A new technique for calculating the Hubble constant has been demonstrated by astronomers, which involves timing the intervals between different photos of a lensed supernova.
The Cepheid variable, a pulsating star in the Andromeda nebula, was first noticed 100 years ago this year by Edwin Hubble. The star's very low luminosity indicated that it must have been very far away and that Andromeda must have been a different galaxy - the first indication that the Milky Way was not the only galaxy in the universe. As Hubble continued to discover more galaxies, he discovered that they were all expanding cosmically and moving away from us, as demonstrated by the so-called Hubble constant.
The expansion has now been studied by astronomers using data from another star, an exploding supernova whose light was deflected on its way to Earth. The team got a lower value of the Hubble constant than estimates based on Cepheids and other distance markers. This value was obtained by determining a temporal delay between various photos of the supernova. More observations will be required before scientists can use lensed supernovae as a precise accelerometer for cosmic expansion because the error bars are high for the new result.
The gravitational force bending the light results in a lensed supernova. Light from a supernova is bent around the galaxy by the galaxy's gravity while behind the galaxy as seen from Earth.
Similar to a magnifying glass, this movement magnifies and distorts the image of the star. This lensing can occasionally produce multiple images of the star, each appearing in a different position in the sky. Such a collection of images causes light to travel to Earth along various routes and reach Earth at various times. The idea of determining the Hubble constant using time delays was introduced in 1964 by astronomer Sjur Refsdal. But finding a multi-image supernova has proven difficult.
Fifty years after Refsdal's proposal, luck finally smiled at him. Patrick Kelly, then at the University of California at Berkeley and now at the University of Minnesota, discovered a four-lens view of the same supernova in an image taken from the Hubble Space Telescope in December 2014.
While the team wasn't able to pinpoint the exact time intervals between these photos, Kelly and her colleagues were able to predict the coming of a fifth image based on previous work in this region of the sky. This assumption was made based on the fact that the detected supernova was hidden behind a galaxy cluster instead of a single galaxy, providing alternative routes to the supernova light to Earth. The fifth image emerged in December 2015, approximately 376 days after the first four images, as astronomers continued their vigil. The massive mass density of the cluster caused this long time delay, which is advantageous for cosmic expansion measurement.
The value of the Hubble constant is inversely proportional to the time delay, Kelly argues, which makes it appropriate to wait a year.
However, the time delay alone is not enough to calculate the Hubble constant. In addition, astronomers need to determine the exact routes that supernova light takes to reach Earth. For this, they use mass distribution models across the galaxy cluster. Because much of this mass is hidden by invisible dark matter, the results of the models are inconsistent. To circumvent this problem, Kelly and colleagues looked at models' predictions for the position of the most recent photo and the relative brightness of subsequent supernova images.
Based on this assessment, they developed a mass distribution optimal model that they then used to calculate the Hubble constant, which is 65 km/s/Mpc, where Mpc stands for megaparsecs.
The margin of error is about 6%, or 4 km/h/Mpc.
Cosmologists are interested in new techniques for measuring the Hubble constant because the results of the two most commonly used approaches are inconclusive. The first technique builds on Hubble's research a century ago, using Cepheids and other well-characterized objects such as masers and type 1a supernovae to determine cosmic distances. According to Nobel laureate Adam Riess of Johns Hopkins University in Maryland, “Hubble would be surprised to see that we are still using Cepheids.”
Such cosmic distance observations allowed Riess and his colleagues to precisely calculate the Hubble constant, which they determined to be 73 km/s/Mpc. This measurement differs from the measurement made using the cosmic microwave background, which results in 67 km/h/Mpc. The Hubble voltage or that 9% difference is still a big mystery.
The Hubble voltage range is low for Kelly et al.'s lensed supernova result. However, this value overlaps with the other two due to the size of the error bars. “I don't think [the lensed supernova measurement] says anything significant about the Hubble constant,” Riess says at the moment.
One way to reduce the error bars in the latest Hubble prediction is to improve mass distribution models in the cluster, which is the lens for the supernova light seen. According to Kelly, future observations by JWST (the telescope that will replace Hubble) could help determine the mass of the cluster. There is also hope of seeing more lensed supernovae. A separate lensed supernova was recently found by scientists using JWST. According to Kelly, "I'm pretty hopeful that this supernova will reveal something interesting."
Future research, such as those planned for the Rubin Observatory in Chile in the coming years, should find hundreds more lensed supernovae, according to Sherry Juice of the Max Planck Institute for Astrophysics in Germany. Lensed supernovae have been extremely rare since then, according to Suyu. “Lentric supernovae herald an exciting era!”
Günceleme: 19/05/2023 15:35