Last Wednesday, Katherine Freese gave a Public Lecture at Perimeter on the topic of Dark Matter and Dark Energy. The talk should be on Perimeter’s YouTube page by the time this post is up.
Answering twitter questions during the talk made me realize that there’s a lot the average person finds confusing about Dark Matter and Dark Energy. Freese addressed much of this pretty well in her talk, but I felt like there was room for improvement. Rather than try to tackle it myself, I decided to interview an expert on the Dark Side of the universe.
Twitter doesn’t know the power of the dark side!
Lord Vader, some people have a hard time distinguishing Dark Matter and Dark Energy. What do you have to say to them?
Fools! Light side astronomers call “dark” that which they cannot observe and cannot understand. “Fear” and “anger” are different heights of emotion, but to the Jedi they are only the path to the Dark Side. Dark Energy and Dark Matter are much the same: both distinct, both essential to the universe, and both “dark” to the telescopes of the light.
Let’s start with Dark Matter. Is it really matter?
You ask an empty question. “Matter” has been defined in many ways. When we on the Dark Side refer to Dark Matter, we merely mean to state that it behaves much like the matter you know: it is drawn to and fro by gravity, sloshing about.
It is distinct from your ordinary matter in that two of the forces of nature, the strong nuclear force and electromagnetism, do not concern it. Ordinary matter is bound together in the nuclei of atoms by the strong force, or woven into atoms and molecules by electromagnetism. This makes it subject to all manner of messy collisions.
Dark Matter, in contrast, is pure, partaking neither of nuclear nor chemical reactions. It passes through each of us with no notice. Only the weak nuclear force and gravity affect it. The latter has brought it slowly into clumps and threads through the universe, each one a vast nest for groupings of stars. Truly, Dark Matter surrounds us, penetrates us, and binds the galaxy together.
Could Dark Matter be something we’re more familiar with, like neutrinos or black holes? What about a modification of gravity?
Many wondered as much, when the study of the Dark Side was young. They were wrong.
The matter you are accustomed to composes merely a twentieth of the universe, while Dark Matter is more than a quarter. There is simply not enough of these minor contributions, neutrinos and black holes, to account for the vast darkness that surrounds the galaxy, and with each astronomer’s investigation we grow more assured.
As for modifying gravity, do you seek to modify a fundamental Force?
If so, you should be wary. Forces, by their nature, are accompanied by particles, and gravity is no exception. Take care that your tinkering does not result in a new sort of particle. If so, you may be unknowingly walking the path of the Dark Side, for your modification may be just another form of Dark Matter.
What sort of things could Dark Matter be? Can Dark Matter decay into ordinary matter? Could there be anti-Dark Matter?
As of yet, your scientists are still baffled by the nature of Dark Matter. Still, there are limits. Since only rare events could produce it from ordinary matter, the universe’s supply of Dark Matter must be ancient, dating back to the dawn of the cosmos. In that case, it must decay only slowly, if at all. Similarly, if Dark Matter had antimatter forms then its interactions must be so weak that it has not simply annihilated with its antimatter half across the universe. So while either is possible, it may be simpler for your theorists if Dark Matter did not decay, and was its own antimatter counterpart. On the other hand, if Dark Matter did undergo such reactions, your kind may one day be able to detect it.
Of course, as a master of the Dark Side I know the true nature of Dark Matter. However, I could only impart it to a loyal apprentice…
Yeah, I think I’ll pass on that. They say you can only get a job in academia when someone dies, but unlike the Sith they don’t mean it literally.
Let’s move on to Dark Energy. What can you tell us about it?
Dark “Energy”, like Dark Matter, is named for what people on your Earth cannot comprehend. Nothing, not even Dark Energy, is “made of energy”. Dark Energy is “energy” merely because it behaves unlike matter.
Matter, even Dark Matter, is drawn together by the force of gravity. Under its yoke, the universe would slow down in its expansion and eventually collapse into a crunch, like the throat of an incompetent officer.
