The Universe's Dark Secret

Fresh hint of dark matter seen in neutrino search - space - 25 February 2014 - New Scientist

Dark Matter is something of a problem for science because it suggests our understanding of the Universe is incomplete. Not just incomplete in the detail, but incomplete at a fundamental level. The problem is, we devised our models, such as the model of fundamental particles, to explain what we could see.

This is not to say we are on the wrong track completely - the discovery of the Higgs Boson which essentially completed the theoretical collection of fundamental particles suggests quite the opposite - but that the track is much longer than we expected.

In some ways it's analogous to the state we were in when Einstein discovered the relationship between matter and energy and showed the velocity of light in a vacuum to be a universal constant independent of the velocity of an observer, and so discovered Relativity. Like just about everyone else at that time, Einstein's conceptual model of the Universe was that it consisted of the Milky Way galaxy and very little else, and that it was static - neither expanding nor shrinking.

Having realised his formula predicted that the Universe should be expanding, Einstein then made what he later called 'the biggest mistake of my life' and included a 'cosmological constant' for no other reason but to remove the expansion which he assumed must be an error. In doing so, he not only failed to be the first to predict that the Universe was expanding but also to make the logical deduction that it must therefore once have been very small, and so to discover the very small silent event mis-named the Big Bang.

The problem with dark matter is that it isn't a small problem. Some theoreticians have calculated that it, together with dark energy which is thought to be related to it, comprises some 95% of the Universe, meaning we only really have much idea about 5% of it.

In several ways, dark matter is paradoxical to our understanding. For example, it is generally accepted that dark matter consists of weakly interactive massive particles (WIMPs) which basically means they have mass (and therefore gravity) but barely interact with the particles we know about. This means they are difficult to detect because we detect things by observing how they interact with other things. This in turn means we would expect not to be able to find much evidence for it, and that, apart from one thing, comes perilously close to arguing that absence of evidence is not only not evidence of absence but confirmation of presence.

The one thing, of course, is the reason we know about it in the first place. We can detect it's effect on other things because it has mass and so exerts a gravitational effect. In fact, it was that which led us to suspect there was a lot of dark matter about. Rotating galaxies rotate at speeds which should make them fly apart according to basic Relativity and Newtonian Laws of Motion, yet we see they don't. Therefore there must be more mass present than we can see. In fact, it's the presence of so much dark matter that causes the rate of rotation in the first place to conserve angular momentum.

The term WIMPs is merely descriptive of what we are fairly sure particles of dark matter will be like. It doesn't tell us what they are nor how they relate to other particles so we don't know what it is that has this mass and in the absence of evidence, all we have is theory and hypotheses - rather like where we were with Higgs Boson.

We tested every single scenario we could come up with and eliminated things very carefully. The upshot is we just don't know what this is. The most exciting explanation is the decay signature of sterile neutrinos.

Esra Bulbul, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA

But now we may have moved a little closer to understanding what this dark matter is, or rather confirming one of the theories about what it's made of. Two teams of researchers acting independently have both detected bursts of x-rays being emitted from crowded clusters of galaxies and their energies appear to be exactly what we would expect of a hypothetical 'sterile neutrino' decaying into x-ray photons and other neutrinos, and these 'sterile neutrinos' are one suspected culprit for the building blocks of dark matter.

Neutrinos are notoriously incapable of interacting with 'normal' matter, hence the difficulty in detecting them. Billions of neutrinos come sleeting through the cosmos to pass right through your body and right through Earth every second, yet leave no trace at all, so their form make them highly suspect as the WIMPs of dark matter. The problem is that the three neutrinos we know about don't have enough mass, hence the hypothecated 'sterile' type which is not only even less interactive that the others but is much more massive. (Incidentally, don't confuse 'massive' with volume; it relates more to weight).

It's intriguing. There's a consistent picture for it being dark matter, but I think confirming it would really require deeper observations of other things.

Kevork Abazajian, University of California, Irvine, USA

The teams were Esra Bulbul of Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA and her colleagues, using observations of 73 galaxy clusters from the European Space Agency's XMM-Newton space telescope and NASA's Chandra X-ray telescope, and Alexey Boyarsky of Leiden University in the Netherlands and colleagues using XMM-Newton observations of the Perseus galaxy cluster and the Andromeda galaxy.

For a contrast with how science copes with something like dark matter and how religion copes with new information which upsets its established dogma, see The Dark Matter of Gods. Imagine, for example, authenticated documentary evidence being found that the 'Gospels' were the work of committees complete with evidence of editing and embellishment, and marginal notes such as "Will people believe this?!!!", so destroying the standard Christian theological model of how the New Testament was written and how reliable it is as history. How would established churches incorporate this into their understanding and teaching? My guess is it would be ignored completely.

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