New study reports innovative method to ascertain the Hubble parameter without the sound horizon

A team of astronomers used archival data from The Baryon Oscillation Spectroscopic Survey (BOSS) and the Pantheon supernovae to obtain stringent constraints on the Hubble parameter at $H_{0}=65.1^{+3.0}_{-5.4} km/s/Mpc$ which differs from the best fit of SH0ES at 95% confidence level. This is the first time the Hubble parameter has been ascertained from the horizon scale at matter-radiation equality and therefore offers a consistency check for standard cosmological physics in the pre-recombination era.

Source: hdqwalls.com

Until now, there has always been a disagreement in the estimated value of the $H_{0}$ from different sources. The so-called "Late universe" measurements that employs the calibrated distance ladder techniques all largely converge to an estimated value of approximately 73 km/s/Mpc. Whereas the "early universe" techniques based primarily on the measurements of the cosmic microwave background radiation (CMBR) have converged towards a smaller value of 67.7 km/s/Mpc. Although both of these techniques have undergone dramatic enhancement, this however has not been able to alleviate the apparent discrepancy in the measured values of the $H_{0}$ and the disagreement is now statistically significant at 99.9% confidence level. This discrepancy termed the Hubble tension is an active area of research in observational astronomy.

Scientists have come up with numerous compelling ways to mitigate this tension which can be broadly classified into two categories: a) lack of precision in observational techniques and b) incompleteness of the standard $\Lambda$CDM model. For the latter, a wide range of theoretical models have emerged which hitherto have not been widely accepted.

The Hubble parameter $H_{0}$ is generally measured from the ongoing galaxy surveys by juxtaposing the angular scale of Baryon Acoustic Oscillations (BAO) with the theoretical size of the sound horizon scale at decoupling. However, a second method also exists which due to some unknown reasons has not been fully exploited. The method is based on the horizon scale at the matter-radiation equality, which in other words refers to the horizon wavenumber at matter radiation equality. The new study has shown that upon employing this compelling and elegant method, tight constraints on $H_{0}$ can be placed which is completely independent of the sound horizon.

The new study therefore is pioneering in the sense that a new method to ascertain this fundamental cosmological parameter would shed some light on the discordance and on the viability of unknown physics beyond the concordance model. The article has been uploaded in arXiv and awaits being peer-reviewed.

Article Information: O. H. E. Philcox et al., "Determining the Hubble Constant without the Sound Horizon: Measurements from Galaxy Surveys", arXiv: 2008.08084

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