How fast is universe expanding?

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These galaxies are selected from an HST programme to measure the expansion rate of the universe. The red giants are identified by yellow circles. Photo: NASA, ESA, W. Freedman, ESO, and the Digitised Sky Survey

UNIVERSITY of Chicago astronomers have made a new measurement of how fast the universe is expanding, using an entirely different kind of star than previous endeavours. The revised measurement, using NASA's Hubble Space Telescope (HST), adds to the ongoing controversy about the universe’s expansion rate. This, the researchers believe, may lead to a new interpretation of the universe's fundamental properties. The measured quantity called the Hubble Constant gives the rate at which the space is stretching, but whose precise value has been elusive because of challenges involved in its measurement, most important of which is an accurate calculation of cosmic distances. In their paper that is due for publication in “The Astrophysical Journal”, Wendy Freedman and her team announced a new measurement of the Hubble Constant using a kind of star known as the red giant. Their observations, made using the HST, indicate that the expansion rate for the nearby universe is just under 70 km/sec/megaparsec (Mpc). One parsec is equivalent to 3.26 light years distance.

This measurement is slightly smaller than the value of 74 km/sec/Mpc recently reported by the Hubble SH0ES (Supernovae H0 for the Equation of State) team using Cepheid variables, stars that pulse at regular intervals that correspond to their peak brightness, which was stated to be the highest precision to date using the Cepheid distance measurement technique. In 2001, the Hubble Space Telescope Key Project team, which included Wendy Freedman, measured the value using Cepheid variables as distance markers. The experiment concluded that the value was 72 km/sec/Mpc. Recently, using a model based on the data from the European Space Agency’s Planck satellite on Cosmic Microwave Background (CMB), the remnant light from the big bang, scientists have calculated the Hubble Constant to be 67.4 km/sec/Mpc, which differs significantly from the above values using Cepheid stars.

“Naturally, questions arise as to whether the discrepancy is coming from some aspect that astronomers don't yet understand about the stars we're measuring, or whether our cosmological model of the universe is still incomplete,” Wendy Freedman said. “Or maybe both…” Her team sought to check their results by using the red giant as the milestone marker for intergalactic distances. These bright red giant stars are the final stages of certain kind of stars, the sun being one such. At a certain point, the red giant undergoes a catastrophic event called a helium flash, in which the temperature rises to about 100 million degrees and the structure of the star is rearranged, which ultimately dramatically decreases its luminosity. Astronomers can measure the apparent brightness of the red giant stars at this stage in different galaxies, and they can use this as a way to tell their distance. The Hubble Constant is calculated by comparing distance values to the apparent recessional velocity of the target galaxies—that is, how fast galaxies seem to be moving away. The team's calculations give a Hubble constant of 69.8 km/sec/Mpc—which falls between those obtained by the Planck and Riess teams.


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