Since the mainstream news media, in their reporting on the new result from the LUX experiment I wrote about Wednesday, insists on confusing the public with their articles and headlines, I thought I’d better write a short post reminding my readers what we do and don’t know about dark matter.

• Do we know dark matter exists?

Scientists are, collectively, pretty darn sure, though not 100% certain. Certainly something is out there that acts a lot like a dark form of matter (i.e. something that gravitates and clumps, but doesn’t shine, either in visible light or in any other form of electromagnetic waves). There have been some proposals that try to get around dark matter, by modifying gravity, but these haven’t worked that well. Meanwhile the evidence that there really is dark stuff out there that really behaves like matter continues to grow year by year, and every claim that it actually isn’t there (such as this one I wrote about — see the second half of the article) has turned out to be wrong.  Dark matter is needed to explain features of the cosmic microwave background, to explain how galaxies form, to explain why we see certain types of gravitational lensing, etc. etc.  No one alternative can explain all of these things.  And dark matter easily arises in many particle physics theories, so it’s not hard to imagine it might be created in the early universe and be a dominant player today.

• Do we know dark matter is made from particles (i.e. ultra-microscopic objects with uniform properties)?

No, that’s not certain. Particles would do the job, but that’s not a proof it is made from particles.

• If dark matter is made from particles, do we know these are Weakly Interacting Massive Particles (WIMPs) — to be precise, particles that interact with the Standard Model via the weak nuclear force or the Higgs force or something else we already know about?

No. Dark matter could be WIMPs. Or dark matter could be made from a very different type of particle called “axions”. Or dark matter could be made from particles that aren’t of either of these types.  This could include particles that only interact with ordinary matter through the force of gravity, which could make them very, very hard to detect.

• Do most scientists believe dark matter is made from WIMPs? (This was claimed to be true in several news articles.)

As far as I can tell, most experts do not know what to think; some have a bias toward one idea or another, but when pressed admit there’s no way to know. Many scientists think WIMPs are a good candidate, but I’ve never heard anyone say they are the only one.

Partly because they can. Sometimes science involves looking under the lamppost for your keys. You look where you can because you can look there, and you may get lucky — it has happened many times before in history.   That’s fine as long as you remember that’s what you are doing.

Not that WIMPs are the only things that people are looking for. They can also look for axions, and there are experiments doing that search too. Looking for other types of dark matter particles directly is sometimes very difficult. Some of these other types of particles could be found by the experiments at the Large Hadron Collider [LHC] (and people are looking.) Others could be found by experiments such as FERMI and AMS, through the effect of dark matter annihilation to known particles (and people are looking; there’s even a hint, not yet shown to be wrong). Still other possible types of dark matter particles are completely inaccessible to modern experiments, and may remain so for a long time to come.

• If we don’t know dark matter is particles, or that those particles are WIMPs, then why do the headlines say “dark matter search in final phase” in reference to the new result from LUX, even though LUX is mainly only looking for WIMPs?

The search for dark matter will end when some type of dark matter is found (or somehow shown convincingly not to exist), not before. The former could happen any day; the latter will not happen anytime soon.  The only thing that is currently approaching its end is the search for WIMPs as the dark matter (and even that search will not, unfortunately, end as soon or as conclusively as we would like.) If WIMPs aren’t found, that just probably means that dark matter is something else on the list I gave you above: some other type of particle, or some other type of thing that isn’t a particle. Or it could mean that dark matter forms clumps, rather than being smoothly distributed through our galaxy, and that we’re unlucky enough to be in an empty zone.  Certainly, if LUX and XENON1T and the other current experiments don’t find anything, we will not be able conclude that dark matter doesn’t exist. Only those who don’t understand the science will attempt to draw that conclusion.

• So why is the LUX experiment’s result so important?

Well, it’s important, but not amazingly important, because indeed, (a) they didn’t find anything, and (b) it’s not like they ruled out a whole class of possibilities (e.g. WIMPs) all at once. But still, (i) they did rule out a possibility that several other experiments were hinting at, and that’s important, because it settles an outstanding scientific issue,  and (ii) their experiment works very, very well, which is also important, because it means they have a better chance at a discovery in their next round of measurements than they would have otherwise. In short: they deserve and will get a lot of praise and admiration for their work… but their result, unlike the discover of the Higgs particle by the LHC experiments, isn’t Nobel Prize-worthy. And indeed, it’s not getting a front-page spread in the New York Times, for good reason.

1. You make me cry! 🙂 Where the heck is Occam’s razor?! Argh… There is *no* need for dark matter or dark energy.

• Carl 'SAI' Mitchell

Occam’s razor is the reason for dark matter and dark energy.
Something causes the effects associated with them. Possibly several somethings.
We know that the Standard Model of particle physics is incomplete, as it does not account for gravity.
Modifying the theories by adding a new particle and a new force is the simplest way to account for the observed effects. That does not mean it’s the only way, or the right way, just that it’s the easiest to work with and test with our current knowledge.

• Agreed with Carl. Before Occam’s razor comes the data. And the simplest way to explain the data is with dark matter and dark “energy” [actually dark “tension”, but that’s a technical issue], which involve very minor modifications of existing equations, rather than a whole-scale reworking of the equations.

• Yeah, but those very minor modifications are needed because underlying equations are not complete. With complete equations every single odd ball in physics go away. I thought this would be no-brainer…

• kevin dowd

Yes well, when you get your new equations to work as well as the SM. So they give the same.verified results, and then you get them to predict something new that can be searched for…

Then we can upend what we have.. until then, we add dark matter and whatever seems to be excellerating the expansion of the universe.

• Apparently you don’t know what you talking about. Read my papers first *then* come back. As a gift for rebel physicists, I’m planning to start an open collaboration in order to create the antimatter “device” 😉 Check out my blog after couple of days.

I’m going to reveal every trick in a book.

• Surreptitious Evil

You do know that William of Ockham invented his “razor” to ‘prove’ the existence of the Christian version of God?

It is a useful rule of thumb – and, in the contra-commonsense world of “quantum”, rarely even that.

• Now I know, thanks 😉

• Tony Mach

Funny thing, he invented his “razor”, and then failed to apply it correctly – any parallels to the first commenter are purely coincidently.

2. Marshall Eubanks

The way I like to think about it is that there is excellent evidence and no real doubt that there is a failure of physics that causes anomalous accelerations, but it is less clear whether the failure is a sign of new microphysics (e.g., WIMPs), or of a modification in the standard model (e.g., Axions or CCOs / CUDOs), or of a modification to General Relativity (e.g., MOND / TeVeS). The cool thing is that all of these options are testable, and that is going to drive a lot of thought and work until someone finally nails this down.

• Sab

By ‘anomalous accelerations’ do you mean acceleration in the expansion rate of the universe? This is NOT related to WIMPs or Axions.

3. But the big bang cannot survive without DM or DE , now suppose it was proved they does not exist , are we going 100 years back ?

• “the big bang cannot survive without DM or DE” — a false statement. The data supports a Big Bang with dark matter and dark energy, but that is not the same statement as saying that it is impossible to imagine a Big Bang without either one.

4. But Carl ; who or what puts the limit for that fractal of particles and forces ?
I mean how can we know where to stop ?

