The Whole Story on Dark Matter

“Science progresses best when observations force us to alter our preconceptions.” -Vera Rubin

I want you to think about the Universe. The whole thing; about everything that physically exists, both visible and invisible, about the laws of nature that they obey, and about your place in it.

It’s a daunting, terrifying, and simultaneously beautiful and wondrous thing, isn’t it?

Image credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team.

After all, we spend our entire lives on one rocky world, that’s just one of many planets orbiting our Sun, which is just one star among hundreds of billions in our Milky Way galaxy, which is just one galaxy among hundreds of billions that make up our observable Universe.

Yes, we’ve learned an awful lot about what’s out there and our place in it. As best as we can tell, we’ve learned what the fundamental laws are that govern everything in it, too!

Image credit: Mark Garlick / SPL, retrieved from the BBC.

As far as gravitation goes, Einstein’s theory of general relativity explains everything from how matter and energy bend starlight to why clocks run slow in strong gravitational fields to how the Universe expands as it ages. It is arguably the most well-tested and vetted scientific theory of all time, and every single one of its predictions that has ever been precision-tested has been verified to be spot-on.

Image credit: Contemporary Physics Education Project.

On the other hand, we’ve got the standard model of elementary particles and interactions, which explains everything known to exist in the Universe, and all the other (nuclear and electromagnetic) forces that they experience. This, also, is arguably the most well-tested and vetted scientific theory of all time.

And you would think that if our understanding of things were perfect, if we knew all about the structure of the Universe, the matter in it, and the laws of physics that it obeyed, we’d be able to explain everything. Why? Because all you’d have to do is start out with some set of initial conditions — immediately following the Big Bang — for all the particles in the Universe, apply those laws of nature that we know, and see what it turns into over time! It’s a hard problem, but in theory, it should be not only possible to simulate, it should give us a sample Universe that looks just like the one we have today.

Image credit: NASA / WMAP Science Team.

But this doesn’t happen. In fact, this doesn’t happen at all. This picture I painted for you above is all true, on the one hand, but we also know that it isn’t the whole story. There are other things going on that we don’t fully understand.

Here, as best as I can present the full history in a single blog post, is the whole story.

Visualizations & Simulations: Ralf Kähler, Tom Abel, and Oliver Hahn (KIPAC).

As we come forward from the event of the Big Bang, our Universe expands, cools, while the entire time experiencing the irresistible force of gravity. Over time, a number of extremely important events happen, including, in chronological order:

  1. the formation of the first atomic nuclei,
  2. the formation of the first neutral atoms,
  3. the formation of stars, galaxies, clusters, and large-scale structure,
  4. and how the Universe expands over its entire history.

If we know what’s fundamentally in the Universe and the physical laws that everything obeys, we’ll arrive at quantitative predictions for all of these things, including:

  1. what nuclei form and when in the early Universe,
  2. what the radiation from the last-scattering-surface, when the first neutral atoms are formed, looks like in great detail,
  3. what the structure of the Universe, from large scales down to small scales, looks like both today and at any moment in the Universe’s past,
  4. and how the scale, size, and number of objects in the observable Universe have evolved over its history.

We have made observations measuring all of these things, quantitatively, extremely well. Here’s what we’ve learned.

Image credit: NASA / Goddard Space Flight Center / WMAP101087.

What we consider to be normal matter, that is, stuff made up of atoms, is highly constrained by a variety of measurements. Before any stars formed, the nuclear furnace of the very early Universe fused the first protons and neutrons together in very specific ratios, depending on how much matter and how many photons there were at the time.

What our measurements tell us, and they’ve been verified directly, is exactly how much normal matter there is in the Universe. This number is incredibly tightly constrained to be — in terms that might be familiar to you — about 0.262 protons + neutrons per cubic meter. There could be 0.28, or 0.24, or some other number in that range, but there really couldn’t be more or less than that; our observations are too solid.

Image credit: Ned Wright.

After that, the Universe continues to expand and cool, until eventually the photons in the Universe — which outnumber the nuclei by more than a billion-to-one — lose enough energy that neutral atoms can form without immediately being blasted apart.

When these neutral atoms finally form, the photons are free to travel, uninhibited, in whatever direction they happened to be moving last. Billions of years later, that leftover glow from the Big Bang — those photons — are still around, but they’ve continued to cool, and are now in the microwave portion of the electromagnetic spectrum. First observed in the 1960s, we’ve now not only measured this Cosmic Microwave Background, we’ve measured the tiny temperature fluctuations — microKelvin-scale fluctuations — that exist in it.

Image credit: WMAP Science Team / NASA.

(For those of you who like your maps shown on Mercator projections, click here for that view.)

These temperature fluctuations, and the magnitudes, correlations and scales on which they appear, can give us an incredible amount of information about the Universe. In particular, one of the things they can tell us is what the ratio of total matter in the Universe is to the ratio of normal matter. We would see a very particular pattern if that number were 100%, and the pattern we do see looks nothing like that.

Here’s what we find.

Image credit: Pavel Kroupa.

The necessary ratio is about 5:1, meaning that only about 20% of the matter in the Universe can be normal matter. This doesn’t tell us anything what this other 80% is. From the Cosmic Microwave Background alone, we only know that it exerts a gravitational influence like normal matter, but it doesn’t interact with electromagnetic radiation (photons) like normal matter does.

You can also imagine that we’ve got something wrong about the laws of gravity; that there’s some modification we can make to it to mimic this effect that we can re-create by putting in dark matter. We don’t know what sort of modification could do that (we haven’t successfully found one, yet), but it is conceivable that we’ve just got the laws of gravity wrong. If a modified theory of gravity could explain the fluctuations of in the Microwave Background without any dark matter at all, that would be incredibly interesting.

But if there really is dark matter, it could be something light, like a neutrino, or something very heavy, like a theorized WIMP. It could be something fast-moving, with a lot of kinetic energy, or it could be something slow-moving, with practically none. We just know that all of the matter can’t be the normal stuff we’re used to, and that we’ve come to expect. But we can learn more about it by simulating how structure — stars, galaxies, clusters, and large-scale structure — forms in the Universe.

Video credit: DEUS Consortium.

Because the types of structures you get out — including what types of galaxies, clusters, gas clouds, etc. — exist at all times in the Universe’s history. These differences don’t show up in the Cosmic Microwave Background, but they do show up in the structures that form in the Universe.

What we do is take a look at the galaxies that form in the Universe and see how they cluster together: how far away from a galaxy do I have to look before I see a second galaxy? How early in the Universe do large galaxies and clusters form? How quickly do the first stars and galaxies form? And what can we learn about the matter in the Universe from this?

