the neutrons are not allowed to decay)

 

[mfb]

Also, hadrons are held together by the strong nuclear force, not by gravitation which affects them far more weakly. And they do interact with EM - free neutrons quicly decay to proton + electron + photon for instance (https://en.wikipedia.org/wiki/Neutron#Free_neutron_decay)

They decay to proton + electron + anti-electronneutrino via the weak interaction. The electromagnetic interaction cannot let neutrons decay as it cannot change quark flavors. Additional photons from the decay are possible but not necessary.

I just read they have @ 600s lifetime. Are there other kinds of uncharged hadrons?

All other uncharged hadrons have much shorter lifetimes.

I don't see any reason to completely rule out neutron stars with particularly thin ionized gas layers.

Microlensing (well, the absence of) rules out stellar-mass objects as significant contribution to dark matter. Also, it would be unclear where all those cold neutron stars would have come from.
Neutrons do interact with photons of sufficient energy - they can scatter at the quarks inside the neutron. At lower energy, you still have the neutron magnetic moment.Concerning the original news, I wonder what previous expectations for the cross-section were. Those limits are orders of magnitude above the limits for dark matter / regular matter interactions, and orders of magnitude above typical weak cross-sections. So what type of interaction was expected?

 

[PWiz]

Neutrons do interact with photons of sufficient energy - they can scatter at the quarks inside the neutron.

Can you please explain this effect in some detail?

 

[PeterDonis]

AFAIK neutron stars are not counted as dark matter, but as ordinary matter

That's correct. See below.

I just can't understand why a photon would interact with a neutron when according to the standard model it is a gauge boson whose interactions are exclusively limited to charged particles

Neutrons are composite objects; they are composed of three quarks. So even though a neutron is electrically neutral, it can still interact with photons because the quarks inside it are electrically charged. For example, the neutron has a nonzero magnetic moment. This is really no more mysterious than the fact that electrically neutral atoms can emit and absorb photons.

I don't think that decay occurs in a neutron star

Correct; neutrons in bound states don't undergo the weak interaction decay, only free neutrons do. (Actually, that's not quite true--there are atomic nuclei that undergo beta decay, which means one of the neutrons inside the nucleus undergoes the weak interaction decay. But that happens because the neutron is very loosely bound in such nuclei. Neutrons in a neutron star are more tightly bound.)

I thought that such invisible, yet massive objects would qualify as dark matter.

There is probably some variation in terminology, but from the standpoint of cosmology and modeling the universe as a whole, the key characteristic of dark matter is not just that it's not visible now, but that it never was visible; i.e., it has always been dark since the early universe. Neutron stars, even if they themselves are not easily visible, are formed from matter that is easily visible (stars). So we can estimate how many neutron stars there are from looking at visible matter--watching how often supernovas happen, etc. (Also, of course, we have detectable evidence of some neutron stars, since that's what pulsars are--see below.) The only way we have of estimating how much dark matter there is is by its gravitational effect.

The argument I've seen simply states that neutron stars are hot so they emit blackbody radiation in the x ray range

This is true when they first form, but AFAIK they cool fairly rapidly. However, there are still ways for neutron stars to emit radiation: pulsars are neutron stars that emit beams of radio waves because they are spinning rapidly and have magnetic fields.

 

[mfb]

Can you please explain this effect in some detail?

Similar to deep inelastic scattering, just more direct. I don't know if it has a special name as the photons need such a high energy that it is impractical to study it in a lab. It was relevant in the very early universe, and above ~200 MeV the energy is sufficient to create new hadrons.

 

[Chalnoth]

Would "uncharged Hadrons held together by g..." be little clumps of neutrons?

I just read they have @ 600s lifetime. Are there other kinds of uncharged hadrons?

Yes, but they all have much shorter lifetimes.

The only long-lived particles with no electric charge in the standard model are neutrinos and photons. Neutrinos are too light, and photons have no mass at all.

Something very much like a neutrino but with more mass would work well as a dark matter particle, however.

I'd appreciate a clarification of just how likely it is, given this new upper constraint on self interaction, that DM is truly non self interacting. I mean is this number truly small/large or just bounding off the normal end. If this stuff is at once non self interacting, though differentiated in space time (identifiably somewhere and not somewhere) then one implication is that it is everywhere in space time only itself - entangled, non-local.

It narrows the available parameter space a little bit.

 

[PWiz]

the key characteristic of dark matter is not just that it's not visible now, but that it never was visible; i.e., it has always been dark since the early universe

That proves to be a very important distinguishing factor in the definition. It considerably alters my view of dark matter. I guess this means that the only possible candidates for non-baryonic dark matter are some hypothetical particles. Thanks.