5. Dan D.

Nice summary. It’s worth pointing out, however, that the leaders of CoGeNT and CDMS aren’t quite willing to throw in the towel on their results just yet, even if they seem much more implausible in light of LUX. At the very least, they don’t seem to agree that the issue is settled.

• Let’s see what they have to say; right now their evidence is rather flimsy and certainly not convincing on its own.

6. dark matter ans dark energy do not exist in the cosmo,the problem is in the foundations of the physics,where some mathematical structures are incomplete. that does appear a “vacuo” in the physics laws that does apear the antimatter,dark energy and dark matter.the problems are in the incomplete resolutions of the mathematical equations to gravity,mass and energy,rotational invariance to cpt ,pt,cp…so as the incresing of symmetries to explain mistakes of physical solutions

7. great post, as usual!

As for “front page of New York Times” – No coverage at all would be better then the garbage that Overbye has published in the NYT…

8. L.. Edgar Otto

Nobel Prizes in post Higgs era? See my blog on this.

9. Jon Lennox

Two questions:

1. If Dark Matter isn’t made of particles, what would it be? Are there any candidate theories for non-particle-based matter?

2. There seems to be a general assumption that Dark Matter doesn’t interact via the Strong Force. Is this actually known for certain (e.g., if it did, we almost certainly would have seen it by now)?

• 1. See for example: http://science.time.com/2013/09/05/the-mystery-of-dark- matter-clarified-a-little/

2. It’s not an assumption, if they are particles. If dark matter particles interacted via the strong force, they would bind to nuclei and we would see heavy atoms, which we don’t observe. The possibility that dark matter might however be something more bizarre involving the strong force has been suggested as a long shot: http://arxiv.org/abs/1305.6318 . This isn’t very plausible but I can’t tell you of a specific reason why it is excluded.

• Paul D.

That possibility may be a long shot, but it would be a really interesting one! It might be possible to locate and capture these putative quark anti-matter nuggets and use them to produce energy.

I second question 1: What exactly would massive non-particular dark matter be like?

(Sorry. You must have answered while I was posting)

10. Marshall Eubanks

@Jon Lennox – On # 1, the only _truly_ non particle suggestions I am aware of are the MOND / TeVeS ideas, which are hard to reconcile with all of the evidence for “dark matter,” but still have their proponents. The CUDO / CCO ideas are generally not _called_ a particle solution, as they involve many-particle condensates, but the mass in these condensed objects is still coming from particles (quarks).

On # 2, I think that there was a lot of excitement over the “WIMP miracle,” which suggested a natural tie between the electroweak interaction and dark matter. I would be curious as to Prof. Strassler’s take on whether the “miracle” is still consistent with the experimental data.

• #1) I don’t think the statement : “the mass in these condensed objects is still coming from particles” should be viewed as correct. In general, the mass can be a much more complicated function of what goes in. For instance, the mass of a black hole need not be merely the sum of the masses of the matter you put into the black hole; in fact you can build a black hole with a mass out of massless particles. So I disagree with the way you’ve phrased this; these are fundamentally non-particle viewpoints. Equally important: particles of a given type are identical, with identical mass and other properties; but if dark matter is a set of larger “condensed” objects, those objects may have a wide distribution of masses and properties — a very different experimental and conceptual situation.

#2) I never thought the word “miracle” was appropriate (and a lot of my colleagues say the same thing.) It is a coincidence (and not a precise one) of two numbers: the amount of dark matter we observe, and the amount of mass in WIMPs that would be left over if they are what as known as a “thermal relic” from the Big Bang. So the WIMP coincidence is cute, but proves nothing, and there are many ways around it. I think this “not-so-miraculous coincidence” is already pretty much ruled out as associated with dark matter, because the interaction strength of dark matter [if it is in WIMP form and smoothly distributed throughout the galaxy] is now constrained by experiment to be too low. But that doesn’t rule out WIMPs, because they don’t have to be thermal relics.

• Jon Lennox

Oh, I see — when you say “not particles”, you mean in the same sense that ordinary matter isn’t particles. It’s made up of particles at the bottom, but it comes in a size and mass spectrum from hydrogen atoms to supergiant stars.

• Well, the variety of possibilities is very large. Certainly if dark matter were a breed of dark star (we’d know this by now, so it isn’t the case) I certainly wouldn’t say dark matter was particles. I’d say it was stars.

11. We got certain aspects of the universe , we designed the DM model to explain them , then we say : those aspects explain our only working model of explaining those aspects……..is this circular reasoning ?
Maybe we are following the wrong concept……..maybe it is how gravity works beyond all of our limited power of reasoning ……
I bet $100 that no DM will be found . • I’ll take your$100 bet, because you’ll never collect. I won’t bet you it will be found in our lifetime. I will bet you that it will take between 1 and 10,000 years to discover it.

As for circular reasoning — there is a difference between circular reasoning and self-consistent reasoning. Buildings and bridges are not built linearly; they are built with temporary structures, and when the temporary structures are removed the different parts support one another. So it is with scientific understanding. We have a consistent picture of the universe; it was not obtained in linear fashion, nor was the logic linear. But the logic was not circular either, just interconnected, with each aspect partially supporting the others.

12. But if a model is the only working one this does not prove it is the correct one.

• A working model is a working model until it is cumulatively verified by experiment. The existing working model of the universe has passed a number of very non-trivial tests of its validity.

• kevin dowd

And it is better than a non-working model! Which itself could be useful if it was the only one you had…

13. LHC and LEP experimental result summaries typically claim to exclude all unknown weakly interacting particles below a certain mass (e.g. half of the Z boson mass (ca. 45 GeV), since all particles observed in its decays are accounted for and the missing mass-energy in its decays can fully be explained by three generations of Standard Model neutrinos). If this is true, it would seem that any WIMP is impossible.

Is this accurate (I would think it is), or is there a lack of sensitively in collider experiments to very low mass weakly interacting particles? Are sub-45 GeV WIMPs only possible if something like R-parity prevents them from being produced in W and Z boson decays?

• 1) WIMPs could be 200 GeV or more, and then there are no constraints from LEP, or from LHC yet.

2) things a bit more weakly interacting than the simplest WIMPs (e.g. something that is 10% WIMP and 90% something that feels no STandard Model forces) would still be fine, even at much lower masses.

• Point taken on (2).

With regard to (i), this would seem to exclude “simple WIMPS” of up to the 5 GeV where LUX has an exclusion, and the LUX exclusion pretty much goes from 5 GeV to 1000 GeV, so this would limit “simple WIMPS” to 1 TeV or so.

14. If Gravity IS geometry of space as per GR , and if DM particles interacts mainly thru gravity , then do we have a theory for geometry-baryons interactions ?if not then what is the form of that interaction?

• There’s no relation of this sort. Gravity interacts with everything. Dark matter is no more special than ordinary matter; it may interact with itself, just as ordinary matter interacts with itself.

15. I mean how can we understand baryonic particles interacting with DM particles not thru any force but thru pure geometry ?
Did any one found a solution for such interaction?

• This is how stars interact with each other — mainly through “pure geometry”. It doesn’t require anything special. And no one said dark matter can’t interact with itself, through other, unknown forces.

