Quasars with a Double-Image Gravitational Lens Could Help Finally Figure out how Fast the Universe is Expanding

How fast is the Universe expanding? That’s a question that astronomers haven’t been able to answer accurately. They have a name for the expansion rate of the Universe: The Hubble Constant, or Hubble’s Law. But measurements keep coming up with different values, and astronomers have been debating back and forth on this issue for decades.

The basic idea behind measuring the Hubble Constant is to look at distant light sources, usually a type of supernovae or variable stars referred to as ‘standard candles,’ and to measure the red-shift of their light. But no matter how astronomers do it, they can’t come up with an agreed upon value, only a range of values. A new study involving quasars and gravitational lensing might help settle the issue.

That the Universe is expanding is not in question. We’ve known this for about 100 years. The light from distant galaxies is red-shifted as they move away from us, and measuring that red-shift has produced different values for universal expansion.

“The Hubble constant anchors the physical scale of the universe.”

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

The rate of expansion is measured in kilometers per-second per-Megaparsec, written as (km/s)/Mpc. So for example, something expanding at the rate of 10 (km/s)/Mpc means that two points in space 1 megaparsec apart (the equivalent of 3.26 million light-years) are racing away from each other at a speed of 10 kilometers per second.

When it was first discovered in the 1920s, the rate of expansion was thought to be 625 kps/Mpc. But starting in the 1950s, better research measured it as less than 100 kps/Mpc. In the last few decades, multiple studies have measured the expansion rate, and come up with speeds between about 67 to 77 kps/Mpc.

But science won’t accept an array of answers for something which should have one value. It wouldn’t be science if it did. So scientists keep trying different ways to measure the Hubble Constant to see if they can get it right, because the Hubble constant is more than just a measurement of the expansion of the universe.

“The Hubble constant anchors the physical scale of the universe,” said Simon Birrer, a UCLA postdoctoral scholar and lead author of the study. Without a precise value for the Hubble constant, astronomers can’t accurately determine the sizes of remote galaxies, the age of the universe or the expansion history of the cosmos. So getting it right is a big deal.

A new study just published in Monthly Notices of the Royal Astronomical Society is trying a novel method of measuring the Hubble Constant. The research is led by a team of astronomers at UCLA, and relies on distant quasars whose light undergoes gravitational lensing before it reaches Earth.

Artist’s impression of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun. Credit: ESO/M. Kornmesser

Quasars are ultra-bright objects. They’re also called active galactic nuclei, because they’re thought to be caused by supermassive black holes at the center of galaxies. The electromagnetic radiation that they emit is caused by the swirling accretion disc around the black hole. As the disc of matter around the hole speeds up, it emits an enormous amount of energy.

Since quasars are so luminous, they can be seen from vast distances. This makes them not only fascinating objects of study, but also useful as markers for studying Hubble’s Law.

Gravitational lensing occurs when light source from an extremely distant object, quasars in this study, encounters an intervening galaxy before it reaches observers on Earth. The extreme mass of the galaxy is sufficient to bend the light, similar to the way a glass lens does. The result is a kind of ‘house of mirrors’ effect. The image below shows what it looks like. The discovery of gravitational lensing is most closely associated with Einstein, though it wasn’t until 1979 that it was observed.

Hubble image of a luminous red galaxy (LRG) gravitationally distorting the light from a much more distant blue galaxy, a technique known as gravitational lensing. Credit: ESA/Hubble & NASA

This study focused on double quasars. A double quasar, sometimes called a twin quasar, isn’t two quasars close to each other, but rather an effect of gravitational lensing. With a double quasar, their light is lensed around an intervening galaxy before reaching Earth, producing two images of the quasar. No previous study has used them to try to determine the Universe’s rate of expansion.

As the light from the quasar is bent around the intervening galaxy, producing two images of the same quasar, it sets up a unique observational opportunity. The light that creates the separate images of the quasar travels a different path to each image. As the light from the quasar fluctuates, there’s a delay between the flicker in each of the two images.

By measuring the time delay between the flickers, and by knowing the mass of the intervening galaxy, the team deduced the distances between Earth, the lensing galaxy, and the quasar. Knowing the redshifts of the quasar and galaxy enabled the scientists to estimate how quickly the universe is expanding.

This study focused on the double quasar called SDSS J1206+4332, and also 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, network. The team spent several years taking daily images of the double quasar, which gave them very precise measurements of the time delay between flickers. When combined with the other data, it gave astronomers one of the best measurements of the Hubble Constant yet.

“The beauty of this measurement is that it’s highly complementary to and independent of others,” said Tommasso Treu, UCLA professor of physics and astronomy and the paper’s senior author.

An image from the paper. The lensing galaxy is G0 in the center of the image. A and B is the double quasar SDSS J1206+4332. G2 is a triplet galaxy, and G3 and G4 are other nearby galaxies. Image Credit: Hubble Space Telescope, Treu et. al. 2019.

So, How Quickly Is It Expanding?

“…the universe is a little more complicated.

Tommasso Treu, UCLA professor of physics and astronomy.

The team came up with a value for the Hubble Constant of 72.5 kilometers per second per megaparsec. This puts it in line with other measurements that used distant supernovae as standard candles to measure the Hubble Constant. But it’s about 7% higher than measurements that rely on the Cosmic Microwave Background to measure it.

This isn’t the end of the debate over Hubble’s Law. There’s still that nagging difference between the measurement methods. What does it mean? “If there is an actual difference between those values, it means the universe is a little more complicated,” Treu said. Treu also said that one of the measurements, or even all three, are wrong.

The team is going to persist with their quasar-lensing measurement method. They’re looking at 40 quadruple quasars to hopefully give them an even more precise measurement of the Universe’s rate of expansion.


Evan Gough

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