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Split images of quasars allow scientists to calculate Hubble constant

"The beauty of this measurement is that it's highly complementary to and independent of others," astronomer Tommaso Treu said.

By
Brooks Hays
Scientists used double quasar images to develop a new method for estimating the Hubble constant. Photo byNASA Hubble Space Telescope, Tommaso Treu/UCLA/Birrer et al
Scientists used double quasar images to develop a new method for estimating the Hubble constant. Photo byNASA Hubble Space Telescope, Tommaso Treu/UCLA/Birrer et al

Jan. 23 (UPI) -- The Hubble constant is the rate at which the universe is expanding. Through the decades, astronomers have developed more reliable ways to estimate the value of the Hubble constant, but discrepancies between models remain.

To estimate the Hubble constant with greater accuracy and consistency, a group of astronomers turned to the light from a unique class of quasars. Scientists selected quasars that appear as a pair of side-by-side images in astronomical observations -- the result of gravitational lensing.

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Quasars are sources of intense radiation and luminosity powered by supermassive black holes at the center of distant galaxies. When their light is bent by the gravity of other galaxies on its path toward Earth, mirror images of the radiation sources appear in telescope lenses.

Scientists have found a way to utilize this astronomical phenomena for the purpose of estimating the Hubble constant.

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Traditional methods for calculating the Hubble use the difference in the redshift of two separate galactic light sources. Redshift is the lengthening of light's wavelengths as they are stretched by the expansion of the universe. By measuring the distance between two objects and calculating the difference in redshift, scientists can determine how quickly they're moving away from each other.

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The new method uses a similar technique, but its two sources are really one -- quasar radiation that has taken alternate paths around an intervening galaxy on its way toward Earth.

The levels of radiation given off by quasar fluctuate as the black hole's accretion disk pulls in fresh gas and dust for consumption. For quasars that appear as double images, changes in luminosity register at different times in the two images. Scientists can use the difference in redshifts between the two images and the difference in their fluctuating radiation to calculate the diverging paths of the split quasar light -- and to ultimately calculate the Hubble constant.

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For the new study, published this week in the journal Monthly Notices of the Royal Astronomical Society, scientists analyzed observations of the quasar SDSS J1206+4332 made by the Hubble Space Telescope, the Gemini and W.M. Keck observatories and from a network of telescopes known as the Cosmological Monitoring of Gravitational Lenses, or COSMOGRAIL.

Several years of data collection allowed astronomers to precisely calculate the time delay between the two quasar images.

"The beauty of this measurement is that it's highly complementary to and independent of others," Tommaso Treu, a UCLA professor of physics and astronomy, said in a news release.

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Astronomers can also calculate the Hubble constant by analyzing the cosmic microwave background, but the estimates using different methods continue to produce different results. Scientists hope their latest efforts will either bring Hubble constant models into agreement or confirm that there is a missing component to astronomers' current understanding of the expansion of the universe.

"If there is an actual difference between those values, it means the universe is a little more complicated," Treu said.

It's also possible that all the current methods for calculating the Hubble constant are incorrect.

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