“While it is certainly possible that axions — a hypothetical elementary particle predicted to be among the lightest particles in the universe–make up dark matter, they also may make up a source of “dark radiation” in our universe which we refer to as the Cosmic Axion Background (CaB), analogous to the observed Standard Model radiation, known as the Cosmic Microwave Background,” Jeff Dror, with the Santa Cruz Institute for Particle Physics, told The Daily Galaxy. “Furthermore,” adds Dror, “experiments designed to search for axion dark matter can be repurposed to search for the CaB and potentially lead to the discovery of the axion, which would also teach us a great deal about the history of our universe.”
The Invisible Universe
If they exist, axions would be virtually invisible, yet inescapable; they could make up nearly 85 percent of the mass of the universe, in the form of dark matter–whose existence was postulated in order to explain why certain subatomic reactions appear to violate basic symmetry constraints, in particular symmetry in time. The 1980 Nobel Prize in Physics went for the discovery of time-asymmetric reactions that led to understanding how the the matter we are made of was once created in a Big Bang and how it could survive the birth pains.
Meanwhile, during the following decades, astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize that most of the matter in the universe was not visible. It was dubbed dark matter, and today’s best measurements find that about 84% of matter in the cosmos is dark.
This component is dark not only because it does not emit light—it is not composed of atoms or their usual constituents, like electrons and protons, and its nature is mysterious. Axions have been suggested as one possible solution. Particle physicists, however, have so far not been able to directly detect axions, leaving their existence in doubt and reinvigorating the puzzles they were supposed to resolve.
Interacting with Magnetic Fields
In 2018, Harvard Center for Astrophysics astronomer Paul Nulsen and his colleagues used a novel method to investigate the nature of axions. Quantum mechanics constrain axions, if they exist, to interact with light in the presence of a magnetic field. As they propagate along a strong field, axions and photons should transmute from one to the other in an oscillatory manner.
Because the strength of any possible effect depends in part on the energy of the photons, the galaxies. They observed X-rays from the nucleus of the galaxy M87, which is known to have astronomers used the Chandra X-ray Observatory to monitor bright X-ray emission from strong magnetic fields, and which (at a distance of only fifty-three million light-years) is close enough to enable precise measurements of variations in the X-ray flux.
Moreover, M87 lies in a cluster of galaxies, the Virgo cluster, which should insure the magnetic fields extend over very large scales and also facilitate the interpretation. Not least, M87 has been carefully studied for decades and its properties are relatively well known.
The search did not find the signature of axions. It does, however, set an important new limit on the strength of the coupling between axions and photons, and is able to rule out a substantial fraction of the possible future experiments that might be undertaken to detect axions. The scientists note that their research highlights the power of X-ray astronomy to probe some basic issues in particle physics, and point to complementary research activities that can be undertaken on other bright X-ray emitting galaxies.
In an email to The Daily Galaxy, Harvard’s Paul Nulsen wrote: “Axion like particles (ALPs) are not really my area of expertise, but it turns out that X-ray data obtained for other purposes could be used to look for them. My colleagues, Helen Russell, then at the Institute of Astronomy, Cambridge (now at the University of Nottingham) and M. C. David Marsh, then at DAMTP (department of applied mathematics and theoretical physics), Cambridge (now at Stockholm University) recognized the opportunity presented by Chandra X-ray data obtained by Helen for the galaxy M87, at the center of the Virgo cluster.
“ALPs interact with magnetic fields in a way that can cause them to oscillate back and forth between their original state and photons,” Nulsen explained. “As a result, the weak magnetic field in the hot plasma that fills the Virgo cluster should cause oscillations in the X-ray spectrum from the bright active nucleus of M87. In practice, we could only place upper limits on the ALP features in the spectrum of M87, giving upper limits on the mass of the ALPs and on the strength of the interaction between ALPs and the electromagnetic field. I have not followed subsequent work on this subject closely, but the papers I have seen only seem to make such limits more restrictive.”
Cosmic axion Background (CaB) –a New Fossil from the Early Universe
No one knows what happened in the universe for its first 400,000 years, but a June 7, 2021 paper, suggests discovering the hypothetical particle axion could be the fossil of the universe researchers have been waiting for to shed light on the early history of the universe. What’s more, current dark matter experiments announced by Kavli Institute for the Physics and Mathematics of the Universe in June 2021 may have already detected it in its data. In their paper, they suggest the possibility of searching for an axion analogue of the CMB, the so-called Cosmic axion Background.
In the new paper, the researchers point out that as experimentalists develop more sensitive instruments to search for dark matter, they may stumble upon another sign of axions in the form of the CaB. But because the CaB shares similar properties with dark-matter axions, there is a risk the experiments would throw the CaB signal out as noise.
Finding the CaB at one of these instruments would be a double discovery. Not only would it confirm the existence of the axion, but researchers worldwide would immediately have a new fossil from the early universe. Depending on how the CaB was produced, researchers could learn about various different aspects of the universe’s evolution never possible before.
“What we have proposed is that, by changing the way current experiments analyze data, we may be able to search for left-over axions from the early universe. Then, we might be able to learn about the origin of dark matter, phase transition, or inflation at the beginning of the universe. There are already experimental groups who have shown interest in our proposal, and I hope we can find out something new about the early universe that wasn’t known before,” said Hitoshi Murayama, MacAdams Professor of Physics at UC Berkeley, senior faculty scientist at LBNL, and principal investigator at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) in Japan;.
“The evolution of the universe can produce axions with a characteristic energy distribution. By detecting the energy density of the universe currently made up of axions, experiments such as MADMAX, HAYSTAC, ADMX, and DMRadio could give us answers to some of the most important puzzles in cosmology, such as: How hot did our universe get? What is the nature of dark matter? Did our universe undergo a period of rapid expansion known as inflation? And was there ever a cosmic phase transition?” said Dror.