If mysterious dark matter is made up of mini black holes formed in the first moments after the big bang, then it may be possible to test this theory and detect these black holes as they collide with stars, argue two postdoctoral researchers from the United States. Tantalisingly, there is a slim possibility we may have already detected them and we just haven’t realised, the evidence sitting in a data archive somewhere, waiting to be unearthed.

Dark matter has been a puzzle at the forefront of cosmology since the 1970s, when Vera Rubin and Kent Ford built on observations made by Fritz Zwicky in the 1930s that galaxies on the outskirts of clusters were moving faster than the gravity from the observed matter within the clusters should dictate. It seemed something else was there, invisible but massive. This ‘dark’ matter has remained elusive, but several explanations are currently vying to explain it.

One of these explanations is black holes. These are not the supermassive black holes at the hubs of large galaxies, or even the stellar mass black holes born from the ashes of supernovae. Instead, the theory describes hypothetical primordial black holes that may have formed in the immense densities of matter and energy that existed in the Universe immediately after the big bang. If Michael Kesden of New York University and Shravan Hanasoge of Princeton University are correct, their existence could be proven by watching for their collisions with stars in our Galaxy.

These collisions would not destroy the black hole or the star. Although their characteristics are uncertain, Kesden and Hanasoge’s simulations posit that primordial black holes could have masses of 10¹⁸ kilograms (a million trillion kilograms, about the mass of a largish asteroid), which is too small to destroy a star as they pass through. “The black hole is like a very dense bullet passing through the Sun, which is like a fluffy feather pillow,” says Kesden.

The black hole’s gravity would perturb the star, first squeezing it slightly as the black hole passes through, then causing it to snap back into shape as it exits. This would create ripples that would cascade through the star and be visible on their surface as slight variations in brightness.

Already there are spacecraft embarking on asteroseismology missions, monitoring the regular oscillations on stars that provide hints of their convection processes, the structure of their interior and their magnetic activity. These spacecraft include Canada’s MOST satellite, the French-led CoRoT spacecraft, and NASA’s Kepler mission, which collects data on the brightness of stars as it searches for exoplanet transits. While Kesden is dubious as to whether these missions could have already seen such an encounter without it being recognised as such, theoretically black hole-induced ripples should stand out, his colleague Shravan Hanasoge tells Astronomy Now.

“The oscillations excited in a star are directly proportional to the mass of the black hole that passes through it,” he says. “We find from our calculations that waves generated by the black hole are of relatively large wavelength and high frequency compared to the ambient noise level.”

Hanasoge and Kesden estimate that ten thousand such encounters occur in our Galaxy each year, but with with several hundred billion stars in the Milky Way it is like looking for a needle in a haystack, with odds of tens of millions to one. However, their calculations suggest that, if primordial black holes make up the entirety of the missing mass in the Universe, then they could be expected to exist at a density of 500 million black holes per cubic light year in the disc of the galaxy, and in even higher concentrations in the MIlky Way’s halo, where many trillions of these tiny black holes (the radius of their event horizon would be merely a billionth of a metre) would exist.

Primordial black holes would certainly be an attractive solution to the dark matter problem. How do they rate against the other dark matter candidates? Here we take a look at the various possible forms of dark matter.

Black holes
The problem with primordial black holes is that we don’t even know if they exist, or whether they could survive for long. “For primordial black holes to form we need very large densities,” says Dr Anne Green of the University of Nottingham, who specialises in dark matter, primordial black holes and particle astrophysics. “Probably the most natural mechanism is the collapse of large density perturbations in the early Universe. These are the same perturbations that galaxies and galaxy clusters form from, but on smaller scales.”

Their mass would be equal to the mass within the observable Universe at the time they form, so the younger the Universe, the smaller the observable horizon and the less massive the black holes would be. We’re talking about during the first second here. Professor Stephen Hawking showed that black holes gradually lose mass, and ultimately evaporate, through the loss of ‘Hawking radiation’. The amount of Hawking radiation is inversely proportional to the mass of the black hole, so the smaller the black hole, the more rapid its evaporation. The earlier that primordial black holes formed, the less massive they will be and the less likely they are to survive the age of the Universe. However, searches for evaporating black holes releasing a final burst of gamma rays have so far come up empty handed.

Black holes would tick the box in other ways. They are invisible, which fits in with the ‘dark’ description of the missing mass, and they require no new physics or new particles to explain dark matter, unlike some other, more popular theories.

WIMPs
Weakly Interacting Massive Particles (WIMPs) are the current front-runner in the dark matter race. These are unknown, theoretical particles that interact through gravity and the weak force (which causes radioactive decay). The Large Hadron Collider (LHC) is searching for evidence of their existence, and theory suggests they fit the bill.

