====== Galaxy mass anomalies ======

The gravitational pull of the combined masses of all the component stars and planets in a spiral galaxy must fall-off away from the centre, simply because there are less of them than in the tightly packed centre. So, before the 1970s, it had been assumed that stars which were further away from the centre of the galaxy would be rotating (around the central axis of the galaxy) at much slower speeds than those towards the centre.

But, in the 1970s, astronomers Vera Rubin and Albert Bosma published observational data which showed that the rotation speed of stars in (many) spiral galaxies appears to be roughly constant - from the centre to the outermost stars, and well beyond their optical discs. See :[[content:physics:cosmology:galaxy_rotation]]

Those new observations, that the rotation speed was constant, led to the idea of [[content:physics:cosmology:dark_matter]] as a way of explaining the anomaly. By suggesting that 'missing mass' provided by the Dark Matter was apparently holding the galaxies together

With regard to our local galaxy - The Milky Way - new (2023) experimental data from the European Space Agency’s //Gaia// satellite appears to show that the outer-most stars are indeed //revolving more slowly around the centre.// Based on this, new calculations infer that the combined mass of the Milky Way is about four to five times less than had been suggested by several other earlier assessments. And so any 'Dark Matter' content would therefore also be considerably less than seems to be present in other galaxies.

There is currently no explanation for the observations.

Various possibilities have been suggested - including :

  * The Milky Way may in some way differ structurally from other spiral galaxies
  * For some reason there's only about half as much dark matter as in other similar galaxies
  * There are problems with the latest measurements
  * There are problems with previous measurements
  * Over cosmological times and distances gravity might not be constant

See : [[https://www.aanda.org/articles/aa/pdf/forth/aa47513-23.pdf|Detection of the Keplerian decline in the Milky Way rotation curve]] {{:oa_padlock_grn.png?16}}//Astronomy & Astrophysics// manuscript no. AA:2023:47513 September 21, 2023