Astronomers have used the NASA/ESA Hubble Space Telescope and ESA’s Gaia space observatory to make the most precise measurements of the expansion rate of the Universe since it was first calculated nearly a century ago. The results, published in the Astrophysical Journal, provide further evidence of the discrepancy between the expansion rate of the nearby Universe and that of the distant, primeval Universe. This ‘tension’ implies that there could be new physics underlying the foundations of the Universe; possibilities include the interaction strength of dark matter, dark energy being even more exotic than previously thought, or an unknown new particle in the tapestry of space.
Using NASA/ESA Hubble and ESA’s Gaia space telescopes, Riess et al have made the most precise measurements to date of the Universe’s expansion rate. This is calculated by gauging the distances between nearby galaxies using special types of stars called Cepheid variables as cosmic yardsticks. By comparing their intrinsic brightness as measured by Hubble, with their apparent brightness as seen from Earth, scientists can calculate their distances. Gaia further refines this yardstick by geometrically measuring the distances to Cepheid variables within the Milky Way Galaxy. This allowed astronomers to more precisely calibrate the distances to Cepheids that are seen in outside galaxies. Image credit: NASA / ESA / A. Field, STScI.
“The tension seems to have grown into a full-blown incompatibility between our views of the early and late time Universe,” said Professor Adam Riess, from the Space Telescope Science Institute and the Johns Hopkins University.
“At this point, clearly it’s not simply some gross error in any one measurement. It’s as though you predicted how tall a child would become from a growth chart and then found the adult he or she became greatly exceeded the prediction. We are very perplexed.”
In 2005, Professor Riess and members of the Supernova H0 for the Equation of State (SHOES) team set out to measure the Universe’s expansion rate with unprecedented accuracy.
In the following years, by refining their techniques, the researchers shaved down the rate measurement’s uncertainty to unprecedented levels.
Now, with the power of Hubble and Gaia combined, they have reduced that uncertainty to just 2.2 %.
They measured the present rate of expansion to be 45.6 miles (73.5 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 45.6 miles per second faster.
However, the results from another space observatory, ESA’s Planck mission, predict the Universe should be expanding today at only 41.6 miles (67 km) per second per megaparsec.
“Planck mapped the primeval Universe as it appeared only 360,000 years after the Big Bang,” the astronomers said.
“The entire sky is imprinted with the signature of the Big Bang encoded in microwaves. Planck measured the sizes of the ripples in this Cosmic Microwave Background (CMB) that were produced by slight irregularities in the Big Bang fireball. The fine details of these ripples encode how much dark matter and normal matter there is, the trajectory of the Universe at that time, and other cosmological parameters.”
These measurements, still being assessed, allow scientists to predict how the early Universe would likely have evolved into the expansion rate we can measure today. However, those predictions don’t seem to match the new measurements of our nearby contemporary Universe.
“With the addition of these new Hubble and Gaia data, we now have a serious tension with the CMB data,” said Planck team member Dr. George Efstathiou, of the Kavli Institute for Cosmology, who was not involved with the new work.
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Adam G. Riess et al. 2018. Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant. ApJ 861, 126; doi: 10.3847/1538-4357/aac82e
