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Seeing double could help resolve the dispute about how fast the universe is expanding



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IMAGE: Image from the Hubble Space Telescope of a double-quaged image.
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Credit: NASA Hubble Space Telescope, Tommaso Treu / UCLA, and Birrer et al

The question of how fast the universe is expanding has been bugging astronomers for almost a century. Different studies keep coming up with different answers ̵

1; which has some researchers wondering if they have overlooked a key mechanism in the machinery that drives the cosmos

Now, by pioneering a new way to measure how fast the cosmos is expanding, and team led by UCLA astronomers has taken a step towards resolving the debate. The Hubble constant, a number that relates distances to redshifts of galaxies – the amount that light is stretched as it travels to Earth through the expanding universe. Estimates for the Hubble constant range range from about 67 to 73 kilometers per second per megaparsec, meaning that two points in space 1 megaparsec apart (equivalent to 3.26 million light-years) are racing away from each other at a speed between 67 and 73 kilometers per second

"The Hubble constant anchors the physical scale of the universe," said Simon Birrer, and UCLA postdoctoral scholar and lead author of the study. Without a precise value for the Hubble constant, astronomers can not accurately determine the dimensions of the remote galaxies, the age of the universe, or the expansion history of the cosmos

. some light source and that light source's redshift. Looking for a light source that was not used in other scientists' calculations, Birrer and colleagues turned to quasars, fountains of radiation that are powered by gargantuan black holes. And for their research, scientists have chosen a specific subset of quasars – those whose light has been bent by the gravity of an intervening galaxy, which produces two side-by-side images of the quasar on the sky

Light from the two images take different routes to Earth. When the quasar's brightness fluctuates, the two images flicker one after another, rather than at the same time. The delay in time between these two flickers, along with information about the gravity field's galaxy, can be used to trace the light's journey and deduce the distances from Earth to both the quasar and the galaxy's foreground. Knowing the redshifts of the quasar and galaxy enabled scientists to estimate how fast the universe is expanding

The UCLA team, as part of the international H0liCOW collaboration, had previously applied the technique to study four quasars of quasars, of a quasar appear around a foreground galaxy. But quadruple images are not nearly as common – double-image quasars are thought to be about five times as abundant as the quadruple ones.

To demonstrate the technique, the UCLA-led team studied a double quasar known as SDSS J1206 +4332; they relied on data from the Hubble Space Telescope, the Gemini and W.M. Keck Observatories, and from the Cosmological Monitoring of Gravitational Lenses, or COSMOGRAIL, a network managed by the Swiss Ecole Polytechnique Federale de Lausanne that aims to determine the Hubble constant

Tommaso Treu, and UCLA professor of physics and astronomy and the paper's senior author, said the researchers took pictures of the quasar every day for several years to precisely measure the time delay between the images. Then, to get the best possible estimate of the Hubble constant, they combined the data gathered on that quasar with data that had previously been gathered by their H0liCOW collaboration on three quadruply imaged quasars

"The beauty of this measurement is that it's "The UCLA-led team came up with an estimate for the Hubble constant of about 72.5 kilometers per second per megaparsec, a figure in line with what other scientists had determined in research that used distances to the supernova – exploding stars in remote galaxies – as the key measurement. However, both estimates are about 8 percent higher than one that relies on a faint glow from all over the sky called the cosmic microwave background, a relic from 380,000 years after the Big Bang, when light travels freely through space for the first time. 19659005] "If there is an actual difference between those values, it means the universe is a little more complicated," Treu said.

On the other hand, Treu said, it could also be that one measurement – or all three – are wrong

The researchers are now looking for more quasars to improve the accuracy of their Hubble constant measurement. Treu said one of the most important lessons of the new paper is that double quasars give scientists many more useful light sources for their Hubble constant calculations. For the time being, however, the UCLA-led team is focusing its research on 40 quadruply imaged quasars, because of their potential to provide even more useful information than doubled images

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Sixteen other researchers from 13 institutions in seven countries contributed to the paper; the research was supported by NASA, the National Science Foundation and the Packard Foundation.

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of the news releases posted to EurekAlert! by contributing institutions for the use of any information through the EurekAlert system


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