Talking About Dark Matter and Dark Energy

Trying to keep these occasional Facebook Live videos going. (I’ve looked briefly into other venues such as Periscope, but FB is really easy and anyone can view without logging in if they like.)

So here is one I did this morning, about why cosmologists think dark matter and dark energy are things that really exist. I talk in particular about a recent paper by Nielsen, Guffanti, and Sarkar that questioned the evidence for universal acceleration (I think the evidence is still very good), and one by Erik Verlinde suggesting that emergent gravity can modify Einstein’s general relativity on large scales to explain away dark matter (I think it’s an intriguing idea, but am skeptical it can ever fit the data from the cosmic microwave background).

Feel free to propose topics for future conversations, or make suggestions about the format.

  1. Proposal for future conversation: the equation of state of dark energy

    What I liked most about today’s video: the very clear and helpful discussion about baryon acoustic oscillations in the early universe. For example, the following 4 points:

    1. the cycle where an overdense region collapses, heats up, then partially restores in a bounce back;

    2. how DM does not bounce or heat up;

    3. even numbered oscillations are damped and odd numbered oscillations are enhanced by the existence of dark matter;

    4. temperature difference dampening: first collapse is big, smaller differences in subsequent cycles

    Format: I like the format as is, no tweaking necessary IMO

    Other comment: regarding your passing reference to the fact that the hot dense smooth early universe is a low entropy state – thank you for going into detail on that in your previous FB video. That may be the single most helpful, confusion-busting explanation I’ve heard in this series of videos.

  2. Please notify subscribers of your website before your talks. I don’t use social media so it’s not possible to get your tweets and watch your talk live.
    Thanks for the interesting talk.

  3. I would be very interested in hearing your opinion, or refutation, of David L. WIltshire’s “Timescape Cosmology” approach to Dark Energy and his assertion that there may be no accelerated expansion of space. To me it makes complete sense that our measured observations ( type 1a SNe, CMB, BAO, etc) may be biased by our location in a matter dense region of the Universe, and if similar measurements could be made from the middle of a large void, far from significant matter and it’s gravitational influence on time, things would “appear” significantly different. In fact, the measure “age” of the Universe there could be closer to 18 billion years. Wiltshire asserts that as of about 5 billion years ago the inhomogeneity of the Universe became dominant enough that the assumption of it being homogenous (within certain scales) is a poor one and should be replaced within the standard cosmological model. There is no need for DE because the expansion is not accelerating. Thoughts?

  4. From ‘standard candle’ paper:

    advantage of our method is that we get a goodness-of-fit
    statistic in the likelihood which can be used to compare
    models or judge whether a particular model is a good
    fit. Note that the model is not just the cosmology, but
    includes modelling the distributions of x1 and c

    In general likelihood is not useful as a ‘goodness-of-fit’. Have these authors defined it in such a way as to make it a reasonable ‘goodness-of-fit’ criteria in this case? It was a common mistake in particle physics to use likelihood as a goodness-of-fit criteria (we have since learned better).

    -Arthur Snyder, SLAC, BaBar and Fermi

  5. Well I don’t really care if these things are live; I just love listening to them. I listen to them here. I don’t do Facebook.

    Ref the topic on which Prof Carrol speaks on this occasion, perhaps he could explain why space absent of ordinary matter is expanding, indeed accelerating, while the space which matter occupies is stable, i.e. not expanding. Thanks.

  6. Thanks for your amazing efforts Sean,

    I was exactly hoping to see your take on the paper by Erik Verlinde, when I read about it.

  7. I’m still struggling through Verlinde’s paper, not in any hope of comprehending it, but rather to understand it from a structural perspective, like diagramming a sentence. He’s referencing papers in areas that themselves are still under very active discussion and research, so I’m trying to assess the degree of reliance on too new or too weak work. That is, which references pose the greatest risk to the foundation upon which his analysis and conclusions rely?

    Others will have to comment on the physics of the theory itself (I’m looking at you, Sean). I, for one, would love to see Verlinde’s work be accepted as a strong theory, and see it become the leading non-DM/non-DE theory. Science works best when there is tension between competing theories, and the higher the level the better (rather than, say, the lower-level competition between theories of DM particles, all of which start with the assumption that particulate DM exists).

    Combined with ER=EPR, emergent gravity seems to be trending. And here I was still struggling with emergent time…

  8. W.R.T: Nielsen, Guffanti, and Sarkar
    is there some where where the basic analysis is described more pedagogically? As basically a partic experimenter with some limited experience now with Fermi Space Telescope, I find it hard to figure out the likelihood equation be used here. The jargon is kind of heavy going for me.

    I’d like to start with the basic formulation of the problem w/o the corrections for color, etc.

  9. Fantastic. Somewhat frustrating, to a point, to think that dark matter may not be measured.

    Does neutrino density have any role to explain dark matter? Assuming 1) they have a non-neglible mass contribution and 2) are affected by gravitational effects?

  10. >Does neutrino density have any role to explain dark matter? Assuming 1) they have a non-neglible mass contribution and 2) are affected by gravitational effects?

    The light neutrinos are not heavy enough or expected in sufficient numbers. Also Galaxy formation simulations need ‘cold dark matter’ which is to say heavy dark matter particles.

    At Fermi we’ve been looking and see some excess production of high energy gamma rays from neat th galactic center where such dark matter particles should accumulate. Unfortunately, we can’t rule out alternative explanation.

