11 February 2021 Volume :9 Issue :2

Astrophysicists Address a Puzzle of Cosmic Expansion during Lockdown

Astrophysicists Address a Puzzle of Cosmic Expansion during Lockdown
Professor Yin-Zhe Ma (left), Dr Wei-Ming Dai and Figure 1.

Professor Yin-Zhe Ma of UKZN’s Astrophysics Research Centre (ARC) recently published research in a prestigious journal that will form the basis of an upcoming talk on a potential solution to the Hubble constant discrepancy, a highly contested point of debate in the astrophysics community concerning the rate of expansion of the Universe.

Published in Physical Review D Rapid Communication, Ma’s work presents a plausible physical solution to the discrepancy, an area of research in the field that has seen a flurry of attention and activity in recent years, including from fellow ARC researchers last year.

The calculation of the age of the Universe is linked to the rate of its expansion, a number quantified by what is known as the Hubble constant, with the accepted estimate of an age of 13.8 billion years derived from the standard model of the Universe that utilises measurements of the oldest light in the universe, observed by the Planck satellite.

In recent years however, as cosmology has entered a “golden age” of precision measurements, new local measurements have challenged accepted measurements of the cosmic microwave background (CMB) radiation that inform this constant, raising questions as to the rate of the Universe’s expansion and consequently its age, the correctness of the measurements, and whether some new kind of physics was responsible for these deviant observations. Notable measurements include CMB and baryon acoustic oscillation surveys that give a low value Hubble constant, while distance ladder measurements that are derived from observations of celestial bodies are much higher.

Together with his co-authors, postdoctoral researcher at UKZN Dr Wei-Ming Dai and Professor Hong-Jian He of Shanghai Jiao Tong University, Ma proposed using holographic dark energy (HDE), which describes the vacuum energy in a cosmic infrared region where the total energy saturates the limit of avoiding collapse into a black hole, to develop a dark energy equation demonstrating that the Universe accelerated more slowly early in its life and faster at later stages.

This presents a physical resolution of the Hubble constant discrepancy, as their HDE prediction of the Hubble constant fits cosmological data at all redshifts, and Ma and colleagues expect that their scenario will be tested in future CMB and large-scale structure surveys.

On 19 February, Dai and Ma will jointly present a data@breakfast talk hosted by UKZN’s Centre for Quantum Technology and the National Institute for Theoretical Physics (NITheP) on the puzzle of the Hubble space constant: a holographic Universe solution. Their presentation will include a review of the measurements from various surveys and will describe their physical and provable solution to the discrepancies between measurements.

Ma, who joined UKZN in 2015, obtained his PhD from the University of Cambridge, conducting postdoctoral research at the Universities of British Columbia and Manchester. His research focuses on observational and theoretical cosmology aimed at understanding the fundamental laws of the Universe and uncovering the nature of dark energy and dark matter. He is a member of the Hydrogen Epoch of Reionization Team (HERA), several working groups for the Square Kilometer Array (SKA), the Planck science team, and the TAIPAN/6dFGS galaxy survey team.

Dai obtained his PhD from the Chinese Academy of Sciences’ Institute of Theoretical Physics in 2018 and has since worked with Ma at UKZN. His research interests include 21-cm intensity mapping, CMB radiation, neutrino cosmology, large-scale structure, and data analysis.

Figure 1: A figure demonstrating the comparison between the data and the theories where the y-axis is the Hubble constant evolution and x-axis is the redshift indicating distance. The grey band is the predicted evolution curve of standard cosmology theory and the blue band is the predicted evolution of holographic dark energy theory. The data point at z=0 is the local distance ladder measurement. The holographic dark energy theory renders the prediction consistent with the data, but standard cosmology theory does not.

Words: Christine Cuénod

Photographs: Supplied


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