We have been spoiled over recent years with first the Hubble Space Telescope (HST) and then the James Webb Space Telescope (JWST.) Both have opened our eyes on the Universe and made amazing discoveries. One subject that has received attention from both is the derivation of the Hubble Constant – a constant relating the velocity of remote galaxies and their distances. A recent paper announces that JWST has just validated the results of previous studies by the Hubble Space Telescope to accurately measure its value.
The Hubble Constant (H0) is a fundamental parameter in cosmology that defines the rate of expansion of the universe. It defines the relationship between Earth and distant galaxies by the velocity they are receding from us. It was first discussed by Edwin Hubble in 1929 as he observed the spectra of distant galaxies. It is measured in unites of kilometres per second per megaparsec and shows how fast galaxies are moving away from us per unit of distance. The exact value of the constant has been the cause of many a scientific debate and more recently the HST and JWST have been trying to fine tune its value. Getting an accurate value is key to determining the age, size and fate of the universe.
A paper recently published by a team of researchers led by Adam G. Riess from John Hopkins University validate the results from a previous HST study. They use JWST to explore its earlier results of the cepheid/supernova distance ladder. This has been used to establish distances across the cosmos using cepheid variable stars and Type 1a supernovae. Both objects can be likened to ‘standard candles’ whose actual brightness is very well understood. By measuring their apparent brightness from Earth, their distances can be calculated by comparing it to their actual brightness, their intrinsic luminosity.
Over recent decades, a number of attempts have been made to accurately determine H0 using a multitude of different instruments and observations. The cosmic microwave background has been used along with the aforementioned studies using cepheid variables and supernovae events. The results provide a range of results which has become known as ‘Hubble tension.’ The recent study using JWST hopes that it may be able to fine tune and validate previous work.
To be able to determine H0 with a level of accuracy using the cepheid/supernova ladder, a sufficiently high sample of cepheids and supernovae must be observed. This has been challenging, in particular of the sample size of supernovae within the range of cepheid variable stars. The team also explored other techniques for determining H0 for example studying data from HST of the study of the luminosity of the brightest red giant branch stars in a galaxy – which can also work as a standard candle. Or the luminosity of certain carbon rich stars which are another technique.
The team conclude that, when all JWST measurements are combined, including a correction for the low sample of supernovae data, that H0 comes out at 72.6 ± 2.0 km s?1 Mpc?1 This compares to the combined HST data which determines H0 as 72.8 km s?1 Mpc?1 It will take more years and more studies for the sample size of supernova from JWST to equal that from HST but the cross-check has so far revealed we are finally honing in on an accurate value for Hubble’s Constant.
The same Cepheid data has been used by Freedman et al to show that the ladder methods combine to show no tension with other methods. “Based on these new JWST data and using three independent methods, we do not find strong evidence for a Hubble tension,” said Freedman, renowned astronomer and the John and Marion Sullivan University Professor in Astronomy and Astrophysics at the University of Chicago. “To the contrary, it looks like our standard cosmological model for explaining the evolution of the universe is holding up.” [Phys.org]
DESI’ s survey results are out and they show that other methods disagree with the above value:
“The addition of the cosmic microwave background (CMB) data tightens these constraints to ?_m = 0.3056 ± 0.0049 and ?8 = 0.8121 ± 0.0053, while further addition of the the joint clustering and lensing analysis from the Dark Energy Survey Year-3 (DESY3) data further improves these measurements, and leads to a 0.4% determination of the Hubble constant, H_0 = (68.40 ± 0.27) km s^?1 Mpc^?1.”