Observational cosmologists are actively searching for a “new physics” that may solve the enduring enigma of our rapidly expanding Universe. Quasars are ancient cores in galaxies where a massive black hole pulls in matter at very high rates from its surroundings. This may be the key to unlocking 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
The leading scenario states that the universe only contains a small percentage 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.
To measure cosmic expansion, we use active galaxies. 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.
Quasars are used to probe the expansion of the universe
Quasars, which are the cores galaxies, are areas where a supermassive dark hole is actively pulling in matter at extremely high rates. These brightly illuminated regions of the electromagnetic spectrum. The black hole absorbs material and forms a swirling disk that emits ultraviolet and visible light. This light heats up nearby electrons and generates 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’.
Type Ia supernova is the most famous class. It involves the demise of white dwarf star stars that have eaten material from a companion Star. This causes explosions of predictable brightness which allow astronomers pinpoint their distance. Observations of these supernovae in the late 1990s revealed the universe’s accelerated expansion over the last few billion years.
The astronomers now have a large sample of quasars to work with, and the results are fascinating.
The XMM-Newton archive was searched and they obtained X-ray data covering over 7000 quasars. They also combined these with optical observations taken from Sloan Digital Sky Survey. They are so far from Earth that visible light is actually derived from their ultraviolet radiation, which has been significantly redshifted by the stretching cosmological growth of the universe. A new set of data was also used by the team, especially obtained using XMM-Newton 2017 to examine very distant quasars. They were observed as they were in 2017, when the universe was just 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.
“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 have been so accurate that the team has even identified two distinct groups. 70 percent of sources shine brightly with low-energy Xrays while the remaining 30% emit lower amounts of Xrays which are characterized as higher energies. The team only kept the first group of sources because it showed a clearer relationship between ultraviolet emission and Xray radiation.
“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.
Is Dark Energy Increasing 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 below shows distances to astronomical object types such as Type Ia Supernovae (cyan symbols), and quasars. These can be used to study expansion history of our 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.”
Astronomers discovered a tension in the standard model for cosmology by looking at this period of cosmic history using quasars. This may require additional parameters to reconcile data and 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.
To improve their findings, the team looks forward to continuing to observe quasars. 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 HillIn an email, The Daily Galaxy Hill uses cosmological data analysis to find evidence of new Physics. His focus is on the cosmic microwave radiation background radiation. Hill is a member 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. I recommend caution for now. Also, I don’t know of any other work that strongly supports the idea of dark energy evolving as suggested in that paper. 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 HillAnd European Space Agency
Image credit 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. G. Lansbury et al; Optical: NASA/STScI/W. Keel et al).
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Maxwell Moe is an astrophysicist and NASA Einstein Fellow at the University of Arizona. Max is often found at Kitt Peak National Observatory two nights per week, exploring the mysteries of the Universe. Max earned his Ph.D. in Astronomy in 2015 from Harvard University.
