Using a new method for estimating cosmic distances based on measuring the average brightness of stars in giant elliptical galaxies, astronomers have obtained an updated value for the expansion rate of the local Universe. The results of the study are published in The Astrophysical Journal.
The expansion rate of the Universe is one of the key parameters of cosmology. With increasing distance from the Earth, it increases due to the action of dark energy, the nature of which is still a mystery. The coefficient that links the distance to any extragalactic object with the speed of its removal is called the Hubble constant, or H0. Physically, it is a local acceleration expressed in kilometers per second per megaparsec.
The Hubble constant is included in the basic equations of cosmology describing the evolution of the Universe, but the problem is that measurements made by different methods give different values of this quantity.
Traditionally, scientists use two calculation methods to estimate the rate of expansion of the Universe: the first is based on relic radiation, and the second is based on the accidental appearance of supernovae in distant galaxies. According to the first method, the value of H0 is approximately equal to 67.4, and according to the second - 74. Naturally, astronomers are concerned about this discrepancy and are constantly looking for new ways to measure the Hubble constant.
The new H0 value emerged as a by-product of the MASSIVE survey of nearby galaxies, in which scientists use space and ground-based telescopes to study in detail the 100 most massive galaxies within about 100 megaparsecs, or 330 million light-years from Earth.
To obtain H0, scientists from the MASSIVE project measured fluctuations in the surface brightness of 63 giant elliptical galaxies and determined the distance to each of them depending on their speed. The authors note that the advantage of the surface brightness fluctuation (SBF) method is that it does not depend on other parameters or methods of observation and can provide more accurate distance estimates than other methods.
"For measuring distances to galaxies as far away as 100 megaparsecs, this is a fantastic method," said research leader, professor of astronomy and physics Ma Chung-Pei Ma in a press release from the University of California at Berkeley. which collects a large homogeneous dataset of 63 galaxies for the study of H0 using the SBF method."
The technique using surface brightness fluctuations is one of the newest. It is based on the fact that giant elliptical galaxies are very ancient, with a constant population of old stars, mostly red giants. Using a wide-angle camera on the Hubble Space Telescope, the researchers captured high-resolution infrared images of each galaxy and calculated how the brightness of each pixel in the image differs from the average infrared brightness for the entire galaxy. It is known that the smoother the fluctuations over the entire image, the farther away the galaxy is from us. After correcting for defects such as bright star-forming regions, the authors obtained the distances to each of 63 objects.
Last year, scientists from the MASSIVE group already tested this method to determine the distance to the giant elliptical galaxy NGC 1453 in the southern constellation of Eridani. The results of the new study allowed calculating the "local" value of the Hubble constant for a large number of galaxies.
It was 73.3 kilometers per second per megaparsec, which is very close to the results of estimates for type Ia supernovae - a method that is considered the gold standard in cosmology. This means that for every megaparsec - 3.3 million light years, or three billion trillion kilometers - the universe is expanding at 73.3 kilometers per second.