New Measurement Of The Expansion Rate Of The Universe Continues To Show Major Discrepancy

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACSJ0717.5+3745. NASA, ESA and the HST Frontier Fields team (STScI)

The value of the Hubble Constant, the rate of the expansion of the universe, seems to have two separate values depending on which method is used to estimate it. This tension continues to be debated and we still don't have a solution.

The discrepancy has caused many scientists to scratch their heads because as far as we can tell, each method is well thought out and reliable. Now, using a new methodology, researchers have obtained a new estimate that's consistent with one of the values. The findings are published in the Monthly Notices of the Royal Astronomical Society.

The researchers found a value for the Hubble Constant of 72.5 kilometers per second per megaparsec (a megaparsec is equivalent to 3.26 million light-years). This means that if two galaxies are 1 megaparsec apart, they would appear to be moving away from each other at a speed of 72.5 kilometers (45 miles) per second. This value is within the uncertainty range of the value obtained using the light of supernova, but both are about 8 percent higher than what is estimated using light from the cosmic microwave background, the echo of the Big Bang.

"The beauty of this measurement is that it's highly complementary to and independent of others," senior author Tommaso Treu of UCLA said in a statement. "If there is an actual difference between those values, it means the universe is a little more complicated."

This measurement was obtained using the light of a gravitationally lensed quasar (the bright nucleus of an active galaxy). The team focused on the image of a particular quasar that's split in two by a massive foreground galaxy, warping space-time around itself. Just like the light in a regular lens, the light from this distant quasar takes different paths to reach us. One path is longer than the other, giving a distinct time delay. Using that, along with knowledge of how far away the lensing galaxy and the quasar are, the researchers were able to estimate how quickly the universe is expanding.

They took images of the quasar every day for several years and combined the estimation with information from other lensed quasars. The team plans to increase the precision of their measurement by studying even more objects.

The work is an important contribution to the discussion of what the discrepancy means. Researchers are very upfront about the fact that one or more measurements might be wrong due to potential flaws in methodology. At the same time, it could be that there’s some new phenomenon that can explain the tension between the approaches. Only more data will help us work out exactly what’s going on.

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