16. lucretius

I have a rather different question. The CDMS observation of WIMPS was claimed to have a 99.8 (3 sigma) confidence level of being a genuine anomaly. Most people probably assume that this confidence level (vastly higher than some other widely cited ones which are claimed to provide sufficient grounds for spending billions of dollars to avoid the predicted consequences) indicates “virtual certainty” but in particle physics we are used to seeing results of observations with this or even higher confidence level overturned by the next, more accurate, experiment. Any comment?

• I’ve commented on this all over this website. Google “strassler 3 standard deviations” and read away.

For one thing, 3 standard deviations either says you have weak evidence for a new, exciting phenomenon OR you have weak evidence for a mistake in your experiment. You can have a 20 standard deviation signal; that proves nothing, unless you did the experiment right. OPERA had a six standard deviation signal for faster-than-light neutrinos.

For another thing, 5 standard deviations is the gold standard in our field, but no fixed number of standard deviations is some magic number after which you believe something and before which you don’t. Belief and trust in a scientific result cannot be captured in a single number.

• lucretius

Of course, I quite agree and I am quite familiar with statistics ( I even teach it sometimes). I just thought you might comment on the difference in this respect between particle physics and the kind of “science” that we keep hearing so much in the media.

• What comment are you fishing for? 🙂

• lucretius

Maybe the time to remind you of this will be the next time you discuss a different science: one where “better understanding” means greater error margins 😉

• kevin dowd

Sounds like a shot.at.global climate.change…

• The Secret Experimentalist

From the CDMS paper (arXiv:1304.4279): “Though this result favors a WIMP interpretation over the known- background-only hypothesis, we do not believe this result rises to the level of a discovery.”

• E.Chaniotakis

In order to detect WIMPS one takes advantage of the expected diurnal (and annual) variation of their flux.
If we had two SIMILAR detectors located in two sites with as same backgrounds as possible – antidiametrically in the earth –
then when one would measure diurnal excess, the other would measure deficit.
Thus by measuring this anticorrelation , wouldn’t we be able to conclude easier about whether there are WIMPs or not?

17. leo vuyk

Matt, Is it excluded that Dark matter is related to Black holes?.
You wrote in september 2013; “Why has the controversy gone on so long? It is because the mathematics required to study these problems is simply too hard — no one has figured out how to simplify it enough to understand precisely what happens when black holes form, radiate particles, and evaporate.”

• No it’s not entirely excluded, though it is constrained; see the link I posted above in an earlier comment.

18. ppnl

Is the distribution of dark matter in the galaxy smooth? I seem to remember years ago that there was a suggestion of an excess of DM on the outer edge of galaxies. It may just be my bad memory.

And is the distribution spherical? I’m trying to imagine the distribution of individual dark matter orbits. Over time they should scatter off of the stares almost as if they collided with them. What kind of orbit distribution would this create?

• It’s surely not exactly spherical since the Milky Way is absorbing other galaxies and their halos over time. How oblate or how clumpy the distribution might be isn’t known. Some people suggest there might be a disk within the sphere. There’s much that isn’t known and that depends on how dark matter interacts with itself and perhaps other unknown particles.

• ppnl

Yeah, a complex dark sector would be cool. But if dark matter interacts with itself you would need some new force carrying particles right? Also it can’t interact very strongly or it would clump into dark matter star like and planet like objects.

Unless it is repulsive maybe.

Knowing the exact distribution would be really interesting I think.

There should be a small population of stars in an out of galactic plain orbit. I wonder what a study of those orbits could tell us about the distribution of dark matter.

19. Matt. ; Maybe I was not clear enough , all WIMPS search are based on DMparticle -baryonic particle interaction as happened in LUX for example , and in this realm is my question , this interaction is not thru any force , it is thru gravity ie. thru geometry ……this is what I wanted to understand: do we have a theory for that ? Or it was assumed that interaction is thru a force of the three forces ? Is this a valid assumption ?

• Daniel Carosone

LUX et. al. are looking for interactions via the weak force, and finding constraints on the possible cross section

• ” all WIMPS search are based on DMparticle -baryonic particle interaction”

That is true of DIRECT searches, like LUX and XENON; it is not true of indirect searches at the LHC or at FERMI and AMS and PAMELA

” this interaction is not thru any force”

Of course it is. It is through the weak nuclear force (“W”IMP) and/or through the Higgs force.

20. David Brahm

Matt, why does LUX use liquid xenon? It’s not exactly a common material.

• chemically inert; possible to make it radioactively pure; heavy and therefore good for detecting collisions with heavy dark matter particles; various other detailed advantages having to do with scintillation light — but I’m not the expert. Some information is here

http://lux.brown.edu/LUX_dark_matter/Experiment.html

and the XENON100 website probably also has some.

• The Secret Experimentalist

Other reasons:

1) It scintillates
2) In two-phase configuration it has decent (but not excellent) background rejection

21. Daniel: Are we sure that DM will interact thru weak force or it is just an assumption ? If it is failed assumption then we will not see any WIMPS .
May be it interact only thru Gravity and that is my question , we do not have a theory for interaction of PARTICLES thru ONLY Gravity /geometry.

• If dark matter is a strict WIMP then BY DEFINITION it interacts by the weak nuclear force. Variations on WIMPs also may interact via the Higgs force.

• S. Dino

aa.sh.,
It has been suggested on this website and elsewhere (I believe even an episode of “Through the Wormhole” made mention) that we may be dealing with GIMPs (Gravitationally Interacting Massive Particles) that ONLY interact through gravity. The detection of GIMPs would be far more difficult than the detection of WIMPs.

What I found interesting about this notion, besides the obvious possibility of a GIMP as a dark-matter candidate, is a kind of hierarchy created in which there would be 1) particles like the GIMP that only interact through gravity, 2) neutrinos – all types – (electron type, muon type, tau type) that interacted only through gravity and the weak force, 3) electrons – all types – (muons, taus) that interact through electromagnetic, weak and gravity and 4) quarks – all types – (u,c,t and d,s,b) that interact through all 4 forces – strong, electromagnetic, weak and gravity.

22. matt, you wrote: in fact you can build a black hole with a mass out of massless particles.
SO? could photons do the job?
However could gravitons leave the BH?

• You can make black holes out of gravitons, or out of photons, in classical general relativity. You can slowly emit gravitons, and photons, in quantum general relativity. They’re completely consistent statements, since in quantum mechanics, if A + A –> B is possible, then B –> A + A must be allowed.

23. The Secret Experimentalist

This is a nice result.  However, whether you believe in “light” dark matter or not, it doesn’t change anything in a fundamental way: there was already tension between XENON and the various WIMP claims.  Though LUX has a lower threshold than XENON, it hasn’t shown a better recoil calibration at low energies.

24. In addition , if WIMP interact via unknown forces then the effect could very possibly be overlooked as the experiment is not designed to detect what is unknown.

25. Oren

Hi David! Long time no see. The LBL website has an explanation of “why xenon” here: http://newscenter.lbl.gov/feature-stories/2012/05/23/lux-lz/.

• David Brahm

Thanks, Oren! Nice to hear from you too.

26. Tony (Rácz) Rotz

Excuse me, but could there be more than one type of Graviton, or gravitational energy?

• Good question, but no; it turns out this gives inconsistencies. This isn’t immediately obvious.

27. And that is at the same time why the understanding of black hole creation is so difficult?

• I’m not sure what you mean; understanding how black holes *form* is no problem. How they *decay* is a much more complicated problem. The first is relatively easy to simulate; the second isn’t even understood mathematically, so we can’t study it.

