Scientists have discovered that unusually massive black holes appear to be absent from the Milky Way’s diffuse outer halo.
The discovery could spell bad news for theories that suggest that the most mysterious form of “stuff” 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 even though it is effectively 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 1 gram of “everyday matter” that makes up stars, planets, moons and humans, there is over 6 grams of dark matter.
Scientists can infer the presence of dark matter by its interactions with gravity and the effect it has on everyday matter and light. Yet, despite this and the ubiquity of dark matter, scientists have no idea what it might be made of.
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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) at the University of Warsaw’s Astronomical Observatory.
“The nature of dark matter remains a mystery. Most scientists believe it is composed of unknown elementary particles,” team leader Przemek Mroz of the University of Warsaw’s Astronomical Observatory said in a statement. “Unfortunately, despite decades of effort, no experiment, including those conducted with the Large Hadron Collider, has detected any 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 found beyond the Milky Way appear more massive than those at the boundaries of our galaxies.
Our primordial black holes are missing!
The team’s 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 discovered a population of unusually large, stellar-mass black holes.
Until the first detection of gravitational waves, which was 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 five and 20 times more than the sun.
Gravitational wave observations of stellar-mass black hole mergers indicate a more distant population of black holes with much greater mass, equivalent to between 20 and 100 Suns. “Explaining why these two populations of black holes are so different is one of the great mysteries of modern astronomy,” Mroz pointed out.
One possible explanation for this larger population of black holes is that they are remnants of a period immediately after the Big Bang that formed not from the collapse of massive stars, but from overdense blobs of primordial gas and dust.
“We know that the early universe was not perfectly homogeneous — small fluctuations in density gave rise to present-day galaxies and galaxy clusters,” Mroz said. “Such density fluctuations, if they exceed the 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 remain frustratingly elusive. This could be because the smaller examples would quickly “leak” a form of heat energy called Hawking radiation and eventually evaporate, meaning they would not exist in the current epoch of 13, The 8 billion year old cosmos. Still, this obstacle hasn’t 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 mass of the Milky Way. 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 matter around them and casting their shadow on it. But just like dark matter, black holes interact with gravity.
In this way, Mróz and his colleagues were able to turn to Albert Einstein’s 1915 theory of gravity, general relativity, and the principle it introduced to search for primordial black holes in the Milky Way.
Einstein extends his hand
Einstein’s theory of general relativity says that objects with mass distort the very fabric of space and time, united as a single entity called “space-time.” Gravity is a result of this curvature, and the more massive an object is, the more extreme distortion of space-time it causes, and therefore the more “gravity” it generates.
This curvature not only tells the planets how to orbit the stars and how 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 the object with mass, the more its path is “curved”.
In this way, different paths of light 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 enhanced 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—are almost perfectly aligned in space,” Andrzej Udalski, OGLE’s principal investigator, 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 from the background source is illuminated depends on the mass of the lensing body that passes between it and the Earth, with objects of greater mass causing longer microlensing events. An object around the mass of the sun should cause brightening for about a week; for lenticular bodies with a mass 100 times that of the Sun, however, the illumination should last as long as several years.
Previous attempts have been made to use microlensing to detect primordial black holes and study dark matter. Previous experiments seem to indicate that black holes are less massive than the sun and may contain less than 10% dark matter. The problem with these experiments, however, was that they were not sensitive to extremely long microlensing events.
Thus, since more massive black holes (like those recently detected with gravitational wave detectors) would cause longer events, these experiments are also not sensitive to this population of black holes.
This team improved the sensitivity to long-duration microlensing events by addressing a 20-year observation of nearly 80 million stars located in a satellite galaxy, or Milky Way, called the Large Magellanic Cloud (LMC).
The data studied, described by Udalsky as “the longest, largest, and most accurate 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 work phase. 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 is composed of primordial black holes.
“If all the dark matter in the Milky Way was composed of 10-solar-mass black holes, we should have detected 258 microlensing events,” Mroz said. “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 numbers of events, the team found only 12 microlensing events in the OGLE data. Further analysis revealed that all these events could be explained by the known stars in the Milky Way and in the LMC itself. After these calculations, the team found that black holes of 10 solar masses can contain at most 1.2% dark matter, smaller black holes of 100 solar masses can account for no more than 3.0% of dark matter, and 1000 solar-mass black holes can contain only 11% dark matter.
“This shows that massive black holes can make up at most a few percent of dark matter,” explained Mroz.
“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,” Udalsky concluded. “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-mass black holes beyond the Milky Way, while physicists continue to puzzle over the true nature of dark matter.
The team’s research was published June 24 in the journals Nature and Astrophysical Journal Supplement Series.
