Cosmology Talks: Mateja Gosença & Bodo Schwabe on Simulating Mixed Fuzzy and Cold Dark Matter

It’s been too long since I shared one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting.

Anyway, although I’ve been too busy to check out the talks much recently I couldn’t resist sharing this one not only because it’s on a topic I find interesting (and have worked on) but also because one of the presenters (Mateja Gosença) is a former PhD student of mine from Sussex! So before I go fully into proud supervisor mode, I’ll just say that the talk is about AxioNyx, which is a new public code for simulating both ultralight (or “Fuzzy”, so called because its Compton de Broglie wavelength is large enough to be astrophysically relevant) dark matter (FDM) and Cold dark matter (CDM) simultaneously. The code simulates the FDM using adaptive mesh refinement and the CDM using N-body particles.

P. S. The paper that accompanies this talk can be found on the arXiv here.

17 Responses to “Cosmology Talks: Mateja Gosença & Bodo Schwabe on Simulating Mixed Fuzzy and Cold Dark Matter”

  1. “So before I go fully into proud supervisor mode”

    Is that one step down from executive mode?

    I’m interested in how many here get the joke.

  2. Hi Peter,
    I think you are referring to the de Broglie, not the Compton wavelength

  3. Anton Garrett Says:

    As FDM comprises bosons I’m surprised that it’s not called fuzzy dark energy. Has any astrophysics been done on ultralight fermions with significant de Broglie wavelength?

    • Yes, and “fuzzy dark matter” is the term used.

      • Correction: Most (all?) fuzzy–dark-matter candidates are bosons. If “fuzzy” refers to low mass and hence long wavelength, then fermions could be fuzzy, but would also be hot, hence not a candidate for CDM, though perhaps for cosmological dark matter.

    • Sorry, misread your comment. It’s not called “dark energy” because dark energy implies negative pressure.

      • Anton Garrett Says:

        Thanks. I had been taking the dark matter/dark energy terminology to refer to fermions/bosons rather than positive/negative pressure. Got a reference for fuzzy dark fermions?

    • It is possible for light bosonic fields to exert negative pressure so they can in principle generate a form of dark energy as well as dark matter.

      The key thing about dark matter is that it has to cluster. With a bosonic particle this is no problem but for fermions you have to take account of the exclusion principle. Very light particles will pick up huge velocities if they are packed in at high spatial density. Neutrinos of course are fermions (with masses around the eV) and they are termed Hot Dark Matter because of this effect. You can’t make them cluster on galaxy scales because of this effect. Any fermionic dark matter candidate would have to be quite massive to get around this and can’t therefore be fuzzy.

      • “It is possible for light bosonic fields to exert negative pressure so they can in principle generate a form of dark energy as well as dark matter.”

        But can they exactly mimic the effect of the cosmological constant (which is still the model which best fits the data)?

        “Any fermionic dark matter candidate would have to be quite massive to get around this and can’t therefore be fuzzy.”

        I guess you mean “any fermionic cold–dark-matter candidate”. As far as large-scale cosmology, i.e. the energy budget of the Universe goes, it doesn’t matter if dark matter is clustered or not. Dark matter on galaxy scales, of course, would have to cluster and hence be cold.

        Theoretically, the cosmologists’ dark matter could not cluster at all, and MOND or something like it explain smaller-scale effects usually attributed to dark matter. (I don’t think that that is particularly likely, but one must be careful what inferences are drawn from which data.)

        In his latest book, Jim Peebles describes the cosmologists’ dark matter, the astronomers’ dark matter, and the particle physicists’ dark matter. To what extent any of those coincide is still an open question.

      • “The key thing about dark matter is that it has to cluster. With a bosonic particle this is no problem”

        Right. By making the particles very light (so Compton wavelength (which just gives a scale here, nothing to do with Compton scattering) of a few parsecs, say), one can avoid the extreme clustering which overpredicts the cuspiness of halos and the number of small galaxies in the standard LambdaCDM picture.

        The terminology is relatively new, but “fuzzy” usually means “not strongly clustered” in order to avoid the overclustering problems mentioned above; it could be bosonic or fermionic.

        Hypothetical sterile neutrinos could be fuzzy baryonic dark matter.

      • Obvious typo in that last sentence. 😦

      • Correction: fuzzy fermionic dark matter. However, most (all?) fuzzy–dark-matter candidates are bosons. If “fuzzy” refers to low mass and hence long wavelength, then fermions could be fuzzy, but would also be hot, hence not a candidate for CDM, though perhaps for cosmological dark matter.

        The case for cosmological dark matter is pretty firm; even many MOND enthusiasts don’t contest this. There is also good evidence for strange behaviour at low-acceleration scales. MOND explains that well at the the cost of completely unknown physics; cold dark matter explains that well at the cost of having to believe in a specific distribution of dark matter on scales much larger than the observed effects it is supposed to explain.

        A separate issue are the problems associated with structure formation, which depend on the details of interactions between non-baryonic dark matter and baryonic matter. Naive predictions don’t seem to be observed, but it is an open question if the observers are missing something, the model is wrong, or too many details have been left out.

        Ideally, keeping Occam’s razor in mind, one would like a density to explain the cosmological dark matter while at the same time explaining MOND phenomenology but avoiding the problems which plague LambdaCDM (using more-conventional CDM candidates, which could be WIMPs, primordial black holes, self-interacting dark matter, or whatever). Superfluid dark matter looks promising here, but as far as I know it is an ad-hoc idea (which doesn’t mean that it must be wrong.

        One would also like to explain the lack of direct detection of dark matter (though I don’t see that as a serious problem). Many non-mainstream CDM candidates, some of which are mentioned above, one would not expect to detect in the lab via current experiments.

        The amount of dark matter in the Universe seems to be a factor of a few larger than that of baryonic matter. To me, the assumption that most of the matter in the Universe is one non-interacting particle, while the small fraction of baryonic (and associated, such as leptonic) matter has such a rich structure, seems a bit odd. Thus, I don’t see self-interacting dark matter as an epicycle to explain the non-detection in the lab, but rather a credible assumption.

      • “Theoretically, the cosmologists’ dark matter could not cluster at all”

        By which I mean that derived from measuring lambda and Omega via classical cosmological tests.

        There is some evidence, independent of observations of galaxies and clusters, that dark matter does cluster, namely the power spectrum of CMB anisotropies.

      • Anton Garrett Says:

        Is the Peebles book “Cosmology’s Century”? How technical is it?

      • Yes, It has equations, but is not overly technical. It’s neither a textbook nor a memoir, but rather his personal view of the last 50 or so years of cosmology (and a bit from the time before that). It concentrates on topics he has worked on, so, for example, there is very little on the Hubble constant (he realizes that, of course, and mentions it in a footnote). Although from a personal perspective, it’s about cosmology; it is neither about Peebles himself nor, in any detail, the people he has worked with. (He does mention, though, that the only programming language he knows is Fortran.)

        Apart from the different types of dark matter (with a chapter on each), perhaps somewhat surprising is a chapter on evidence for homogeneity and isotropy from the time before Peebles arrived on the scene. Of course, in the old days, most work was on homogeneous and isotropic models, which at the beginning was a practical assumption. With time, the observational evidence for this has become more and more secure, though there is still some debate. However, he concentrates on the evidence for homogeneity and isotropy found during the first half of cosmology’s century, writing that he did so because he couldn’t find a good overview anywhere else.

        The century is not quite 100 years, essentially starting at Einstein’s first cosmology paper and ending with the establishment of the concordance model.

      • Anton Garrett Says:

        Interesting; thanks.

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