BYLINE: Russ Nelson
Researchers at The University of Alabama in Huntsville (UAH), a part of The University of Alabama System, have published a series of two papers in the Monthly Notices of the Royal Astronomical Society that resolve one of three major outstanding puzzles in cosmology: the “missing baryon problem,” a discrepancy between the amount of baryonic matter detected from shortly after the Big Bang when compared with recent epochs. Dr. Massimiliano “Max” Bonamente, a professor of physics and astronomy, along with Dr. David Spence and international colleagues, used X-ray radiation from quasars to determine the “missing” particles reside in the warm-hot intergalactic medium, or WHIM, a state of matter characterized by low density and high temperatures.
“This is the result of over 10 years of work at UAH, primarily by myself and a recent graduate of ours, David Spence, also in collaboration with several scientists worldwide,” Bonamente says. “With three major problems in modern cosmology – missing baryons, identification of dark matter, identification of dark energy – it’s case closed on the first.”
Baryons are subatomic particles, most commonly protons and neutrons, that comprise the bulk of visible matter in the universe. The WHIM is a crucial component of the cosmic web, the large-scale structure of the universe composed of enormous filaments of dark matter and gas that connect galaxies to one another.
“The universe is believed to start off as a ‘ball of fire,’ the Big Bang; it then cools and forms structures on various scales: stars, galaxies, filaments of galaxies,” Bonamente notes. “The gas is attracted by the gravity of these filaments that stretch hundreds of millions of light-years, and the gas heats back up as it falls towards the WHIM. This is something any graduate student in physics would learn in their classical dynamics course, so astronomers were confident in this simple picture.”
Current observations suggest baryons make up about five percent of the total energy density of the universe. Yet, a significant fraction of present-day baryons remained unaccounted for in deep far-ultraviolet (FUV) searches. A census of baryons in the recent observable universe found that the observed baryonic matter accounts for about one half the expected amount.
“The location of the missing baryons near these WHIM structures was proposed back in 1999 in a seminal paper by Princeton scientists, and then consistently seen ever since in all subsequent simulations,” the researcher says. “But simulations aren’t real, and we needed to look in the real, uppercase Universe.”
To accomplish this, Bonamente, Spence and their colleagues analyzed X-ray sources from the European Space Agency’s orbiting X-ray telescope XMM-Newton and NASA’s Chandra Observatory to augment the FUV findings to make the breakthrough.
The core of the discovery lies in studying the cosmological density of missing baryons by analyzing X-ray absorption lines in quasars, focusing on the warm-hot intergalactic medium. In quasars, X-ray absorption lines arise when X-rays emitted from the quasar’s central black hole pass through intervening gas clouds, either within the quasar’s host galaxy or even farther away in the intergalactic medium
Staying grounded
The study made a systematic search for the absorption lines of highly ionized oxygen atoms in the spectra of 51 XMM-Newton and Chandra background quasars. These show up as “dark lines” in the X-ray spectrum, created when specific wavelengths are absorbed by atoms in the gas. By learning the properties of these absorption lines, like their strength and velocity, astronomers can learn about the physical conditions and amount of the absorbing gas.
“For nearly 20 years, astronomers used a single source – or a few at most – to study this effect. This results in a strong bias of results, and the problem that one source (or few) are not representative of the whole universe,” Bonamente notes. “So, we wanted to amend that problem, and the only way to do it is to go ‘big’ with the largest sample we could come up with. Statistics demands a large sample in order to make the best possible estimates, so we did just that.”
The results of this analysis contributes to the characterization of the missing baryons, indicating they are associated with the high–temperature portion of the WHIM, and possibly with large-scale WHIM filaments traced by galaxies, as predicted by numerical simulations and by other independent probes.
“It is only in X-rays that the relevant absorption lines occur,” Bonamente says. “This is something that is dictated by the atomic structures. The calculations of the wavelengths where the absorption lines occur – precisely in X-rays – are reliable, and make use of standard quantum-mechanics calculations that have been confirmed in our laboratories. So, we had no other choice: hot gas at those temperatures are only ‘visible’ in X-rays.”
Looking to the future of this research, Bonamente says there are still plenty of questions to answer.
“There is work to do to improve the characterization of the WHIM and the missing baryons,” the researcher says. “What is exactly the temperature of the WHIM? How do these baryons distribute in the cosmos – closer to galaxy clusters or more in the intergalactic space? Are they rich in ‘metals’ or mostly hydrogen and helium? Answers to these questions will reduce the error bars on our measurements.”
With regards to solving the big challenges in cosmology in general, Bonamente likes to take a well-grounded approach.
“The other problems in cosmology are more speculative, in my opinion, and it may even be that there is no dark matter or dark energy after all. But it’s good to know that the ordinary matter is as it should be. Cosmologists – and many other scientists – like to look at headline-grabbing, but speculative topics such as dark energy, when in fact there are problems more down-to-earth – pun intended – that need to be solved first. Wouldn’t you want to make sure your shoelaces are tied before you run the 100 meters at the Olympics?”
Bonamente adds that an effort of this magnitude could not be done without the support of many other dedicated scientists who bought into the idea and shared in the hard work.
“Throughout the years, I’ve had the good fortune of being surrounded by great colleagues who made this project happen,” the researcher concludes. “It’s my pleasure to thank and acknowledge Drs. Jussi Ahoranta and Kimmo Tuominen from Helsinki University, Dr. Natasha Wijers of Northwestern University and Dr. Jelle de Plaa of SRON Utrecht, who co-authored the papers describing these findings. And my good friend Dr. Jukka Nevalainen, who has also been a long-term collaborator on this and many other projects.”