Scientists have discovered that unusually massive black holes appear to be absent from the Milky Way’s diffuse outer halo.
The discovery could be bad news for theories suggesting that the most mysterious form in the universe, dark matter, is made up of primordial black holes that formed in the first moments after the Big Bang.
Dark matter is puzzling because, although it is invisible because it does not interact with light, this substance makes up about 86% of the matter in the known universe. This means that for every gram of “common matter” that makes up stars, planets, moons and humans, there are more than 6 grams of dark matter.
Scientists can infer the presence of dark matter by observing its interactions with gravity and the influence it exerts on everyday matter and light. Yet despite this and the ubiquity of dark matter, scientists have no idea what might be in it.
Related: If the Big Bang created miniature black holes, where are they?
The new findings on dark matter come from a look back over 20 years of observations carried out by a team of scientists from the Optical Gravitational Lensing Experiment (OGLE) survey at the University of Warsaw Astronomical Observatory.
“The nature of dark matter remains a mystery. Most scientists believe that it is composed of unknown elementary particles,” team leader Przemek Mróz of the University of Warsaw Astronomical Observatory said in a statement. “Unfortunately, despite decades of effort, no experiment, including experiments conducted at the Large Hadron Collider, has found new particles that could be responsible for dark matter.”
The new findings not only cast doubt on 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 within the confines of our galaxies.
Our primordial black holes have disappeared!
The team’s hunt for black holes in the Milky Way’s halo has its origins in the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its sister gravitational-wave detector, Virgo, which appear to have discovered an unusually large population of stellar-mass black holes.
Until the first detection of gravitational waves, produced by LIGO and Virgo in 2015, scientists had found that our galaxy’s population of stellar-mass black holes, born from the gravitational collapse of massive stars, tended to have masses between five and 20 times that of the sun.
Observations of gravitational waves of mergers between stellar-mass black holes indicate a population of more distant black holes with much larger masses, equivalent to 20 to 100 suns. “Explaining why these two populations of black holes are so different is one of the greatest mysteries of modern astronomy,” Mróz emphasized.
One possible explanation for this larger population of black holes is that they are remnants of a period just after the Big Bang that formed not from the collapse of massive stars but from overly dense areas of primordial gas and dust.
“We know that the early universe was not ideally homogeneous: small fluctuations in density gave rise to today’s galaxies and galaxy clusters,” Mróz said. “Similar density fluctuations, if they exceed a critical density contrast, can collapse and form black holes.”
These “primordial black holes” were first postulated by Stephen Hawking more than 50 years ago, but have remained unfortunately elusive. This could be because smaller examples would quickly “leak” a form of thermal energy called Hawking radiation and eventually evaporate, meaning they would not exist in the current epoch of the cosmos 13.8 billion years old. Yet this obstacle has not stopped some physicists from considering primordial black holes as a possible explanation for dark matter.
Dark matter is estimated to make up 90 to 95 percent of the Milky Way’s mass. This means that if dark matter is made up of primordial black holes, our galaxy should contain a large number of these ancient bodies. Black holes do not emit light because they are bounded by a light-trapping surface called an “event horizon.” This means that we cannot “see” black holes unless they feed on the matter around them and cast their shadows on it. But, just like dark matter, black holes interact with gravity.
Mróz and his colleagues were thus able to turn to Albert Einstein’s 1915 theory of gravity, general relativity, and a principle it introduced to hunt primordial black holes in the Milky Way.
Einstein lends a helping hand
Einstein’s theory of general relativity states that objects of mass distort the very fabric of space and time, united into a single entity called “spacetime.” Gravity is the result of this curvature, and the more massive an object, the more extreme the warping of spacetime it causes and, therefore, the greater the “gravity” it generates.
This curvature not only allows planets to orbit stars and stars to move around the center of their home galaxy, but it also bends the path of light from stars and background galaxies. The closer the light gets to the mass object, the more its path is “bent.”
Different light paths from a single background object can thus be curved, shifting the apparent location of the background object. Sometimes the effect can even make the background object appear in multiple places in the same sky image. Other times, the light from the background object is amplified and that object is enlarged. This phenomenon is known as “gravitational lensing” and the intermediate body is called gravitational lensing. Weak examples of this effect are called “microlensing.”
If a primordial black hole in the Milky Way passes between Earth and a background star, then we should observe microlensing effects on that star for a brief period of time.
“Microlensing occurs when three objects – an observer on Earth, a light source, and a lens – align in a nearly ideal way in space,” Andrzej Udalski, principal investigator of the OGLE study, said in the statement. “During a microlensing event, light from the source can be deflected and amplified, and we observe a temporary brightening of the light from the source.”
The length of time that the light from the background source is brightened depends on the mass of the lensing body passing between it and the Earth, with higher mass objects inducing longer microlensing events. An object around the mass of the sun should cause brightening for about a week; However, for lens bodies with a mass 100 times that of the sun, the brightening is expected to last several years.
Previous attempts have been made to use microlensing to detect primordial black holes and study dark matter. Previous experiments seemed to show that black holes were less massive than the sun and could contain less than 10% of dark matter. The problem with these experiments, however, was that they were not sensitive to microlensing events on an extremely long time scale.
So, since 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 sensitivity to long-lived microlensing events by turning to a 20-year survey of nearly 80 million stars located in a satellite galaxy or the Milky Way called the Large Magellanic Cloud (LMC).
The data studied, described by Udalski as “the longest, largest and most precise photometric observations of LMC stars in the history of modern astronomy”, were collected by the OGLE project from 2001 to 2020 during its third and fourth phases of operation. The team compared the microlensing events observed by OGLE to the theoretically predicted amount of such events, assuming that the Milky Way’s dark matter consists of primordial black holes.
“If all the dark matter in the Milky Way consisted of 10-solar-mass black holes, we should have detected 258 microlensing events,” Mróz said. “For 100-solar-mass black holes, we would have expected 99 microlensing events. For 1,000-solar-mass black holes, we would have expected 27 microlensing events.”
Contrary to these estimates of the number of events, the team found only 12 microlensing events in the OGLE data. Further analysis revealed that all of these events could be explained by the 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% dark matter, that smaller black holes of 100 solar masses could contain only 3.0% dark matter, and that black holes of 1,000 solar masses could contain only 11% dark matter.
“This indicates that massive black holes can make up, at most, a few percent of dark matter,” Mróz explained.
“Our observations indicate that primordial black holes cannot constitute a significant fraction of dark matter and, simultaneously, explain the observed black hole merger rates measured by LIGO and Virgo,” Udalski concluded. “Our results will remain in astronomy textbooks for decades.”
This forces astronomers to go back to the drawing board to explain the observation of excessive stellar-mass black holes beyond the Milky Way, while physicists continue to wonder about the true nature of dark matter.
The team’s research is published June 24 in the journals Nature and Astrophysical Journal Supplement Series.