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How fast is the universe expanding?

The Hubble constant is a central quantity in cosmology: It describes how quickly the distances between galaxies in today's space increase. Researchers have now determined this expansion rate for the first time on the basis of detailed analyzes of colliding neutron stars in distant galaxies. The astrophysicists combined various measurements of gravitational waves, light, radio and X-rays with theoretical approaches from nuclear physics. As the scientists report in the journal “Science”, the behavior of the extremely dense matter inside neutron stars can be determined more precisely than ever before.

There are now several methods of determining the Hubble constant. But even if each of them provides quite precise results, they cannot be reconciled within the scope of their respective error limits. For example, measurements based on cosmic background radiation - i.e. the radiation echo of the Big Bang - yield a value around 68, while analyzes of supernovae yield a significantly higher value around 74. The numerical values ​​indicate by how many kilometers the distance between two objects that are 3.26 million light years apart increases per second. For years, celestial researchers have been trying to explain the discrepancy between the measurements in the young universe and in the local cosmos.

In another, very promising process, astronomers are trying to determine the Hubble constant from the brightness of kilonovae, i.e. from the explosive merging of two neutron stars. To do this, however, their actual brightness would have to be known - which in turn results from the behavior of the extremely dense matter inside the neutron stars. In principle, physicists describe such properties with the help of so-called equations of state, each of which depends on the volume, pressure and temperature of the matter. However, the equation of state of matter inside neutron stars was not yet exactly known.

But Tim Dietrich from the University of Potsdam and his colleagues have now succeeded in determining this equation of state more precisely than ever before. First, they collected different observations of two different neutron star collisions and of pulsars - that is, rapidly rotating neutron stars. They supplemented these analyzes with theories from nuclear physics and concluded from them the equation of state for matter inside neutron stars. The brightness of the radiation flash when neutron stars collided could then be calculated and the Hubble constant could be determined.

"I was genuinely surprised by the result," admits Dietrich. Because with a value of 66 it agrees well with the Hubble constant determined from the background radiation. “The neutron star collisions we used took place in the local cosmos, which is why we had expected a higher value.” However, it is still too early to discard other local measurement methods. "The errors in our process are still too big for that," says Dietrich. In order to get a more precise value, the researchers first have to apply the method to many more collisions of neutron stars. So the discrepancy between the various values ​​of the expansion rate cannot yet be explained.