Image credit: E.M. Huff; SDSS-III; South Pole Telescope / Zosia Rostomian.

Because if the dark matter — which doesn’t interact with light or normal matter — has lots of kinetic energy, it will delay the formation of stars, galaxies, and clusters. If the dark matter has some but not too much, it makes it easier to form clusters, but still hard to form stars and galaxies early on. If the dark matter has virtually none, we should form stars and galaxies early. Also, the more dark matter there is (relative to normal matter), the more smooth the correlations will be between galaxies on different scale, while the less dark matter there is means that the differences in correlations between different scales will be very stark.

The reason for this is that early on, when clouds of normal matter starts to contract beneath the force of gravity, the radiation pressure increases, causing the atoms to “bounce back” on certain scales. But dark matter, being invisible to photons, wouldn’t do this. So if we see how big these “bouncing features” are, known as baryon acoustic oscillations, we can learn whether there’s dark matter or not, and — if it’s there — what its properties are. The thing we construct, if we want to see this, is just as powerful as the graph of the fluctuations in the microwave background, a couple of images above. It’s the much lesser-known but equally important Matter Power Spectrum, shown below.

Image credit: W. Percival et al. / Sloan Digital Sky Survey.

As you can clearly see, we do see these “bouncing” features, as those are the wiggles in the curve, above. But they’re small bounces, consistent with 20% of the matter being “normal” matter and the vast majority of it being smooth, “dark” matter. Again, you might wonder if there isn’t some way we could modify gravity to account for this type of measurement, rather than introducing dark matter. We haven’t found one yet, but if such a modification were found, it would be awfully compelling. But we’d have to find a modification that works for both the matter power spectrum and the cosmic microwave background, the way that a Universe where 80% of the matter is dark matter works for both.

This is from the structure data on large scales; we can also look on small scales, and see whether small clouds of gas, in-between us and very distant, bright objects from the early Universe, are thoroughly gravitationally collapsed or not; we look at the Lyman-alpha forest for this.

Image credit: Bob Carswell.

These intervening, ultra-distant clouds of hydrogen gas teach us that, if there is dark matter, it must have very little kinetic energy. So this tells us that either the dark matter was born somewhat cold, without very much kinetic energy, or it’s very massive, so that the heat from the early Universe wouldn’t have much of an effect on the speed it was moving millions of years later on. In other words, as much as we can define a temperature for dark matter, assuming it exists, it’s on the cold side.

But we also need to explain the smaller-scale structures that we have today, and examine in gory detail. This means when we look at galaxy clusters, they, too, should be made of 80% dark matter and 20% normal matter. The dark matter should exist in a big, diffuse halo around the galaxies and the clusters. The normal matter should be in a couple of different forms: the stars, which are extremely dense, collapsed objects, and the gas, diffuse (but denser than the dark matter) and in clouds, populating the interstellar and intergalactic medium. Under normal circumstances, the matter — normal and dark — is all held together, gravitationally. But every once in a while, these clusters merge together, resulting in a collision and a cosmic smash-up.

Image credit: NASA/CXC/CfA/M.Markevitch et al.; NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

The dark matter from the two clusters should pass right through one another, because dark matter doesn’t collide with normal matter or photons, as should the stars within the galaxies. (The stars not colliding is because the cluster collision is like firing two guns loaded with bird-shot at one another from 30 yards away: every single pellet should miss.) But the diffuse gas should heat up when they collide, radiating energy away in the X-ray (shown in pink) and losing momentum. In the Bullet Cluster, above, that’s exactly what we see.

Image credit: NASA/CXC/STScI/UC Davis/W.Dawson et al., retrieved from Wired.

Ditto for the Musket Ball Cluster, a slightly older collision than the Bullet Cluster, that’s just recently analyzed. But others are more complicated; cluster Abell 520, for example, below, appears to have too much gravity associated with a location that ought to have only normal matter and not dark matter.

Image credit: NASA / CXC / CFHT / UVic. / A. Mahdavi et al.

If we look at the individual components, you can see where the galaxies are (which is also where the dark matter ought to be), as well as the X-rays, which tell us where the gas is, you’d expect the lensing data — which is sensitive to the mass (and hence, dark matter) — to reflect that.

Image credit: NASA, ESA, CFHT, CXO, M.J. Jee and A. Mahdavi.

Instead, we see evidence for the gas creating a large amount of lensing, which shouldn’t be. So, perhaps something funny is going on here. Maybe this is evidence in favor of modified gravity and against dark matter, as some contend. Or, perhaps, there’s an explanation consistent with dark matter, and we simply have an unusual mass distribution in this type of smash-up.

But we can go to even smaller scales, and look at individual galaxies on their own. Because around every single galaxy, there should be a huge dark matter halo, comprising approximately 80% of the mass of the galaxy, but much larger and more diffuse than the galaxy itself.

Image credit: ESO/L. Calçada.

Whereas a spiral galaxy like the Milky Way might have a disc 100,000 light-years in diameter, its dark matter halo is expected to extend for a few million light-years! It’s incredibly diffuse because it doesn’t interact with photons or normal matter, and so has no way to lose momentum and form very dense structures like normal matter can.

What we don’t yet have any information about, however, is whether dark matter interacts with itself in some way. Different simulations give very different results, for example, as to what the density of one of these halos ought to look like.

Image credit: R. Lehoucq et al.

If the dark matter is cold and doesn’t interact with itself, it should have either an NFW or a Moore-type profile, above. But if it is allowed to thermalize with itself, it would make an isothermal profile. In other words, the density doesn’t continue to increase as you get close to the core of a dark matter halo that’s isothermal.

Why a dark matter halo would be isothermal isn’t certain. Dark matter could be self-interacting, it could exhibit some sort of exclusion rule, it could be subject to a new, dark-matter-specific force, or something else that we haven’t thought of yet. Or, of course, it could simply not exist, and the laws of gravity that we know could simply need modification. On galactic scales, this is where MOND, the theory of Modified Newtonian Dynamics, really shines.

Image credit: University of Sheffield.

While the NFW and Moore profiles — the ones that come from the simplest models of Cold Dark Matter — don’t really match up with the observed rotation curves very well, MOND fits individual galaxies perfectly. The isothermal halos do a better job, but lack a compelling theoretical explanation. If we only based our understanding of the “missing mass” problem — whether there was extra, “dark” matter, or whether there was a flaw in our theory of gravity — on individual galaxies, I would likely side with the MOND-ian explanation.

