I just noticed an interesting item on the ESA website about results described in a paper by Jiao et al. (on the arXiv here) relating to the rotation of the Milky Way as determined by Gaia’s Data Release 3.
The key result in the paper is summarized in this diagram:
A galaxy rotation curve like this is a diagnostic of the radial distribution of mass. If all the mass were concentrated at the centre, the galaxy would behave like the Solar System (in which most of the mass is contained within the Sun). In such a Keplerian profile the rotation speed falls off with distance, just as the outer planets move more slowly in their orbits than the inner ones. According to modern cosmology, however, there is dark matter not concentrated in the centre, in which case the rotation curve does not decline with distance and may even rise. According to theory, at large distances, the rotation curve of a spiral galaxy should be roughly flat.
The new results seem to contract this notion. The Figure shows a rotation curve that declines for distances about 15 kpc from the Galactic Centre; for reference the Sun orbits at a radius of about 10 kpc.
One of the problems in constructing a rotation curve of our own Galaxy is that we are inside it so it isn’t possible to make measurements across the entire system like we can with other galaxies. Using the Gaia measurements and a plausible model, however, the authors find much less dark matter than anticipated.
With a bit of extrapolation using a model, this measurement leeds to a reduction in the estimated total mass of the Milky Way. The value usually bandied about is around 1012 Solar Masses, while the new measurements imply a much lower mass of about 2 × 1011 Solar Masses.
A factor of five reduction is quite a dramatic change and I’m sure this result will be challenged by those of an orthodox persuasion while also providing encouragement to dark matter sceptics. We’ll just have to wait and see how this pans out.
In the Dark 