Since the Big Bang, the universe has continued to expand. But the measured rate of this cosmic expansion does not correspond to the predictions of the cosmological standard model – the expansion of the cosmos is now happening faster than it should. Astronomers have now checked the previous measurements using the James Webb Space Telescope. They used it to determine the distances of supernovae, variable stars, red giants and carbon-rich giant stars in various galaxies. These new, partly more precise measurements confirm the discrepancy between the observations and the cosmological standard model. The Hubble constant they determined – a measure of cosmic expansion – is significantly higher than the value determined using models and data from the early cosmos, as the team reports.
Since our universe was created in the Big Bang some 13.8 billion years ago, it has continued to expand. The astronomer Edwin Hubble discovered this cosmic expansion in 1929 based on the redshift of distant galaxies. The constant that describes the rate of this expansion in Einstein’s field equations is named after him – the Hubble constant, or H0 for short. Then, in the 1990s, astronomers discovered that, contrary to previously expected, this cosmic expansion had accelerated over time. A possible driving force for this is dark energy – a force that has not yet been precisely defined and that counteracts the attractive effect of gravity. The cosmological standard model takes this into account and combines observational data on the early cosmos in the form of the cosmic background radiation with theoretical models of cosmic evolution.
Hubble measurement data reviewed
But there are also discrepancies with this cosmological standard model: the cosmic expansion is now proceeding faster than the models suggest. According to the standard model, the Hubble constant should be around 67 kilometers per second per megaparsec. But over the last two decades or so, astronomical measurements have regularly shown significantly higher values. For such measurements, astronomers determine the distance and redshift of supernovae, variable stars (Cepheids) or red giants. One of the most comprehensive data sets to date comes from the Hubble Space Telescope and includes 42 type 1a supernovae in 37 different galaxies, whose distances have also been compared with Cepheids. These and other observations result in an average Hubble constant of 73 kilometers per second per megaparsec – and thus a significantly higher value that deviates from the model by around five standard deviations.
Now a team of astronomers led by Nobel Prize winner Adam Riess from Johns Hopkins University in Baltimore has used the James Webb Telescope to measure the expansion rate again. “The new capabilities of the Webb telescope offer us the opportunity to carry out additional checks by comparing the distances it measures to supernova host galaxies with the results from the Hubble telescope,” explain the astronomers. For this study, they determined the distances of 16 supernovae – a good third of the Hubble data set, as well as Cepheids, red giants and a class of carbon-rich giant stars, which are also considered suitable distance markers. “The Webb data significantly improves the signal-to-noise ratio and shows us the universe in HD, so to speak,” explains Riess’ colleague Siyang Li.
Discrepancy remains
The evaluation confirmed the discrepancy in the measurements of cosmic expansion compared to the model: the various distance markers each gave Hubble constants between 72.1 and 73.4. “The result from the James Webb Telescope combined for all methods is 72.6 kilometers per second per megaparsec,” report Riess and his team. This value confirms the previous measurements with the Hubble Space Telescope for the measured galaxies and at the same time shows that the deviations from the Standard Model cannot be due to measurement uncertainties or errors of the Hubble Telescope. “The discrepancy between the observed expansion rate of the Universe and the predictions of the Standard Model suggests that our understanding of the Universe may be incomplete,” says Riess. “With two NASA flagship telescopes confirming each other’s results, we must take the problem of variations in the Hubble constant very seriously – it is a challenge but also an incredible opportunity to learn more about our universe.”
It is still unclear what causes the deviations and at what point the cosmological standard model could be incomplete. “A possible explanation for the discrepancy in the Hubble constant would be that something is missing from our understanding of the early universe, for example a new component such as early dark energy, which would have given the universe an unexpected impulse after the Big Bang,” he does not explain Cosmologist Marc Kamionkowski from Johns Hopkins University was involved in the study. “But there are also other ideas, such as yet unrecognized properties of dark matter, exotic particles, changing electron masses or primordial magnetic fields, that could solve the problem. Theorists can be very creative here.”
Source: Adam Riess (Johns Hopkins University, Baltimore) et al., The Astrophysical Journal, doi: 10.3847/1538-4357/ad8c21