Very recently, there was the fascinating news of new measurements being done by LIGO, in this case detecting the gravitational waves of two Neutron stars colliding. These detections prompted the astronomers to also look for the emitted electromagnetic waves of the aftermath, given that now they knew in which direction to look for them thanks to the GW measurements.
These discoveries make for really exciting times in physics, I personally get goosebumps when I read about them. Now admittedly, it is very difficult to grasp the physics that is going on here for someone not involved in the field, but it would be incredibly valuable if, briefly, some light could be shed on how these new discoveries relate to the problem of rate of expansion of the universe, potentially enabling physicists to further test the Hubble number. Such that one can have a rough idea at least at a conceptual level what the key idea behind this relation may be.
As a small preliminary question, in our current understanding, what are the main elements that play a role in the balance between the expansion and contraction of the universe? What one often hears, is that there's a strong interplay between how matter, EM radiation and potentially dark energy influence the rate of expansion of the univserse.
How does the recent detection of GW's connect to the study of expansion rate of the universe?
This is merely an attempt to get more input in order to understand some of the main ideas involved, as this is very exciting news given the long history behind these problems and predictions, and all the debates around them.