Blindingly bright black holes could help cosmologists see deeper into the universe's past

Astronomers keen to solve some of the universe’s most fundamental mysteries have their eyes on a new target: quasars, tempests of matter that swirl violently around colossal black holes and pierce the cosmos with their brilliant light. 

© NASA, ESA and J. Olmsted (STScI) An illustration of a quasar, the chaotic bodies that swirl around black holes.

These poorly understood galactic cores outshine just about every other object in the universe. Visible across unthinkable distances, they could turn out to be just what researchers need to understand certain traits of the entire cosmos, including its expansion. Early attempts to harness the mighty spotlights have been plagued with uncertainties, but a new analysis finds that the objects just might shine consistently enough for researchers to use them to fill a yawning hole in cosmic history. 

“There is a very huge gap,” says Susanna Bisogni, an astrophysicist at the National Institute for Astrophysics in Milan, Italy. “Quasars have the potential to span this range.”

In recent decades, the gold standard for measuring vast distances has been one variety of stellar explosion: the type 1a supernova. These supernovae typically detonate with the same brightness, so astronomers know that more brilliant ones must be closer while dimmer ones must be farther away. These so-called “standard candles” have revealed that the universe is expanding faster and faster, implying that a mysterious “dark energy” is driving galaxies apart. 

But individual stars, even exploding ones, eventually peter out as astronomers peer deeper into the darkness. With current telescopes, researchers can’t see type 1a supernovae beyond nine to ten billion years ago (because light takes billions of years to reach earth, looking out into space also means looking back in time.) Without any visible supernovae, cosmologists—researchers who specifically study the evolution of the cosmos as a whole—are left largely in the dark as to what went on during the universe’s first four billion years. 

That’s where quasars come in. A supermassive black hole drags gas toward itself with such intensity that the matter gets white hot, outshining the entire galactic system that surrounds it. 

Since astronomers can pick out the blaze of quasars during the universe’s first billion years, could these objects serve as brighter, more penetrating standard candles? 

Some astronomers believe that they can, thanks to one crucial property. Quasars pump out ultraviolet light, and some of these ultraviolet rays smash into a surrounding cloud of hot electrons, unleashing higher energy X-rays. Because the ultraviolet light makes X-rays in a predictable way, a quasar’s X-ray brightness is tied to its ultraviolet brightness in a fixed manner, no matter how far away the galaxy is.  By comparing the ultraviolet and X-ray emissions with how bright or dim a quasar appears overall, astronomers can use it as a cosmic mile marker. 

[Related: One of Einstein’s predictions on black holes has finally been confirmed]

Or at least that’s the theory. It has appeared to hold up for many relatively nearby quasars, but the many details of how the objects emit ultraviolet light and X-rays remain unknown. Some researchers have questioned whether quasars in the early universe behaved in the same way they do now. 

To find out, a team of Italian astronomers combed through legacy observations and looked further back in time. They used data from the Sloan Digital Sky Survey to find quasars shining in the ultraviolet, and data from the Chandra X-ray Observatory to find quasars shining in X-rays and compared the two groups. They found that the relationship between the two emissions held all the way back to about 1.3 billion years after the Big Bang. In other words, the quasars burned steadily throughout the universe’s history, as good standard candles should. 

“This was a necessary check for us to be able to use this method for measuring distances, and to be sure that we were not using a tool that’s changing in time,” Bisogni says. 

The group published a preprint of their research, which has been accepted by the journal Astronomy & Astrophysics, on September 7. 

The astronomers suspect that their ancient quasars are already hinting that theorists’ account of the cosmos might need major edits. When they calculated the distances to the oldest quasars in 2019, their results clashed with the leading “Standard Model” of cosmology, with one potentially groundbreaking interpretation being that dark energy has changed over time. “We think it’s real,” says Francesca Civano, an astrophysicist with the quasar team who works at the Center for Astrophysics, which is jointly run by Harvard University and the Smithsonian. “The difference is quite significant.”

Bold claims require rock solid evidence, however, and cosmologists need more convincing. Dan Scolnic, a cosmologist at Duke University who uses type 1a supernovae to make precise measurements of the universe’s expansion and was not involved with the quasar research, praised the group as “one of the top teams for understanding the physics of quasars,” and said they’re “doing the right steps” to test quasars’ potential as standard candles. 

[Related: Astronomers may have found the surprisingly elusive medium-sized black hole]

Yet he doesn’t believe current quasar observations are mature enough to dethrone supernovae, the locations of which astronomers can pinpoint five times more accurately than they can quasars. The Italian astronomers’ latest work overcomes this drawback by analyzing a mountain of quasars big enough to easily pass statistical tests. But Scolnic worries, for instance, that different varieties of quasars could be hiding in the comparatively noisy data. 

“What makes me a bit nervous is that, when you have individual measurements that are not terribly precise,” he says, “you have to wonder what systematic uncertainties are lurking in that data.”

The situation will become clearer in the coming years. A recently launched X-ray space telescope known as eROSITA is expected to turn up millions of nearby quasars, which could validate their usefulness as standard candles in the better-understood local universe, while additional surveys will likely discover more of the objects in the shrouded, ancient universe.  

“Cosmologists, they need to take quasars seriously for cosmological measurements,” Civano says. “They’re a very good resource.”

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