The term “black hole” likely summons an image of a titanic, cosmic monster hungrily swallowing all matter unfortunate enough to venture too close to it, but not all black holes are created equally, or in the same way.

Astronomers are well aware of stellar mass black holes with masses between three to 100 times that of the sun, that are born when stars run out of fuel and collapse under their own gravitational influence. Scientists have even managed to image supermassive black holes at the heart of galaxies with masses in excess of a million or even billions of times that of the sun, that grow when chains of stellar mass black holes merge over billions of years.

It has long been suspected that alongside these cosmic titans exist much more diminutive black holes that do not form as the result of star collapse, but are instead formed when dense regions of space collapsed in the early universe. The tiniest of these aptly named “primordial black holes,” with masses around the size of large asteroids and no wider than an atom, are hypothesized to date back to seconds after the Big Bang, the rapid inflation of space that occurred at the beginning of our universe.

That means if primordial black holes exist, they did so before stars or the first black holes could have populated the cosmos.

The Cosmic Hunt for Primordial Black Holes

The theoretical existence of these mini black holes was first calculated in depth by Stephen Hawking and Bernard Carr in the early 1970s. But if they do exist, they have remained frustratingly elusive.

Searching for primordial black holes has become a pressing concern as not only could they teach us about conditions in the early universe and how galaxies have evolved, but they could also be the source of dark matter. This is the mysterious substance believed to make up around 85 percent of the matter in the universe despite not interacting with light and thus being almost invisible, with astronomers only able to infer dark matter through its influence on gravity.

Now, a new paper published in the journal Monthly Notices of the Royal Astronomical Society has suggested a novel way of detecting primordial black holes via their interaction with another of the universe’s most exotic objects: neutron stars.

“My theoretical work is essentially focused on the interaction of a particular kind of primordial black hole, one of the tiniest with a mass equivalent to Mount Everest but compressed to the size of an atom, with one of the densest objects in the universe — a neutron star,” Oscar del Barco, research author and physicist at the Universidad de Murcia’s Center for Research in Optics and Nanophysics in Spain, told Popular Mechanics.

the history of the universe with primordial black holes
European Space Agency, CC BY-SA 4.0//Wikimedia Commons


Playing Catch With Neutron Stars

Neutron stars are born in a similar way to stellar mass black holes but lack the mass to cause the complete gravitational collapse that births a black hole.

Instead, they are left as a body with a mass similar to that of the sun squashed into the diameter of a city here on Earth like Manhattan, just around 12 miles. As a result, neutron stars are made of material so dense that a teaspoon of it would weigh 4 billion tons, equivalent to around 10,000 Empire State Buildings.

As young objects, neutron stars spin at incredibly rapid rates, blasting out radiation that can be seen when they are directed at Earth almost like a “cosmic lighthouse” but officially known as a pulsar. As neutron stars age, they slow their rotation and stop emitting bright radiation.

As atom-sized primordial black holes wander the universe, one would expect them to occasionally bump into an old neutron star that has lost most of its spin. Two things could happen during these encounters: the primordial black hole could be captured by the neutron star, known as a capture event, or if approaching from a large distance it could circle the neutron star and then be launched back out into space, known as a scattering event.

Depending on the orbit of the primordial black hole during the latter event, del Barco calculated that the interaction could generate a very specific and unique signal in high-energy light called gamma rays.

merging neutron stars, illustration
MARK GARLICK/SCIENCE PHOTO LIBRARY//Getty Images
An illustration of two neutron stars merging, creating gamma ray bursts.

Astronomers are already pretty adept at spotting gamma-ray bursts, which are considered the most energetic events in the universe, despite the fact they last only for a period of a few milliseconds to several hours and are launched from sources located billions of light years away.

Short gamma-ray bursts lasting less than two seconds are thought to be launched by the merger of neutron stars or black holes, or from a “mixed merger” between the two types of object. Longer bursts lasting more than two seconds, on the other hand, are associated with the supernovae that accompany the death of massive stars and the creation of a neutron star or black hole.

The interaction of a primordial black hole and a neutron star would produce a gamma-ray burst lasting around 35 seconds with a specific profile, which del Barco describes as a “smooth and sustained emission, followed by an abrupt and rapid decrease in just a few hundredths of a second.”

“If observed by modern telescopes, this unique gamma-ray burst may serve as an indicator of the presence of such tiny primordial black holes,” del Barco said.

A Helping Hand From Hawking

The physicist explained that this detection technique is very different than the method the Event Horizon Telescope has used to image the supermassive black hole at the heart of the galaxy Messier 87 and at the center of our own galaxy, the Milky Way, as it relies on very different emissions.

“Atom-sized primordial black holes should be remarkably difficult to detect, due to their limited size,” del Barco said. “While supermassive black holes, such as the recently photographed Sagittarius A*, can be detected due to the emissions from discs of extremely hot gasses surrounding them, tiny primordial black holes might not possess such a luminous accretion disk.”

new black hole images released
Handout//Getty Images
The first photo ever of a black hole, captured by the Event Horizon Telescope and released on May 2022. The supermassive black hole pictured, called Sagittarius A*, is at the center of our galaxy.

He pointed out that nevertheless, atom-sized primordial black holes have the advantage of emitting energetic radiation by themselves, a hypothetical form of radiation that black holes may emit called “Hawking radiation.”

Hawking radiation was suggested to solve the mystery of what happens to information carried by matter when that matter is swallowed and destroyed by a black hole. Because quantum physics says that information can’t be destroyed, Stephen Hawking realized it must be able to eventually escape a black hole. He suggested that black holes may “leak” a type of thermal radiation and that information is rescued from black holes via this “leaking.”

Theoretically, as black holes leak Hawking radiation they lose mass, very gradually evaporating and then ending their lives in an explosion. This process is so slow that black holes with mass many times that of the sun would take longer than the projected age of the universe to meet this fate.

The smaller the black hole is, however, the more rapidly it leaks Hawking radiation, meaning a tiny atom-sized primordial black hole should emit Hawking radiation quickly and evaporate. This would mean the Hawking radiation they produce is much more intense than that of larger black holes, perhaps powerful enough to detect.

“Due to its extremely high temperature, this Hawking emission should be the clue to detecting the presence of primordial black holes in the universe,” del Barco continued, adding that modern gamma-ray telescopes such as the future e-ASTROGAM or AMEGO instruments should be quite sensitive to the primordial Hawking radiation.

“Nevertheless, it would be a difficult task to discover them, especially the tiniest ones, or equivalently the oldest ones, mainly due to the huge distances that might weaken the gamma-ray burst signal,” del Barco concluded. “Only time will tell if we can find primordial black holes in our lifetime.”

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Robert Lea

Robert Lea is a freelance science journalist focusing on space, astronomy, and physics. Rob’s articles have been published in Newsweek, Space, Live Science, Astronomy magazine and New Scientist. He lives in the North West of England with too many cats and comic books.