So when you see a recent headline like Serious blow to dark matter theories?, you already have a hint that they’re looking at individual galaxies. Let’s see what this is about.

Image credit: ESO/L. Calçada.

A paper released just two days ago took a look at stars relatively close to our solar neighborhood, and looked for evidence of this inner distribution of mass from the theoretical dark matter halo. You’ll notice, looking a couple of images up, that only the simplest, completely collision-less models of Cold Dark Matter give that large effect in the cores of dark matter halos.

So let’s take a look at what the survey shows.

Image credit: C. Moni Bidin et al., 2012.

Indeed, the simple (NFW and Moore) halo profiles are highly disfavored, as many studies before have shown. Although this is interesting, because it demonstrates their insufficiency on these small scales in a new way.

So you ask yourself, do these small-scale studies, the ones that favor modified gravity, allow us to get away with a Universe without dark matter in explaining large-scale structure, the Lyman-alpha forest, the fluctuations in the cosmic microwave background, or the matter power spectrum of the Universe? The answers, at this point, are no, no, no, and no. Definitively. Which doesn’t mean that dark matter is a definite yes, and that modifying gravity is a definite no. It just means that I know exactly what the relative successes and remaining challenges are for each of these options. It’s why I unequivocally state that modern cosmology overwhelmingly favors dark matter over modified gravity. But I also know — and freely admit — exactly what it will take to change my scientific opinion of which one is the leading theory. And you’re free to believe whatever it is you like, of course, but there are very good reasons why the modifications to gravity that one can make to have gravity succeed so well without dark matter on galactic scales fail to address the other observations without also including dark matter.

And we know what it isn’t: it isn’t baryonic (normal matter), it isn’t black holes, it isn’t photons, it isn’t fast-moving, hot stuff, and it probably isn’t simple, standard, cold and non-interacting stuff either, like most WIMP-type theories hope for.

Image credit: Dark Matter Candidates, retrieved from IsraCast.

I think it’s likely to be something more complicated than the leading theories of today. Which isn’t to say that I think I know exactly what dark matter is or how to find it. I’m even sympathetic to certain degrees of skepticism expressed on that account; I don’t think I would claim to be 100% certain that dark matter is right and our theories of gravity are also right until we can verify dark matter’s existence more directly. But, if you want to reject dark matter, there’s a whole host of things you’ll need to explain some other way. Don’t completely ignore large-scale structure and the need to address it; that’s a surefire way to fail to earn my respect, and the respect of every cosmologist who studies it.

And that’s, as best as I can express it in a single blog post, the whole story on dark matter. I’m sure there are plenty of comments; let the fireworks begin!

Comments

  1. #1 Michael Richmond
    April 19, 2012

    Thanks very much for writing this entry — it summarizes a great deal of information in a short, readable form, AND has nice figures. Excellent!

    If I wanted to quibble about anything, I’d say that the figure you provide for relative abundances of light elements may suggest a smaller range of uncertainties in the values than is actually observed. One relatively recent review (2004 — maybe out of date by now) is http://prd.aps.org/abstract/PRD/v70/i2/e023505. Figure 18 of that paper shows the uncertainties which we knew back then.

    But that quibble does not at all alter the conclusions of your post, with which I agree completely.

  2. #2 Dan MacDuff
    April 19, 2012

    Things were a lot simpler when all we needed to do was ask Gawd!

  3. #3 The Bobs
    April 19, 2012

    All that and no hint as to what you think dark matter may be. Now that’s a mystery!

    I take it from previous posts that you think strangelets are right out as a possibility?

  4. #4 George Monser
    April 19, 2012

    This is indeed a neat mystery story, and I enjoy reading your very clear explanation of the case. Has anyone tried to combine the two theories (CDM and MOND) in someway that supports all of the observations to date?

  5. #5 makeinu
    April 19, 2012

    One question comes to mind. First, please understand, I’m not a scientist of any kind, not really, just someone smart enough to know how little he really knows.

    Reading this article at Nature Survey finds no hint of dark matter near solar system, I’m wondering about the mathematics of gravitational theory.

    As I understand it, the theory was developed initially using our local environment observations and further refined via astrophysical measurements. What I’m curious about is how much of the baseline of gravitational theory might have been influenced by local dark matter and it’s gravitational interactions with the normal, baryonic, matter in the solar system, and whether that might not put some bias into any follow-on mathematics, which wouldn’t necessarily be accounting for that influence.

    I’m sure I’m missing something, so don’t expect a wild theory about invisible gravity monsters and general relativity any time soon, m’kay? :P

  6. #6 quen_tin
    April 20, 2012

    I heard theories involving matter with negative mass that would be present in the empty space between galaxies and clusters and “push away” normal matter (instead of attracting it). Is it a serious option?

  7. #7 Impaler
    April 20, 2012

    Reading the new paper it says that DM is merely excluded from being concentrated in the galactic disk or in a spherical halo, and if their is a Halo at all it would need to be highly ‘prolate’ (elongated along the galactic axis) such that their is little or no missing mass in the disk itself.

    This seems like a rather odd configuration for even Isothermal DM to create with regard to a Spiral Galaxy, though it would probably be a good match for an elliptical galaxy.

    Also I am curious as to why the ‘flat’ rotation rate seems to be such an issue. I recall distinctly that a uniform spherical shell of mater exerts NO NET gravitational attraction to an object within the shell, but to an object outside the shell the attraction is as if the entire mass of the shell were concentrated at its center. Thus it is possible to abstract a spherical body as a point by treating it as an set of such shells and that works as long as your outside all the shells.

    But once you start talking about a Galaxy in which the mass is distributed across a disk and/or Halo then its perfectly natural for objects at the edge to move as if they ‘feel’ more matter, indeed they should feel the net attraction of all the matter inside their orbital radius. Conversely the stars closer to the center don’t feel the pull of the outer stars and just feel the mass of the core. So we should always expect to see a flat or semi-flat rotation curve when looking at a swarm of bodies.

    Now of course if the stars ALL THROUGH OUT the Galaxy are going too fast then you have a problem, which from what I’ve read is the real argument for missing mass. But for some reason this flat rotation thing gets mixed up with it.

  8. #8 Wow
    April 20, 2012

    re 6: there does seem to be a set of “holes” that cause the galactic superclusters to distend over surfaces, rather than randomly or homogenously. Rather like soap bubbles, where the soap surface is forced toward other bubbles and away from the center because of the air pressure in the middle of the bubble.

    It’s not something that Dark Matter really bothers explaining. Whether that is valid or not depends on what that Dark Matter actually needs to be. At the very least it cannot disagree with this configuration, therefore it can’t accumulate in those areas unless its presence does something to Visible Matter over its other required effects.

