Measurements of today’s expansion rate do not match the rate that was expected based on how the Universe appeared shortly after the Big Bang over 13 billion years ago. Astronomers using the NASA/ESA Hubble Space Telescope have significantly lowered the possibility that this discrepancy is a fluke.
This image shows the entire Large Magellanic Cloud, with some of the brightest objects marked. Image credit: Robert Gendler / ESO.
The inconsistency is between Hubble measurements of today’s expansion rate of the Universe and the expansion rate as measured by ESA’s Planck satellite, which observes the conditions of the early Universe just 380,000 years after the Big Bang.
For years, astronomers have been assuming this discrepancy would go away due to some instrumental or observational fluke. Instead, as Hubble astronomers continue to ‘tighten the bolts’ on the accuracy of their measurements, the discordant values remain stubbornly at odds.
The latest Hubble data lower the possibility that the discrepancy is only a fluke to 1 in 100,000. This is a significant gain from an earlier estimate, less than a year ago, of a chance of 1 in 3,000.
These most precise Hubble measurements to date bolster the idea that new physics may be needed to explain the mismatch.
“The Hubble tension between the early and late Universe may be the most exciting development in cosmology in decades,” said Nobel Laureate and the project’s leader Professor Adam Riess, from the Space Telescope Science Institute and Johns Hopkins University.
The new estimate of the Hubble constant is 46 miles (74.03 km) per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it appears to be moving 46 miles per second faster, as a result of the expansion of the Universe.
The number indicates that the Universe is expanding at a rate about 9% faster than that implied by Planck’s observations of the early Universe, which give a value for the Hubble constant of 41.6 miles (67 km) per second per megaparsec.
“This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This disparity could not plausibly occur by chance,” Professor Riess said.
The team analyzed the light from 70 Cepheid variables in a neighboring satellite galaxy known as the Large Magellanic Cloud, now calculated to be 162,000 light-years away.
Because these stars brighten and dim at predictable rates, and the periods of these variations give us their luminosity and hence distance, astronomers use them as cosmic mileposts.
Professor Riess and colleagues used an efficient observing technique called Drift And Shift (DASH) using Hubble as a ‘point-and-shoot’ camera to snap quick images of the bright stars. This avoids the more time-consuming step of anchoring the telescope with guide stars to observe each star.
The results were combined with observations made by the Araucaria Project, a collaboration between astronomers from 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.
“Previously, theorists would say to me, ‘It can’t be. It’s going to break everything.’ Now they are saying, ‘we actually could do this’,” Professor Riess said.
Various scenarios have been proposed to explain the discrepancy, but there is yet to be a conclusive answer. An invisible form of matter called dark matter may interact more strongly with normal matter than astronomers previously thought. Or perhaps dark energy, an unknown form of energy that pervades space, is responsible for accelerating the expansion of the Universe.
Although the astronomers don’t have an answer to this perplexing disparity, they intend to continue using Hubble to reduce the uncertainty in their measure of the Hubble constant, which they hope to decrease to 1%.
The team’s results were published in the Astrophysical Journal.
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W. D’Arcy Kenworthy et al. 2019. The Local Perspective on the Hubble Tension: Local Structure Does Not Impact Measurement of the Hubble Constant. ApJ 875, 145; doi: 10.3847/1538-4357/ab0ebf
