Sunday, 14 August 2011

Dark matter may be an illusion

Again, with teaching, PhD reviewing, public talks etc, things have been very busy, but should calm down a little in the next few weeks. So, again, a brief post this week. An as I am feeling grumpy, I'm going to comment on

Is dark matter an illusion created by the gravitational polarization of the quantum vacuum? (original paper here).

We are all aware that we don't know what dark matter is, but there are a couple of things we do know; 1) we know that we need something to explain the way we observe the Universe 2) that something may be something material (like a dark matter particle, or left over black holes) or even a modification to our laws of physics, such as modified Newtonian Dynamics or (cue weird music) leaky dimensions, or other such stuff.

Now, I will say this again. We know this. IMHO, most astronomers don't worry too much about this and they go with the simplest assumption, that dark matter is just that, matter which is dark. Most papers report very simple numbers, such as a Mass-to-Light ratio, or if there are more direct probes, such as gravitational lensing, you may get the shape of the dark matter.

But if someone said tomorrow that Dark Matter is definitely not material, but is something else, something weird, most people would say "OK" and get on understanding their results within this framework. I don't think people will be really shocked.

So most astronomers don't really bat an eye-lid at papers like this one. They are quite common, maybe dark matter is this, or dark matter is that, or unicorns, or whatever. But they often claim to explain everything, but have very few (if any) predictions that can truly differentiate the new idea from plain old vanilla dark matter. And so on we carry, quoting ML and other simple quantities.

To be clear, I am not attacking this current paper. The author, at the end, states that;
"In conclusion, we have revealed the first indications that what we call dark matter may be consequence of the gravitational repulsion between matter and antimatter and the corresponding gravitational polarization of the quantum vacuum by the existing baryonic matter. Of course, this is not a claim, just possibility. A lot of work would be needed before such a claim would be eventually possible."
Basically, the paper is one of a myriad of speculations of what dark matter might be.

No, my reason for posting is what happens when this stuff gets into the media. Of course, the speculation often gets firmed up in the press release. And then the discussion on blogs etc begin. Just check out SlashDot! The first comment is
"I hope so. Dark matter is the ugliest kludge to the standard model ever."
and so on and on. Apparently,
 "Average Slashdotters have been more skeptical of they dark matter theory than physicists, from what I've seen."
This is the kind of comment that really gets to me. No, what is actually happening is that average slashdotters like ideas with lost of mysterious sounding, but actually extremely skeptical, ideas, than the simplest working model.

 It's actually the opposite of Occam's razor, the wish to employ the most complex and whizz-bang over the straight-forward. Of course, this actually happens in science, when we have to let go of old ideas and adopt new ones, but things like quantum mechanics were not adopted because they were whacky and out-there, but because the weight of observational evidence crushed classical ideas.

Currently, all observations are consistent with dark matter being just that. When we have the observational evidence that does not agree with this simplest of hypotheses, then it will be discarded. And this will not happen until the alternatives can tell us precisely what tests we need to do to single out their theories as the best. Today is not the day that this is going to happen.

Anyway, looking over the comments, it's clear that many slashdotters actually understand this. But to the others, this is for you;


  1. I've always wondered why dark matter is seen as a kludge at all. That only makes sense if one believes that, by default, everything must be luminous. What it actually is is a different question.

  2. I just have to mention that the captcha for my last comment was "angsz".

  3. I just had a brief look at the paper. The author talks of particles and antiparticles gravitationally repelling each other rather than attracting each other. He is on leave at CERN. IIRC, at CERN an experiment was done to see if antimatter falls downward (it does) instead of upward.

    A PDF only paper? Reminds me of a business which accepts only cash (in advance). :-)

  4. >> That only makes sense if one believes that, by default, everything must be luminous.

    I gave a public talk last night - and I stressed this, but lots of people think the concept of dark matter is "illogical" or some such, but apparently the logic of vacuum polarized anti-matter makes sense. Makes me groan.

    >> "angz"

    Excellent (as I mentioned previously, I still have a copy of angsiz.for :)