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Home / Science / Quasars with a double-image gravitational lens could eventually help to figure out how fast the universe is expanding

Quasars with a double-image gravitational lens could eventually help to figure out how fast the universe is expanding



How fast is the universe expanding? Astronomers could not answer this question exactly. You have a name for the expansion rate of the universe: the Hubble constant or the Hubble law. However, the measurements are always based on different values, and astronomers have been debating this topic for decades.

The basic idea for measuring the Hubble constant is to look at distant light sources, usually a type of supernova or variable star called "standard candles" and measure the redshift of their light. No matter how astronomers do it, they can not deal with an agreed value, just with a set of values. A new study using quasars and gravitational lenses could help solve the problem.

That the universe is expanding is out of the question. We have known this for about 100 years. The light from distant galaxies will shift red as they move away from us, and the measurement of this redshift has given different values ​​for universal expansion.

"The Hubble constant anchors the physical scale of the universe."


Simon Birrer, postdoctoral fellow of UCLA and lead author of the study.

The rate of expansion is measured in kilometers per second per megaparsec. written as (km / s) / Mpc. For example, something that expands at a speed of 10 (km / s) / Mpc means that two points in space are separated by a distance of 1 megaparsec (that is 3.26 million light years) at a speed of 10 kilometers Secondly.

When it was first discovered in the 1920s, an expansion of 625 kps / Mpc was assumed. However, from the fifties, research was measured at less than 100 kps / Mpc. In recent decades, several studies have measured the rate of expansion and are at speeds between about 67 and 77 kps / Mpc.

But science does not accept answers for something that should have value. It would not be science if it did. As a result, scientists repeatedly try to measure the Hubble constant differently to see if they can do it right, because the Hubble constant is more than just a measure of the expansion of the Universe.

"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. So it's a big deal to get it right.

A new study recently published in Monthly Notices by the Royal Astronomical Society attempts a novel method for measuring the Hubble constant. The research, led by a team of astronomers at UCLA, is based on distant quasars whose light is exposed to gravitational lenses before it reaches the Earth.

  Artistic impression of ULAS J1120 + 0641, a very distant quasar driven by a black hole two billion times the size of the sun. Credit: ESO / M. Kornmesser
Artistic impression of ULAS J1120 + 0641, a very distant quasar driven by a black hole two billion times the size of the sun. Credit: ESO / M. Kornmesser

Quasars are ultrabright objects. They are also referred to as active galactic nuclei because they are thought to be caused by supermassive black holes in the center of galaxies. The electromagnetic radiation they emit is caused by the swirling accretion disk around the black hole. When the disk of matter accelerates around the hole, it emits a lot of energy.

Because quasars are so bright, they can be seen from great distances. This makes them not only fascinating objects of study, but also as markers for the study of the Hubble law.

Gravitational lenses appear when the light source strikes an intermediate galaxy from an extremely distant object, quasars in this study, before it reaches observers on Earth. The extreme mass of the galaxy is enough to bend the light, much like a glass lens. The result is a kind of "mirror house" effect. The picture below shows what it looks like. The discovery of the gravitational lens is most closely associated with Einstein, although it was first observed in 1979.

  Hubble image of a glowing red galaxy (LRG) gravitationally distorting the light of a far-flung blue galaxy known as gravitational lensing. Credit: ESA / Hubble & NASA
Hubble image of a glowing red galaxy (LRG) that gravitationally distorts the light of a far farther blue galaxy, a technique known as gravitational lensing. Photo credits: ESA / Hubble & NASA

This study focused on double quasars. A double quasar, sometimes referred to as a double quasar, is not two nearby quasars, but rather an effect of gravitational lenses. With a double quasar, their light is focused around an intervening galaxy before reaching the Earth, creating two images of the quasar. No previous study attempted to determine the rate of expansion of the universe.

As the light of the quasar is bent around the intervening galaxy and produces two images of the same quasar, this opens up a unique opportunity for observation. The light that creates the individual images of the quasar wanders a different path to each image. As the light of the quasar fluctuates, there is a delay between the flicker in each of the two images.

By measuring the time delay between flickers and knowing the mass of the intervening galaxy, the team deduced the distances between the Earth, the lens galaxy, and the quasar. Knowing the redshifts of quasar and galaxy enabled the scientists to estimate how fast the universe spreads.

This study focused on the double quasar called SDSS J1206 + 4332 and also relied on data from the Hubble Space Telescope, the Gemini and WM Keck Observatories and from the Cosmological Monitoring of Gravitational Lenses or COSMOGRAIL, Network. For several years, the team took daily shots of the double quasar, which accurately measured the time delay between the flickers. In combination with the other data, the astronomers received one of the best measurements of the Hubble constant so far.

"The beauty of this measurement is that it is highly complementary to and independent of others," said Tommasso Treu, a professor of physics at UCLA and astronomy and the lead author of the newspaper.

A picture from the newspaper. The lens galaxy is G0 in the center of the picture. A and B is the double quasar SDSS J1206 + 4332. G2 is a triplet galaxy and G3 and G4 are other nearby galaxies. Picture credits: Hubble Space Telescope, Treu et. al. 2019.

How fast is it expanding?

"… the universe is a bit more complicated.


Tommasso Treu, Professor of Physics and Astronomy at UCLA.

The team had a value for the Hubble constant of 72.5 kilometers per second per megaparsek. This aligns it with other measurements that used remote supernovae as standard candles to measure the Hubble constant. However, it is about 7% higher than measurements that rely on cosmic microwave background for measurement.

This is not the end of the Hubble Law debate. There is still this nagging difference between the measurement methods. What does that mean? "If there is an actual difference between these values, it means that the universe is a bit more complicated," Treu said. Treu also said that one of the measurements or even all three are wrong.

The team will maintain its Quasarl lens measurement method. They are looking at 40 quadruple quasars to hopefully get an even more accurate measure of the expansion rate of the universe.

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