    PS the pioneer anomaly has now been tracked at least 80% due to anisotropic radiation, rather than any oddity in the gravitational attraction.

    Which means that there can’t be much DM around the solar system out to a few hundred AU at least, but that there can’t be a modification to GR on that scale either.

    To within less than 1/5th the anomaly seen in the pioneer trajectory.

  9. #9 Simon Flagstaff
    April 20, 2012

    Basically what you are saying then seems to be that since you can’t imagine a different model than the current one that simply desperately needs the DM hypothesis to sustain itself, you are ready to reject direct observational evidence like the Moni-Bidin article shows. Or did I misinterpret you?

    Both BAO and CMB spectrum peaks interpretation are highly theoretical model-dependent as you know and therefore if the hypothesis “DM doesn’t exist” was correct (wich you apparently don’t rule out completely in the spirit of scientific honesty you referred to in your last blog entry), those should be interpreted in some other way, so they don’t make a case against or in favour of the rejection of the hypothesis a priori, only when a certain interpretation is chosen.
    You point out that only in the galaxy scale there are indications against DM, but at the same time you mention Abell520 wich should be at least as important a hint as it is the Bullet cluster.
    There are reasons to be puzzled at the very least, and I agree that there should be serious attempts to explain the big picture with an alternative model but that shouldn’t make us reject or dismiss or simply ignore obvious observational results just because they don’t fit in the nice big theory we are so proud of.

  10. #10 Alan L.
    April 20, 2012

    Unless I’ve been misinformed, Einstein is quoted as having stated that:

    ‘Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field.’

    And:

    ‘Space and time are modes in which we think, not conditions in which we exist.’

    By which I understood Einstein to have admitted that he did not know the actual cause of gravity. This is a separate issue to relativity theory which is 100% correct and needs no ‘modifications’ as far as I can tell.

    One thing puzzles me more than anything else about the DM question.

    If Dark Matter has one or more special properties that makes a fudge factor like MOND appear to work well or, in your words: ‘On galactic scales, this is where MOND, the theory of Modified Newtonian Dynamics, really shines…’, then why can’t the special properties or attributes that Dark Matter particles needs must possess to make MOND appear to be plausible, be worked out by the experts?

  11. #11 Wow
    April 20, 2012

    I think the biggest leap forward for the DM ur-theory (we need something that is between theory [probably true] and hypothesis [probably false], that was my attempt) would be some proposals as to what it does and what consequential evidence separate from its gravitational effect could be discerned.

    A consequential effect of a “spread probability” electron as opposed to a constrained orbiting electron is the lamb shift.

    A consequential effect of GR over Newtonian gravity was the bending of a star’s image around our sun.

    DM that is put forward as the best explanation for most of the effects remains begging the question of its existence until someone comes up with some explanation of what DM “actually is” that makes it matter, not a fudge in spacetime (gravity).

    Works better than any other ur-theory? Not a problem. It makes it a better fit than others, therefore “more correct”, but IMO we can’t call it “real” until we know what form its reality takes other than the effects we invented it to explain in the first place. That was what was wrong with the wave theory of light.

  12. #12 Eric Lund
    April 20, 2012

    impaler @7 asks: Also I am curious as to why the ‘flat’ rotation rate seems to be such an issue.

    If you assume that the observed luminosity is a good proxy for the distribution of mass in the galaxy (a reasonable thing to do if you don’t know better), you will find that the rotation speed should fall off with distance. As you correctly point out, it won’t fall off as fast as for Keplerian orbits, but it should still fall because the falloff in luminosity is fast enough. The observations show that the rotation speed does not fall off with distance.

    Possible conclusion #1: The observed luminosity is not a good proxy for the distribution of mass. Which means one or both of two things: (1) gas and dust are less tightly bound to the galaxy than stars, so that stars do not reflect the distribution of baryonic matter, or (2) there is some form of non-baryonic matter which would not be expected to have the same spatial distribution as stars. The first of these seems unlikely: stars are still forming within our galaxy, and they form from gas and dust, so the distribution of gas and dust cannot, over scales of ~1 kPc or larger, differ greatly from the distribution of stars (more precisely, O and B stars, but those stars have most of the luminosity anyway). That leaves us with dark matter.

    Possible conclusion #2: General relativity does not correctly describe how gravity works at scales of several kPc or larger, so you need a modified theory of gravity. You can indeed describe galactic rotation curves with such theories. But as the original post points out, the theories of this type which have been developed thus far fail to explain any of the other features of the universe which dark matter can explain.

  13. #13 Marcel S. Pawlowski
    April 20, 2012

    I fully agree with Simon Flagstaff’s comment (9), but not only is Abell 520 a counter-example to the Bullet Cluster which should be taken serious, the velocity with which the two ‘Bullet Clusters’ collide is in itself a problem for LCDM: They are much faster than predicted in the model.

    It might be interesting to mention another recent paper: “Missing Dark Matter in the Local Universe” by Karachentsev. I have blogged about the different places in which dark matter has gone missing recently: http://www.scilogs.eu/en/blog/the-dark-matter-crisis/2012-04-19/dark-matter-gone-missing-in-many-places-a-crisis-of-modern-physics

    And, Ethan, I am surprised that you do not discuss the numerous other problems for cold or warm dark matter that have been listed in Pavel Kroupa’s recent paper paper (http://adsabs.harvard.edu/abs/2012arXiv1204.2546K). Especially because it is the paper from which you took the figure on the CMB spectrum. Which, by the way, shows that both LCDM and the combination of MOND and sterile neutrinos (a form of hot dark matter) can fit the CMB spectrum equally well.

  14. #14 Wow
    April 20, 2012

    Extinction may be other than presumed, Eric. Luminosity may not evolve quite as we think in those galaxies. More gas than we suppose is not being illuminated in the disk. We only recently reconsidered our galaxy to be a barred spiral, rather than a spiral galaxy. THAT is how badly we can see our own galaxy.

    One problem several have pointed out is that we can’t find any of this non-baryonic matter around us. Why, then, do we see it spread throughout these other galaxies, but not ours?

  15. #15 Dan
    April 20, 2012

    Can we think of Dark Matter as being the canvas were the paint is the “normal” matter?

  16. #16 Dan
    April 20, 2012

    Can we think of Dark Matter as being the canvas were the paint is the “normal” matter?

