Darker Matter: Observation Reveals Anomalous Dark Matter Distributions, Spells Trouble for Cosmologists

The mystery deepens. Scientists have found distribution of dark matter in dwarf galaxies, which is completely anomalous to the current theory of cold dark matter distribution in the Universe. The AMS observation from aboard the ISS has shown that dark matter is spread uniformly throughout the dwarf galaxy, whereas one would expect it to clump around the centre and then thin around towards the edges. This pattern, based on the prediction of the cold dark matter model, is seen in bigger galaxies like our own. Dwarf galaxies provide an exception and no one knows why.

A web of dark matter throughout the Universe?

What really is Dark Matter?

Dark matter refers to the attractive positive density matter that supposedly makes up as much as 23%-26% of the Universe, as compared to the 4% of visible matter (all stars, planets, galaxies everything that we know about!). The rest is dark energy and it is the energy of the vacuum, tending to tear to Universe apart making it accelerate its expansion. Dark matter was proposed by Fritz Zwicky and was initially invoked to explain the high speed of the outer spirals of a galaxy. They were spiraling too fast to be held by gravity of the visible matter. An invisible matter, interacting only through gravity had to be present. Gradually, it was recognized that dark matter could explain other things as well, like the observed cosmic microwave background radiation, unexplained gravitational lensing etc. Matter is believed to reside in the space-time trough created by lumps of dark matter.

It is to explain the motion of such galaxies that dark matter was hypothesized in the first place

The most successful version of dark matter has been the cold dark matter’ model, which says that the dark matter particles, whatever they may be, are moving very slowly with respect to light.

The Study and an Anomaly

The study involved observing the Fornax and Sculptor galaxies, which orbit the Milky Way. These are dwarf galaxies and are thought to be primarily made up of dark matter. What was expected was that the center would be rich in dark matter and then the distribution would thin out towards the edges. What was seen instead, was a uniform distribution, confounding scientists to no end. Matt Walker, the leader for this study conducted by the Harvard-Smithsonian Center for Astrophysics, says

Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data.

Dark matter distributions can be inferred from the motion of stars. The presence of matter curves space and matter particles follow curves on this curved space, called geodesics. The geodesics would be significantly modified by the presence of dark matter. This gives an estimate to the dark matter present. The team investigated the motion of about 2000 stars and found this anomaly.

Matt Walker says

After completing this study, we know less about dark matter than we did before

Well, nature has never been kind, has it?

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.