“My feeling, and the general consensus, is that WIMPs are the most likely dark matter candidate,” says Green. “If they exist they are generically produced with roughly the right abundance, and extensions of the standard model of particle physics, such as supersymmetry, typically contain concrete WIMP candidates.”

Kesden agrees. “The majority of the cosmology community, myself included, would say that our best guess for the identity of dark matter is the lightest supersymmetric particle,” he says, explaining that supersymmetry is a model that attempts to explain the relatively large range in masses between fundamental particles, and that it predicts that every particle has a supersymmetric partner particle. “In variants of this theory the lightest supersymmetric particle is stable, massive and non-interacting, a perfect dark matter candidate.”

There is caution in the wind, however. Supersymmetry is not necessarily correct, and the existence of WIMPs is being put to the test by the LHC. “If the LHC doesn’t see some sign of physics beyond the standard model in the next few years, then [WIMPs’] popularity will wane,” says Green.

MACHOs
If the WIMPs fail, then the MACHOs may see a resurgence. These are Massive Compact Halo Objects and would include things like neutron stars, brown dwarfs, white dwarfs and the like that may lurk undetected in the haloes of galaxies. The problem is that there is no conceivable way that enough of these regular dark bodies could accumulate to solve the mystery of dark matter.

“The constraints on the density of baryonic [normal] matter from nucleosynthesis and the cosmic microwave background radiation tells us that if MACHOs exist they have to be non-baryonic,” says Green. Furthermore, notes Kesden, searches for microlensing events (where the gravity of a smaller foreground body such as a star or planet briefly magnifies the light of a more distant object, much like gravitational lensing on the larger scale of galaxy clusters) in the Magellanic Clouds rule out any unseen bodies larger than ten millionths of the mass of the Sun. However, for what it’s worth, these constraints do support primordial black holes.

“Primordial black holes are the most plausible non-baryonic MACHO candidate, but there is no known reason for them to be produced in the right density to be dark matter,” says Green. “Therefore I’d currently rate them as substantially less likely than WIMPs.”

MOND
Suppose dark matter isn’t real, but an illusion, a figment of our imagination caused by a lack in our understanding of how gravity works. This is the central concept of Modified Newtonian Dynamics, developed by Mordehai Milgrom of Israel’s Weizmann Institute, which argues that at extremely low gravitational acceleration – such as what the traditional theory of gravity first developed by Isaac Newton suggests should be found on the outskirts of galaxies and galaxy clusters – gravity does not behave as we would expect, leading to galaxies orbiting on the edges of clusters faster than they should, which is what we observe. But MOND says there is no invisible matter there acting on the stars and speeding them up; rather gravity itself operates differently here.

MOND has been able to explain some aspects of dark matter, but received criticism by its inability to easily explain the Bullet Cluster. This was a merging between two clusters, imaged by the Hubble Space Telescope and the Chandra X-ray Observatory. Hubble was able to detect the visible matter in the form of galaxies, and Chandra was able to see the hot X-ray emitting gas of the clusters. However, the combined mass of the Bullet Cluster was also gravitationally lensing more distant galaxies, and the extent of the lensing could only be explained if two clouds of dark matter had passed straight through each other to end up on opposite sides of the Bullet Cluster. According to MOND the greatest lensing should have taken place close to the highest concentration of normal matter in the cluster, namely the X-ray emitting intra-cluster gas, but instead the strongest lensing signal was found where the dark matter would be. Similar cluster mergers have shown the same results.

All or nothing
All the current explanations for dark matter, be they hypothetical particles, unseen black holes, clouds of brown dwarfs and neutron stars or tinkering with Newtonian gravity, are as yet unproven. Certainly hypothetical particles are no less likely than altering classical physics or the existence of clouds of myriad tiny, unseen black holes. But is it possible that all could contribute to a combined dark matter solution?

The simplest explanation of dark matter is that there is one answer, not many. “People sometimes argue that because the observable matter comes in so many different forms we shouldn’t expect the dark matter to be in a single form,” says Green. “I’m not sure I buy that argument because I think you’d need some underlying physical connection for different dark matter candidates to be produced with similar densities.”

Still, even though the simplest explanation – that dark matter is made of just one thing – is the likeliest, agrees Kesden, he points out that until we know more about what dark matter is, we shouldn’t be too quick to rule out any possibility. What we do know is that we now have the tools to investigate dark matter more thoroughly, whether it is deep within the bowels of the world’s largest particle accelerator or deep in space, watching for encounters between stars and tiny black holes or the interactions of galaxy clusters. The scientific community is in it for the long haul, but perhaps we’ll be surprised and a turning point discovery might result in an answer swifter than we expect. How could that answer be any more dramatic than tiny black holes punching through giant stars?