 

[Lord Crc]

How does their cross-section limit compare to that of neutrinos?

 

[Jimster41]

I see there is an experiment ongoing (ice cube) that is looking for certain kinds of neutrinos. I had heard of it a while back but didn't think it was related at all to the search for DM.

Are there any other major experiments or studies going on in the DM effort? Is Dark Matter thought to be a component of the dynamics of a gravitational system like a planetary disk or ring system, or is it just way way too weak and diffuse for even a hope of detection at those scales?I've read (tried to read) a couple things in the past about what dynamic structures in Galaxies are hard to explain from visible matter, something to do with density waves in spiral arms. I got all bothered about how well we understand things like Saturn's Rings, whether or not a system like those, might display exotic g-field effects, like DM or g-waves. Is the eLISA experiment expected to add to the DM data?

 

[wabbit]

There was a discussion about this in pf not long ago, apparently it is diffuse - around galaxies it forms a halo rather than something more structured, and at the scale of the solar system that would be just uniform. At large scales however my understanding is that it forms the filaments etc. that are the scaffolding of large scale features in the distribution of galaxies.

 

[Lord Crc]

Are there any other major experiments or studies going on in the DM effort?

There are several direct searches, if that what you're asking for. I have a buddy who's involved with the DEAP-3600, due to start data acquisition soon.

As an aside, the detector itself is a piece of art in my humble opinion, see the image on the Wikipedia page :)

 

[wabbit]

As an aside, the detector itself is a piece of art in my humble opinion, see the image on the Wikipedia page :)

Beautiful. (I hope they wouldn't feel insulted if I said it looks a bit like a huge virus?)

 

[Lord Crc]

Beautiful. (I hope they wouldn't feel insulted if I said it looks a bit like a huge virus?)

Hopefully they know what they've created

Here's a short article, including pictures of the assembly: http://www.symmetrymagazine.org/article/shh-deap-is-hunting-dark-matter

As a piece of small trivia, my buddy used the open-source physically based renderer I've been involved with to determine that the "weird" patterns they saw in their light guides were to be expected due to reflection of the components and not due any issues with production or assembly.

 

[Jimster41]

As an aside, the detector itself is a piece of art in my humble opinion, see the image on the Wikipedia page :)

Yeah, that is so cool. I get a sense of vertigo when I see something man-made really trying (hard) to arrange itself around the infinitesimal, take on the appearance of... familiar natural things. It's obviously not a coincidence IMHO.

I subscribed up to that Symmetry eMag (for free!)

 

[wabbit]

things like Saturn's Rings, whether or not a system like those, might display (...) or g-waves.

To me that would be surprising: gravitational waves are very hard to detect with sophisticated purpose-built instruments, so for them to have a detectable macroscopic effect there (at the same distance to the source as we are) seems difficult to imagine.

 

[Jimster41]

To me that would be very surprising: gravitational waves are very hard to detect with sophisticated purpose-built instruments, so for them to have a detectable macroscopic effect there (at the same distance to the source as we are) seems difficult to imagine.

I can totally imagine you are right. I don't have much concrete to base my intuition on scale-wise. I was just struck (totally naively) by the apparent similarity of orbital ring structures, like Saturn's, Neptune's and others to Poisson spot interference phenomena. Complete coincidence, surely maybe (really?).

Maybe such relationships could be easily ruled out on scale arguments. But just going from my sense of how human beings sometimes miss things that are right in front of us, I can imagine there is something mysterious in the way that periodicity appears in familiar orbital systems, that we just think of as "well that's just what gravity does", understandably missing some sense of astonishment - that it does what it does and not something else, a fact with implications. Just reading that interview featured today about the black holes discovered in galactic globular clusters (strange orbital phenomena in their own right), the professor described disagreement or uncertainty about why the mass to light ratio of the clusters looked the way it did. That depressed me a little, because it gave me a sense that such questions are really a long way from being answered - and as PWiz said, I'd like to know before I kick the bucket...

anyway, drifting off topic. Sorry. Fun stuff.

 

[wabbit]

There's a whole field of study in such dynamics based as far as I know on Newtonian mechanics (with perhaps a tiny bit of GR corrections in some cases), its complex enough (chaotic in many aspects) as it is, I'm not sure very small corrections due to exotic effects woud have a significant effect here.

http://www.cambridge.org/us/academic/subjects/astronomy/astronomy-general/solar-system-dynamics

 

[Jimster41]

Thanks wabbit. Looks like a great thing to add to my stack. I have been looking for something to add to my weak grasp of the subject.

