The latest Dark Matter developments
Researchers continue to discover more about the elusive substance that comprises 90% of the mass of the Universe. March saw the discovery of two galaxies lacking dark matter and constraints placed on dark matter properties. Whilst a possible candidate — the axion — is still missing.
Two more dark matter-free galaxies discovered
In two separate studies, researchers have begun to further develop our knowledge of dark matter — the substance that makes up between 70–90% of the mass of the visible Universe.
In what seems like an ironic development, researchers have discovered a further two galaxies that seem not to contain any dark matter. Something that actually bolsters the case for the existence of the substance — rather than explaining its properties with a revised model of gravity.
The research — published in the Astrophysical Journal Letters — builds upon the observation last year of a galaxy with no apparent dark matter content. The discovery had left astronomers sceptical, to say the least, as it was the only observation of its kind.
Astronomer Pieter van Dokkum of Yale University, who led last year’s study, noted: “If there’s only one object, you always have a little voice in the back of your mind saying, ‘but what if you’re wrong?’
“Even though we did all the checks we could think of, we were worried that nature had thrown us for a loop and had conspired to make something look really special whereas it was really something more mundane.”
The new work focuses on a ghostly galaxy 60 million light-years from our solar system — NGC 1052-DF2 (DF2)— which has no discernable dark matter. In addition to this study, another study published in the same journal divulged details of DF4, yet another galaxy — this one dim and diffuse — with no apparent dark matter.
The research implies a larger population of galaxies, for which, dark matter is not required to grant them stability. The research gives new insight into the nature of dark matter and hints that there is much more to learn about the evolution of galaxies.
Both DF2 and DF4 are part of a relatively new class of galaxies called ultra-diffuse galaxies (UDGs). They are as large as the Milky Way but have between 100 to 1,000 times fewer stars. This makes them appear fluffy and translucent — and difficult to observe.
Shany Danieli, a graduate student at Yale University and lead author of the DF2 study, says: “The fact that we’re seeing something that’s just completely new is what’s so fascinating.
“No one knew that such galaxies existed, and the best thing in the world for an astronomy student is to discover an object — whether it’s a planet, a star, or a galaxy — that no one knew about or even thought about.”
As mentioned above, the irony of these discoveries, researchers point out actually strengthens the case for the existence of dark matter. This is because it shows that the effects of dark matter are not coupled to normal matter — as we would expect if these properties were simply quirks in our understanding of gravity.
Danieli is leading a wide area survey with the Dragonfly Telephoto Array — a telescope designed by van Dokkum — to look for more examples in a systematic way, then observe candidates again using the Keck telescopes.
They conclude: “We hope to next find out how common these galaxies are and whether they exist in other areas of the universe.
“We want to find more evidence that will help us understand how the properties of these galaxies work with our current theories. Our hope is that this will take us one step further in understanding one of the biggest mysteries in our universe — the nature of dark matter.”
Whilst these studies focused on galaxies missing dark matter, another study — also released this month — concentrates on eliminating possible dark matter candidates.
Physicists place constraints on dark matter
Researchers from Russia, Finland, and the U.S. have put a constraint on the theoretical model of dark matter particles by analyzing data from astronomical observations of active galactic nuclei. The new findings provide an added incentive for research groups around the world trying to crack the mystery of dark matter: No one is quite sure what it is made of. The paper was published in the Journal of Cosmology and Astroparticle Physics.
The question of what particles make up dark matter is a crucial one for modern particle physics. Despite the expectations that dark matter particles would be discovered at the Large Hadron Collider, this did not happen.
A number of then-mainstream hypotheses about the nature of dark matter had to be rejected. Diverse observations indicate that dark matter exists, but apparently, something other than the particles in the Standard Model constitutes it.
Physicists thus have to consider further options that are more complex. The Standard Model needs to be extended. Among the candidates for inclusion are hypothetical particles that may have masses in the range from 100 to 10 times the mass of the electron such as axions mentioned above. That is, the heaviest speculated particle has a mass 40 orders of magnitude greater than that of the lightest.
One theoretical model treats dark matter as being made up of ultralight particles — such as axions, the focus of the next section. This offers an explanation for numerous astronomical observations. However, such particles would be so light that they would interact very weakly with other matter and light, making them exceedingly hard to study. So, researchers turn to astronomical observations.
Sergey Troitsky, a co-author of the paper and chief researcher at the Institute for Nuclear Research of the Russian Academy of Sciences, says: “We are talking about dark matter particles that are 28 orders of magnitude lighter than the electron. This notion is critically important for the model that we decided to test.
