There is Less “Tension” In The Expansion of the Universe (Astronomy / Cosmology)

A new method for measuring the Hubble constant presented by the  work of the international team led by Nandita Khetnadnan, PhD student at the Gran Sasso Science Institute and associated with INFN, published in Astronomy & Astrophysics.

How fast does the Universe expand? The question, which astronomers have been trying to answer for almost a century, continues to stimulate the research of cosmologists, and is the basis of the work of an international team led by Nandita Khetan.

The study is part of the ongoing debate on the value of the Hubble constant, the parameter that measures the current expansion rate of the Universe and which today represents the main anomaly between observations and cosmological models, otherwise in good agreement with each other. . According to distance measurements made in the local Universe – using type Ia supernovae as “standard candles”, that is, explosions of white dwarfs in double star systems – today’s cosmic expansion would occur significantly faster, by about 10 percent , compared to what was predicted by the evolution of the early Universe, obtained on the basis of observations of the cosmic background radiation. What lies behind this “tension”: a fascinating new physics or interesting systematic effects?

Measuring distances in astronomy is a complex problem that requires the combination of different techniques and observations to estimate the intrinsic brightness of the sources in question. In the case of supernovae Ia, the most used calibration method is based on observations of a certain type of variable stars, the Cepheids, in the same galaxies that host supernova explosions. The study, published today in the journal Astronomy & Astrophysics, proposes a new method to calibrate the distances of supernovae, using the surface brightness fluctuations (or SBF, from the English acronym surface brightness fluctuations) of the host galaxies.

“Using a sample of 24 supernovae, all of which exploded into galaxies whose distance is known from SBF measurements, to calibrate a larger cosmological sample of 96 supernovae, the team obtained a value for the Hubble constant of 70.5. km per second per megaparsec “- says Nandita Khetan -” This estimate is positioned halfway between the most recent value obtained from measurements of supernovae calibrated in a classical way with the Cepheids (73.2 km / s / Mpc) and the one based on observations of the cosmic background radiation of the Planck satellite (67.4 km / s / Mpc) “

Even if, considering the respective uncertainties, the new measurement is compatible with the two extreme values, this result highlights an important difference between the supernovae calibrated with the two different methods. This difference could derive from the environment in which the supernovae selected in the two samples are found, since the two types of calibration are based on observations made in different galaxies: the Cepheids, in fact, are found in spiral galaxies and not in elliptical galaxies. where the SBF method is generally used.

“The origin of this tension on the measurement of the Hubble constant could be due to the action of systematic errors: the intrinsic brightness of supernovae could be different for a different environment of the explosion or for a different progenitor” says Luca Izzo, researcher of the Dark Cosmology Center in Copenhagen.

The work demonstrates the effectiveness of using the SBF method to calibrate supernovae, opening promising new avenues for this type of research. In the coming years, the advent of tools such as the James Webb Space Telescope and the Vera Rubin Observatory will make this method a key tool for precision cosmology.

“This technique, not based on unpredictable events – such as the explosion of a supernova – or on long observation campaigns – as in the case of the study of Cepheid variables – will allow on the one hand the measurement of distances to very distant galaxies, where the Hubble flux it dominates the peculiar speeds, on the other hand it will allow us to measure distances for an incredible number of nearby galaxies and therefore increase the number of calibrators ”, observes Michele Cantiello, researcher at INAF and expert in the SBF method.

It should therefore not surprise us that studies of this type are, in perspective, particularly fruitful in results: “the detailed study of the galaxies that host supernovae will shed light on the possible presence of hidden systematic effects that could resolve the tension on the constant of Hubble, or reveal unexpected properties and important information on the astrophysical nature of what we now consider the best standard candles ”comments Marica Branchesi, professor at GSSI, associate of INFN and president of the INAF Scientific Council, supervisor of Nandita.
“By bringing this result back to its nominal value, we confirm the trend observed so far: a new measurement of the Hubble constant, based on calibrators observed in the ‘local’ universe, finds a value of H0 marginally higher than the value obtained by Planck. But it is certainly early to explain this effect with the intervention of a ‘new’ physics ”, concludes Massimo Della Valle of INAF.

The team involves scientists expert in supernova observations, SBF measurement and theory who work at the Gran Sasso Science Institute, the National Institute of Astrophysics, the National Institute of Nuclear Physics – Gran Sasso National Laboratories, DARK – Niels Bohr Institute of the University of Copenhagen, the Center for Astrophysics and Supercomputing of the University of Swinburne, the Las Cumbres Observatory and the University of California Santa Barbara and University of California Davis.

Reference: Nandita Khetan, Luca Izzo, Marica Branchesi et al., “A new measurement of the Hubble constant using Type Ia supernovae calibrated with surface brightness fluctuations”, A&A 647, A72 (2021). Article link

Provided by GSSI

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