Dark matter search narrows

Fornax dwarf galaxy

This faint smattering of stars is actually a small galaxy. Scientists have been unable to spot evidence of certain kinds of dark matter particles within this galaxy and nine others.

STUDIES DECADES AGO OF THE ROTATION of galaxies, and of the movement of groups of galaxies, led scientists to conclude that the universe contained more matter than could be detected in the normal ways.

Being unseen at visible wavelengths, and with its nature unknown, the putative matter was dubbed “dark matter“, and according to popular models it comprises over 80 per cent of all the matter in the universe.

In the early years of investigation into this strange phenomenon, two broad candidates emerged—MACHOs and WIMPS.

MACHOs were hypothetical “massive compact halo objects”, ie. large bodies such as dim stars, black holes or large free-floating planets that would inhabit the outer or “halo” regions of a galaxy. WIMPs are hypothetical “weakly interacting massive particles”, ie. sub-atomic particles that could pervade space but not interact much with normal forms of matter.

Artist's impression of NASA’s Fermi Gamma-ray Space Telescope

The research used two-years of data collected by NASA’s Fermi Gamma-ray Space Telescope (artist's impression).

Research programmes failed to find evidence of MACHOs, so dark matter investigations now focus on WIMPs.

WIMPs could take many forms—perhaps as one or more of the familiar particles, such as neutrinos, or maybe as-yet-unknown particles.

In new research using two-years of data from NASA’s Fermi Gamma-ray Space Telescope, a team that includes astrophysicist Jennifer Siegal-Gaskins (Caltech) has been able to rule out certain kinds of WIMPs.

According to some models, when two WIMPs collide, they can annihilate each other and produce a burst of gamma rays with specific wavelengths. Such energy bursts would be detectable with Fermi.

The scientists studied 10 small galaxies that circle our Milky Way galaxy, looking for telltale gamma ray signs of WIMP collisions within them. They didn’t spot any.

This negative result will help scientists by eliminating particular kinds of WIMPs from the field of candidates, and will enable them to focus on searches for other kinds.

More information: New Insights on Dark Matter

Story by Jonathan Nally. Images courtesy NASA / Sonoma State University / Aurore Simonnet / ESO / Digital Sky Survey 2.

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