“The gravitational interaction is what betrays the presence of dark matter. If we explain all the observed dark matter mass in terms of ultralight particles, that would mean there is a tremendous number of them. But with particles as light as these, the question arises: How do we protect them from acquiring effective mass due to quantum corrections?”
Calculations show that one possible answer would be that these particles interact weakly with photons — that is, with electromagnetic radiation. This offers a much easier way to study them: by observing electromagnetic radiation in space.
When the number of particles is very high, researchers can treat them as a field of certain density permeating the universe. This field coherently oscillates over domains that are on the order of 100 parsecs — about 325 light years — in size.
What determines the oscillation period is the mass of the particles. If the model considered by the authors is correct, this period should be about one year. When polarized radiation passes through such a field, the plane of radiation polarization oscillates with the same period. If periodic changes like this do in fact occur, astronomical observations can reveal them. And the length of the period — one terrestrial year — is very convenient, because many astronomical objects are observed over several years, which is enough for the changes in polarization to manifest themselves.
The authors of the paper used data from Earth-based radio telescopes because they return to the same astronomical objects many times during a cycle of observations. Such telescopes can observe remote active galactic nuclei — regions of superheated plasma close to the centres of galaxies. These regions emit highly polarized radiation. By observing them, one can track the change in polarization angle over several years.
Troitsky continues: “At first it seemed that the signals of individual astronomical objects were exhibiting sinusoidal oscillations. But the problem was that the sine period has to be determined by the dark matter particle mass, which means it must be the same for every object. There were 30 objects in our sample. And it may be that some of them oscillated due to their own internal physics, but anyway, the periods were never the same.
“This means that the interaction of our ultralight particles with radiation may well be constrained. We are not saying such particles do not exist, but we have demonstrated that they don’t interact with photons, putting a constraint on the available models describing the composition of dark matter.”
Yuri Kovalev, a co-author of the study and laboratory director at the Moscow Institute of Physics and Technology and Lebedev Physical Institute of the Russian Academy of Sciences, is extremely excited at the prospect.
He says: “Just imagine how exciting that was! You spend years studying quasars when one-day theoretical physicists turn up, and the results of our high-precision and high angular resolution polarization measurements are suddenly useful for understanding the nature of dark matter.”
In the future, the team plans to search for manifestations of hypothesized heavier dark matter particles proposed by other theoretical models. This will require working in different spectral ranges and using other observation techniques.
According to Troitsky, the constraints on alternative models are more stringent: “Right now, the whole world is engaged in the search for dark matter particles. This is one of the great mysteries of particle physics.
“As of today, no model is accepted as favoured, better-developed, or more plausible with regard to the available experimental data. We have to test them all. Inconveniently, dark matter is “dark” in the sense that it hardly interacts with anything, particularly with light.”
In some scenarios, it could have a slight effect on light waves passing through. But other scenarios predict no interactions at all between our world and dark matter, other than those mediated by gravity.
“This would make its particles very hard to find,” concludes Troitsky.
None the less, this difficulty has not persuaded an MIT-led team to stop searching within a specific mass-range.
Dark matter experiments find no evidence of axions
Physicists have performed the first run of a new experiment to detect axions — hypothetical particles predicted to be among the lightest particles in the universe. If they exist, axions would be virtually invisible, yet inescapable — forming 85% of the mass of the universe, in the form of dark matter.
Axions are particularly unusual in that they are expected to modify the rules of electricity and magnetism at a minute level. In a paper published in the journal Physical Review Letters, the MIT-led team reports that in the first month of observations the experiment detected no sign of axions within the mass range of 0.31 to 8.3 nano electronvolts.
This means that axions within this mass range — equivalent to about one-quintillionth the mass of a proton — either don’t exist or they have an even smaller effect on electricity and magnetism than previously thought.
Lindley Winslow, the principal investigator of the experiment, says: “This is the first time anyone has directly looked at this axion space.
“We’re excited that we can now say, ‘We have a way to look here, and we know how to do better!’”
While they are thought to be everywhere, axions are predicted to be virtually ghost-like, having only tiny interactions with anything else in the universe.
Winslow, the Jerrold R. Zacharias Career Development Assistant Professor of Physics at MIT, adds: “As dark matter, axions shouldn’t affect your everyday life.
“But they’re thought to affect things on a cosmological level, like the expansion of the universe and the formation of galaxies we see in the night sky.”
