Cosmological theory says that the relationship between the mass of a galaxy cluster and its age is a test of the value of the density parameter of the universe. The density parameter is, in turn, a measure of just how much normal matter, dark matter, and dark energy there are. Because the mass of a cluster is difficult to measure directly, astronomers have to infer it from computer models, which tell them how the temperature of the gas in a cluster depends on that cluster’s mass.
Even measuring the temperature of a cluster is difficult, though. What is easy to measure is its luminosity. That should be enough, since luminosity and temperature are related. All one needs to know are the details of the relationship; and, by measuring luminosity, one can backtrack to temperature and then to mass.
That has been done for nearby clusters, but not for distant ones which, because of the time light has taken to travel from them to Earth, provide a snapshot of earlier times. Thus, Dr. Vauclair and his colleagues used XMM-Newton, a European X-ray-observation satellite that was launched in 1999, to measure the X-ray luminosities and the temperatures of eight distant clusters of galaxies. They then compared the results with those from closer (and therefore apparently older) clusters.
The result was that the relationship between mass and age did not match the predictions of conventional theory. It did, however, match an alternative model with a much higher density of ordinary matter in it.
