Crunch time for Dark Matter?

Gratuitous picture of the cluster Abel 2218, showing numerous gravitational lensing arcs

I was reading through an article by Philip Ball in the Grauniad this morning about likely breakthroughs in science for the forthcoming year. One of the topics discussed therein was dark matter. Here’s an excerpt:

It’s been agreed for decades that the universe must contain large amounts of so-called dark matter – about five times as much as all the matter visible as stars, galaxies and dust. This dark matter appears to exert a gravitational tug while not interacting significantly with ordinary matter or light in other ways. But no one has any idea what it consists of. Experiments have been trying to detect it for years, but all have drawn a blank. The situation is becoming grave enough for some researchers to start taking more seriously suggestions that what looks like dark matter is in fact a consequence of something else – such as a new force that modifies the apparent effects of gravity. This year could prove to be crunch time for dark matter: how long do we persist in believing in something when there’s no direct evidence for it?

It’s a good question, though I have to say that there’s very little direct evidence for anything in cosmology: it’s mostly circumstantial, i.e. evidence that relies on an inference to connect it to a conclusion of fact…

Anyway, I thought it would be fun to do a totally unscientific poll of the sort that scientists find  fun to do, so here’s one. It’s actually quite hard to make this the topic of a simple question, because we know that there is ordinary (baryonic) matter that we can’t see, and there is known to be some non-baryonic dark matter in the form of a cosmic neutrino background. What the question below should be interpreted to mean, therefore, is  `is there a dominant component of non-baryonic dark matter in the Universe in the form of some as-yet undiscovered particle?’ or something like that.

For the record, I do think there is dark matter but less convinced that it is simple cold dark matter. On the other hand, I regard its existence as a working hypothesis rather than an article of faith and do not lose any sleep about the possibility of that hypothesis turning out to be wrong!



19 Responses to “Crunch time for Dark Matter?”

  1. Reblogged this on Disturbing the Universe and commented:
    Might as well throw this open to more people…

  2. knudjahnke Says:

    Which is why I really like that the Euclid mission is also officially testing MOND models. Not just as a hushed-up inofficial aside.

  3. Most discussions of dark matter are hung up on the idea that it is a WIMP. There are still some mass ranges for primordial black holes which are not ruled out observationally.

  4. I don’t think it’s a question of plumbing for a simple yes or no. As always, I feel this particular question being posed the wrong way around. Surely the real question is: Do we have any reason to assume that the dominant form of matter interacts with the electromagnetic interaction? And what is the basis of this assumption?

    • I agree. This is my biggest criticism of some of the MOND proponents: the cast it as bizarre that most matter is not electromagnetically detectable. There is no justification for the expectation.

      (My biggest criticism of MOND critics is that they do not know as much traditional astronomy and astrophysics as most MOND enthusiasts.)

    • Michel C. Says:

      There is another possibility. We could have many indepedent sets of electromagnetic interactions. For example, each set could be based on a specific finite length. There would be only one length for gravity, the Planck length.

      • telescoper Says:

        The properties of electromagnetism are extensively tested in laboratory experiments.

      • The electromagnetic interactions of ordinary matter would not interact with other sets, which are DM for us. In other words, there are many sets of DM, each one having its own separated EM interaction but a common gravity.

      • All these type of DMs were produced at the big bang and therefore there is no way to get any EM interaction with it.

      • The other EM basic lengths would be like hidden dimensions, analogous to the string theory.

  5. It’s all beginning to sound like ‘epicycles’ – increasingly complicated layers of explanation that fail to explain away the observed discrepancies in the received theory.

    It isn’t standing up to observation, and every new elaboration as to what the missing mass may be is seen – sooner than the last – to be inconsistent with the data.

    Trouble is, the alternative explanation – that there’s something wrong in our gravitational theory – descends and degenerates into epicycles of elaboration when we try to ‘fix’ the calculations.

    Nevertheless, this is the obvious avenue of investigation. And even if it doesn’t fix this problem in cosmology, I don’t see any downside from increasingly intensive investigation of gravity.

    As a layman, I will make a prediction: the successful answer will be simpler than anything offered so far.

    • Read the papers by Justin Khoury and collaborators on superfluid dark matter. This seems to combine the advantages of CDM and MOND.

      • telescoper Says:

        There was a nice talk at the RAS on Friday about axion dark matter. It seems a promising idea, but there’s also a fine-tuning issue with the need to make the effective de Broglie wavelength a kiloparsec scale.

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