  17. #17 Ethan Siegel
    April 20, 2012

    Marcel,

    You can point out 10,000 different examples of how the standard collisionless, cold dark matter theory gives results that are not in extremely good agreement with detailed observations on small cosmological scales. That’s not really the point of the dark matter problem; harping on one piece of evidence rather than addressing the full suite of it is a surefire way to be led astray.

    I notice that you do not discuss large-scale structure and the matter power spectrum in your link; what does MOND (or MOND + HDM) have to say about that?

    The CMB is a red herring for the temperature/type of dark matter; it’s been well-known since before I was a graduate student that the CMB is completely insensitive to those things, but it is sensitive to the ratio of dark-to-normal matter, which even in Pavel’s paper (which my graph is taken from) is in that mandatory 5:1 ratio.

  18. #18 llDayo
    April 20, 2012

    I’m just a casual lunchtime reader of this blog and do not fully grasp all of the maths and models (but I try and find it interesting!). I do have an idea as to how dark matter exists and is growing within our current known universe.

    Imagine our universe as existing within a larger sphere that is composed entirely of dark matter. It started as a small point in the center of this sphere (which would explain the near non existence of dark matter at the beginning if my memory is correct on that subject) and as it expanded and continues to do so it is including more and more of the dark matter from the larger sphere.

    Now, this dark matter can exist both in/outside of our detectable universe at the same time. It doesn’t directly affect gravity but instead actually IS gravity. Think of our universe as drops of oil inside an expanding bubble inserted into a large container of water of which the water particles can pass directly through the bubble. The oil stays together in small clumps because it is forced to by the water particles, however it doesn’t completely interact with it. At the same time the water can push the clumps further away from each other as more is added to the bubble.

    Eventually, the oil clumps would be forced further and further apart from each other (much like galaxies are moving apart) and the continuous addition of water eventually moves the matter in the galaxies apart as well. The universe would then not be considered expanding but instead is being forced apart.

    Now, hopefully I didn’t sound too loony during that explanation and that I was able to describe what I was thinking without too much confusion. I can picture it in my head but I can’t seem to find a good way to put it into words. Go easy on me, I’m still learning :)

  19. #19 Boniface Muggli
    April 20, 2012

    “All that and no hint as to what you think dark matter may be. Now that’s a mystery!”

    Yep! We’re all in the dark as to what all that matter out there might be!

  20. #20 Denier
    April 20, 2012

    With regards to the Bullet Cluster gravitational evidence, what would a gravity wave look like from a great distance? Could that be evidence of a gravity wave instead of dark matter?

  21. #21 Juice
    April 20, 2012

    Dark matter seems in all cases to be a fudge factor. “Well, the theory matches observation if we just add much more matter.” What if it’s neither matter nor gravity, but somehow we’re missing some insight into the nature of space over large distances and spans of time? ie We’re not doing relativity correctly. That might be associated with the lensing problem. Do people use Newtonian dynamics on galactic scales? And I thought a few body problem was tough. What’s up with a billion body problem like the rotation of a galaxy? Or the collision of galaxy clusters? Just asking questions here. Don’t know if they’re stupid ones.

  22. #22 mick
    April 20, 2012

    Quantum events happen by probability, therefore, if you knew the starting position of all matter you still would not be able to predict all outcomes. This was the point of Stephen Hawking’s last book but it was all lost in a silly argument about God.

  23. #23 Impaler
    April 20, 2012

    Eric Lund: Possible conclusion #1: The observed luminosity is not a good proxy for the distribution of mass. Which means one or both of two things: (1) gas and dust are less tightly bound to the galaxy than stars, so that stars do not reflect the distribution of baryonic matter, or (2) there is some form of non-baryonic matter which would not be expected to have the same spatial distribution as stars. The first of these seems unlikely: stars are still forming within our galaxy, and they form from gas and dust, so the distribution of gas and dust cannot, over scales of ~1 kPc or larger, differ greatly from the distribution of stars (more precisely, O and B stars, but those stars have most of the luminosity anyway). That leaves us with dark matter.

    The former conclusion seems far more reasonable then the latter. We still have lots to learn about galaxy and star formation so a conclusion of a 1:1 relationship between luminosity and mass distribution seems naive. The argument about star formation rates ignores the fact that new gas is constantly falling into the galaxy. Likewise much of this gas is plasma and we know their are large magnetic-fields which could easily have major effects on mass distribution or the ability of that mass to condense into a star and become luminous.

  24. #24 Alex Besogonov
    April 20, 2012

    BTW, you’ve missed some types of possible dark matter.

    For example, it can consist of small spheres of strange matter. They’ll be approximately baseball-sized and with density just shy of critical.

    They’ll behave like cold dark matter for all purposes – their cross-section is negligible and because their density would be about one sphere per 1 cubic light-year.

  25. #25 Chelle
    April 21, 2012

    Great article, made me think of the preEinsteinian time when there where a dozen of Aether theories, and Albert saved the day by coming up with a nice and easy solution. Wouldn’t it be for redshift and the optical illusion that *Stars* are drifting away, than …

  26. #26 Sinisa Lazarek
    April 21, 2012

    I have noticed that in the last image (showing different possible candidates) there is an image of EM radiation.

    It has crossed my mind before, but guess this is the right moment to ask. Could the whole EM radiation of the whole universe account for missing mass? I mean, even CMB has to have some small relativistic mass value.

    Has someone done the studies on how EM would impact the mass of the galaxy? How much is the mass contribution in such a system? Not just for visible light, but the whole EM spectrum?

  27. #27 Nathan Myers
    April 21, 2012

    Most possibilities are taboo even to mention.

    Variations in amounts and kinds of matter are fashionable just now, variations in gravitation a bit less so. Known forces that involve nonlinear dynamics, and systematic inaccuracies in favored measurement systems such as red shift, CMB, or supernova brightness, are right out. What we can be sure of is that soon today’s brightest ideas will be as interesting as phlogiston.

    There was never any objective reason to assume planetary orbits must be circular, so no reason to demand extraordinary evidence for elliptical orbits. Ellipses were just unfashionable at the time.

  28. #28 James Salsman
    April 21, 2012

    Primordial intermediate mass black holes of about 100,000 stellar masses aren’t ruled out by anything, including microlensing studies or the orbits of wide binaries. If the inflationary epoch was not a linear expansion, but sped up before it slowed down, then there would be plenty of density for the observed baryon and nucleosynthesis ratios. The WIMP theorists don’t have a shred of empirical evidence compared to the dozens of intermediate mass black holes confirmed in the past couple years, and WIMPs can’t explain cuspy halos or the dwarf galaxy distribution. Furthermore, nobody has a theory of supermassive black hole formation which doesn’t involve agglomeration of intermediate mass primordial black holes implying that orders of magnitude more of the latter, again, of about 100,000 stellar masses, don’t still exist today.