However, the universe is not collapsing, but accelerating, galaxies torn away from each other by a force that must compose more than two thirds of the universe. It is rather like the Yuuzhan Vong, a mysterious force from outside the galaxy that scouts persistently under- or over-estimate.
Umm, I’m pretty sure the Yuuzhan Vong don’t exist anymore, since Disney got rid of the Expanded Universe.
That perfidious Mouse!
Well folks, Vader is now on a rampage of revenge in the Disney offices, so I guess we’ll have to end the interview. Tune in next week, and until then, may the Force be with you!
4 gravitons 
“Only the weak nuclear force and gravity affect it.”
The case for dark matter that interacts via the weak nuclear force is increasingly weak.* If it did, it would make up some of the decays of W or Z bosons (which there is strong evidence that it does not), unless it was too heavy to do so. It would also have a cross-section of interaction with ordinary matter comparable to that of a neutrino, which has been ruled out by direct dark matter detection experiments over a very broad range of masses. Moreover, weakly interacting dark matter particles heavy enough to not be ruled out by either of these sets of experimental data are strongly disfavored by astronomy data because “cold” dark matter gives rise to too much galactic scale structure, has too much scatter from the Tully-Fischer relation to match reality, and produces NFW shaped dark matter halos when the real world produces rugby ball shaped isothermal halos. (The lamdaCDM model’s definition of cold dark matter includes pretty much any particle with more than 10s of eVs of mass, a definition that includes both “warm” and “cold” dark matter according to dark matter theorists’ terminology.)
The case that dark matter (if it exists and is not simply a tweak of the gravitational weak field) is “sterile” (i.e. does not interact via the Standard Model’s weak nuclear force or any other force other than gravity) or has non-gravitational interactions only with other dark matter (i.e. self-interacting dark matter) is much stronger. Those are really the only two classes of dark matter models that can fit the data.
Also, we know from simulations and from modified gravity theories that replicate the phenomenology of dark matter that the dark matter sector, or at least the part of the dark matter sector that we can observe experimentally and observationally, must be very simple, because the observed phenomenology can be described with very few (not more than 2-3) degrees of freedom in addition to those already present in general relativity, all of the way from the millimeter scale to the scale of galactic clusters. Chi square per degree of freedom tests of fits of simulations to observed dark matter phenomenology almost always favor either one kind of dark matter particle, or one dark matter fermion and one massive scalar that carries the dark matter self-interaction and gives rise to a Yukawa force between dark matter fermions.
If dark matter is “sterile” the astronomy data favor a mass on a keV order of magnitude. Self-interacting dark matter models and non-thermal dark matter (where the amount of dark matter in the universe is the product of a dynamic process than creates and destroys it, rather than being fixed in quantity and not decaying at all following a “freeze out”) are less tightly constrained in mass ranges but have their own issues.
“Dark energy”, of course, is perfectly described to the limits of experimental accuracy even from Planck, as merely a cosmological constant term in general relativity representing an intrinsic curvature of space-time. No evidence supports the argument than reifying it as a pervasive, probably constant scalar, energy field (which would be mathematically equivalent) is a better description, although the virtue of a dark energy hypothesis is that it allows for more variations on the theme than a simple cosmological constant would permit.
True believers think that the discrepancies between cold dark matter models and the phenomenology are pathologies of inferior simulation models rather than of the cold dark matter paradigm itself, and that models with cold dark matter will start acting more like the real world as the simulations become more realistic. But, while some claims to that effect have been made in recent moths, they aren’t terribly convincing yet, IMHO.
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Agreed that weakly interacting dark matter is disfavored at this point, though I don’t think it’s in the same ballpark as a proposal for strongly- or electromagnetically- interacting dark matter would be.
Wasn’t aware that people had done statistics favoring simpler dark matter models. It doesn’t surprise me, though “the part relevant for observations is simple” isn’t really the same claim as “the dark sector as a whole is simple”. You don’t need to tell me about keV scale dark matter, regardless, it’s all the rage at PI these days.