One comment though... don't we know that Newton's mechanics, didn't do it?

 

[wabbit]

One comment though... don't we know that Newton's mechanics, didn't do it?

Sure, there's the famous precession of Mercury's perihelion, so we do know that for some aspects GR corrections come into play - but at the scale of the solar systems given the masses involved, they're fairly small corrections (after all Newtonian predictions aren't that bad even for Mercury) and well known too, so they can be included when needed. I haven't studied that though, so this is just a layman opinion based on a small sample of information.

 

[Jimster41]

I think what I'm confused about, are any of the features of something like Saturn interesting in a different way when viewed as manifestations of a QM gravitation process (not a Newtonian one), like statistical periodicity in space-time structure?

 

[Lord Crc]

I think what I'm confused about, are any of the features of something like Saturn interesting in a different way when viewed as manifestations of a QM gravitation process (not a Newtonian one), like statistical periodicity in space-time structure?

Not sure I fully understand you. AFAIK Saturn and the formation of it's features can be described just fine without quantum gravity. However if you go further back, to the big bang, then quantum gravity is believed to set its mark. This was what BICEP2 was trying to detect. A nice layman-level article about that can be found here: http://profmattstrassler.com/2014/0...t-gravitational-waves-directly-or-indirectly/ (you can skip to the third section if you don't feel like reading the whole shebang).

 

[wabbit]

Why would QG effects be at play here ? Again, there are a huge number of things that can and have been studied about such systems involving only ordinary gravity, which is known to be far far more potent at the scales and densities concerned. Any QG effect would most likely be a very small correction to these.

To take a comparison going to a widely greater scale, in cosmology, QG effects are estimated to be very subtle already a tiny fraction of a secong after the bang or bounce - there are predicted effects that should de observable today when analysing subtle features of the CMB, or perhaps in explaining why this or that feature of our current universe is what it is - but these come from the progapation in time of something that happened near the bang/bounce, not from effects generated afterwards - and this is a far cry from QG effects happening today within the solar system.

Finding detectable predictions of QG is in fact a challenge, if it were detectable so easily we'd probably already have a well established QG theory by now.

 

[Jimster41]

I just don't understand how we can relegate the process of QM gravitation to some small distant corner, when everything that happens in the next proper instant, literally everywhere, somehow has to flow through that process.

Sorry guys, I really appreciate your patience. It's been a real help to be able to voice these questions and confusions. I'm going to go back to studying my Susskind etc and hope I'm not too embarrassed later, when it finally clicks.

 

[wabbit]

It's an interesting question : why is the QG scale so small / the QG density so high ?

Experimentally it's just a fact : it it was much larger, we'd have noticed it as say deviations to NG or GR already - or we'd have labs running experiments and studying those effects like say CERN does for the Standard Model.

But other than that, is there a fundamental reason ? I really don't know at all, perhaps the experts here would. The QG scale is somewhat naturally expected to be of the same order of magnitude as a combination of the gravitational constant and Planck constant etc.., but that doesn't really answer anything : )

 

[Jimster41]

why is the QG scale so small / the QG density so high ?

Not sure what you mean by QG density? I get the small scale.

 

[mfb]

How does their cross-section limit compare to that of neutrinos?

Let's take the sigma/M=.5cm/g value and assume a dark matter mass of 1 keV (a larger mass gives a larger cross-section). Then we get ~10^33 m^2 as cross-section. That is several orders of magnitude above the limits for the interaction of dark matter with regular matter, and ~7 orders of magnitude above typical neutrino cross-sections at 1-10 MeV.

Is Dark Matter thought to be a component of the dynamics of a gravitational system like a planetary disk or ring system, or is it just way way too weak and diffuse for even a hope of detection at those scales?

The local density of dark matter is too low. There are upper limits on invisible mass in the solar system, but those measurements are not sensitive to the small expected amount yet.
I don't see how eLISA would contribute to dark matter research in any way. For gravitational waves you need massive amounts of stuff accelerated quickly - dark matter does not do that.

I think what I'm confused about, are any of the features of something like Saturn interesting in a different way when viewed as manifestations of a QM gravitation process (not a Newtonian one), like statistical periodicity in space-time structure?

That has nothing to do with quantum mechanics.

 

[Lord Crc]

Not sure what you mean by QG density? I get the small scale.

If you just have a buch of particles in a small volume, then their gravitational attraction is negligible. This means the spacetime curvature can be accurately approximated as being static, and regular QM can be used.

For QG to be relevant you need enough particles in a small enough volume for their gravitational attraction to be significant, in other words that they affect the spacetime curvature. Then neither GR or QM is sufficient on their own.