American astronomer Edwin Hubble and others discovered in the 1920s that the Universe is expanding by showing that most galaxies are receding from the Milky Way — and the farther away they are, the faster they are receding. The Hubble constant is a unit that describes how fast the Universe is expanding at different distances from a particular point in space.

The data collected in recent years by Hubble and other telescopes show a discrepancy between the expansion rate measured in the local Universe compared to independent observations. However, what causes this difference remains elusive.

The data from Hubble, including objects that act as distance markers, support the idea that something weird is going on, possibly involving brand-new physics.

Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) said, “You are getting the most precise measurement of the expansion rate for the universe from the gold standard of telescopes and cosmic mile markers.”

SH0ES, which stands for Supernova, H0, for the Equation of State of Dark Energy, is a research partnership directed by Riess investigating the Universe’s expansion rate.

Riess said, “This is what the Hubble Space Telescope was built to do, using the best techniques we know to do it. This is likely Hubble’s magnum opus because it would take another 30 years of Hubble’s life even to double this sample size.”

A new paper by the Riess’s team completed the biggest and likely last major update on the Hubble constant. The findings are more than double the prior sample of cosmic distance markers. Scientists also reanalyzed all of the prior data, with the whole dataset now including over 1,000 Hubble orbits.

Cepheids have long been the gold standard of cosmic mile markers. They are the stars that brighten and dim periodically. There have been several difficulties associated with using Cepheids as distance indicators. Hence, scientists started using Type Ia supernovae to calculate much greater distances.

Combined, these objects built a “cosmic distance ladder” across the Universe and are essential to measuring Hubble constant. That value is critical to estimating the age of the Universe and provides a basic test of our understanding of the Universe.

Dr. Licia Verde, a cosmologist at ICREA and the ICC-University of Barcelona, speaking about the SH0ES team’s work, said, “The Hubble constant is a very special number. It can be used to thread a needle from the past to the present for an end-to-end test of our understanding of the Universe. This took a phenomenal amount of detailed work.”

The team measured 42 of the supernova milepost markers with Hubble.

Riess said, “We have a complete sample of all the supernovae accessible to the Hubble telescope seen in the last 40 years. Like the lyrics from the song “Kansas City” from the Broadway musical Oklahoma, Hubble has “gone about as fur as it can go!”

Weird Physics?

The Hubble Constant was predicted to be slower than what Hubble sees. By combining the Standard Cosmological Model of the Universe and measurements by the European Space Agency’s Planck mission, astronomers predicted a lower value for the Hubble constant: 67.5 plus or minus 0.5 kilometers per second per megaparsec, compared to the SH0ES team’s estimate of 73.

Riess said“Given the large Hubble sample size, there is only a one-in-a-million chance astronomers are wrong due to an unlucky draw, a common threshold for taking a problem seriously in physics. This finding is untangling what was becoming a nice and tidy picture of the Universe’s dynamic evolution. Astronomers are at a loss for an explanation of the disconnect between the expansion rate of the local Universe versus the primeval Universe. Still, the answer might involve additional physics of the Universe.”

Journal Reference:

  1. Adam G. Riess, Weblong Yuan, et al. A comprehensive Measurement of the Local Value of the Hubble Constant with 1 km s−1 Mpc−1 Uncertainty from the Hubble Space Telescope and the SH0ES Team. arXiv:2112.04510v2

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