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Splitting Images Of Quasars Helps Researchers Understand Expansion Of The Universe

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Researchers from the University of California, Los Angeles devise a way to measure the rate in which the universe is expanding: quasars.

The team argued that the split images of quasars, a fountain of radiation powered by black holes, can be used to produce an estimate of the Hubble Constant or the measurement used to describe the expansion of the universe.

The study was published in Monthly Notices of the Royal Astronomical Society.

The Hubble Constant Debate

Scientists know that the universe is expanding, but how fast it is happening and, by extension, how much time has passed since the Big Bang, remains in contention. At the heart of the debate is the Hubble Constant, the number that relates to the distances of redshifts — when the light is stretched as it travels to Earth across the expanding universe — of galaxies.

"The Hubble constant anchors the physical scale of the universe," explained Simon Birrer, a postdoctoral scholar and the lead author of the study. 

Previous estimates of the Hubble Constant ranges from 67 to 73 kilometers per second per megaparsec. Most methods used to produce an estimate of the Hubble Constant involve two factors: the distance to the source of light and the source of light's redshift. 

The Key To The Expanding Universe

However, the researchers proposed a new ingredient that has not been used in the calculations of other scientists to infer the Hubble Constant. In their study, they chose quasars whose light has been bent by the pull of an intervening galaxy, producing two side-by-side images. 

The light between these two images take different paths before they arrive on Earth so, when the brightness of the quasar fluctuates, the images do not flicker at the same time. This delay, along with the information on the intervening galaxy's gravitational field, can be used to deduce the distances of the quasar and the foreground galaxy to Earth. 

By knowing the redshift of a quasar, the researchers claim that it can be used to produce a new estimate of how fast the universe is expanding. 

The demonstrate the technique, they observed the split images of the quasar SDSS J1206+4332 using the data collected by the Hubble Space Telescope, the Gemini Observatory, W.M. Keck Observatory, and the Cosmological Monitoring of Gravitational Lenses (COSMOGRAIL) network. They took photos of the quasar every day for several years to measure the time delay between the split images. 

They combined the best estimate of the Hubble Constant from the quasar with previous data on three quadruply imaged quasars gathered by their H0liCOW collaboration. They came up with the Hubble Constant estimate of about 72.5 kilometers per second per megaparsec. 

The researchers are still looking for more quasars to improve the precision of the technique. The team is looking at 40 quadruply image quasars, which are more difficult to find, to see what else they can learn. 

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