The stars within the Milky Way are all moving, some astonishingly fast, but they change their speeds very slowly. Those changes reveal the forces acting upon them. Astronomers have measured the accelerations of nearby pulsars, creating a resource that can be used to explore the distribution – and possibly nature – of dark matter.
In a galaxy far, far away from any others, stars might orbit the galactic center in a perfect plane. The Milky Way's reality is messier. Past mergers with smaller galaxies and interactions with those that retain their independence have produced local hotspots of dark matter, causing stars to bob up and down below the galactic plane for part of their orbit, like a near-drowning swimmer in a whirlpool. One highly disputed theory holds that the Sun's passage through the plane every 27 million years causes mass extinctions.
Measurements of these movements, and therefore the gravitational force that drives them, could reveal not only the average density of dark matter in the plane, but also local clumping indicating the presence of former galaxies.
Astronomers have used main sequence stars' movements to calculate the average mass density of the plane since 1932, but their results have been drastically different depending on which star type they use. When the movement of G-type stars (like the Sun) gives a value almost ten times lower than bright A-type stars, something else is needed. Moreover, previous measures relied on the assumption the galaxy is in equilibrium when we know it is in fact always changing.
Dr Sukanya Chakrabarti of Rochester Institute of Technology used pulsars instead, measuring changes in the velocities of 14 pulsars with precisely known orbital periods. By eliminating the influences of things we can see, such as companion stars, the forces that explain the remaining acceleration must be things we can't see, particularly dark matter.
"Our analysis not only gives us the first measurement of the tiny accelerations experienced by stars in the galaxy, but also opens up the possibility of extending this work to understand the nature of dark matter, and ultimately dark energy on larger scales," Chakrabarti said in a statement.
The accelerations are astonishingly small in comparison to stellar speeds because the forces these stars experience almost entirely cancel out. The team measured changes in speed of just a few centimeters per second, which they note is the speed a baby crawls.
In the Astrophysical Journal Letters (preprint on ArXiv.org) Chakrabarti and co-authors report a value much closer to that previously measured for G-type stars. More importantly, they find hints of lumpiness. If that can be confirmed by larger studies, the authors hope it will assist the process of “galactic archeology”, helping us understand the galaxies the Milky Way swallowed. Better still, the more we know about dark matter's distribution, the greater our chance of finding some – or at least working out what it is.
In related work published in the same edition of the journal, Dr Joseph Simon of the University of Colorado Boulder used observations of pulsar movements to look for low-frequency gravitational waves.