Michael Brooks

Here comes the science bit

Does dark matter exist?

After 80 years of agreement about the dark stuff, opinions may be changing.

By Michael Brooks Published 15 May 2012 11:41

Images of giant galaxy clusters, said to be mainly made up of dark matter. Photograph: Nasa/Getty Images

The peasants are revolting. Last night the Flamsteed Astronomical Society met at the National Maritime Museum to hear a debate on the existence (or not) of dark matter. In a vote at the end, the audience decided it probably doesn’t exist.

The idea of dark matter has been around since 1933, when a Swiss astronomer called Fritz Zwicky found that centrifugal forces should have been tearing spinning galaxy clusters apart – but weren’t. The answer, he suggested, was that there was extra stuff in there, whose gravitational pull was holding everything together.

Astronomers now believe this stuff makes up around a quarter of the universe, if you take into account all the mass and energy in the cosmos. Ignore the pure energy, and dark matter accounts for 80 per cent of the universe’s mass. Which makes it a little embarrassing that we have never seen any.

Neither do we know what it looks like. We’ve been groping around for dark matter since about 1970. Various predictions have been made: in 1980, astronomer Vera Rubin said it would be found within 10 years. In 1990, astronomer royal Martin Rees said the dark matter mystery would be solved by the turn of the century. In 1999 Rees was aware he had been too hasty, and said we would know what dark matter is by 2004. Last January, CERN theoretical physicist and Gandalf lookalike John Ellis gave the physicists another decade.

But patience is starting to wear thin. At last night’s debate, Oxford physicist and co-presenter of The Sky at Night Chris Lintott made the case for dark matter; astronomy writer Stuart Clark argued that a modification to the laws of gravity, which are dictated by Einstein’s general relativity theory, held more promise for explaining the (apparently) missing mass. At the end of the evening, the audience sided with Clark and modifying gravity.

That’s not going to have dark matter astronomers quaking in their boots. But it is nonetheless indicative of a change of mood. Take what went on at the Cosmic Variance blog last week. Sean Carroll, the blog’s host, has always been bullishly pro dark matter. But it seems he has started to hedge a bit.

In a fascinating post, he published the trialogue he had been conducting with astronomer Stacey McGaugh, the original proponent of the modified gravity idea (it’s called MOND: modified Newtonian Dynamics) and German astrophysicist Rainer Plaga. Right at the top, Carroll concedes that “it may very well turn out that the behavior of gravity on large scales does not precisely match the prediction of ordinary general relativity”. In other words, he is saying, we might well have to modify gravity.

It’s worth pointing out a couple more reasons it’s OK to harbour doubts about the dark stuff. Last September, Durham astronomer Carlos Frenk admitted he was “losing sleep” over the results of his own computer simulations. His work had showed that the way simulated dwarf galaxies – mainly composed of dark matter – form in a halo around our own galaxy doesn’t tally with what we observe. His conclusion was that the standard theory of dark matter is almost certainly wrong, adding that searches for the stuff at the LHC in Geneva would therefore prove fruitless.

Then last month two groups of astronomers announced that dark matter wasn’t where it should be. The sun is meant to be surrounded by a halo of dark matter, and it isn’t.

If there really is no dark matter, that won’t be a mainstream view for decades to come. Once it’s got some momentum, it takes a lot of effort to change direction in science. But it does seem that, after 80 years, someone’s found the handbrake on the dark matter juggernaut.

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18 comments

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Wed, 2012-06-20 11:29 — steel pipe (not verified)

|steel pipe|e data, even with gravity modifications.
sIt's a bit like finding a patient with a clear stab wound. You might ars

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Sat, 2012-06-02 17:54 — srh2174 (not verified)

Dark matter is just a theory. We will never be able to prove or disprove its existence. Kind of like man made global warming.

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Sat, 2012-05-26 11:04 — thinkingloud (not verified)

Many astronomers were feeling uneasy about the Moni Bidin et al. claim (http://adsabs.harvard.edu/abs/2012ApJ...751...30M) that there is no dark matter near the Sun, and it is good to see these being rectified by Bovy & Tremaine (http://adsabs.harvard.edu/abs/2012arXiv1205.4033B). The Moni Bidin et al. result would have ruled out MOND as much as dark matter if it had been correct.

The MOND _prediction_ for a dark matter effect near the Sun is very close to what Bovy & Tremaine compute by adopting Newtonian dynamics. That is, rather than getting dark matter back, it is not there, and all one is seeing is the MOND "phantom" dark matter. Phantom dark matter is the "dark matter" a Newtonian observer "sees" in a galaxy which obeys MOND. Phantom dark matter is completely different though from the cold or warm dark matter which the standard cosmological model needs.

Given other recent claims about the non existence of dark matter, this MOND prediction is seen in Fig.12 of this paper: http://adsabs.harvard.edu/abs/2012arXiv1204.2546K (see the description on SciLogs: http://www.scilogs.eu/en/blog/the-dark-matter-crisis/2012-04-19/dark-mat...). This means that the true (MONDian) rotational velocity must be larger than the one derived just from the normal matter by about 20 per cent.

A recent (robust) claim that dark matter is missing significantly in the Local Universe can be found here: http://arxiv.org/abs/1204.3377. This appears to be a robust result as it is simply based on adding up the galaxies there are. Further, that the standard model of cosmology fails on massive galaxy clusters is shown here rather convincingly:
http://arxiv.org/abs/1205.3788 - citing: "For standard LCDM structure formation and observed background field galaxy counts this lens system should not exist."

Sat, 2012-05-26 01:38 — Donx2014 (not verified)

I prefer to believe there is dark matter exist...

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Wed, 2012-05-23 17:04 — charlesfrith

LHC - Waste of money while science ignores serious problems on planet earth with real humans.

Wed, 2012-05-16 23:43 — Tevong (not verified)

I believe what Sean Carroll was writing was that no matter what other modification may or may not exist, the evidence is clear that something with the properties of dark matter must exist. This doesn't rule out other possibilities, but there must be some form of non-baryonic matter to account for the data, even with gravity modifications.

It's a bit like finding a patient with a clear stab wound. You might argue that he died from blunt trauma to the head and find evidence for this which is reasonable, but the evidence still requires a knife whether or not it's the only cause of death or just part of the explanation.

The problem is when people read "dark matter must exist" in a polemic way and assume it's attacking their own personal pet theory as being false. This isn't football, one team doesn't "win" and the other "loses". There's data and matching with theoretical predictions, with varying degrees of confidence and plausibility which only more data can disentangle.

Wed, 2012-05-16 23:43 — Tevong (not verified)