Matt: I am interested in the comment “light is reflected by a very rapidly rotating black hole instead of being absorbed” in the popular article you quoted. I have not heard of this before.Where is this proved? Can you understand this intuitively?
Thanks.

29. Marshall Eubanks

If people want a technical review of current state of non-WIMP dark matter, this just appeared today http://arxiv.org/abs/1310.8642 – it’s from a Snowmass-2013 Cosmic Frontier 3 Working Group

• It is really something of a pity for it to come out just a few days before LUX data can be included. Notably, it suggests quite low cross-sections of interaction for neutrinos as a baseline comparison and tracks the increasing precision of the measurements over time in a nice chart that (alas) excludes LUX.

• The Secret Experimentalist

There are quite a few LUX members in that Working Group …

• Paul D.

Fairly soon experiments will begin coming up against the background from scattering of solar and cosmic ray neutrinos. To reduce the solar background, future generations may decide to put detectors farther from the sun (inside asteroids, perhaps?).

I wonder if detectors could be made more sensitive to WIMPs by accelerating the detector, so the WIMPs strike the detector with greater energy. Future interstellar world ships, cruising to nearby stars at .05c, might have WIMP detectors built into them.

30. Marshall: at page 27 of the article it is said: “However, if BH’s formed by
a diﬀerent method before the epoch of matter domination, they could make up the entirety of the DM.
Primordial black holes (PBH) could form in the Early Universe from overdensities in the matter/radiation
ﬂuid [186, 187]. ” YES!!

• Marshall Eubanks

Note that there are a variety of constraints on PBH as dark matter, especially from Gravitational Microlensing, which rules them out (as dark matter) from roughly the mass of the Moon up to several dozen Solar masses. It is hard to see how PBH weighing more than the Sun could form cosmologically, but that still leaves a possibility of PBH dark matter in the asteroid mass range. (PBHs, or any other dark matter, in that mass range would have a significant probability of impacting the Earth over geologic time, so any PBH theory also needs to explain why the solar system hasn’t been eaten by black holes.)

31. telescoper

Reblogged this on In the Dark and commented:
Following on from yesterday’s post about the LUX Dark Matter experiment, here is a reblog of excellent overview of the current state of the field…

32. However this could mean that the Big bang was more or less a splitting super Black hole, leaving non-SM BH splinters behind as creators of fast star- galaxy formation and the Lyman Alpha forest.
An example of how this could have happened: see
Dark Matter Black Holes and a New Dark Energy Higgs Field, lead to
a Bouncing CP symmetrical Multiverse, and new experiments

• Second warning: this is not an advertising site for your personal ideas.

33. Zoltán Suhajda

Just a thought: lets assume that the dark matter phenomena is just curvature of space, and it is interferance pattern of gravitational waves at galactic scale – which organize the matter to astrologycal structures. It will be unvisible for the particle physics, it is a large scale structure, and could be evidence for gravitational waves at large scale, perhaps a remnant of the big bang.

• But the data contradicts that assumption. Curvature of space will not behave the way matter does; the equations are different, and the universe would look very different.

34. Kevin

Give the proportion of 5 time more dark matter than ordinary matter, dark matter should accelerate a lot stellar object accretion through gravity ? Do simulation models of universe evolution observe that. Do clues in star formation observations point to that. If there are halos of dark matter, there should have a strong self repulsion that prevent the halo to always collapse into a black hole ?

• No, that’s not the way it works; the dynamics of free particles is more complicated than that, and your intuition for how things behave is off.

Remember that there’s no self-repulsion necessary to prevent the solar system from collapsing; gravity pulls, angular momentum conservation effectively repels. The same applies to dark matter; no repulsion is needed, because angular momentum conservation is enough.

Ordinary matter clumps and forms stars only because it can cool by emitting photons. This is not an option for dark matter, so it may not clump.

35. Probably no dark matter at all, only dark energy because of no pair of it in the diffraction pattern of the Planck Distribution Law. The only way this dark energy can interact is the Weak Interaction, when changing the temperature and part of it changes its frequency, that already could participate in the diffraction patterns. http://www.academia.edu/4168202/Theory_of_Everything_-_4_Dimensional_String_Theory#

• This is not an advertising site for your personal ideas; sorry.

36. S. Dino

Do we know EXPERIMENTALLY that neutrinos have a ratio of gravitational mass to inertial mass = 1?

• No, we do not know that experimentally at this time, at least not as precisely as for some other particles.

However, remember that electrons and neutrinos are very tightly tied together in the Standard Model https://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/the-known-particles-if-the-higgs-field-were-zero/ so it is not easy to play games with them. During the discussion of the OPERA experiment, it was similarly difficult to imagine neutrinos traveling faster than [standard] light speed, because to do that without screwing up what we know about electrons is mathematically and physically highly constrained by the structure of the Standard Model.

• S. Dino

Yes, that whole OPERA FTL episode was a nasty ride of dashed hopes for me…but I’ll tell you this: I have not seen a better or more complete coverage as to what was actually going on as I found on “Of Particular Significance” by not only yourself (just outstanding) but also by many of the comments from readers which showed expertise in engineering as well. I have been hooked ever since…

37. After a deep search in the Web , I came to adopt the view of Feng and others that WIMP can decay to what they call superWIMP that interact only
Gravitionally and as such WIMP will not show itself ever.
I wonder that among tens of possible DM (things) only WIMP was chosen for experimental confirmation based on nothing but bias .
To Matt. : I elevate my bet to the \$1000 level that NO WIMP will be found
And that DM effects are due to Gravitational effects thru particles or geometry or some new fields/ forces that we not even imagined.

38. I assure you Matt. , I will collect even after 1000 years.!!

39. If it did not show up then my last option may be the one : some new fields or forces that we did not even imagined…..you bet.

40. Please note that I said : no WIMP will ever show up , I did not mention superWIMP or hidden valley or You name it DM cannot show up.

• Well, then you haven’t defined your bet (and I won’t take it anyway.) What *precisely* is the definition of WIMP?

41. As per your definition : Something that interacts with baryonic matter via weak force.
As per Feng ; this will decay to superWIMP that interacts only via gravity.
My stand is for the later , why ?
Because I adopt the view that gravity as a force not geometry is what builds all cosmic structures , but not Newton,s gravity but a MOND of TeVeS type or even something more exotic that do All what the imaginary WIMP do.
Remember : Any type of DM needs some kind of template to build the hierarchy of cosmic structures , when N and E gravity failed , DM was introduced but still DM needs “guiding principles” or blueprint .
Remember :I am talking about a cosmic structure not a mere collection of celestial bodies.

• You’re not correct about super-WIMP’s interacting only via gravity; in general that’s not true.

42. Please note that when I say “template” I mean something similar to all the
Governing principles of QM …….UCP , PEP , quantization itself …..etc
Why should the micro-cosmos be strictly governed by principles while the universe structure itself is void of governing principles ?
Remember that all laws are general equations for even 10 to the power 1000 cases , while our universe is one particular solution of unlimited landscape , without “template” that particular solution cannot materialize ever ……..that is why I take WIMP as mere fictional brick in the grand structure of the cosmos.
We need lot more to grasp its magnificent aspects on the top of which is the infinite feelings landscape of the human self.
Thanks

43. Superb summary. Can one argue that DM must exist on the basis of astronomical observations of the collision of galaxies like the bullet cluster, or are there alternate explanations?

