Quasars could help reveal the story of the early universe

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

These misunderstood galactic nuclei outperform just about every other object in the universe. Visible from unimaginable distances, they could turn out to be exactly what researchers need to understand certain traits of the entire cosmos, including its expansion. Early attempts to harness the powerful searchlights were marred by uncertainties, but new analysis reveals the objects could shine bright enough for researchers to use them to fill a gaping hole in cosmic history.

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

Special supernovas light up an expanding universe

In recent decades, the gold standard for measuring great distances has been a variety of stellar explosions: the type 1a supernova. These supernovae usually explode with the same brightness, so astronomers know that the brightest need to be closer while the fainter ones need to be farther away. These so-called “standard candles” revealed that the universe is expanding faster and faster, implying that a mysterious “dark energy” is separating the galaxies.

But individual stars, even exploding ones, eventually die out as astronomers peer deeper into the darkness. With current telescopes, researchers cannot see Type 1a supernovae beyond nine to ten billion years (because light takes billions of years to reach Earth, looking into space also means look back.) Without any visible supernovae, cosmologist-researchers who specifically study the evolution of the cosmos as a whole – are left largely in the dark about what happened for the first four billion years. of the universe.

A new standard candle

This is where quasars come in. A supermassive black hole pulls gas towards it with such intensity that matter becomes white-hot, eclipsing the entire galactic system around it.

Given that astronomers can spot the burning of quasars in the universe’s first billion years, could these objects serve as brighter, more penetrating standard candles?

Some astronomers believe they can, thanks to a crucial property. Quasars emit ultraviolet light and some of these ultraviolet rays break into a surrounding hot electron cloud, releasing higher energy x-rays. Because ultraviolet light produces x-rays in a predictable way, the brightness of a quasar’s x-rays is linked to its ultraviolet brightness in a fixed fashion, regardless of the distance from the galaxy. By comparing ultraviolet and x-ray emissions with the overall brightness or brightness of a quasar, astronomers can use it as a marker of a cosmic kilometer.

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Or at least that’s the theory. It appeared to withstand many relatively close quasars, but the many details of how objects emit ultraviolet light and x-rays remain unknown. Some researchers have wondered if the quasars of the early universe behaved the same way they do today.

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

“It was a necessary check so that we could use this method to measure distances and to be sure that we were not using a tool that changes over time,” says Bisogni.

The group published a pre-publication of their research, which was accepted by the journal Astronomy & Astrophysics, September 7.

A first look at ancient history

Astronomers suspect that their former quasars are already hinting that the cosmos theorists’ account may require major changes. When they calculated the distances to the oldest quasars in 2019, their results clashed with the main “standard model” of cosmology, a potentially revolutionary interpretation being that dark energy has changed over time. “We think it’s real,” says Francesca Civano, an astrophysicist for the quasars team at the Center for Astrophysics, which is jointly run by Harvard University and the Smithsonian. “The difference is quite significant.

Bold claims require solid evidence, however, and cosmologists need more conviction. Dan Scolnic, a cosmologist at Duke University who uses Type 1a supernovae to make precise measurements of the expansion of the universe and has not been involved in quasar research, praised the group as ” one of the best teams at understanding quasar physics, ”and said they are“ doing the right things ”to test the potential of quasars as standard candles.

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Still, he doesn’t think current quasar observations are mature enough to dethrone supernovae, which astronomers can locate five times more accurately than quasars. The latest work by Italian astronomers overcomes this drawback by analyzing a mountain of quasars large enough to easily pass statistical tests. But Scolnic fears, for example, that different varieties of quasars may be lurking in the relatively noisy data.

“What makes me a little nervous is that when you have individual measurements that are not very precise,” he says, “you must be asking yourself what systematic uncertainties are lurking in this data.”

The situation will become clearer in the years to come. A recently launched X-ray space telescope, known as eROSITA, is expected to reveal millions of nearby quasars, which could validate their utility as standard candles in the better understood local universe, while further investigation will likely discover more objects in the shroud. , old universe.

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