Active galaxies point to new physics of cosmic expansion



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The artist's impression of the quasars, the nuclei of galaxies where an active supermassive black hole is pulling matter from its surroundings at very intense rates, located at ever greater distances from us. Credit: ESA (artist's impression and composition); NASA / ESA / Hubble (background galaxies); CC BY-SA 3.0 IGO

Investigating the history of our cosmos with a large sample of "active" distant galaxies observed by ESA's XMM-Newton, a team of astronomers found that there may be more in the initial expansion of the universe than predicted by the standard cosmology model.


According to the main scenario, our universe contains only a few percent of the common matter. A quarter of the cosmos is made of indescribable dark matter, which we can feel gravitationally but not observe, and the rest is the still more mysterious dark energy that is driving the current acceleration of universe expansion.

This model is based on an infinity of data collected over the last two decades, from the cosmic microwave background, or CMB – the first light in the history of the cosmos, released only 380,000 years after the Big Bang and observed in unprecedented detail by ESA's Planck Mission – for more local & # 39; observations. The latter include supernova explosions, clusters of galaxies and the gravitational distortion imprinted by dark matter in distant galaxies, and can be used to track cosmic expansion in recent times in cosmic history – in the last nine billion years.

A new study, led by Guido Risaliti of the Università di Firenze in Italy and Elisabeta Lusso of the University of Durham in the United Kingdom, points to another kind of tracer cosmic quasars that would fill part of the gap between these observations by measuring the expansion of universe up to 12 billion years ago.

Quasars are the nuclei of galaxies where an active supermassive black hole is pulling matter from its surroundings at very intense speeds, shining through the electromagnetic spectrum. When the material falls into the black hole, it forms a spiral disc that radiates in visible and ultraviolet light; this light, in turn, heats the nearby electrons, generating X-rays.

Three years ago, Guido and Elisabeta realized that a well-known relationship between the brightness of ultraviolet and X-ray quasars could be used to estimate the distance to these sources – something that is notoriously complicated in astronomy – and, ultimately, to probe the history of expansion of the universe.

Astronomical sources whose properties allow us to measure their distances are called "standard candles."

The most notable class, known as the Ia-type supernova, consists of the spectacular death of white dwarf stars after they have overfilled the material of a companion star, generating predictable brightness explosions that allow astronomers to identify the distance. Observations of these supernovae in the late 1990s revealed the accelerated expansion of the universe in the last few billion years.

The artistic impression of a quasar, the core of a galaxy where an active supermassive black hole is pulling matter from its surroundings at very intense rates. When the material falls into the black hole, it forms a spiral disc that radiates in visible and ultraviolet light; this light, in turn, heats the nearby electrons, generating X-rays. The relationship between ultraviolet brightness and x-ray quasars can be used to estimate the distance to these sources – something that is notoriously complicated in astronomy – and ultimately to fathom the history of expansion of the Universe. A team of astronomers applied this method to a large sample of quasars observed by ESA's XMM-Newton to investigate the history of our cosmos up to 12 billion years ago, finding that there may be more in the initial expansion of the universe than predicted by the model standard of cosmology. Credit: ESA – C. Carreau

"Using quasars as standard candles has great potential, since we can observe them at much greater distances from each other than Type Ia supernovae, and use them to investigate earlier times in the history of the cosmos," explains Elisabeta.

With a sizable sample of quasars at hand, astronomers have already put their method into practice, and the results are intriguing.

By scouring the XMM-Newton archive, they collected X-ray data for more than 7000 quasars, combining them with ultraviolet observations from the Sloan Digital Sky Survey. They also used a new dataset, especially obtained with XMM-Newton in 2017 to examine very distant quasars, observing them as they were when the universe was only two billion years old. Finally, they supplemented the data with a small number of even more distant quasars and some relatively close ones observed at NASA's Chandra and Swift X-ray observatories, respectively.

"Such a large sample allowed us to closely examine the relationship between X-rays and ultraviolet emission of quasars, which greatly refined our technique to estimate their distance," says Guido.

The new XMM-Newton observations of distant quasars are so good that the team even identified two different groups: 70% of the sources shine brightly on low-energy X-rays, while the remaining 30% emit smaller amounts of X-rays. characterized by higher energies. For further analysis, they only maintained the previous group of sources, in which the ratio between X-ray and ultraviolet emissions seems clearer.

"It is remarkable that we can discern this level of detail in sources so far away from us that its light has traveled for over ten billion years before it reaches us," says Norbert Schartel, ESA's XMM-Newton project scientist.

After traversing the data and bringing the sample to about 1600 quasars, astronomers were left with the best observations, leading to robust estimates of the distance to those sources that they could use to investigate the expansion of the universe.

"When we combine the quasar sample, which covers almost 12 billion years of cosmic history, with the most local sample of type Ia supernovas, covering only the last eight billion years, we find similar results in overlapping times," says Elisabeta

Graph showing distance measurements of astronomical objects such as supernovae type Ia (cyan symbols) and quasars (yellow, red and blue symbols) that can be used to study the history of universe expansion.

"However, in the early stages we can only plumb with quasars, we find a discrepancy between the observed evolution of the universe and what we could predict based on the standard cosmological model."

Examining this previously unexplored period of cosmic history with the help of quasars, astronomers have revealed a possible tension in the standard model of cosmology, which could require the addition of extra parameters to reconcile the data with theory.

"One possible solution would be to invoke evolving dark energy with a density that increases over time," says Guido.

Incidentally, this particular model would also alleviate another tension that kept cosmologists busy lately, relative to Hubble's constant-the current rate of cosmic expansion. This discrepancy was found between estimates of the Hubble constant in the local universe, based on supernova data – and independently on clusters of galaxies – and those based on Planck's observations of the cosmic microwave background in the primordial universe.

"This model is quite interesting because it can solve two puzzles at once, but the jury has definitely not come out and we will have to look at many other models in great detail before we can solve this cosmic enigma," added Guido.

The team is eager to observe even more quasars in the future to further refine their results. Other clues will also come from ESA's Euclid mission, scheduled for a launch in 2022 to explore the last ten billion years of cosmic expansion and investigate the nature of dark energy.

"These are interesting times to investigate the history of our universe, and it is exciting that XMM-Newton can contribute to looking at a cosmic era that has remained largely unexplored so far," concludes Norbert.


Explore more:
A new technique to evaluate the distant universe

More information:
G. Risaliti et al. Hubble cosmological constraints of quasars in high redshifts, Astronomy of Nature (2019) DOI: 101038 / s41550-018-0657-z

Journal Reference:
Astronomy of Nature

Provided by:
European Space Agency

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