• One could probably fit the bullet cluster to relativistic extensions of MOND plus “dim matter” (neutrinos and baryonic dark matter like interstellar gases that exist in the Standard Model) of kinds that are much more common in galactic clusters than elsewhere – in part because our understanding of the structure and composition of galactic clusters is a bit fuzzy.

One certainly can argue very convincingly that some new physics (either a new force or new particles) must exist to explain phenomena attributed to dark matter. DM phenomena are pretty much the only experimental/observational data out there which can be explained purely with the SM + GR + minor experimental errors.

• There are many decent arguments, but no one argument is airtight. The problem is that any alternative explanation — any attempt to suggest that dark matter doesn’t exist — has to simultaneously explain away all of these decent arguments. So it is best to say: dark matter is the only explanation that explains many observations — galactic, intergalactic, and cosmic — all at once.

• I disagree strongly. I can explain away all arguments pro DM. So don’t forget me. Ignoring my model ain’t professional option.

• Of course it isn’t. I’m not ignoring it. There are thousands of models out there.

• lucretius

Actually ignoring models that violate what we already know is true is *the professional option*. This is why “professionals” don’t examine every design of a perpetual motion machine (a number of them are designed every year) to know that they are all nonsense. They just quote to the laws of thermodynamics.

• That’s not ignoring them; it is dealing with them, collectively.

• @lucretius You should know better.

@Matt Very funny, clap clap..

44. 18 Years ago Professor Bernard d’spagnat in his book ( of physics and philosophy ) wrote that science can never cross the dam beyond which a Veiled Reality exists , 9 years later you discussed the concept of what you called Veiled Sector , then you changed it to Hidden Valey hoping that The LHC will show all or part of the anticipated hidden Valey …….
9 years later nothing veiled was shown !!
Now within 7 years veiled reality will be a fact after the incomplete SM
Is proven to remain incomplete in principle .
Remember this if we reach the year 2020.
Thanks.

• I don’t know whether they will find something or not; but there are many searches they have not even done yet, so no conclusions are possible one way or the other at this time.

• lucretius

You are misquoting d’Espagnat. He was writing about the dispute about the interpretation of Quantum mechanics after the violations of Bell’s inequalities were confirmed by experiment. He stated that there were two choices left: either accepting Niels Bohr’s and the majority of quantum theorists view that “objective reality” independent of an observer makes no sense in quantum mechanics (the view I agree with and I suspect so does Matt) or cling on, for reasons of psychological comfort, to the view that there is an “independent reality” but it is irrevocably veiled. In no sense can this be taken as a “proof” of the existence of “veiled reality”.

• Not to mention that there is no connection between this notion of “veiled reality”, which is a conceptual issue in quantum mechanics, and any “veiled” or “hidden” sector of particles, which are just as much a part of ordinary reality as anything that we can see and touch… in short, two completely unrelated things are being conflated here just because someof the words happen to be similar.

45. Veiled Reality is all around us , the explanations of all the SM parameters are Veiled Reality meaning that it seems we can never reach detailed mechanisms to generate them ……
Why particular values of constants and coupling strengths ? Why three generations ? Even why QM ? ……..
I understand the difference between ontological V.R. And what you mean by veiled sector Matt.
As for d’Espagnat philosophy refer to his speech as he received Templton’s prize for advocating spiritual views .
Both of you should not declare that what I mean must be what you want me to mean.
V.R. as per d’Espagnat — a really great thinker — is an ontological view where QM gives some scientific foundation to adopt it but is independent of it………You should know better.
Question : Why science of post-modern Era “hates” seeing a wider reality while fighting to stablish a very narrow viewing perspective ?
I tell you why , scientists are in perpetual scape from the concept of the Divine………shame.

@aa.sh
I have been reading with lot of interest your debates with Matt and others. It is not true that all scientists are ”anti-divine”! Some 30-40% of scientists have no problem with concept of divine. Personally I do not care for physicists who use their eminence in physics to tirade against religion. However, Matt’s blog is not a place for this. I read this blog to understand what is going on in high energy physics. Also for hundreds of years it is accepted in physics that unless you show something mathematically, which is relevant to the experimental data, wordy arguments have no place in physics. So let us not clutter up Matt’s blog with these arguments. As I said, I am interested in mystical or divine ideas. But there are other places to discuss these! Although modern physics is close to being mystical , it is not clear if that will solve any real problems for modern physics!

• “god” and the “universe “are the same thing.the physics laws could be confused with a creator and that with physics laws

but what would be god in essence?

46. lucretius

It is not “science of post-modern Era” but science (at least) since the famous conversation between Napoleon and Laplace:

http://www.eoht.info/page/Napoleon+Laplace+anecdote

Laplace and the scientists following him were right: the concept of “the Devine” in any form has no place in science because it “explains everything” (which is the same as explaining nothing). Introducing it into science or even sneaking it in, would remove the incentive to look for solutions to problems strictly within the empirical and mathematical realm and it would completely destroy science. Some, like perhaps yourself, may find this limiting but the fact is that the enormous success of science since the 19th century owes more to this limitation (and especially to its strictest form know as “positivism” or “instrumentalism” – as represented by Einstein and Niels Bohr) than to any philosophical idea.
On a personal level a scientist is, of course, free to be a religious believer. Most often, as in the case of Max Planck, Wolfgang Pauli, Fred Hoyle etc., this takes a form of some sort of Deism, but it can also be quite conventional “established” religion. The crucial point is that it does not directly appear in the scientists own work otherwise he no longer is a scientist (it can, of course, be a source of inspiration – but that is a different matter).
This was also true of Bernard D’Espagnat: you can read his *scientific* views here:

http://www.unz.org/Pub/Encounter-1981jan-00066

47. As I understand it, the model does not work well without dark matter/dark energy. Am I right?

48. 1- This site is named: conversations about science , and this is exactly what we do here.
2- To confine your mind ONLY in the mass or properties of such and such particles , is an unbelievable waste of yourself as a Human being .
3- This post is about some UNKNOWN thing with unknown properties having unknown identity leaving unknown effect ……..something we only know it’s subjective definition , something that when /if we really know may change all our concepts of reality……….
So , what is off topic in our discussions ? Unless you are of the type ( do not bother me with facts , I made up my mind ).
As for Laplace , who told you that he is an authority on this matter ?
Believing in the divine will enrich science since narrow minded scientist is as bad as any ignoramus , we need and want broad minded researchers seeking for Reality without any predetermined borders or restrictions , ,….
From where you got the fallacy that introducing the divine will halt scientific investigations .? No Sir : introducing the Devine will enrich your inner self with the awe and wonder of us and our cosmos .
As for Lucretius reference , I ask him : you referring me to Arther kaostler as editor , did you read his great book of 1969 (Beyond Reductionism)
Attacking the four pillars of un wisdom / foolishness ; Freud , Darwin ,Marx ,and Durkheim ……?
Was that unscientific waste of time,effort and money ?
Did that great man wasted his life writing ( The Roots Of Coincidence)?

• Nonsense

It is quite obvious you are a troll whose only objective is to spam this blog with dumbfounded speculations and distorted personal opinions. There are many other sites for this type of discussions, so would you do us all a favor and leave?

