Scientists have discovered that unusually massive black holes appear to be missing from the Milky Way’s diffuse outer halo.
The discovery could spell bad news for theories that suggest the most mysterious form of cosmic “matter,” dark matter, consists of primordial black holes that formed in the first moments after the Big Bang.
Dark matter is mysterious because, despite being effectively invisible because it does not interact with light, it makes up about 86% of the matter in the known universe. This means that for every 1 gram of “everyday matter” that makes up stars, planets, moons and people, there are more than 6 grams of dark matter.
Scientists can infer the presence of dark matter by its interactions with gravity and its effect on everyday matter and light. Despite this and the ubiquity of dark matter, scientists have no idea what it might be made of.
Related: If the big bang created miniature black holes, where are they?
The new dark matter results come from a look back at 20 years of observations by a team of scientists from the Optical Gravitational Lensing Experiment (OGLE) at the Warsaw University Astronomical Observatory.
“The nature of dark matter remains a mystery. Most scientists think it consists of unknown elementary particles,” team leader Przemek Mróz of the University of Warsaw’s Astronomical Observatory said in a statement. “Unfortunately, despite decades of effort, no experiment, including those performed at the Large Hadron Collider, has found new particles that could be responsible for dark matter.”
The new findings don’t just challenge black holes as an explanation for dark matter; they also deepen the mystery of why stellar-mass black holes detected beyond the Milky Way appear to be more massive than those found at the boundaries of our galaxies.
Our primordial black holes are missing!
The search for black holes in the Milky Way’s halo owes its origins to the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its sister gravitational wave detector Virgo, which appear to have uncovered a population of unusually large, stellar-mass black holes.
Until the first detection of gravitational waves produced by LIGO and Virgo in 2015, scientists found that the population of stellar-mass black holes in our galaxy, born from the gravitational collapse of massive stars, tended to have masses between 5 and 20 times the sun.
Observations of gravitational wave mergers between stellar-mass black holes suggest a more distant population of black holes with much more mass, equivalent to 20 to 100 Suns. “The explanation of why these two populations of black holes are so different is one of the biggest mysteries of modern astronomy,” pointed out Mróz.
One possible explanation for this larger population of black holes is that they are remnants from the period just after the Big Bang, formed not by the collapse of massive stars, but by overly dense blobs of primordial gas and dust.
“We know that the early universe was not ideally homogeneous – small density fluctuations gave rise to present-day galaxies and galaxy clusters,” said Mróz. “Such density fluctuations, if they exceed a critical density contrast, can collapse to form black holes.”
These “primordial black holes” were first postulated by Stephen Hawking more than 50 years ago, but have remained frustratingly elusive. This could be because the smaller examples would quickly “escape” in a form of thermal energy called Hawking radiation and eventually evaporate, meaning they would not exist in the current epoch of the 13.8 billion year old universe. Still, this obstacle has not stopped some physicists from positing primordial black holes as a possible explanation for dark matter.
Dark matter is estimated to make up 90 to 95% of the Milky Way’s mass. This means that if dark matter is made up of primordial black holes, our galaxy should contain many of these ancient bodies. Black holes do not emit light because they are bound by a light-trapping surface called the “event horizon”. This means that we cannot “see” black holes unless they are feeding on the matter around them and casting their shadow on it. But like dark matter, black holes interact with gravity.
This allowed Mróz and colleagues to turn to Albert Einstein’s 1915 theory of gravity, general relativity, and the principle it introduced in the hunt for primordial black holes in the Milky Way.
Einstein extends his hand
Einstein’s theory of general relativity states that objects of matter distort the very fabric of space and time, united as a single entity called “space-time”. Gravity is the result of this curvature, and the more massive an object is, the more extreme distortion of space-time it causes, and thus the more “gravity” it generates.
Not only does this curvature tell planets how to orbit stars and tell stars to race around the centers of their home galaxies, but it also bends the path of light coming from background stars and galaxies. The closer the light travels to a material object, the more its path is “bent”.
Different paths of light from a single background object can thus be bent and shift the apparent location of the background object. Sometimes this effect can even cause the background object to appear in multiple places in the same sky image. Other times, light from a background object is enhanced and that object is magnified. This phenomenon is known as “gravitational lensing” and the intermediate body is called a gravitational lens. Weak examples of this effect are called “microlensing”.
If a primordial black hole in the Milky Way passes between Earth and a background star, we should observe microlensing effects on that star for a short time.
“Microlensing occurs when three objects—the observer on Earth, the light source, and the lens—align up virtually perfectly in space,” said Andrzej Udalski, principal investigator of the OGLE survey. “During microlensing, the source light can be deflected and magnified, and we observe a temporary brightening of the source light.”
How long it takes for the light from the background source to brighten depends on the mass of the lens that passes between it and the Earth, with objects of greater mass giving rise to longer microlenses. An object around a solar mass should cause brightening for about a week; however, for lenses with a mass 100 times the mass of the Sun, the brightening should take up to several years.
Previous attempts have been made to use microlenses to detect primordial black holes and to study dark matter. Previous experiments seemed to show that black holes are less massive than the Sun and may contain less than 10% dark matter. However, the problem with these experiments was that they were not sensitive to extremely long microlensing events.
Because more massive black holes (similar to those recently detected by gravitational wave detectors) would cause longer events, these experiments were not sensitive to this population of black holes either.
This team improved the sensitivity of long-time microlensing by turning to a 20-year-long observation of nearly 80 million stars located in a satellite galaxy or Milky Way called the Large Magellanic Cloud (LMC).
The data studied, which Udalski described as “the longest, largest, and most accurate photometric observations of stars in the LMC in the history of modern astronomy,” were collected by the OGLE project between 2001 and 2020 during its third and fourth operational phases. The team compared the microlensing events observed by OGLE with the theoretically predicted number of such events, assuming that the Milky Way’s dark matter is made up of primordial black holes.
“If all the dark matter in the Milky Way was composed of black holes with 10 solar masses, we should detect 258 microlenses,” Mróz said. “For 100 solar masses of black holes, we expected 99 microlensing events. For 1000 solar masses of black holes — 27 microlensing events.”
In contrast to these estimated event abundances, the team found only 12 microlensing events in the OGLE data. Further analysis revealed that all of these events could be explained by known stars in the Milky Way and the LMC itself. After these calculations, the team found that black holes of 10 solar masses could contain at most 1.2% of dark matter, smaller black holes of 100 solar masses could account for no more than 3.0% of dark matter and 1000 black holes of of solar radiation could contain only 11% of dark matter.
“This suggests that massive black holes can make up at most a few percent of dark matter,” explained Mróz.
“Our observations suggest that primordial black holes cannot make up a significant fraction of dark matter and at the same time explain the observed black hole merger rates measured by LIGO and Virgo,” Udalski concluded. “Our results will remain in astronomy textbooks for decades.”
Astronomers can thus go back to the drawing board to explain the observation of supermassive stellar-mass black holes beyond the Milky Way, while physicists are still puzzling over the true nature of dark matter.
The team’s research was published June 24 in Nature and the Astrophysical Journal Supplement Series.