In the photo: four images of the same supernova next to a lensing cluster of galaxies
Physics Today magazine published an article about how astronomers observed the same supernova in several places at once, which allowed them to independently calculate the expansion rate of the universe.
A team of astronomers at the University of California at Berkeley observed the Refsdal supernova in another galaxy 14 billion light-years away. Back in 2014, it was noticed in images taken by the Hubble Space Telescope that the supernova is visible in multiple copies due to the effect of gravitational lensing. About a year after the start of observations, the same supernova flared up at a slightly different point in the starry sky.
Gravitational lensing is an effect caused by the deflection of light in a gravitational field, causing a massive object to act as a huge "lens" .
Images of the supernova did not light up at the same time due to the fact that the light traveled along different trajectories. Thanks to this, scientists have recently been able to calculate the distance from a supernova to a cluster of galaxies, which acted as a gravitational lens. Knowing the redshift of the supernova, scientists were able to calculate with good accuracy the value of the Hubble constant, which determines the rate of expansion of the universe. It turned out to be equal to 66.6 km / s / megaparsec with an error of 3-4 km / s / megaparsec. This is close to the value of the Hubble constant, which is determined from cosmic microwave radiation (67.4 ± 0.6 km/s/megaparsec) and differs somewhat from the values of the Hubble constant, which are determined from “standard candles” (73 ± 1 km/s/megaparsec). megaparsec).
The Hubble constant is a measure of how fast the universe is expanding. Observations show that the farther a galaxy is from us, the faster we are moving away from each other. The speed at which a galaxy is moving away is proportional to its distance. The coefficient of this proportionality is called the Hubble constant.
One way to determine the Hubble constant is as follows. The speed at which galaxies are receding can be determined from the redshift of their spectra. The distance to (not very distant) galaxies was estimated from the so-called "standard candles" – objects whose luminosity is known, and therefore their distance can be determined from their observed brightness.
Another way to determine the Hubble constant is to study the CMB spectrum.
CMB radiation is radiation in the microwave range, it comes to us from all directions in the sky. It is very isotropic, that is, it shines from all directions almost equally. However, small deviations from isotropy, combined with other observed data, allow scientists to calculate cosmological parameters.
The difference between the values of the Hubble constant, measured in different ways, is one of the great mysteries of cosmology. It may ultimately come down to the discovery of errors in measurements, or maybe to a new understanding of the differences between the universe in the era of recombination (the moment when the CMB was formed) and in subsequent cosmological epochs. Scientists now hope that with the help of the James Webb telescope they will be able to make measurements based on standard candles with greater accuracy and ultimately solve the cosmological mystery.
