by Clara Moskowitz
One of the greatest mysteries of our time is the question of dark matter. What is this invisible stuff that seems to make up 27 percent of our universe? This question has been hanging over astrophysicists for decades, but the plot has thickened recently, and I'm hopeful that 2018 will prove to be an important time for progress in figuring out this hidden constituent of the cosmos.
Evidence shows that there must be more than the visible matter we see holding the stars together in galaxies and the galaxies together in clusters — otherwise these things would likely fly apart. Theorists' best guess is that undiscovered "cold dark matter" particles make up this missing bulk, and that we cannot see them or detect other signs of their presence because they almost never interact with normal matter. Yet decades of searching for the most plausible dark matter candidates has turned up nothing but increasingly strong limits on where these hidden particles could be hiding. Many in the field have become more and more frustrated at our seeming inability to make headway on this problem. Several developments have occurred recently, however, that offer scientists new clues in this outstanding quandary.
One comes from a surprising observation of a signal from the early universe. Astronomers using the EDGES radio antenna in Australia measured radio waves dating from just 180 million years after the Big Bang. This emission originated when the first stars lit up and their light bombarded the hydrogen atoms that filled space back then. When electrons in the hydrogen atoms absorbed this energy, they emitted photons with a wavelength of 21 cm. The EDGES antenna observed these same photons, redshifted, of course, due to the expansion of the universe.
Astronomers fully expected to find this relic from the first starlight, but what they didn't expect was to find so much of it. One explanation is that the hydrogen gas filling the early universe was colder than we thought. And the one thing theorists could think of to make the hydrogen that cold is dark matter. Perhaps, researchers proposed, cold dark matter particles--which would be even chillier than the hydrogen--were bumping into the hydrogen atoms and stealing some of their heat. If this was the case, however, the dark matter particles would have to be no heavier than several proton masses, which would be surprising indeed.
Such a lightweight particle would cast doubt on one of theorists' favored candidates for dark matter--a class of particles called weakly interacting massive particles, or WIMPs. Though no one has ever seen such entities, they seem to make sense and could tie into other popular theories, such as the idea of supersymmetry, which posits a partner particle for every known particle in the universe. Physicists like the idea of WIMPs so much that they have built increasingly sensitive detectors over several decades designed to search for them, but none have succeeded so far. WIMPs, however, are predicted to be about 100 times heavier than protons--that is, much too heavy to have bounced off hydrogen in this manner.
The new 21-cm emission results, published at the end of February in two papers in Nature by Judd D. Bowman of Arizona State University et al. and Rennan Barkana of Tel Aviv University, still need to be verified with future measurements. And theorists will have to mull over the implications for dark matter more deeply--some response papers suggest that the EDGES finding may have nothing to do with dark matter after all. But the discovery is nonetheless intriguing and potentially significant.
Another recent finding throws a wrench into a tale that many of us dark matter enthusiasts have been following since 2014. At that time, astronomers announced they had observed an excess of x-ray light at an energy of 3.5 kiloelectron volts (keV) coming from the Perseus galaxy cluster. This extra light isn't expected from any normal astrophysical objects, and researchers thought immediately that it might be coming from dark matter particles. This first observation was made by the Chandra and XMM-Newton space telescopes, but in 2016 Japanese astronomers launched the Hitomi telescope, which was specifically designed to look for this kind of light feature. Hitomi, however, saw no sign of any excess 3.5 KeV light. At the time, many scientists assumed the initial observations were wrong and crossed that signal off the list of intriguing dark matter leads.
But a new burst of optimism came our way last December when Joseph P. Conlon of the University of Oxford and his colleagues published a study in Physical Review D suggesting an explanation for all of these observations that revives the dark matter interpretation. These researchers reanalyzed the 2014 observations and other older Chanda data, and found that 3.5 KeV light actually decreases in a small area around a supermassive black hole at the center of one of the galaxies in Perseus called NGC1275. Something there must be absorbing these X-rays. This dip in light near the black-hole could explain why Hitomi found no extra 3.5 KeV photons--the dip was cancelling out the excess in other areas.
The researchers propose so-called "fluorescent dark matter" particles that have two states--a lower state and an excited state. These particles could absorb a photon with an energy of 3.5 KeV to enter the excited state, and then re-emit that photon to drop back down to their ground states. To confirm the latest analysis, we'll have to wait for better observations of the Perseus cluster, and specifically of the black hole at the center of the NGC1275 galaxy.
Another new finding seems to dash the hopes of dark matter hunters--at least the subset of them aiming to find evidence that dark matter interacts with itself.
For a few years astronomers have been intrigued by the galaxy cluster Abell 3827, where four galaxies are in the process of colliding. Observations in 2015 suggested that the dark matter in these galaxies was lagging behind visible matter in the cosmic pile-up, but new-and-improved measurements from the Atacama Large Millimeter-Submilliter Array (ALMA) and the Very Large Telescope (VLT), show the dark matter centered on the galaxies, as conventionally expected. (How, you may ask, can astronomers tell where dark matter is when it's invisible? Its gravity gives it away: telescopes can pinpoint where most of the gravity is coming from by looking at how it bends the light of galaxies through a process called gravitational lensing.)
That finding is bad news for models of "self-interacting" dark matter. Dark matter clearly does not interact much--if at all--with regular matter through the normal forces of electromagnetism, the strong or the weak force. If it did, it would be much easier to detect. But some scientists had thought dark matter might interact with itself through some "dark force" that regular matter doesn't feel. If so, there must also be "dark boson" particles to carry this force, just as electromagnetism is carried by boson particles called photons.
Astronomer Richard Massey of Durham University in England reported the latest observations April 6 at the European Week of Astronomy and Space Science in Liverpool, England (and posted the findings to the preprint server arXiv). He and his team, who were also behind the 2015 observations, had originally thought that clumps of dark matter might be interacting with other clumps of dark matter, causing it to lag behind the visible matter, which, since it doesn't interact with dark matter except through gravity, would zoom on by. Yet the latest measurements indicate that nothing special is going on in Abell 3827, and scientists must go back to the drawing board to look for evidence of self-interacting dark matter. Massey, at least, plans to do just that. He and collaborators will launch a balloon experiment called SuperBIT from New Zealand later this year to study other galaxy clusters for any hints of lagging dark matter.
Who's to say if any of these efforts will find the smoking-gun that researchers long for, but all of them offer the exciting promise of more clues one way or the other coming soon.