The expansion rate of the universe is estimated using a quasar
The Hubble constant is not a fundamental constant, it is fairly accurately known about it that it changes with time. However, at the current time, it is the same for the entire Universe and, in this sense, is constant.
The value of the constant has been estimated in recent decades repeatedly using various techniques. In general, estimates range from 67 to 73 (km / s) / Mpc.
A group of astronomers from the USA, Switzerland and the UK tried to estimate the Hubble constant from observations of quasars, the image of which, due to gravitational lensing, is split into two. Recall that quasars are objects that have a very large brightness and are located very far away. Another characteristic of them is the periodic fluctuations in brightness - its brightness changes dozens of times.
If the light of a quasar passing through its obscuring galaxy is split into two streams, then both the images received by the earth observer do not flicker simultaneously, but with a certain time interval, depending on which of the light paths was shorter and how much. The time delay between these two scintillations, along with information about the gravitational field of the interfering galaxy, can be used to track the path of light and determine the distances from the Earth to the quasar and the foreground galaxy. Knowing the redshifts of the quasar and galaxy allowed scientists to evaluate how quickly the universe is expanding.
Quasar SDSS J1206 4332 acted as an “experimental rabbit” for scientists. Observations of it made it possible to determine the Hubble constant as 72.5 (km / s) / Mpc.
It should be noted that the constant assessment systematically faces one problem, which scientists cannot solve or at least explain. Observations of any astronomical objects, such as quasars, supernovae, and galaxies give values of about 72–73. At the same time, observations of CMB lead to the conclusion that the Hubble constant is noticeably lower - about 66 (km / s) / Mpc. The reasons for the discrepancy are not yet clear.
You can get acquainted with the details in an article by scientists published in the Monthly Notices of the Royal Astronomical Society.