• lucretius

I mentioned Laplace, of course, not because I view him (or any one else) as an “authority” but because his answer “we do not need this hypothesis” remains as valid now as then.

As for your question about Arthur Koestler – you are really dragging the discussion further and further off topic and so that I fear Matt will soon loose patience with it and perhaps with the participants too.
But the brief answer: yes I have read this and lots of other things by Koestler. As for what I think about it I will say only this: in all the matters that counted most to me he and I were always on the same side, so I am reluctant to express my true opinion of those other things (this comment also applies to a certain well known physics blogger of whom I will only say that he is not Matt).

• the universe would exist without the human consciouness as it is? or the universe would be other thing ou wouls have other meaning?

• lucretius

In Science we certainly have to assume that the universe existed before human consciousness since we do study it’s history by scientific (and mathematical) methods.

On the other hand, the formalism of Quantum Mechanics certainly did not exist before men invented it (fairly recently). However, there is no doubt that if anyone who understands QM was somehow transported one million years back into the past and was able to perform experiments, he would have found that it worked just as well then as it does now. In other words, it’s a timeless “mathematical tool”.

Of course all of this is also off topic.

49. What can anyone expect from Nonsense ?

50. Being Nonsense per se it far far beyond your capacity to understand full sense matters ….pity.

51. There is Frog’s Eye View , there is Bird’s Eye View …..
Some like it FROG’s !!!

• Nonsense

Same idiotic responses, nothing new from aa.sh.!

52. jal

I found the following paper.
It seems appropriate for the discussion.

http://arxiv.org/abs/1202.5097

Matter Non-conservation in the Universe and Dynamical Dark Energy
Harald Fritzsch, Joan Sola
(Submitted on 23 Feb 2012 (v1), last revised 30 Aug 2012 (this version, v3))
In an expanding universe the vacuum energy density \rho_{\Lambda} is expected to be a dynamical quantity. In quantum field theory in curved space-time \rho_{\Lambda} should exhibit a slow evolution, determined by the expansion rate of the universe H. Recent measurements on the time variation of the fine structure constant and of the proton-electron mass ratio suggest that basic quantities of the Standard Model, such as the QCD scale parameter \Lambda_{QCD}, may not be conserved in the course of the cosmological evolution. The masses of the nucleons m_N and of the atomic nuclei would also be affected. Matter is not conserved in such a universe. These measurements can be interpreted as a leakage of matter into vacuum or vice versa. We point out that the amount of leakage necessary to explain the measured value of \dot{m}_N/m_N could be of the same order of magnitude as the observationally allowed value of \dot{\rho}_{\Lambda}/\rho_{\Lambda}, with a possible contribution from the dark matter particles. The dark energy in our universe could be the dynamical vacuum energy in interaction with ordinary baryonic matter as well as with dark matter.

53. Curious George

A superb article, thanks. What is the main reason why the dark matter can not be composed of low energy neutrinos?

• Antti Kangasvieri

Low energy neutrinos as DM is in conflict with large scale structure formation.

• S. Dino

Curious George,

Not all that long ago, in the scheme of things, say 30 years, neutrinos WERE considered a dark matter candidate; provided they had a rest energy of a few electron volts (eV). (This, despite the fact that many people at the time made theoretical arguments of why neutrinos must, like photons, have zero rest energy. In fact, I still have a few old textbooks that make this argument). However, since then certain observations coupled with certain theories indicate neutrinos should be less massive than 0.3 eV and neutrino oscillation theory indicates less mass than even this.

Now you may wonder why neutrinos of this mass or even less couldn’t account for dark matter by simply packing them more densely. Ah, but there’s the rub – neutrinos (like electrons) are spin ½ particles and as such there are limits as to how densely they can be packed together as dictated by h/mc = hc/E. Bottom line, once the energy gets below ~1 eV, neutrinos just can’t cut it as a major contributor to dark matter. So with a rest energy of < 0.3 eV they just don’t deliver enough gravitational bang for the buck. So is it case closed for neutrinos as dark matter? Not quite yet…

Purely by experiment, leaving theory out of it completely, all that is known is that the minimum neutrino rest energy < 2.2 eV – and that’s just for the electron neutrino – still high enough for neutrinos to be a dark matter candidate. In 2015 the KATRIN experiment will seek to push this down to < 0.2 eV. If nothing is found, that will pretty much put the final nail in the coffin for neutrinos as dark matter…

Unless of cause something really outrageous is discovered about them that we don’t even suspect now and would consider "impossible" if we did (hint – see my question about gravitational mass and inertial mass way above) that again brings them into the dark matter spotlight…

54. LAST WORD :
( We do not need this hypothesis)
This is the most Nonsense , non proved , mere hallucination statement in the human history , period.
Bye .

55. Richard H

What else could dark matter be composed if not “particles”?

• the particles are quasi-paricles or geometrical deformation of spacetime in the topology of smooth 4-dimension manifolds.conection in 4-dimensions or quantum topological fields-do not dark matter-there are connections of dimensions.

• lucretius

Sorry, but this is pure gibberish as English and as mathematics (of that I can be sure) and hence probably also as physics.

56. Markus Harder

In this discussion there seems to be the underlying, non-trivial assumption that there is only one type of dark matter, and once it is found, bingo, the problem is solved. I get this impression from, for example, the statement:
“The search for dark matter will end when some type of dark matter is found (or somehow shown convincingly not to exist), not before.”
There may be many types of dark matter, and the dark matter might have a complex structure and many, non-trivial properties. Of course it is the simplest assumption that there is only on kind of DM, and that it behaves rather simple. On the other hand, by analogy: The normal, visible matter is really complex, so doesn´t it look probable that also the dark matter may be complex?

So I would expect that if one kind of dark matter would be found, this should trigger more research if more kinds of it are around (except if there are very strict theoretical reasons that forbid other kinds of dark matter).

• Absolutely, dark matter may not be of one type, and there are dozens (hundreds?) of papers in the literature about that. People do look for things that are dark but are not all the dark matter; and they reinterpret existing experiments in multiple ways. Our minds have to remain completely open on this matter.

57. Sigil

Nice summary. My personal bias is toward non-particulate dark matter-energy. 1. we can’t calculate vacuum energy correctly, 2. we have what behaves like extra energy and mass but is not associated with known particles. It seems likely those problems are related. Personally I think our definition of matter is too simplistic. I suspect what we call particles are actually more like tips of the icebergs floating in the quantum vacuum. With each particle state of the field there is associated a large (in astronomical terms) and delocalised field configuration that 1. gravitates; 2 creates what we consider quantum vacuum.
It could solve the vacuum energy problem because it means vacuum energy density isn’t the same everywhere but depends on the local (in astronomical terms) matter content.

• The words you say are fine in principle. But it’s nothing until there are equations. Of course, many scientists have tried to link the observations of dark matter and dark energy. No one has actually succeeded yet. One challenge is that dark matter is very important in the evolution of the early universe, while dark energy is only becoming important now, in the recent period of the universe. They don’t evolve, over time, the same way. In other words, today dark energy is about 7/10 of the energy budget of the universe while dark matter is 1/4. But that wasn’t true 10 billion years ago, much less 13.6 billion years ago.

