by 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 the universe’s most mysterious form of “stuff,” dark matter, consists of primordial black holes formed in the first moments after the Big Bang.
Dark matter is obscure 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 1 gram of “everyday 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 through its interactions with gravity and its effects on everyday matter and light. But, despite this and the ubiquity of dark matter, scientists have no idea what it might be.
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The new dark matter findings come from a 20-year look back at observations made by a team of scientists from the Optical Gravitational Lensing Experiment (OGLE) survey at the Warsaw University Astronomical Observatory.
“The nature of dark matter is still a mystery. Most scientists think it is composed of unknown fundamental particles,” team leader Przemek Mróz, from the Astronomical Observatory of the University of Warsaw, said in a statement. “Unfortunately, despite years of effort, no experiment, including experiments performed with the Large Hadron Collider, has discovered 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 larger than those within the confines of our galaxy.
Our primordial black hole is missing!
The team’s search for black holes in the Milky Way’s halo appears to have been discovered by the Laser Gravitational-Wave Observatory (LIGO) and its sister gravitational-wave detector, Virgo. .
Until the first detection of gravitational waves, produced by LIGO and Virgo in 2015, scientists were finding that our galaxy’s population of stellar mass black holes, born from the gravitational collapse of massive stars, had a tendency of between five and 20 five and 20 years. that hour of the sun.
Gravitational observations of stellar-mass black hole mergers reveal a more distant population of black holes with much larger masses, equivalent to between 20 and 100 suns. “Explaining why these two populations of black holes are so different is one of the greatest mysteries of modern astronomy,” said Mróz.
One possible explanation for this larger population of black holes is that they were left over from a period immediately after the Big Bang that arose 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 caused the current density of galaxies and galaxy clusters,” said Mróz. “Similar density fluctuations, if they exceed a critical density contrast, could collapse and form black holes.”
These “primordial black holes” were first pointed out by Stephen Hawking more than 50 years ago but they are still unquestioned. That could be because smaller samples would quickly “leak” a type of thermal energy called Hawking radiation and eventually escape, meaning they wouldn’t exist in the current 13.8 billion cosmos era. year old. However, this obstacle has not stopped some physicists from positing primordial black holes as a possible explanation for dark matter.
Dark matter is estimated to comprise between 90% and 95% of the mass of the Milky Way. That means, if dark matter is made from primordial black holes, there should be many of these ancient bodies in our galaxy. Black holes do not emit light because they are bound by a light-trapping surface called the “event horizon.” That means we can only “see” “black holes” if they feed on matter around them and cast their shadow on it. But, just like dark matter, black holes interact with gravity.
Thus Mróz and his colleagues were able to tackle Albert Einstein’s 1915 theory of gravity, general 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 the objects of mass are warped in the framework of space and time, united as a single entity called “space-time.” That curve results in gravity, and the bigger the object, the more space-time is collapsing and, therefore, the more “gravity” it generates.
This curvature not only tells the planets how to orbit the stars, and tells the stars how to race around the centers of their home galaxies, but it bends the path of light coming from the background stars and from galaxies. The closer to an object of mass that light travels, the more its path is “bent.”
Thus different paths of light from a single background object can be bent, shifting the apparent position of the background object. Sometimes, the effect can even cause the background object to appear in different places in the same image of the sky. Other times, light from the background object is amplified, making that object larger. This phenomenon is called “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—align perfectly in space,” OGLE survey Principal Investigator Andrzej Udalski said in the statement. “During a microlensing event, the source light may be deflected and magnified, and we observe a temporary brightness of the source light.”
How far light from the background source is brightened depends on the mass of the lensing body that passes between it and the Earth, with more massive objects sustaining longer microlensing events. An object around the mass of the sun should cause brightening for about a week; for lensing bodies with a mass 100 times that of the sun, however, the brightness 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 seemed to show that black holes are not as massive as the sun and may contain less than 10% dark matter. The issue with these experiments, however, was that they were not sensitive to extremely long microleaching events.
Therefore, because more massive black holes (such as those detected recently by gravitational-wave detectors) would cause longer events, these experiments were not sensitive to that population of black holes but the so much.
This team improved sensitivity to long-period microlensing events by undertaking 20-year monitoring of nearly 80 million stars located in a satellite galaxy or Milky Way known as the Large Magellanic Cloud (LMC).
The data studied, called “the longest, largest and most accurate photometric observations of stars in the LMC in the history of modern astronomy” by Udalski, were collected by the OGLE project from 2001 to 2020 during its third and fourth operating stage. The team compared the microlensing events observed by OGLE to the amount of such events predicted theoretically, assuming that the dark matter of the Milky Way is composed of primordial black holes.
“If all the dark matter in the Milky Way consisted of black holes of 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 sizes, the team found only 12 microleaching events in the OGLE data. Further analysis revealed that the known stars in the Milky Way and in the LMC itself could explain all these events. After these calculations, the team found that black holes of 10 solar masses could contain at most 1.2% of dark matter, while 100 solar mass black holes could contain no more than 3.0% of dark matter. involved and that 1000 solar mass black holes could only include 11. % dark matter.
“That shows that supermassive black holes can compose a few percent of dark matter at most,” Mróz explained.
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“Our observations suggest that primordial black holes cannot constitute a significant fraction of dark matter and, at the same time, explain the observed black hole merger rates measured by LIGO and Virgo,” said Udalski. “Our results will remain in astronomy textbooks for years to come.”
This leaves astronomers back to the drawing board to explain the observation of supermassive black holes outside the Milky Way as physicists continue to ponder the true nature of dark matter.
The team’s research is published on June 24 in the journals Nature and the Astrophysical Journal Supplement Series.
