Our galaxy may have slightly less dark matter than previously estimated. Measurements of how pulsars 鈥� rapidly spinning stars that emit beams of light 鈥� move as they orbit the centre of the Milky Way have demonstrated that the dark matter density in the plane of the galaxy鈥檚 disc is lower than expected.
Most estimates of the galaxy鈥檚 dark matter abundance are based on the assumption that the Milky Way is in equilibrium, meaning its overall structure isn鈥檛 significantly changing. However, that isn鈥檛 necessarily a safe assumption: several recent studies have shown that past interactions with other galaxies are still affecting the Milky Way鈥檚 structure, stretching and warping it.
Sukanya Chakrabarti at the Institute for Advanced Study in New Jersey and her colleagues used measurements of 14 pairs of pulsars within about 3000 light years of the sun to make the first direct calculation of the average acceleration of stars in our galaxy.
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That acceleration is caused by the gravitational pull of matter of all types. We have a good idea of how much regular matter is in the galaxy, and knowing this, the researchers can calculate the amount of dark matter pulling on the stars, and hence its density.
鈥淭his [acceleration] is a very, very small number that we鈥檙e trying to measure,鈥� said Chakrabarti, who presented the work at a virtual meeting of the American Astronomical Society on 11 January. 鈥淚n terms of the change in velocity, it is a few centimetres per second, so that鈥檚 roughly the speed of a crawling baby 鈥� and not a very fast baby at that.鈥�
The dark matter density they found is very slightly below previous measurements 鈥� on the order of a few ten-billionths of a gram per cubic kilometre 鈥� but it could help us understand how the Milky Way has evolved over time, and probe how dark matter is distributed throughout the galaxy.
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