What is the Bullet Cluster?
In the 1970s, the astronomer Vera Rubin was observing how the rotation of spiral galaxies changed as she looked along the radius, from the centre to the extreme edge.
With a bit of basic Newtonian mechanics — which is sufficient for studying galactic dynamics like this, without venturing into general relativity — the rotation speed is predicted to decrease as you move further from the centre. Yet, instead, Rubin found that the speed remained constant towards the outskirts of the galaxy.
This meant one of two things: either the laws of gravity need to be modified or there is far more mass around galaxies than we expect. If it’s the latter, it has to be completely invisible and only interact with gravity.
This invisible matter came to be known as dark matter.
Yet, the case for a modified Newtonian gravity (such as Modified Newtonian Dynamics, MOND) still remains, with proponents arguing that we haven’t actually ever observed dark matter, nor do we have any idea what it is.
This is a rather considerable shortfall of what is otherwise an incredibly successful theory. Whilst the mainstream view of cosmology is a universe filled with…