58. Indeed if the Higgs Vacuum Field is related to dark energy it could gravitate!!

• dark matter doesn’t exist.might change ours conception about of gravity and inertia

i think that the extradimesions could to help that mistake about the dark matter

59. I’m with Sigil. Start off with the bowling-ball in the rubber-sheet analogy. The universe expands, so the rubber sheet gets bigger and thinner, so the bowling ball sags down lower. But note that space expands between the galaxies but not within. So in our analogy, the rubber sheet doesn’t get thinner where the bowling ball is. It’s like the bowling ball ends up cupped in thick heavy rubber sheet, so it sags even lower. In terms of gravity, the bowling ball is now punching above its weight, and inhomogeneous vacuum energy has a mass equivalence. See http://arxiv.org/abs/1209.0563 which I think is kind of on the right lines. Matt, in comment three you said tension. A better analogy would involve a block of gin-clear rubber being pushed outwards, but a halfway house is the balloon analogy. IMHO dark energy ought to be represented by the positive pressure x volume within the balloon, and the negative pressure or tension provided by the balloon skin ought to represent the strong force. Only the balloon is made of bubble gum. As it gets bigger the skin gets weaker, so it expands even more. But conservation of energy does apply.

60. Stuart

Here is why I think WIMPs do not exist.Imagine these massive particles whirling around the galactic halo like moths around a lantern at high speeds. Because they carry gravitational charge they will generate intense (because they outweigh baryonic matter 5 to 1) but low frequency (because of the large orbital radii) gravity waves. This would dissipate energy and over the billions of years cause them to collapse at the galactic core right But this is not tge case because they do nit exist

• Wrong. This is why we do calculations, rather than spouting words; if you do the calculation you will easily see you will not get gravitational waves of any significance.

• Stuart

If WIMPs travel at relativistic speeds then the G waves would be significant. But since they are cold then the G waves would be insignificant. Thanks Matt for the clarification.

Matt: I thought Mond was ruled out by terrestrial experiments. But some people think it can be still modified to fit both galactic and terrestrial data. What do you think?

• I don’t think it is ruled out by terrestrial experiments. Maybe some variants of it are… honestly I can’t recall.

62. Is there any relation between the temperature of the universe and DM/DE? Decreasing temperature, decreasing DM/DE; increasing temperature , increasing DM/DE?

63. Stuart

The fued between extended theories of gravity such as MoND and the Dark Matter hypothesis is a manifestation of the age old war between idealistic and materialistic intepretations of the world. Most physicist favour the materialistic view and are willing to spend colosal amounts of funds to prove its dominion even if it takes them a 1000 years to realise they were mistaken. No WIMPs or other exotic particles exist period.

• Stuart

That being said, the crises in physics over the Dark Sector is really a crises of the materialistic view which so far has only lead us into understanding only 5% of the universe. This brings one to pose the question; is it ideas that generate a crises or it is a crises that generates ideas?

• This “materialistic view” you refer to also predicted, and discovered, the Higgs particle, just last year. If the materialistic view is in crisis, I wish all my views had such crises.

• I don’t get why you bother to make a claim that is counter to all the thread. Matt, and other commenters, have put in immense effort to make it clear that DM is the natural and easiest to explore theory, and why it is an open question.

Matt is a professional, you are trolling. Whose opinion do you think readers will note and whose will they ignore?

• Let him speak freely; I learn things from others’ misconceptions and preconceptions.

• I don’t think you understand the debate. MOND (modified Newtonian gravity) is just as materialistic as dark matter; fields, after all, which is what MOND concerns itself with, are very much material, since matter is nothing but ripples in fields. The inventors of MOND don’t pose it as a matter of idealism; they pose it with equations, just like the dark matter people do. We don’t do physics the way you think we do; the MOND people and the Dark Matter people are from the same community, have serious, detailed discussions about the observations at conferences, and are hardly coming from antithetical philosophical standpoints.

• Stuart

MoND assumes an idealistic character in the sense that it calls upon a law governing the dynamics of matter below Milgroms acceleration constant instead of conjuring particles like you mentioned the Higgs was proposed to solve the problem of how particles acquire mass. Dark matter particles are proposed to explain formation of large scale structures and galaxy rotational curves.Einstein also proposes principles in his relativity theories which is an appeal to the ideal than the materialistic including use of thought experiments.

• Stuart

In short I am advocating for the proposal of far reaching fundamental principles of nature wherin lies the new physics rather than rushing to propose new exotic particles. This approach led to the formulation of the ground breaking relativity theories

• You can give examples where this approach worked (relativity) and approaches where it failed utterly. For instance, postulating neutrinos [very lightweight invisible particles] worked, while postulating violations of energy and momentum failed. Postulating something radical beyond quantum field theory to explain hadrons failed; using a quantum field theory rather similar to the ones already known worked. You can’t know in advance which approach is the right one. You have to try both. And of course people do try both. But the dark matter hypothesis agrees with the vast majority of the observational data, while it is hard to come up with good, convincing MOND equations. And so the “new exotic particles” approach has gradually become the significantly favored one.

It’s not about people “rushing to propose new exotic particles”. People rushed to propose MOND too. It’s just that dark matter works better, when faced with real data.

p.s. my first self-motivated calculation as a young scientist (a junior in college) was to test a theory of MOND that a fellow junior was working on with one of our faculty members. I still have the piece of paper I did it on. Later, one of the graduate students showed this variant of MOND was inconsistent with observational data about galaxies.

• How about new version of MOND? Who could check it out? I just happen to have working one.

• There are a number of MOND experts around; perhaps you should communicate with one of them? By the way, what about the existing versions of MOND? You should probably calculate what is different about yours from the ones that are most popular.

• Could you name one with open mind? Thanks advance!

• I don’t think that any of my current and recent colleagues at Harvard or at Rutgers have closed minds… sorry.

• A field is a state of space, or a “modification” of space, and IMHO there is a way in which they’re both kind of right.

64. Bob Zannelli

Can anyone answer the question, where do we stand with sterile neutrinos as dark matter? We have some tentative evidence for their existence by some neutriono flavor osl experiments and recoil effects observed during pulsar formation.

65. Alan L.

The DAMA collaboration using a detector based on a cryogenically frozen crystalline solid – 4 degrees kelvin or thereabouts – so it would close to its lowest possible energy state, meaning that its atoms and associated nucleons are almost absolutely motionless relative to their immediate surroundings and have, therefore, zero degrees of freedom.

This can be likened to a DM particle trying to skewer a dead minnow in a tank of clear gel.

DAMA has detected DM with something well in excess of a 9 sigma confidence level.

The LUX detector uses Xenon chilled to about 175 degrees kelvin, above the freezing point of xenon, but the xenon is still in a liquid form and its atoms retain their 3 degrees of freedom.

This can be likened to a DM particle trying to skewer a vigorous minnow in a tank of clear water.

LUX has failed to detect any hint of a DM candidate particle and ruled out the type of DM detected by DAMA with what appears to be something close to a similar confidence level. (I can’t find any actual figures on this aspect).

Has any physicist ever dared to consider the possibility that DM particles are gravitationally repulsive to baryonic particles, at least at the nucleon level?

(Just as an aside, I believe that gravity must have a cause or source and is not “just geometry”).

