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Double vision could help resolve disputes about how fast the universe expands


IMAGE: Image from the Hubble space telescope of a double-mapped quasar.
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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 bothering astronomers for almost a century. Different studies provide different answers over and over again ̵

1; some researchers wonder if they have overlooked a key mechanism in the machine that powers the cosmos.

Now, by presenting a new method for measuring the growth rate of the cosmos, a team headed by UCLA astronomers has taken a step towards solving the debate. The group's research findings are published today in Monthly Notices of the Royal Astronomical Society .

At the center of the dispute is the Hubble constant, a number that relates the distances from the redshifts of the galaxies. The light is stretched as it passes through the expanding universe to Earth. Hubble constant estimates are between about 67 and 73 kilometers per second per megaparsek. This means that two points in space at a distance of 1 mph (equivalent to 3.26 million light years) race at a speed between 67 and 73 kilometers away from each other each second.

"The Hubble constant anchors the physical scale of the universe," said Simon Birrer, UCLA's postdoctoral fellow and lead author of the study. Without an exact value for the Hubble constant, astronomers can not accurately determine the size of distant galaxies, the age of the universe, or the expansion history of the cosmos.

Most methods for deriving the Hubble constant have two components: a distance to a light source and redshift of that light source. Searching for a light source that was not used in other scientists' calculations, Birrer and colleagues turned to quasars, radiation fountains driven by huge black holes. For their research, scientists chose a particular subgroup of quasars – those whose light was bent by the gravitational force of an intervening galaxy, creating two adjacent images of the quasar in the sky.

Light from The two images take different paths to Earth. When the brightness of the quasar fluctuates, the two images flicker one after the other and not at the same time. The time delay between these two flickers as well as information about the gravitational field of the embedding galaxy can be used to track the journey of the light and to deduce the distances from Earth to the quasar and the foreground galaxy. Knowing the redshifts of quasar and galaxy enabled the scientists to estimate how fast the universe spreads.

The UCLA team had already used the technique in the international collaboration of H0liCOW to examine quasars with four images of a quasar appearing around a foreground galaxy. Fourfold images, however, are not nearly as common – double-image quasars are probably about five times as common as quadruplicate images.

To demonstrate the technique, the team led by UCLA examined a double-mapped Quasar +4332, designated SDSS J1206; They relied on data from the Hubble Space Telescope, Gemini and W.M. Keck Observatories and from the Cosmological Monitoring of Gravitational Lenses or COSMOGRAIL, a network – a program managed by the Swiss Ecole Polytechnique Federale de Lausanne to determine the Hubble constant.

Tommaso Treu, professor of physics and astronomy at UCLA The newspaper's lead author said the researchers took daily shots of the quasar for several years to accurately measure the time lag between images. To obtain the best possible estimate of the Hubble constant, they combined the data collected on this quasar with data previously collected by their H0liCOW collaboration on three quadruple quasars.

"The beauty of this measurement is that it is so highly complementary and independent of others," said Treu.

The UCLA-led team estimated the Hubble constant to be about 72.5 kilometers per second per megaparse, a number that is consistent with the other researchers in the research. The distance used to supernovas – exploding stars in distant galaxies – as key measurement. However, both estimates are about 8 percent higher than one based on a dim glow of the cosmic microwave background from the entire sky, a relic of 380,000 years after the Big Bang, when light flew freely through space for the first time. 19659005] "If there is an actual difference between these values, it means that the universe is a little bit more complicated," said Treu.

On the other hand, so good faith, it could also be the one measurement – or all three – are wrong.

Researchers are now looking for more quasars to improve the accuracy of their Hubble constant measurement. Treu said one of the key lessons of the new paper is that double-imaged quasars provide scientists with many useful sources of light for their constant Hubble calculations. However, the UCLA-led team is currently focusing on 40 quasars with four-exposure images, as they may provide more useful information than doubly imaged images.


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

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