Scientists have discovered that unusually large black holes appear to be absent from the Milky Way’s diffuse outer halo.
The discovery could spell bad news for theories that suggest the most mysterious form of “stuff” in the universe, dark matter, consists of primordial black holes that formed in the first moments after the Big Bang.
Dark matter is puzzling because, despite being effectively invisible since it doesn’t interact with light, it makes up about 86% of the matter in the known universe. That 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 from its interactions with gravity and the influence it has on everyday matter and light. But 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 are the result of a review of 20 years of observations conducted by a team of scientists from the Optical Gravitational Lensing Experiment (OGLE) study at the University of Warsaw 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 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 the possibility that black holes can explain dark matter, they also add to the mystery of why star-mass black holes detected outside the Milky Way appear to be more massive than those within the boundaries of our galaxies.
Our primordial black holes are gone!
The team’s search for black holes in the Milky Way’s halo originated with the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its sister telescope, Virgo, which appear to have discovered a population of unusually large, stellar-mass black holes.
Until the first detection of gravitational waves, produced in 2015 by LIGO and Virgo, scientists discovered that the population of stellar black holes in our Milky Way, which arise from the gravitational collapse of massive stars, typically have a mass between five and is twenty times larger than that of the sun.
Gravitational wave observations of stellar-mass black hole mergers indicate a more distant population of much more massive black holes, 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,” Mróz says.
One possible explanation for this larger population of black holes is that they are leftovers from a period just after the Big Bang, which formed not from the collapse of massive stars, but from dense patches of primordial gas and dust.
‘We know that the early universe was not ideally homogeneous; small fluctuations in density gave rise to the formation of current galaxies and galaxy clusters,” Mróz said. “Similar 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. That could be because smaller examples would quickly “leak” a form of thermal energy called Hawking radiation and eventually evaporate, meaning they wouldn’t exist in the current era of the 13.8-billion-year-old cosmos. Still, that obstacle hasn’t stopped some physicists from postulating 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 Milky Way should contain many of these ancient bodies. Black holes do not emit light because they are bound to 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. However, like dark matter, black holes do interact with gravity.
Mróz and his colleagues were thus able to rely on Albert Einstein’s theory of gravity from 1915, the general theory of relativity and a principle he introduced to search for primordial black holes in the Milky Way.
Einstein lends a hand
Einstein’s theory of general relativity states that objects with mass distort the structure of space and time, uniting them into a single entity called “spacetime.” Gravity is a consequence of that curvature, and the more massive an object is, the more extreme the curvature of spacetime it causes and therefore 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 it also bends the path of light coming from background stars and galaxies. The closer to the object of mass that light travels, the more its path becomes “bent.”
Thus, different light paths from a single background object can be bent, changing the apparent location of the background object. Sometimes the effect can even cause the background object to appear in multiple places in the same image of the sky. Other times the light from the background object is amplified and that object is magnified. This phenomenon is known as ‘gravitational lens’, 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, we should observe 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 ideally aligned in space,” OGLE study principal investigator Andrzej Udalski said in the statement. “During a microlensing event, the light from the source can be bent and magnified, and we see a temporary brightening of the light from the source.”
How long the light from the background source brightens depends on the mass of the lensing body passing between the background source and Earth, with larger mass objects causing longer microlensing events. An object around the mass of the Sun should brighten for about a week; however, for lensing bodies with a mass 100 times that of the Sun, the brightening should last for several years.
Previous attempts have been made to use microlensing to detect ancient black holes and study dark matter. Previous experiments suggested 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 extremely long-timescale microlensing events.
Because more massive black holes (similar to the black holes recently detected with gravitational wave detectors) would produce longer events, these experiments were also not sensitive to that population of black holes.
This team improved sensitivity to long-term microlensing events by monitoring nearly 80 million stars in a satellite galaxy or Milky Way, the so-called Large Magellanic Cloud (GMW), for 20 years.
The data studied, described by Udalski as “the longest, largest, and most precise photometric observations of stars in the LMC in the history of modern astronomy,” were collected by the OGLE project from 2001 to 2020 during its third and fourth operational phases. The team compared the microlensing events observed by OGLE with the theoretically predicted rate 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 ten solar mass black holes, we should have detected 258 microlensing events,” Mróz said. “For 100 solar mass black holes, we expected 99 microlensing events. For 1,000 solar mass black holes – 27 microlensing events.”
In contrast to these estimated event rates, 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 in the LMC itself. After these calculations, the team found that black holes of 10 solar masses could account for at most 1.2% of the dark matter, smaller black holes of 100 solar masses could account for no more than 3.0% of the dark matter, and black holes of 1,000 solar masses could account for only 11% of the dark matter.
“That indicates that massive black holes could account for at most a few percent of dark matter,” Mróz explains.
“Our observations indicate that primordial black holes cannot account for a significant portion of the 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 to come.”
This leaves astronomers to return to the drawing board to explain the observation of stellar-mass black holes outside the Milky Way, while physicists continue to puzzle over the true nature of dark matter.
The team’s research will be published June 24 in the journals Nature and the Astrophysical Journal Supplement Series.