66. Dark Matter based on small Machos is ruled out by e.g.:http://arxiv.org/abs/astro-ph/0607207, However Primordial Big Bang Black Holes should be still in the race. This means that the Big Bang was perhaps a splitting Giant black Hole?

67. Surpisingly , in that case inflation should be replaced by Primordial Big Bang Black Hole splitting inflation responsible for the Lyman Alpha structure of the universe and fast Star and Galaxy formation.

68. Bob B

This subject has reminded me of Messier 94. A galaxy thats interesting for a few reasons (and beautiful to boot.) but one of them is that its been hypothesised that it has very little if any Dark Matter.

As far as I can find there is no follow up to the speculatory paper on this, Im not sure if thats because it is considered uninteresting for reasons I dont know of or if confirmation is simply time consuming.

The reason I find it interesting as a layman is that while it gives us a big problem to solve with regard to Dark Matter. (How can a galaxy even exist in that state?) It surely deals some fairly fatal blows to most theories correcting the maths.

I could of course be wrong on this but my reasoning is that if you can find two galaxies appearing to consist of much the same matter and they are behaving very differently gravitationally then it seems rather difficult to reconcile the difference with a consistent mathematical model without of course adding in something not so readily apparent.

Id be interested if anyone could explain why this is wrong headed or if they know any further details on M94.

• Marshall Eubanks

I am not sure that the mere existence of a galaxy with a low amount of DM would be fatal to any DM theory (although it might be fatal to MOND). Consider the Bullet Cluster, where two clusters have slammed into each other (and assume we are talking about some sort of dark _matter_, not MOND). The gas is stopped in the middle by gas pressure, while the stars and the DM keep on going. So, this means that the new cluster resulting from the collision can form (on the outside) galaxies and smaller clusters with a higher than normal amount of DM, and (on the inside) galaxies and smaller clusters with almost no DM. The Bullet cluster shows us events happening ~ 3.4 billion years ago. A similar collision 10 billion years ago could have left a nearly DM free cloud of gas that by today could have formed into a normal galaxy. Now, I wasn’t aware of the M94 paper (http://arxiv.org/pdf/astro-ph/0611113v3.pdf) before just now so I can’t comment on it, but I don’t in general find the idea of occasional galaxies with more or less DM than usual to be very troubling.

69. E.Chaniotakis

Dear professor,
Why the high energy physics community spends so much money to build experiments that MIGHT detect something that MIGHT be dark matter?(23 direct dark matter search experiments globally as far as I know)
The insight came to me as I read a document about searches at CERN, which stated that:
“Over the past several decades a dominant
paradigm for dark matter has emerged at the
weak scale. In theories that stabilize the Higgs
mass at the weak scale, there are often new symmetries
that give rise to stable particles. Computing
the thermal relic abundance of the weak scale
mass particles gives rise, in many of these models,
to a dark matter density in accord with what is
observed. This remarkable coincidence has been
termed the “WIMP miracle,” and is perhaps the
most compelling reason to focus theoretically and
experimentally on dark matter at the weak scale.”

It goes on though saying that:
“It has been realized in recent years, however,
that extensions to the Standard Model can be
weakly interacting with the Standard Model while
the masses of such states are much lighter than
the weak scale, and that in these models the
phenomenology can be quite distinct and difficult
to uncover at the LHC. This was the focus
of the Hidden Valley models, where a light gauged hidden sector communicates the the Standard
Model through weak scale states,”

It goes on getting even more complicated to me for the time being. Yet , googling, I found out that the so called “hidden sector” might provide some explanation for the PAMELA data (and AMS I guess), and at J-Lab they are currently setting up experiments to find low mass (~1 GeV) “dark photons” etc.
I was wondering whether you could help us understand this new, puzzling yet exciting concept of the dark sector!

Thank you very much
E.Chaniotakis

70. DrDave

To say or imply that dark matter exists–because there may exist something that acts like it is illogical. If the scientific reality is that there is no direct evidence, just say that. If there is direct evidence, say that. If you want to redefine a large mass of indirect evidence as direct evidence, why bother?

• Stuart

If they do not find dark matter particles they will speak of yet another energy scale or valley etc to be tested. This is called ‘saving the phenomenon’.

• E.Chaniotakis

The point is that there is a good reason to believe in WIMPS theoretically
as there is a good reason to believe in SUSY, GUTs, M-theory theoretically.
Since it’s testable we ought to exercise every ounce of our strength to find it- or exclude it

• You’re so naive sometimes…

• Bob B

You have this the wrong way around, there is an effect we can see and the current most likely explanation has been named dark matter. Nobody has claimed dark matter exists and then pointed to an effect that indicates it could.

Also, Its the dark matter hypothesis, nobody said or even implied it categorically exists. (Professor Strassler notes this specifically near the start of the article.) The entire point of all of this research is to turn hypothesis to theory by investigating the most likely forms the effects we observe could take.

I have no idea where you are getting the idea that people are redefining the evidence unless you get all of your information from news papers that are considerably less precise in how they phrase things.

• The line between “indirect” and “direct” evidence is elusive. When you ‘see something’ with your eyes, you are seeing it indirectly, via the photons that reflect off of it. No one has ever “seen” an atomic nucleus; all the information is indirect, in some sense, yet we build nuclear power plants with that information. So be sure you know exactly what you mean when you try to make simple distinctions.

71. James Beard

What do you think of the idea that dark matter is not a particle at all, but just ordinary matter that is too dark to see? I’m thinking specifically about those objects in what we call the Oort Cloud. We cannot detect ANY of them until they descend into the Solar System as comets. Astronomers posit that the Oort Cloud extends “almost” to the nearest stars, but there is no way to know that for sure. Maybe they extend all the way. Why not everywhere between all the stars in the galaxy? In fact, why not such “clouds” of dirty snowballs in all galaxies at densities proportional to the ordinary matter we see in them? They do not reflect any wavelengths that we can detect even close to the Solar System, nor do they glow like gasses, or react to electric/magnetic fields. So in effect they are invisible except for how they behave gravitationally. Classic Dark Matter. Their shear numbers can easily account for “missing” matter in the Universe.

• This kind of ordinary matter would have affected big bang nucleosynthesis, and thus would have affected the ratio of helium to hydrogen that we observe. So that causes more problems than it solves.

72. I love reading through a post that will make men and women think.
Also, thanks for allowing for me to comment!

73. Eusa

Could it be the vacuum energy flux that need causal interactions with gravitating bodies? A mass particle needs vacuum energy for its inertia and space ether would be a dynamic process of vacuum energy streaming towards centers of mass fields of particles?

All the space would be full of vacuum energy streams that were depending of information from gravitating bodies propagated at speed of causality c. Because of time delay the distant bodies falling free with vacuum energy flux seem to have extra acceleration. Is that the possible ay to explain DM?

• No, it is not. Vacuum energy does not flow.

• Leo Vuyk

However, the vacuum could oscillate with different ( Casimir) energy in different directions. If the earth is able to tranform this outgoing energy in a smaller push, ( the graviton) than we may have a dual pushing attraction force called quantum gravity.

74. Eusa

Then… What’s the mechanism for the balance between mass energy and space tension? We know the tensors are algebraic metric tools, not a good dynamic model like an inclined plane. I think the clue for understanding DM will be lying just next to understanding the mechanism of the gravity/inertia.

75. Très bon article, persiste comme ça

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