A supernova explosion and space-time fluctuations generated by the merger of two neutron stars have helped scientists to accurately measure the expansion rate of the universe. Future measurements of this kind will help resolve the main paradox of cosmology, scientists say in the journal Nature Astronomy.
Back in 1929, the famous astronomer Edwin Hubble proved that our Universe does not stand still, but is gradually expanding. At the end of the last century, astrophysicists discovered, observing type I supernovae, that it expands not with a constant speed, but with acceleration. The reason for this today is considered to be "dark energy" - a mysterious substance that makes space-time stretch faster and faster.
In June 2016, Nobel laureate Adam Riess and his colleagues, who discovered this phenomenon, calculated the exact rate of expansion of the universe today using variable Cepheid stars in the Milky Way and neighboring galaxies, the distance to which can be calculated with ultra-high precision.
This refinement gave an extremely unexpected result - it turned out that two galaxies, separated by a distance of about 3 million light years, scatter at a speed of about 73 kilometers per second. This year, they published updated results of observations, in which this value became even higher - 74 kilometers per second.
The new measurements by Riesz and his colleagues turned out to be almost 10% higher than the data obtained using the WMAP and Planck orbiting telescopes - 69 kilometers per second, and it cannot be explained using our current ideas about the nature of dark energy and the mechanism of the birth of the Universe.
These discrepancies have led cosmologists to think about two possible ways to explain this anomaly. On the one hand, it is quite possible that the measurements by Planck or Riesz and his colleagues are erroneous or incomplete. On the other hand, it is quite admissible that a third "dark" substance, different from dark matter and energy, could exist in the early Universe, as well as that the latter could be unstable and gradually decay.
Kenta Hotokezaka of Princeton University (USA) and his colleagues made this problem even more acute and controversial by making the first relatively accurate measurements of the expansion rate of the Universe using the LIGO gravitational observatory and a number of "conventional" optical telescopes.
The first measurements of this kind, as the astrophysicist notes, scientists carried out at the end of 2017, when LIGO recorded a burst generated by the merger of two neutron stars, and hundreds of ground-based and space telescopes were able to localize its source in the galaxy NGC 4993 in the constellation Hydra.
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The first LIGO measurements were close to the data obtained by Riesz's team, which many scientists considered further evidence that the expansion rate of the universe could change markedly. Hotokezaka and his colleagues have found that this is not necessarily the case by tracking not only gravitational waves, but also the flash of light and the release of matter generated by this cataclysm.
In these observations, scientists were helped by the fact that this stream of incandescent plasma, a jet in the language of physicists, was directed not directly at the Earth, but somewhat away from it. Thanks to this, it seems to observers on our planet that it moves about four times faster than the speed of light, "violating" the theory of relativity, like a sunbeam or a shadow.
This property of emissions, coupled with measurements of the "thickness" of the jet at its starting point, make it possible to very accurately determine which direction it was directed in relation to the Earth and measure its speed. All these data, in turn, allow us to specify the distance to the source of gravitational waves and more accurately calculate how much they "stretched" during the travel from the galaxy NGC 4993 to Earth.
Such refinements, as Hotokezaka notes, brought a great surprise - the value of the Hubble constant became closer not to the measurements of Riesz and his colleagues, but to the results of Planck and other telescopes observing the microwave echo of the Big Bang.
On the one hand, this can really mean that the Nobel laureate and his colleagues are mistaken, but on the other hand, the accuracy of "gravitational" measurements is still noticeably lower - it is about 7% than that of those and other participants of this universal dispute (less than 2%). The current results, the scientist emphasizes, correspond to both theories, but the situation will change in the very near future.
According to the current estimates of the scientific teams of LIGO and its Italian "cousin" ViRGO, both gravitational observatories should find about ten such events a year. Accordingly, in the next 2-3 years, we can hope that observations of mergers of neutron stars will help us to unequivocally find out whether there is a "new physics" in the expansion of the Universe or not, the authors of the article conclude.
