Is Dark Energy Evolving?

The Teacup Quasar


Observational cosmologists are actively searching for a “new physics” that may solve the enduring enigma of our rapidly expanding Universe. Quasars, the ancient cores of galaxies where a supermassive black hole is actively pulling in matter from its surroundings at very intense rates, may hold the clue to solving the mystery.

Luminous quasars –like the cosmic squall shown above known as the “Teacup” buried at the center of its host galaxy– outshine their host, and can easily be detected at extreme distances in the early universe. Investigating the history of our cosmos with a large sample of distant ‘active’ galaxies observed by ESA’s XMM-Newton in 2019, a team of astronomers found there might be more to the early expansion of the universe than predicted by the standard model of cosmology.

The Mystery Driving the Universe’s Expansion

According to the leading scenario, our universe contains only a few percent of ordinary matter. One quarter of the cosmos is made of the elusive dark matter, which we can feel gravitationally but not observe, and the rest consists of the even more mysterious dark energy that is driving the current acceleration of the universe’s expansion.


Using active galaxies to measure cosmic expansion. ESA (artist’s impression and composition); NASA/ESA/Hubble (background galaxies); CC BY-SA 3.0 IGO

This model is based on a multitude of data collected over the last couple of 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 – to more ‘local’ observations. The latter include supernova explosions, galaxy clusters and the gravitational distortion imprinted by dark matter on distant galaxies, and can be used to trace cosmic expansion in recent epochs of cosmic history – across the past nine billion years.

A 2019 study, led by Guido Risaliti of Università di Firenze, Italy, and Elisabeta Lusso of Durham University, UK, points to another type of cosmic tracer – quasars – that would fill part of the gap between these observations, measuring the expansion of the universe up to 12 billion years ago.

Using Quasars to Probe the Expansion of the Universe

Quasars are the cores of galaxies where an active supermassive black hole is pulling in matter from its surroundings at very intense rates, shining brightly across the electromagnetic spectrum. As material falls onto the black hole, it forms a swirling disc that radiates in visible and ultraviolet light; this light, in turn, heats up nearby electrons, generating X-rays.

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

Astronomical sources whose properties allow us to gauge their distances are referred to as ‘standard candles’.

Is Dark Energy a Uniform Force Across Space and Time?

The most notable class, known as Type Ia supernova, consists of the spectacular demise of white dwarf stars after they have gorged on material from a companion star, generating explosions of predictable brightness that allows astronomers to pinpoint their distance. Observations of these supernovae in the late 1990s revealed the universe’s accelerated expansion over the last few billion years.

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

Digging into the XMM-Newton archive, they collected X-ray data for over 7000 quasars, combining them with optical observations from the ground-based Sloan Digital Sky Survey. The quasars are so distant that the visible light detected here on Earth actually corresponds to their ultraviolet radiation that has been substantially redshifted due to the stretching cosmological expansion of the universe. They also used a new set of data, specially obtained with XMM-Newton in 2017 to look at very distant quasars, observing them as they were when the universe was only about two billion years old. Finally, they complemented the data with a small number of even more distant quasars and with some relatively nearby ones, observed with NASA’s Chandra and Swift X-ray observatories, respectively.


Quasar Measurement

“Such a large sample enabled us to scrutinize the relation between X-ray and ultraviolet emission of quasars in painstaking detail, which greatly refined our technique to estimate their distance,” says Guido.

The XMM-Newton observations of distant quasars are so good that the team even identified two different groups: 70 percent of the sources shine brightly in low-energy X-rays, while the remaining 30 percent emit lower amounts of X-rays that are characterized by higher energies. For the further analysis, they only kept the earlier group of sources, in which the relation between X-ray and ultraviolet emission appears clearer.

“It is quite remarkable that we can discern such level of detail in sources so distant from us that their light has been travelling for more than ten billion years before reaching us,” said Norbert Schartel, XMM-Newton project scientist at ESA.

After skimming through the data and bringing the sample down to about 1600 quasars, the astronomers were left with the very best observations, leading to robust estimates of the distance to these sources that they could use to investigate the universe’s expansion.

Does Dark Energy Increase as Time Goes By?

“When we combine the quasar sample, which spans almost 12 billion years of cosmic history, with the more local sample of Type Ia supernovae, covering only the past eight billion years or so, we find similar results in the overlapping epochs,” says Elisabeta.

The graph above shows measurements of the distance to astronomical objects such as Type Ia supernovae (cyan symbols) and quasars (yellow, red and blue symbols) that can be used to study the expansion history of the universe.

“However, in the earlier phases that we can only probe with quasars, we find a discrepancy between the observed evolution of the universe and what we would predict based on the standard cosmological model.”

Looking into this previously poorly explored period of cosmic history with the help of quasars, the astronomers have revealed a possible tension in the standard model of cosmology, which might require the addition of extra parameters to reconcile the data with theory.

“One of the possible solutions would be to invoke an evolving dark energy, with a density that increases as time goes by,” says Guido.

Incidentally, this particular model would also alleviate another tension that has kept cosmologists busy lately, concerning the Hubble 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 galaxy clusters – and those based on Planck’s observations of the cosmic microwave background in the early universe.

“This model is quite interesting because it might solve two puzzles at once, but the jury is definitely not out yet and we’ll have to look at many more models in great detail before we can solve this cosmic conundrum,” adds Guido.

The team is looking forward to observing even more quasars in the future to further refine their results. Additional clues will also come from ESA’s Euclid mission, scheduled for a 2022 launch to explore the past ten billion years of cosmic expansion and investigate the nature of dark energy.

The Last Word: Columbia University’s Colin Hill

“I am a bit cautious about the interpretation of these results. In particular, I don’t think it is yet widely accepted that quasars can serve as standard (or standardizable) candles, which would be a necessary step for their use as a probe of the cosmic expansion history, ” wrote Columbia University cosmologist, Colin Hill, in an email to The Daily Galaxy. Hill analyzes cosmological data to search for evidence of new physics, focusing on the cosmic microwave background radiation. He is a member of the Atacama Cosmology Telescope, Simons Observatory, and CMB-S4 collaborations.

“If such a standardization of their absolute luminosities can be robustly shown,” Hill explained, “then the results would be of interest. For now, I would exercise some caution. I also am not aware of any subsequent work that has robustly shown evidence for the type of evolving dark energy that was suggested in that work.  Nevertheless, we are continuing to look for clues toward any such evidence of new physics!”

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via J.Colin Hill and European Space Agency

Image credit: Located about 1.1 billion light years from Earth, this Chandra image shows an actively growing quasar’s host galaxy], originally discovered in visible light images by citizen scientists in 2007 as part of the Galaxy Zoo project, using data from the Sloan Digital Sky Survey. (X-ray: NASA/CXC/Univ. of Cambridge/G. Lansbury et al; Optical: NASA/STScI/W. Keel et al).

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