Astrophysicists at John Hopkins University have measured a value for the Hubble constant that clashes with the expected value based on early-universe data, meaning that our fundamental understanding of the expansion of the universe could be incomplete.
The Hubble constant is a measure of the rate of expansion of the universe and has applications in studying both the history and the future of the universe. If our understanding of the universe is correct, data obtained from the ESA’s Planck Satellite in 2013 puts the value of the constant at around 67.1 kilometres per second per mega- parsec. However, there has been an ongoing discrepancy between this value and the value measured using data from the Hubble telescope, known as the ‘Hubble Tension’. It was previously thought that this was most likely due to errors in measurements from the Hubble telescope, but according to recent research led by Adam Riess this may not be the case.
Riess’s analysis of data recently obtained from the James Webb Space Telescope has agreed with the Hubble telescope’s readings, putting the value at around 73 kilometres per second per megaparsec, meaning that the discrepancy could lie instead with our understanding of the universe itself. Riess’s analysis used pulsating stars called Cepheid variables as ‘standard candles’ – light sources of known luminosity that can be used to measure large astronomical distances. As JWST produces higher quality results than the Hubble telescope, Riess was able to reject the idea that there were errors in photometric measurements of these Cepheids when using the Hubble telescope that impacted the value of the constant enough to cause the discrepancy between values.
Data from the Planck satellite was used differently, producing a ‘map’ of cosmic microwave background radiation in the early stages of the universe, dating from about 380,000 years after the big bang. If the value of the constant obtained from these calculations turns out to be incorrect, this may mean that our understanding of the way that the universe has expanded since this early stage is flawed. While the discrepancy between values seems small, the fact that astrophysicists are consistently coming up against the same issue suggests that there is something fundamental that we currently don’t understand.
However, the JWST data may not necessarily give a definitive value for the Hubble constant. In April, astronomers from the University of Chicago led by Wendy Freedman presented preliminary results at a conference in London that put the constant at 69.1 kilometres per second per megaparsec, right between the competing values. Freedman’s results have not been officially published and so cannot yet be properly analysed, but they do deepen the mystery of the constant, as both Freedman and Riess used the same methods to obtain their values, and so should theoretically agree on the value of the constant. If Freedman is correct, we might be able to accept our current ideas about the expansion of the uni- verse, but this inconsistency in results raises even more questions about the Hubble Tension.
Far more research using new methods needs to be done into the Hubble constant to find its true value, but the availability of new data from JWST spells an exciting era for new cosmological discoveries. Ideas about the expansion of the universe themselves are fairly new: Riess himself was awarded a Nobel Prize for his work in 1998 regarding the discovery that the expansion of the universe is accelerating rather than slowing down, meaning that it is unlikely to collapse in on itself in the so called ‘Big Crunch’. The previously thought. The implication continuing debate about the Hubble constant proves that there is more to learn about the future of the uni- verse than our current models allow, as astrophysicists have not been able to devise an adequate alternative that explains the discrepancy.
JWST was only launched in December 2021, so findings from its data are still in their initial stages. However, these early discoveries show the promising start the telescope is having. The most powerful telescope ever built, JWST is already making waves in our understanding of the universe, and will hopefully lead to more discoveries in the future.
