The dark secret

Posted on January 13, 2012

Dark Matter MapStep by slow step we may be getting closer to an answer, but still we don’t really have much of an idea what 83% of the universe is. We may be getting more and more data about the 17% or so we can see and measure, but dark matter remains elusive.Finally though, it does seem to be giving up a few of its secrets even if we really can’t pin down what it is and how it might work.

The gravity map is superimposed on a Hubble im...

What we know – or think we know – about dark matter has been fairly simple. It has mass, much like ordinary matter since the original “discovery” of dak matter was simply because we needed some form of mass to make our current theories of gravitation explain why the stars at the far boundaries of galaxies did not fly off into intergalactic space. Enter dark matter – even though it could not be seen – and we can magically keep the stars withing the galactic whirlpool of stars.

Since that first description of dark matter as some form of mass we can neither see nor detect directlybut can measure by its effect on large collections of stars we have gradually pinned down some probable characteristics it must have – if, of course, it really exists at all.

One is that if it is actual particles then they will eventually hit each other and if they do then they could decay along a particular path which would produce gamma rays. Now, two analyses of the gamma rays in space have pinned down a minimum mass for the dark matter particles involved: they must be at least 44 times the mass of a proton. Savvas Koushiappas, part of one of the two tems involved in the analyses and based at Brown University of Providence, Rhode Island says that, “This is the strongest limit to the mass that we have so far.”

Actual gravitational lensing effects as observ...Unfortunately, as pointed out by Dan Hooper of Fermilab, these results appear to contradict earlier results that suggested dark matter particles have less than a quarter of that mass. In simple terms, the new results presume a certain path to result in the gamma rays that were analysed, but the actual path could be a different one and neither the new results nor the older one help in determining which path is corect. As hooper said, “They are ruling out certain kinds of particles – but the are not ruling out mine.”

Even the newer maps of dark matter, drawn by observing the effects of gravitational lensing of distant galaxies to produce dark matter maps on a scale of billions of light years does little to clear the confusion and just produces what we knew in the first place: something out there seems to have mass that we still cannot see.

Perhaps, for now, that will just have to remain all we can say for sure about this mysterious stuff that makes up almost verything there is out there.