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 the most mysterious form of “stuff” in the universe, dark matter, is made up of ancient black holes that formed in the first moments after the Big Bang.
Dark matter is an enigma because, despite being effectively invisible because it does not interact with light, this substance makes up about 86% of the matter in the known universe. That means that for every gram of “everyday matter” that makes up stars, planets, moons and people, there is 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.
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The new dark matter results come from a look back at two decades 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 black holes as an explanation for dark matter; they also deepen the mystery of why stellar-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 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 in 2015 by LIGO and Virgo, scientists found that the population of stellar-mass black holes in our Milky Way, which are created by the gravitational collapse of massive stars, typically have masses between five and 20 times 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, when they were not formed by the collapse of massive stars, but from extremely dense chunks of primordial gas and dust.
“We know that the early universe was not ideally homogeneous — small density fluctuations gave rise to 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, this barrier hasn’t stopped some physicists from postulating primordial black holes as a possible explanation for dark matter.
It is estimated that dark matter makes up 90% to 95% of the Milky Way’s mass. That 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 to a light-trapping surface called the “event horizon.” That means we can’t “see” black holes unless they feed on the matter around them and cast their shadows on it. But just 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 general theory of relativity states that massive objects warp the fabric of space and time, united as a single entity called “spacetime.” Gravity is a result of that curvature, and the more massive an object, 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 also bends the path of light coming from background stars and galaxies. The closer the light is to the mass object, the more its path is ‘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 nearly 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 diffracted 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 appeared to show that black holes were less massive than the Sun and could comprise less than 10% of the dark matter. The problem with these experiments, however, was that they were not sensitive to extremely long-time scale microlensing events.
Because more massive black holes (similar to those 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 the sensitivity to long-term microlensing events by focusing on two decades of monitoring of nearly 80 million stars in a satellite galaxy of the Milky Way, the Large Magellanic Cloud (LMC).
The studied data, 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 its third and fourth phases of operation. 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 consists of primordial black holes.
“If all the dark matter in the Milky Way consisted of black holes of 10 solar masses, then we should have detected 258 microlensing events,” Mróz said. “For black holes of 100 solar masses, we expected 99 microlensing events. For black holes of 1,000 solar masses — 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.
“This indicates that massive black holes can only account for a few percent of dark matter,” Mróz explains.
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“Our observations indicate that ancient black holes cannot constitute 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 be in astronomy books 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.