New estimate of Hubble constant derived using a doubly imaged quasar

The Hubble constant is a number that relates distances in the universe to the redshifts of galaxies

Astronomers from the University of California - Los Angeles (UCLA) have estimated a new value of the Hubble constant using a double imaged quasar.

The Hubble constant is the unit of measurement used to describe the expansion of the universe. It relates distances in the universe to the redshifts of galaxies, that is, the amount of stretching of light as it travels through the expanding universe before arriving at Earth.

Earlier, scientists have estimated that the value of the Hubble constant ranges from about 67 to 73 kilometres per second per megaparsec. In simpler words, it means that two points in space located at a distance of 1 megaparsec (3.26 million light-years) are moving away from each other at a speed between 67 and 73 kilometres per second.

Calculating the accurate value of the Hubble constant is important for astronomers to precisely determine the age of the universe, sizes of remote galaxies, or the expansion history of the universe.

In the current study, the UCLA team investigated a doubly imaged quasar known as SDSS J1206+4332 using data gathered by the W.M. Keck and Gemini observatories, the Hubble Space Telescope, and the Cosmological Monitoring of Gravitational Lenses (COSMOGRAIL) network.

Doubly images quasars are the quasars whose light is bent by an intervening galaxy. This bending of light produces two side-by-side images of the quasar in the universe.

Researchers captured images of SDSS J1206+4332 on a daily basis for several years, which enabled them to accurately measure the time delay between the images.

Then, they combined the data gathered with the data by H0liCOW collaboration on three quadruply imaged quasars. Using the new data, they estimated the value of the Hubble constant to be about 72.5 kilometres per second per megaparsec.

According to researchers, the new estimate is in agreement with an earlier estimate calculated using distances to supernovas as the key measurement. However, these two estimates are about eight per cent higher than another figure derived using the cosmic microwave background (faint glow from all over the sky) - a relic from about 380,000 years after the Big Bang.

"If there is an actual difference between those values, it means the universe is a little more complicated," said Tommaso Treu, professor of physics and astronomy at UCLA, and the senior author of the study paper.

Treu says it is also possible that any one or all three measurements are incorrect.

The researchers are now investigating quasars - specifically quadruply imaged quasars - to improve the accuracy of their Hubble constant measurement.

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