Regarding dark energy, we have to reify it somehow, that’s how physics works. The only question is what form the reification will take.
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Corrected version of final two paragraphs:
“Dark energy”, of course, is perfectly described to the limits of experimental accuracy even from Planck, as merely a cosmological constant term in general relativity representing an intrinsic curvature of space-time. No evidence supports the argument that reifying it as a pervasive, (constant) scalar energy field (which would be mathematically equivalent) is a better description, although the virtue of a dark energy hypothesis is that it allows for more variations on the theme (e.g. scalar energy fields that are not constant) than a simple cosmological constant would permit.
True believers think that the discrepancies between cold dark matter models and the phenomenology are pathologies of inferior simulation models rather than of the cold dark matter paradigm itself, and that models with cold dark matter will start acting more like the real world as the simulations become more realistic. But, while some claims to that effect have been made in recent months, they aren’t terribly convincing yet, IMHO.
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One more point.
The most exciting hypothesis that I’ve seen is that all of the dark matter phenomenology, and much of the dark energy phenomenology, is due to a discrepancy between general relativity and a more correct quantum gravity theory that arises from the nature of the graviton self-interaction in systems that are not spherically symmetric which has been suggested by Alexandre Deur by analogy to the behavior of self-interacting gluons in QCD, and supported observationally by the relationship between the extent to which ellipitical galaxies are not spherical and the amount of apparent dark matter that is present in such systems. If this hypothesis is correct, the non-abelian properties of the graviton that we know and love without any additional kinds of particles can account for all dark matter phenomena at all scales with no new physical constants.
Back of napkin estimates of the effects suggest that unlike the most famous effort to modify gravity phenomenologically (MOND), that the magnitude of the effect stays correct up to the galactic cluster scale, and of course, it is also relativistic, doesn’t have perceptible effects in gravitational strong field, and theoretically well motivated since some of the theoretically distinctive features of general relativity’s treatment of the self-interaction of gravitational fields (e.g. gravitational fields can’t be localized, gravitational fields aren’t a source in the stress-energy tensor, gravitational fields can go to infinity) are fundamentally inconsistent with any graviton based quantum gravity theory.
In this analysis, the stronger than expected weak field behavior which dark matter phenomena are equivalent to is due to gravitons being pulled towards other gravitons in non-spherically symmetric systems, and dark energy effects arise at least in part (e.g. in the case of a spiral galaxy) from gravitons that are attracted to gravitational fields which make gravity effectively stronger in one direction, not being available and effectively making gravity weaker in other directions (something that explains the “coincidence” that the amount of “dark matter” that apparently exists in the universe and the amount of “dark energy” that apparently exists in the universe are of the same order of magnitude).
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Your comments are way too long, Jedi.
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Interesting. Experience at PI has made me fairly leery of attempts to make quantum gravity phenomenologically relevant, though. It’s far too easy to take a few philosophical points about what a theory of quantum gravity should look like and end up with some vaguely plausible argument for whatever you want, as without an actual proposed theory there’s more than enough freedom to only consider those phenomena that make the desired point.
That’s not to say that’s what Deur is doing, but it’s something to keep in mind.
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The failure of modern physics to predict the existence of 95% of the stuff in the universe is one of its largest failures in the last century.
It was however not a surprise as everyone knows that physics has lost true predictive power. Instead physics has turned into a 70 – handled machine which can explain anything – in hindsight. High Tc Superconductivity, neutrino mass, dark matter, dark energy, the cosmological constant, three particle generations. The list is of course much longer.
On the bright side:
Observational astronomy and a few other experimental areas are challenging physics, hopefully one day pushing physics where it needs to go next.
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I’m not sure that physics has really been any different. We’ve got a rosy view of the past mostly focusing on nice simple ideas that survived and were confirmed, but physics in practice has always been a messy business, with at least one lever to pull per physicist.
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Nicely done, Lord Vader!
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