The story begins with a question. “So what are these three points that weren’t there before? » Before that, it was 2015. When the Hubble Space Telescope looked toward a cluster of galaxies in the constellation Ursa Major called PLCK G165.7+67.0 (G165, for short). The three points are luminous objects revealed in an image of the same region recently returned by the James-Webb space telescope.
The answer, the astronomersastronomers found it. These three points correspond to a supernova. Three points for a single supernova? Absolutely. Because between the supernova and us, there is a cluster of galaxies. And, by gravitational lens effect, its mass curves the lightlight of the supernova so as to offer us three images at three different moments of its explosion. A little, say the researchers, as if three mirrorsmirrors gave, for the first, the image of a person raising their comb, for the second, that of the person combing their hair and for the third, that of the person putting down their comb.
A type Ia supernova with gravitational lensing
In a series of articles published in The Astrophysical Journalthe researchers first tell how they located this supernova in a galaxy located far behind the cluster G165. In a galaxy that already existed just 3.5 billion years after the Big Bang. And this makes this supernova the most distant type Ia supernova from Earth ever observed.
Because this is another piece of information provided by the researchers. This supernova is indeed type Ia. Understand that it is the result of the explosion of a woman blanchewoman blanche. And astronomers call this type of supernova a “standard candle.” Because the brightnessbrightness intrinsic of type Ia supernovae is known. And that measurements of their apparent brightness can therefore make it possible to deduce the distance at which they are located.
A supernova to measure the Hubble constant
All this makes this supernova flushed out by the télescope spatial James-Webbtélescope spatial James-Webb particularly interesting. Because the supernovaesupernovae gravitational lensing – from the moment astronomers have information that the time lags between images, the distance of the supernova and the properties of the gravitational lens – can give a precise measurement of the Hubble constantHubble constant. Understand, vitessevitesse to which theUniverseUniverse extends. This is why the supernova was named SNSN H0pe (SN for supernova and H0 for the Hubble constant, H0. H0pe for the hope it gives to finally understand this fundamental parameter of our Universe).
Is this the end of the “crisis” in cosmology with the Hubble tension thanks to the James-Webb telescope?
Because over the past ten years, a tension has appeared around this constant. Astronomers speak of it as « tension de Hubble ». It arose because two ways of calculating the value of the Hubble constant give different results. By measuring the fluctuations of cosmic microwave backgroundcosmic microwave background – what remains of the first lights that came on in the Universe, some 380,000 years after the Big BangBig Bang only -, astronomers find a value of approximately 67 kilometers per second per megaparsec for H0. In agreement with the predictions of the standard model of cosmology. But based on measurements ofstarsstars pulsating, les CepheidsCepheidsthe researchers arrive at a value of approximately 73 kilometers per second per megaparsec. A difference which may seem small, but which could be sufficient to put the standard modelstandard model in question.
A tension on the cosmological model reactivated
And precisely, the value calculated (this work has not yet been validated by peers) by the researchers for the Hubble constant from the gravitational lensing supernova is 75.4 kilometers per second per megaparsec. A value which therefore corresponds more to that given by the measurements carried out in the local universe. But still somewhat « a tension » with the values obtained when our Universe was young.
Already, astronomers are working to obtain more details about SN H0pe. And they hope to be able to uncover, thanks to the James-Webb telescope, other supernovae with gravitational lensing which would allow them to repeat their measurements of H0.
