The Hubble Space Telescope has in 2019 provided data that significantly reduces the chance that the current expansion rate of the Universe, as measured by Hubble, does not match the expected rate based on conditions from shortly after the Big Bang over 13 billion years ago.The NASA/ESA Hubble Space Telescope has revolutionized the way we calculate accurate distances to galaxies, by observing Cepheid variables in the Large Magellanic Cloud. These pulsating stars act as milepost markers, which researchers can use to measure how fast the Universe is expanding over time – a value known as the Hubble constant. Before Hubble was launched in 1990, estimates of the Hubble constant varied greatly, but thanks to its precision observations, the uncertainty was reduced to 10% by the late 1990s. Subsequent refinements have further reduced that uncertainty to a remarkable 1.9%, allowing scientists to gain a greater understanding of the cosmos.
This research suggests that the discrepancy between measurements of the Universe’s expansion rate today and what was expected based on observations of the early Universe’s expansion is highly unlikely to be a fluke, with just a 1 in 100,000 chance. This estimate is a significant improvement from a previous one last year, which was 1 in 3,000. Lead researcher and Nobel Laureate Adam Riess of the Space Telescope Science Institute and Johns Hopkins University calls it the most exciting development in cosmology in decades. The team’s calculations of the Hubble constant remain inconsistent with the expected value derived from Planck’s observations of the early Universe’s expansion, with the new estimate being 74.03 kilometres per second per megaparsec – 9 percent faster than what Planck gave.
Adam Riess and his team used an efficient observing technique called Drift And Shift (DASH) to analyse the light from 70 Cepheid variables in the Large Magellanic Cloud. By observing the periodic variations of these stars, astronomers can use them as cosmic mileposts to calculate their luminosity and hence distance. Riess and his team combined their results with those of the Araucaria Project, a collaboration between astronomers from institutions in Europe, Chile, and the United States, to measure the distance to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in a binary-star system. The results of this study have perplexed astronomers because they conflict with cosmological models which suggest that observed values of the expansion of the Universe should be the same as those determined from the Cosmic Microwave Background. As such, there is still no conclusive answer as to why this discrepancy exists – various scenarios such as dark matter or dark energy have been proposed, but are yet to be proven. Riess and his team intend to continue using Hubble to reduce the uncertainty in their measure of the Hubble constant in the hopes of decreasing it to 1 percent.