    When will Occam’s Razor return to cosmology?

    http://iopscience.iop.org/1475-7516/2010/04/023
    http://www.sciencedirect.com/science/article/pii/S0927650510002173
    http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2011.18890.x/full

  29. #29 Jay Cross
    April 22, 2012

    I see lots of papers about Axions as CDM, but the suggested mass of an Axion is perhaps a millionth that of a neutrino (and hence have positional uncertainty about the size of the Earth). I get the impression that part of the argument for them is that they are easily bound to each other by quantum properties… which I haven’t stumbled on a more detailed explanation of yet. Do you know if this is a possible explanation for how DM in the inner 10kpc of our galaxy could be thermalized? If so, wouldn’t that make the Bullet Cluster (et al.) harder to explain?

  30. #30 ObjectiveSee
    April 22, 2012

    The more I read about Dark Matter the more I’m convinced it doesn’t exist. It appears to have been concocted to explain another flawed theory Gravity. Both have lead us on a wild goose chase to discover the Higgs field which also doesn’t exist. At least we will be able to prove this later this year and put all these flawed theories behind us. If I had to vote for a unified theory it would be electromagnatisum.

  31. #31 SCHWAR_A
    April 22, 2012

    Hi Ethan,
    You said that, when
    “…we look at galaxy clusters, they, too, should be made of 80% dark matter and 20% normal matter. The dark matter should exist in a big, diffuse halo around the galaxies and the clusters.”
    and also that
    “around every single galaxy, there should be a huge dark matter halo, comprising approximately 80% of the mass of the galaxy”.

    How could I derive a fixed ratio, learned from the CMB, being valid for all scales?
    As we have seen, there is a lot of wiggling in all curves.

    In globular clusters like NGC2787 we even see ratios of up to ~100:1.

    Similarly with huge galaxies like IC1101 up to ~80:1 for outer region stars.

    Somewhere I have read that stars of galaxy-cores’ boundary region also could reach that huge ratio of up to 100:1.

    All these scales are heavily separated by factors ~200 respective ~1000. In between the ratios seem to be small, downto 5-10:1.

    I think that the ratio 5:1 (80% vs. 20%) is only valid regarding the whole universe, i.e. the largest scale, integrated over all actual ratios, which seem to range between 1:1 and ~100:1.

  32. #32 Tony Mach
    April 22, 2012

    Billions of years later, that leftover glow from the Big Bang — those photons — are still around, but they’ve continued to cool, and are now in the microwave portion of the electromagnetic spectrum. First observed in the 1960s, we’ve now not only measured this Cosmic Microwave Background, we’ve measured the tiny temperature fluctuations — microKelvin-scale fluctuations — that exist in it.

    I’ve once read that the frequency shift of the cosmic microwave background radiation (“The leftover glow of the Big Bang”) is due to the expansion of space itself (and not due to “cooling”) – so it’s just the most extreme case of red shift (as there is no older/father away observable radiation due to opacity in the early universe AFAIU). Could you clarify what is correct?

  33. #33 Sili
    April 22, 2012

    The more I read about Dark Matter the more I’m convinced it doesn’t exist. It appears to have been concocted to explain another flawed theory Gravity.

    For a flawed theory, GR sure does make it hard for us to falsify it.

    What, pray tell, experiment is not in accord with GR? This issue of “missing mass” appears to be the only one, so the prudent option is to trust to the theory that works, and look for missing matter, rather than dismiss the theoretical framework that gives so many correct results (unlike the Luminiforous Aether, say).

    Incidentally, just what does GR and DM have to do with the Higgs mechanism? Or did you just throw that in there to show how iconoclastic you are?

  34. #34 Edwin Reeves
    April 22, 2012

    The unchallenged assumption in all of this is the cosmic length scale. I know there have been several papers recently about independent confirmation of the Hubble constant, but much of Cosmology relies on distance data from type 1a supernovae. The assumptions about intrinsic brightness without evidence about what causes them (much less a verified model) bring to mind the phrase ‘house of cards’

  35. #35 Anthony J. Nania
    April 22, 2012

    There is no such thing as dark matter.
    Newton and Einstein were wrong.
    There was no big bang
    Gravity is not the force that binds the Universe, nor is the sun a fusion reactor.

    The universe is 99.9% plasma.
    It’s motive is electric; and if our science weren’t our culture, then our scientist would take less than the next century to understand that they are just spinning in circles like Ptolemaic lemmings.

  36. #36 Shane
    April 22, 2012

    Anthony J. Nania – The universe is 99% ionised gas?? Which one? How come we cant actually SEE it?? Newton and Einstein were wrong? Then how come the maths has been able to explain and predict all the phenomena we have discovered ?

    Perhaps if you added a few citations and examples it might sound less like – well bullshit…

  37. #37 Marcus Ranum
    April 22, 2012

    Anthony Nania writes:
    The universe is 99.9% plasma.
    It’s motive is electric; and if our science weren’t our culture, then our scientist would take less than the next century to understand that they are just spinning in circles like Ptolemaic lemmings.

    Did you just read:
    http://www.thunderbolts.info/wp/2012/04/19/dark-and-dead/
    and conclude that because it’s on the Internet it must be true?

  38. #38 David
    April 22, 2012

    Saw a great presentation on possible thermo explainations…

    CalTech, Feb 4 / 2011, 11am
    Erik Verlinde (Amsterdam)
    The Lost Phase Space of the Universe

  39. #40 Marcel S. Pawlowski
    April 23, 2012

    Ethan,

    thanks for your reply. Of course standard CDM cosmology is good at describing the large scales, that is what it was made for, that is where its parameters are determined from. But to test a theory, it has to predict something. And the predictions of CDM can be well tested on smaller scales, especially because there observations are more detailed.

    If it only were about observations being “not in extremely good agreement” with theoretical predictions, nobody would care that much. But there is a large number of really serious problems, (mostly) on the smaller scales. And many of these can not be discussed away by a protective belt of auxiliary hypotheses, as Imre Lakatos would put it.

    Unfortunately, these discussions soon turn into ones of ‘Cold Dark Matter’ vs. ‘MOND’. That is misleading. The failure of one theory does not make an alternative one ‘more true’, it only shows that something is wrong with the first. Therefore, we might have to admit something to ourselves: we to not have as much a clue of what is going on as we would like, or had thought.

