CrazyNeutrino said:
but when they look at the stars and galaxies they can only account for about half this ordinary matter.
They concluded the “missing” half must be made up of individual atoms floating in the spaces between galaxies.
The burst of energy involved is known as a fast radio burst (FRB), the result of an unknown cosmic event that creates an intense radio beam which lasts a few thousandths of a second.
They’ve only ever found sixteen, and until recently they’d never tracked down the source of any of them.
The source was a quiet old galaxy located around 6 billion light years away, and the FRB contained a detailed record of all the particles of matter it had encountered on its journey.
Armed with the distance the burst had travelled, and the amount of matter encountered on the journey, scientists extrapolated to find the amount of ordinary matter in the empty spaces of the entire universe, and it agreed with current models.
A few comments.
1. Quasars have been used to do this for many years. Perhaps there’s some advantage to using a
FRB rather than a quasar, but I don’t know what it is.
2. If the advantage of an
FRB over a quasar is that an has a narrow bandwidth, then that could help to explain previously unexplained events detected by
SETI at home.
3. I can see why finding the source is extremely important. The source gives the distance and the signal gives the absorption, so together that gives the density of the absorbing matter. With the quasar method that didn’t matter so much because the redshift and hence distance of the absorbing matter can be determined directly from the spectrum.