Because of their interaction with electromagnetism, axions are theorized to have a surprising behaviour around magnetars — a type of neutron star that churns up a hugely powerful magnetic field. If axions are present, they can exploit the magnetar’s magnetic field to convert themselves into radio waves, which can be detected with dedicated telescopes on Earth.
In 2016, a trio of MIT theorists drew up a thought experiment for detecting axions, inspired by the magnetar. The experiment was dubbed ABRACADABRA, for the A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus, and was conceived by Thaler, who is an associate professor of physics and a researcher in the Laboratory for Nuclear Science and the Center for Theoretical Physics, along with Benjamin Safdi, then an MIT Pappalardo Fellow, and former graduate student Yonatan Kahn.
The team proposed a design for a small, doughnut-shaped magnet kept in a refrigerator at temperatures just above absolute zero. Without axions, there should be no magnetic field in the centre of the doughnut, or, as Winslow puts it, “where the munchkin should be.” However, if axions exist, a detector should “see” a magnetic field in the middle of the doughnut
After the group published their theoretical design, Winslow, an experimentalist, set about finding ways to actually build the experiment.
She says: “We wanted to look for a signal of an axion where, if we see it, it’s really the axion.
“That’s what was elegant about this experiment. Technically, if you saw this magnetic field, it could only be the axion, because of the particular geometry they thought of.”
It is a challenging experiment because the expected signal is less than 20 atto-Tesla. For reference, the Earth’s magnetic field is 30 micro-Tesla and human brain waves are 1 pico-Tesla.
In building the experiment, Winslow and her colleagues had to contend with two main design challenges, the first of which involved the refrigerator used to keep the entire experiment at ultracold temperatures. The refrigerator included a system of mechanical pumps whose activity could generate very slight vibrations that Winslow worried could mask an axion signal.
The second challenge had to do with noise in the environment, such as from nearby radio stations, electronics throughout the building turning on and off, and even LED lights on the computers and electronics, all of which could generate competing magnetic fields.
The team solved the first problem by hanging the entire contraption, using a thread as thin as dental floss. The second problem was solved by a combination of cold superconducting shielding and warm shielding around the outside of the experiment.
“We could then finally take data, and there was a sweet region in which we were above the vibrations of the fridge, and below the environmental noise probably coming from our neighbours, in which we could do the experiment.”
The researchers first ran a series of tests to confirm the experiment was working and exhibiting magnetic fields accurately. The most important test was the injection of a magnetic field to simulate a fake axion, and to see that the experiment’s detector produced the expected signal — indicating that if a real axion interacted with the experiment, it would be detected. At this point, the experiment was ready to go.
Winslow says: “If you take the data and run it through an audio program, you can hear the sounds that the fridge makes
“We also see other noise going on and off, from someone next door doing something, and then that noise goes away. And when we look at this sweet spot, it holds together, we understand how the detector works, and it becomes quiet enough to hear the axions.”
In 2018, the team carried out ABRACADABRA’s first run, continuously sampling between July and August. After analyzing the data from this period, they found no evidence of axions within the mass range of 0.31 to 8.3 nano electronvolts that change electricity and magnetism by more than one part in 10 billion.
The experiment is designed to detect axions of even smaller masses, down to about 1 femto electronvolts, as well as axions as large as 1 micro electronvolt.
The team will continue running the current experiment, which is about the size of a basketball, to look for even smaller and weaker axions. Meanwhile, Winslow is in the process of figuring out how to scale the experiment up, to the size of a compact car — dimensions that could enable detection of even weaker axions.
Winslow concludes: “There is a real possibility of a big discovery in the next stages of the experiment.
“What motivates us is the possibility of seeing something which would change the field. It’s high-risk, high-reward physics.”
‘A Second Galaxy Missing Dark Matter in the NGC 1052 Group’ https://iopscience-iop-org.libezproxy.open.ac.uk/article/10.3847/2041-8213/ab0d92
‘Still Missing Dark Matter: KCWI High-resolution Stellar Kinematics of NGC1052-DF2’ https://iopscience.iop.org/article/10.3847/2041-8213/ab0e8c
‘Constraining the photon coupling of ultra-light dark-matter axion-like particles by polarization variations of parsec-scale jets in active galaxies’ https://iopscience.iop.org/article/10.1088/1475-7516/2019/02/059/meta
‘First Results from ABRACADABRA-10 cm: A Search for Sub-μeVAxion Dark Matter’ https://journals-aps-org.libezproxy.open.ac.uk/prl/abstract/10.1103/PhysRevLett.122.121802