    About the large-scale structure in MOND: simulations are more complicated for modified gravities, so calculations of structure formation models as complex as the CDM models are not possible yet. There are people working into this direction, and it seems as if MONDian structure formation might solve the problem that observed structures seem from too fast compared to predictions in the standard CDM approach. The large scales are thus not ignored, but one has to wait for more detailed results.

  40. #41 Sinisa Lazare
    April 23, 2012

    Just read in the paper today about a team of physicists from ESO which tried to measure the amount of DM around our Sun and in our “near” vicinity, and couldn’t find anything.

    The whole article is here: http://www.eso.org/public/news/eso1217/

    Guess the war between those in favor and those against will keep on going :) Or maybe it’s not that uniform spread after all. No DM in our stores.. check in the galaxy next door :D

  41. #42 Connie
    April 26, 2012

    Thank you for this wonderful article. I enjoyed reading it. You put things into understandable terms for the novice without sacrificing the integrity of the material. Bravo!

  42. #43 Wow
    April 26, 2012

    Sinisa, it rather depends on what Dark Matter “actually is”. Is it something repelled by ordinary matter? Is it found mostly interstellar? Is it mostly a halo object (therefore of a different density profile than the visible matter, hence under-represented in our locality in the disk).

    Until DMers get a handle on what it really is and start looking for consequential results of that theory, I don’t count it much better than saying “The GR figures and visible matter don’t match, therefore there must be something making the difference” and calling it Dark Matter.

  43. #44 Wow
    April 26, 2012

    “what if it’s neither matter nor gravity, but somehow we’re missing some insight into the nature of space over large distances and spans of time?”

    That’s pretty much what MOND tries to do.

    All methods of finding an elegant (i.e. one that doesn’t multiply the problems) solution haven’t managed to do so so far. I still hold out for this because we already know GR is flawed (in the sense in which pi is not 22/7, or 3.142 or any other typeable number: close but not perfect), therefore finding how it meshes with QM may well show how it could or should be modified on the huge scales of the universe.

  44. #45 SCHWAR_A
    April 28, 2012

    @Wow (44):
    “…finding how it [effect of DM] meshes with QM…”

    Just knocking out a QG? Any ideas (“Ansatz”)?

    Greetings.

  45. #46 julian luque
    April 29, 2012

    in Spanish

    LA GRAVEDAD NO ES INERENTE DE LA MATERIA, ES EL EFECTO DINERCIA. Cuando un cuerpo acelera en el espacio, crea el efecto de inercia en sentido opuesto en la dirección donde se acelera, le llamo punto de gravedad.
    Si es el espacio el que se acelera, creará el mismo efecto(gravedad).
    En una esfera que aumente su radio a una velocidad constante, el espacio que alberga es cada vez mas vacío de una forma exponencial, en el universo es igual, por lo tanto una ecuación exponencial de un vacío constante. es un aceleración constante.(gravedad)
    Ejemplo, en el primer segundo del universo, la esfera universal mediría 300.000 kilómetros de radio,en el 2º segundo 600.000 en el 3º segundo 900.000 kilómetros de radio, siendo la masa la misma, en una esfera que aumenta su volumen de vacío al cubo en densidad negativa. Por lo tanto es una aceleración hacia el vacío. El vacío no es constante, y es acelerado por el radio de la esfera universal en cada momento. EL vacío acelerado va hacia fuera, y el efecto va hacia dentro de cada maza. (gravedad) Principio de causa, efecto.
    Radio Volumen
    1
    2
    3 113
    4
    5
    6 905
    7
    8
    9 3053
    10
    11
    12 7238
    (Radio) x 100,000 (Volumen, o espacio vacío) x 100,000
    El efecto de inercia es constante todo el tiempo que aceleramos, como si desaceramos, siempre en sentido opuesto a la dirección.
    (La gravedad es un efecto de inercia) Debido al volumen constante del universo, la capacidad de la esfera universal aumenta al cubo en cada momento. Aritméticamente es una ecuación acelerada. (La aceleración es igual a inercia).(la inercia es igual a gravedad)
    Todo el volumen de vacío del universo se propaga en un medio mas vacío, de lo contrario se contraería.
    (Si el universo no se expandiera, la gravedad desaparecería como por arte de magia) *(La Gravedad no es inherente de la materia, es la consecuencia del efecto de inercia). (Unificando conceptos de entre la ley de Pascal, Galileo Galilei, Isaac newton, Albert Einstein, y el volumen constante del vacío del universo la teoría queda completa).

    Yo creo que la teoría general de la relatividad, se quedó corta.
    Quiero explicarle algunos razonamientos a tener en cuenta.
    Imaginemos un giróscopo o peonza girando, donde uno de sus puntos señale al sol,
    y el punto contrario señale al centro de la tierra, quitemos el sol, y la peonza seguirá inmutable con respecto al centro de la tierra, quitemos la tierra y el giróscopo seguirá girando en una posición fija en el espacio, resintiéndose a torsionarse aún quitemos todas las galaxias.
    Razonando el espacio tiene un ENTE propio.
    Otro ejemplo,
    A una velocidad constante en el universo sin puto de referencia, dos cohetes que pasasen juntos uno con respecto al otro, nunca sabremos cual de los dos se mueve.
    Diferente es, ese mismo cohete que empiece a acelerar, entonces sabremos que nos movemos
    y en que dirección con respecto al espacio o vacío.
    EL VACIO CUENTA, Y LO ESPLICO.
    Imaginemos el espacio como un globo de 10 centímetros, tiene una capacidad x
    otro de 20 centímetros, la capacidad no es el doble creo es elevada al cubo
    otro de 20 centímetros elevada al cubo del cubo, y así sucesivamente.
    En el espacio sucede lo mismo, pero no en volumen sino en VACIO.
    El vacío no es constante, y es una aceleración hacia mas vacío.
    Si el cohete se aceleraba en un espacio acelerado, crea el efecto de inercia en sentido opuesto a la disección donde se acelera en el espacio.
    Si es el espacio el que se acelera al rededor de un cuerpo fijo creará el mismo efecto pero en todos sus puntos, pero hacia el centro de cada maza.

    Es solo un razona miento, supongo y espero que sea así, que el espacio vacío va en aumento, a razón de la velocidad de la luz. Es el mismo principio que la ley de Pascal, pero al revés.
    Sinceramente pienso que con los principios heredados de los grandes, tenemos argumentos, para demostrar la ley de la gravedad.