  11. Thank you very much for this.
    When I first heard about the article about type 1a supernovas I went and bought your course on dark energy and dark matter on the great courses plus, and literally just finished it yesterday.

    By the end, I understood everything you just explained here but it is still a nice confirmation to get from you that I understood everything correctly.

    Keep up the great work, as well as your public outreach, it is greatly appreciated!

  12. Thanks for doing this Prof Carroll.
    It is absolutely amazing that I can watch it from Western Australia.
    One puzzle, it seems you live in an anti-universe with left-right swapped around!
    Eg the Guy Toubon art work, your hair parting, are the worng way round . . .
    Do you advise not shaking hands should the opportunity arise?

  13. Wonder if there’s some chance of coming up with something akin to E=mc² for Dark Energy and Dark Matter. Or is that a wrong direction altogether?

  14. Sean, many thanks really interesting. I would like to hear more about the very early universe and its low entropic state. I find this difficult to get my head around!

  15. If dark matter is made of black holes (, B. Carr, and A. Kashlinsky, and wormholes represent the entanglement of two black holes, questions:
    1) Why not a wormhole network linking all black holes via ER=EPR, all being dark matter?
    2) Any other candidates ?
    3) If the graviton itself is one such candidate, could it relieve Newton’s anxiety about action at a distance?
    In any case, all are entities that interact only gravitationally, and not eletromagnetically etc.


  16. Wondering if there’s a metric for observational ontological support, such that for instance we could say electrons are real, no question, but dark matter is somewhat more ontologically in doubt, and dark energy even more dubious, given the observational evidence, e.g., their purported effects on ordinary matter. Is there any agreement on what counts as an observation of a phenomenon that qualifies it as indubitably real?

  17. Just watched the video of Sean’s FQXi talk ( Closest thing to a “Physics Slam” I’ve ever seen. I’m not sure Sean ever inhaled.

    I think I heard something about “emergent gravity” in there, along with something about “Mad-Dog Everettian”. Sounded a bit like Sean in Wonderland.

    Don’t recall any mention of wormholes. Though deriving the (finite) Hamiltonian dimensionality from the entropy of a black hole seemed slick.

    I’d like to see how Sean’s finite Hamiltonian approach eventually compares (in terms of fruitfulness) to Verlinde’s. Assuming, in my ignorance, that they are at all comparable.

    Anyone else care to take a look?

  18. The problem I have with….well actually it’s the way General Relativity is being implemented at the moment. The paradoxes, such as the one about an alien invasion from Andromeda, showing up two weeks ago, or in two weeks time depending whether you pause momentarily while walking the dog, are explained as paradoxes and not inconsistencies with relativity-of-simultaneity….but that doesn’t actually work out, and so the back up reason is that GR is a theory for nearby spacetime: it works for the solar system and it works up to the scale of a cluster of stars. It works well up to there. But beyond there nobody knows, until we get to distances that are very large where we already know from the paradoxes it doesn’t work.
    No problem with any of that, but why then is it deployed as an explanation at similarly enormous distances, in other contexts? That’s my problem, or it’s Albert Einstein’s, so it’s mine.
    So shoot me like a sick animal or tell me where my thinking goes off the rails, or propose marriage or whatever the culture is where you are.

  19. I’ld like to know whether Polchinski’s forewalls are consistent with Verlinde’s idea of entropic gravity. I like to know this because I think Polchinski is right (for thermodynamical reasons).

  20. Tom Clark,

    Interesting point. Following your train of thought, would like to revisit “A property that is not measured need not exist (be having ontological import)” (My comment on October 10 in

    By “observational ontology”, I assume that puts us in the post-measurement realm. If so, dark matter has fairly solid support through its gravitational manifestations. For example, see Sean’s description of dark matter halo in spiral galaxies. (Not to mention Vera Rubin’s classic galaxy rotational curves.)

    I happen to be interested in dark matter’s pre-measurement ontology. Just like wondering where is a photon (and what is a photon) before measurement. Thoughts? Could it have, like a photon, some kind of non-physical ontology? (Please see same October 10 comment noted above.)

    After all, Einstein had thought black holes (today’s dark matter?) were non-physical (in ontology).


  21. Hey Sean!
    Thanks! That was really good, mostly comprehensible at my level (MS, engineering, but supporting astrophysics).
    OK to follow up with some high-level equations ( with terms defined, for those of us looking in from outside phd level physics). That’s just a suggestion, but since you’re such a good science communicator, you might be interested to include some basic quantitative theory principles that could help the rest of us non pros and students who are watching and listening to build up our understanding.

  22. KC: “…dark matter has fairly solid support through its gravitational manifestations.”

    One thing I picked up from this talk is the difference between direct and indirect observation. Sean says that although he thinks dark matter and dark energy are real, they haven’t yet been observed directly, as for instance in a lab. We can infer their existence (that is, they are indirectly observed) by their effects on ordinary matter. This counts as a “good, data-based reason” to believe they are real, Sean says, but we haven’t yet “found” (directly observed) them, as we have recently the Higgs boson.

    So what are the criteria for saying we directly observe a phenomenon such that we can claim we’ve found it? So that we can claim with near (if not total) 100% confidence that it’s real? Whatever the criteria are for the Higgs and other accepted sub-atomic particles I guess, but I don’t know if these criteria have been enumerated or formalized.

  23. I wonder what is the reasoning behind the statement that in expansion of the universe, dark energy has to counteract the pull of gravity. I don’t see why gravity should act towards contraction of the universe, space has no mass on which gravity could pull.