Scientists have discovered that extremely 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 universe’s most mysterious form of “stuff,” dark matter, is composed of primordial black holes that formed in the first moments after the Big Bang.
Dark matter is strange because, despite being effectively invisible because it doesn’t interact with light, this substance makes up about 86% of the matter in the known universe. That is, for every 1 gram of “everyday matter” that makes up the stars, planets, moons and humans, there are over 6 grams of dark matter.
Scientists can infer the presence of dark matter from its interactions with gravity and the effect it has on everyday matter and light. However, despite this and the ubiquity of dark matter, scientists have no idea what it might be made of.
Connected: If the Big Bang created miniature black holes, where are they?
The new dark matter results come from a look back over 20 years of observations by a team of scientists from the Optical Gravitational Lensing Experiment (OGLE) survey at the Astronomical Observatory of the University of Warsaw.
“The nature of dark matter remains a mystery. Most scientists think it consists of unknown elementary particles,” team leader Przemek Mróz, from the Astronomical Observatory of the University of Warsaw, said in a statement. “Unfortunately, despite decades of effort, no experiments, including those performed at the Large Hadron Collider, have found new particles that could be responsible for dark matter.”
The new findings don’t just cast doubt on black holes as an explanation for dark matter; they also deepen the mystery of why stellar-mass black holes discovered beyond the Milky Way appear to be more massive than those within the confines of our galaxies.
Our primordial black holes are missing!
The team’s hunt 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 discovered a population of extremely large black holes. large with stellar mass.
Until the first detection of gravitational waves, which was 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.
Gravitational wave observations of mergers between stellar-mass black holes show a more distant population of black holes with much more mass, equivalent to 20 and 100 Suns. “Explaining why these two populations of black holes are so different is one of the great mysteries of modern astronomy,” noted Mróz.
One possible explanation for this larger population of black holes is that they are remnants from a period immediately after the Big Bang that formed not from the collapse of massive stars, but from extremely dense patches of primordial gas and dust.
“We know that the early universe was not ideally homogeneous – small fluctuations in density created the current galaxies and galaxy clusters,” said Mróz. “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 over 50 years ago, but have remained frustratingly elusive. This may be because the smaller examples would rapidly “leak” a form of thermal energy called Hawking radiation and eventually evaporate, meaning they would not exist in the current 13.8-billion-year age of the cosmos. – year old. However, this obstacle has not stopped some physicists from proposing primordial black holes as a possible explanation for dark matter.
Dark matter is estimated to make up 90% to 95% of the mass of the Milky Way. That is, if dark matter consists of primordial black holes, our galaxy should contain many of these ancient bodies. Black holes do not emit light because they are bounded by a light-catching surface called the “event horizon.” This means that we cannot “see” black holes unless they feed on the matter around them and cast their shadow on it. But like dark matter, black holes interact with gravity.
So Mróz and colleagues were able to turn to Albert Einstein’s 1915 theory of gravity, general relativity, and a principle he introduced to hunt for primordial black holes in the Milky Way.
Einstein gives him a hand
Einstein’s theory of general relativity states that objects of mass distort the fabric of space and time, united as a single entity called “space-time”. Gravity is the result of that curvature, and the more massive an object is, the more extreme the distortion of space-time it causes, and thus the greater the “gravity” it generates.
This curvature not only tells planets how to orbit stars and tells stars how to race around the centers of their home galaxies, but also bends the path of light coming from stars and background galaxies. The closer the light travels to the object of mass, the more its path “bends”.
Different light paths from a single background object can be bent, shifting the apparent location of the background object. Sometimes, the effect can even cause the background object to appear in multiple places in the same sky image. Other times, the light from the background object is amplified and that object is magnified. This phenomenon is known as “gravitational lensing” and the intervening 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, then we should see microlensing effects on that star for a short period of time.
“Microlensing occurs when three objects—an observer on Earth, a light source, and a lens—almost ideally line up in space,” OGLE survey Principal Investigator Andrzej Udalski said in the statement. “During a microlensing event, the source light can be deflected and magnified, and we observe a temporary brightening of the source light.”
How long the light is illuminated by the background source depends on the mass of the lensing body passing between it and the Earth, with more massive objects giving rise to longer microlensing events. An object about the mass of the sun should cause a glow for about a week; for lensed bodies with masses 100 times that of the Sun, however, the illumination should last up to several years.
Previous attempts have been made to use microlensing to detect primordial black holes and study dark matter. Previous experiments seemed to indicate that black holes are less massive than the sun and may make up less than 10% of dark matter. The problem with these experiments, however, was that they were not sensitive to extremely long microlensing events.
Thus, because more massive black holes (similar to those recently detected with gravitational wave detectors) would cause longer events, these experiments were not even sensitive to that population of black holes.
This team improved the sensitivity to long microlensing events by going back to 20 years of monitoring almost 80 million stars located in a satellite galaxy or Milky Way called the Large Magellanic Cloud (LMC).
The data studied, described by Udalski 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 from 2001 to 2020 during the third phase and fourth of operation. The team compared the microlensing events seen by OGLE with 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 was composed of black holes with 10 solar masses, we should have detected 258 microlensing events,” said Mróz. “For 100 solar-mass black holes, we expected 99 microlensing events. For 1000 solar-mass black holes – 27 microlensing events.”
In contrast to these estimated event quantities, the team found only 12 microlensing events in the OGLE data. Further analysis revealed that all these events can be explained by known stars in the Milky Way and in the LMC itself. After these calculations, the team found that black holes with 10 solar masses can make up at most 1.2% of the dark matter, black holes smaller than 100 solar masses can make up no more than 3.0% of the dark matter and 1,000 solar-mass black holes can make up only 11% of dark matter.
“This shows that massive black holes can make up, at most, a few percent of dark matter,” explained Mróz.
“Our observations show 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,” concluded Udalski. “Our results will remain in astronomy textbooks for decades to come.”
This leaves astronomers back to the drawing board to explain the observation of supermassive stellar black holes beyond the Milky Way, while physicists continue to puzzle over the true nature of dark matter.
The team’s research is published June 24 in the journals Nature and Astrophysical Journal Supplement Series.