    Julián Luque c/escuelas 24 c/escuelas 24
    montilla cp14550 (Córdoba) España
    teléfono 957 651734 móvil 638017324

    Si es de su interés puedo explicarle mucho mas Gracias por su atención

  46. #47 julian luque
    April 29, 2012

    in Spanish

    LA GRAVEDAD NO ES INERENTE DE LA MATERIA, ES EL EFECTO DINERCIA. Cuando un cuerpo acelera en el espacio, crea el efecto de inercia en sentido opuesto en la dirección donde se acelera, le llamo punto de gravedad.
    Si es el espacio el que se acelera, creará el mismo efecto(gravedad).
    En una esfera que aumente su radio a una velocidad constante, el espacio que alberga es cada vez mas vacío de una forma exponencial, en el universo es igual, por lo tanto una ecuación exponencial de un vacío constante. es un aceleración constante.(gravedad)
    Ejemplo, en el primer segundo del universo, la esfera universal mediría 300.000 kilómetros de radio,en el 2º segundo 600.000 en el 3º segundo 900.000 kilómetros de radio, siendo la masa la misma, en una esfera que aumenta su volumen de vacío al cubo en densidad negativa. Por lo tanto es una aceleración hacia el vacío. El vacío no es constante, y es acelerado por el radio de la esfera universal en cada momento. EL vacío acelerado va hacia fuera, y el efecto va hacia dentro de cada maza. (gravedad) Principio de causa, efecto.
    Radio Volumen
    1
    2
    3 113
    4
    5
    6 905
    7
    8
    9 3053
    10
    11
    12 7238
    (Radio) x 100,000 (Volumen, o espacio vacío) x 100,000
    El efecto de inercia es constante todo el tiempo que aceleramos, como si desaceramos, siempre en sentido opuesto a la dirección.
    (La gravedad es un efecto de inercia) Debido al volumen constante del universo, la capacidad de la esfera universal aumenta al cubo en cada momento. Aritméticamente es una ecuación acelerada. (La aceleración es igual a inercia).(la inercia es igual a gravedad)
    Todo el volumen de vacío del universo se propaga en un medio mas vacío, de lo contrario se contraería.
    (Si el universo no se expandiera, la gravedad desaparecería como por arte de magia) *(La Gravedad no es inherente de la materia, es la consecuencia del efecto de inercia). (Unificando conceptos de entre la ley de Pascal, Galileo Galilei, Isaac newton, Albert Einstein, y el volumen constante del vacío del universo la teoría queda completa).

    Yo creo que la teoría general de la relatividad, se quedó corta.
    Quiero explicarle algunos razonamientos a tener en cuenta.
    Imaginemos un giróscopo o peonza girando, donde uno de sus puntos señale al sol,
    y el punto contrario señale al centro de la tierra, quitemos el sol, y la peonza seguirá inmutable con respecto al centro de la tierra, quitemos la tierra y el giróscopo seguirá girando en una posición fija en el espacio, resintiéndose a torsionarse aún quitemos todas las galaxias.
    Razonando el espacio tiene un ENTE propio.
    Otro ejemplo,
    A una velocidad constante en el universo sin puto de referencia, dos cohetes que pasasen juntos uno con respecto al otro, nunca sabremos cual de los dos se mueve.
    Diferente es, ese mismo cohete que empiece a acelerar, entonces sabremos que nos movemos
    y en que dirección con respecto al espacio o vacío.
    EL VACIO CUENTA, Y LO ESPLICO.
    Imaginemos el espacio como un globo de 10 centímetros, tiene una capacidad x
    otro de 20 centímetros, la capacidad no es el doble creo es elevada al cubo
    otro de 20 centímetros elevada al cubo del cubo, y así sucesivamente.
    En el espacio sucede lo mismo, pero no en volumen sino en VACIO.
    El vacío no es constante, y es una aceleración hacia mas vacío.
    Si el cohete se aceleraba en un espacio acelerado, crea el efecto de inercia en sentido opuesto a la disección donde se acelera en el espacio.
    Si es el espacio el que se acelera al rededor de un cuerpo fijo creará el mismo efecto pero en todos sus puntos, pero hacia el centro de cada maza.

    Es solo un razona miento, supongo y espero que sea así, que el espacio vacío va en aumento, a razón de la velocidad de la luz. Es el mismo principio que la ley de Pascal, pero al revés.
    Sinceramente pienso que con los principios heredados de los grandes, tenemos argumentos, para demostrar la ley de la gravedad.

    Julián Luque c/escuelas 24 c/escuelas 24
    montilla cp14550 (Córdoba) España
    teléfono 957 651734 móvil 638017324

    Si es de su interés puedo explicarle mucho mas Gracias por su atención

  47. #48 John
    May 5, 2012

    Re: 41 (Sinisa Lazare @) .. Actually the link you gave shows how they were unable to detect DM nearby around these stars (or perhaps, in broader terms, within the galaxy?). This does not disprove DM. They mention in the linked article that current models assume DM is nearby to the stars (deep within the galaxy?), but this new insight shows that this assumption is incorrect. All it means, as I see it, is that hte distribution of DM has to be adjusted. And they will need to find observational evidence of this new distrubtion.

  48. #49 John
    May 5, 2012

    … (continued here)

    Indeed, this blog mentions this when Ethan says: “Whereas a spiral galaxy like the Milky Way might have a disc 100,000 light-years in diameter, its dark matter halo is expected to extend for a few million light-years! It’s incredibly diffuse because it doesn’t interact with photons or normal matter, and so has no way to lose momentum and form very dense structures like normal matter can.” He expands on this in other comments.

  49. #50 John
    May 6, 2012

    If dark matter doesn’t interact with normal matter then why does dark matter surround galaxies in a halo? Why not just be everywhere in a random distribution? What sort of physics or structural mechanic allows for DM to specifically anchor itself to galaxies in the strange halo mentioned in the article?

  50. #51 Bill
    Michigan, USA
    September 4, 2012

    One interesting aspect is that, given accurate photometric data, MOND has never been known to fail. It is therefore predictable, in contrast to the DM hypothesis which makes few predictions.

    In fact, the DM model-builders are allowed to place the stuff wherever they want to make their gravitational equations come out right. Is that really science?

  51. #52 Wow
    September 4, 2012

    That’s a bit harsh on DM and a bit generous to MOND, Bill.

    If DM exists and is gravitationally like normal matter, then, just like normal matter it will clump.

    And, just like the normal matter that planets are made of, if you can’t see them directly, you look at the effects they produce.

    As far as I’m concerned the first step on proving DM is working out what it actually, when collected into a flask, is.

    At the moment, it’s a hypothesis with some evidence for “something that acts a bit like clumps of matter”, but without the definition of that matters properties, not much more than a working hypothesis.