Gamma-ray bursts could help understand the nature of dark energy

The speed of the expansion of cosmos observable has been accelerating for a few billion years as if it contained a pressure resulting from an energy density exotic which is called thedark energy and which does not dilute with expansion, unlike the matter. The discovery of this accelerated expansion, while the standard cosmological model preceding the end of the 1990s predicted the opposite after the big Bangwas made possible by using phenomena astrophysics that could largely be considered as standard candles, in particular the supernovae SN Ia.

Determining the nature of dark energy depends on determining the fate ofUniverse observable. But for that, it is necessary to increase the detection of supernovae NS Ia or other cosmic events to calculate distances and spectral shifts. It turns out that an international team ofastrophysicists conducted by Maria Dainotti, assistant professor at the National Astronomical Observatory of Japan (Naoj), has just announced via an article on arXivthat we could use for that some jerks gamma. In fact, this is the confirmation more solid results achieved over many years.

To understand what it’s all about, let’s go back to what Futura explained about standard candles and how they build on each other to make up what is called the cosmic distance scale.

The problem of the scale of cosmic distances

Here are some reminders giving additional explanations regarding the video below, which illustrate what is explained below.

The Cepheids are variable stars individuals who see their brightness change over time with a given period. It was in 1912, while studying the Cepheids of the Small Magellanic Cloud, that Henrietta Leavitt discovered that this period was correlated with their apparent magnitude mean. The brighter they are, the slower they vary. One could therefore hope to deduce the intrinsic luminosity of a Cepheid by measuring its period.

The method was calibrated thanks to the nearby Cepheids whose distance could be evaluated by the parallax method which makes it possible to estimate the distances of the closest stars in the Milky Way. It was therefore possible to deduce the distance of more distant Cepheids directly from their rate of brightness variation. Indeed, the further a star is, the less luminous it appears, but if we know its intrinsic luminosity, we can estimate its distance.

The Cepheids have thus become a sort of standard candle making it possible to assess the distance separating the Milky Way from the galaxiesagain the closest, such as Andromeda or the Large Magellanic Cloud. Edwin Hubble used Henrietta Leavitt’s relationship, first to discover the expansion of the Universe and then to calibrate the Hubble-Lemaître law relating the distance of a galaxy with its spectral shift.

To measure even further distances in the distant Universe, it is possible to use another type of star, these are not exactly standard candles but they can serve as good distance indicators. These are the SN Ia supernovae.

These stars result from the explosion of white dwarfs in a binary system. The luminosity of an SN Ia cannot deviate much from a certain average value, and since it can represent that of hundreds of billions of stars, they can be seen from afar. By using the Cepheids, we can calibrate a relationship giving the apparent luminosity of an SN Ia with its distance. Knowing its redshift, we can then relate its distance to this shift and, by the Hubble-Lemaître law, to the speed of expansion of the Universe at a given date in its history — since observing far , is to observe early.

It is by erecting the curve linking the spectral shift of SN Ia with their luminosity apparent that Riess, Perlmutter and their colleagues discovered theaccelerated expansion of the Universe in 1998 and 1999.

We could do the same with the GRBswhich can help in particular to solve the current problem with measurement of the Hubble-Lemaître constant.

Gamma-ray bursts as standard candles

Let’s now return to the work of Maria Dainotti and her 23 colleagues. The discovery of gamma-ray bursts decades ago came as a surprise because they were obviously extremely energetic events to produce the bursts of gamma energy initially detected by military satellites in space tasked with monitoring illegal weapons tests. nuclearair free or in space.

To account for the intensity and energies of gamma radiation observed with the Gamma Ray Burt (GRB) as we say in English, two classes of processes have been proposed. One involves collisions ofneutron stars and we know that this scenario explains certain GRBs well since we demonstrated it a few years ago by combining the detection ofgravitational waves with that of the electromagnetic waves emitted by a kilonova.

The other class involves the explosion of very massive stars which gravitationally collapse giving a black hole. However, now Maria Dainotti and her peers think they have highlighted a second division with the GRBs by consulting the data recorded since the 1990s both with ground-based telescopes operating in the visible, such as the Subaru in Hawaii, and with space telescopes operating in both the gamma and X domains, notably with the Neil Gehrels satellite Swift Observatory.

Finally, it was the observations in the visible of 500 GRB which were combed through and it turned out that the curves of light of 179 of them had characteristics allowing it to be used as a standard candle. As GRBs are particularly luminous, they can be seen from afar, which makes it possible to probe the evolution of the rate of expansion of the observable Universe up to more than 11 billion years ago in the past.