The mystery of the expansion rate of the universe

In astronomy there are two measurements that indicate the expansion of the universe, known as the “Hubble constant”. One is calculated from observations of supernovae and the second from the “cosmic microwave background,” that is, the radiation that began to flow freely through the universe shortly after the Big Bang. However, these two measurements differ from each other by approximately 10%, which has sparked a wide debate in the community of physicists and astronomers. If both measurements are accurate, it would mean that the current theory of experts on the composition of the universe is incomplete.

Thanks to the data obtained from a supernova enlarged in four images and through the use of a pioneering technique, some scientists have measured the expansion speed of the universe and the result reopens an old debate that could help the scientific community to determine with greater precision the age of the universe and to know the true rate at which the universe is expanding.

The work, divided into two studies, has been carried out by a team led by researchers from the University of Minnesota in the United States, with the participation of José María Diego, a researcher at the Institute of Physics of Cantabria (IFCA), a joint center of the Superior Council of Scientific Research (CSIC) and the University of Cantabria (UC), all these entities in Spain. In total, scientists from 28 institutions spread over four continents (America, Europe, Asia, and Oceania) have participated.

“If new independent measurements confirm this disagreement between the two measurements of the Hubble constant, it would become a chink in the armor of our understanding of the cosmos,” says Patrick Kelly, lead author of both studies and a professor in the School of Physics and Astronomy from the University of Minnesota. “The big question is whether there is a problem with one of the measurements or with both. Our research addresses this using a completely different and independent way of measuring the expansion rate of the universe,” he adds.

The team has been able to calculate this value using data from a supernova discovered by Kelly in 2014: this is the first case of a multi-imaged supernova, meaning the telescope captured four different images of the same cosmic event. In the current work, the IFCA researcher has modeled one of the types of lenses used in a previous project (WSLAP+). “A couple of years ago we carried out a very similar previous work, where Jesús Vega-Ferrero, a former colleague from IFCA, was the main author”, explains Diego.

These four images of the supernova have been obtained because the light from the supernova was gravitationally attracted to a cluster of galaxies, a phenomenon in which the mass of the cluster bends and amplifies the light from the observed source. Thanks to the time elapsed between the appearance of the 2014 and 2015 images, the researchers were able to measure the Hubble constant using a theory developed in 1964 by the Norwegian astronomer Sjur Refsdal, which until then had been impossible to put into practice.

Image of the galaxy cluster Abell 370. (Photo: ESA/Hubble/NASA)

“The researchers’ findings don’t settle the debate,” Kelly says, but they do provide more insight into the problem and move physicists closer to obtaining the most accurate measure of the age of the universe. “Our measurement favors the value of the cosmic microwave background, although it cannot exclude the value obtained with closer supernovae,” she clarifies. “If observations of future supernovae that are also gravitationally bound by clusters yield a similar result, then it would identify a problem with the current value of supernovae, or with our understanding of dark matter in galaxy clusters.”

One of the studies has been published in the academic journal Science. The other, in the academic journal The Astrophysical Journal. (Source: Rebeca García / IFCA / CSIC)