Dying black hole may have fired a ghost particle at Earth

For the first time, scientists think they might have caught a sign of something that has only existed in theory: a dying black hole exploding in its final moments. The signal? A ghost particle with energy so high that it doesn’t fit into anything we have seen before.

Back in February, researchers using the KM3NeT telescope – a massive neutrino detector buried deep in the Mediterranean Sea – reported something unusual. They detected a neutrino with an energy level above 100 petaelectronvolts (PeV). That’s more energy than any particle accelerator on Earth can produce.

No one knows exactly where it came from, and the numbers don’t line up with anything we’d expect from a distant galaxy or a supernova. Now, a new study suggests this strange neutrino might be the last breath of a tiny black hole formed at the beginning of time.

Ghost particle mystery

Neutrinos are called ghost particles because they’re almost invisible. Trillions pass through your body every second without leaving a mark. They rarely interact with anything. Most neutrinos come from the Sun or cosmic rays, and scientists are used to tracking those.

But this one was different. Its energy was so extreme that researchers couldn’t explain it with the usual suspects.

The IceCube Observatory – another giant neutrino detector buried in the ice at the South Pole – has picked up a few similar high-energy neutrinos in the past. But nothing this strong.

The two detectors weren’t telling the same story. IceCube’s past findings suggested ultra-high-energy neutrinos should be rare. But if KM3NeT really caught one, then something strange had to be going on. That’s where the idea of a primordial black hole comes in.

Testing a black hole theory

We’re used to thinking of black holes as massive – millions of times heavier than the Sun. But primordial black holes are different. They’re tiny, and scientists think they may have formed right after the Big Bang.

These small black holes don’t suck in matter the way their larger cousins do. Instead, they slowly lose energy through something called Hawking radiation, named after physicist Stephen Hawking. As they shrink, they get hotter. And right before they disappear completely, they explode in a final burst of high-energy particles.

If primordial black holes really exist – and if they make up most of the dark matter in the universe – then some of them could be reaching the end of their lives today. A few could even be exploding near our part of the galaxy. That’s exactly what physicists at MIT wanted to test.

Black hole explosion with big impacts

The experts ran the numbers. They determined that if primordial black holes make up most of the dark matter, then about 1,000 of them should explode every-year in each cubic parsec of our galaxy. (One parsec is about 3 light years across.)

That’s a lot of tiny black holes exploding. But space is huge, so even with all those explosions, only a few particles from any one event would reach Earth.

Still, if even one primordial black hole exploded within about 200 billion miles of our solar system – which is about 2,000 times the distance from Earth to the Sun – the particles it released could hit our detectors. One of them might have been the ultra-high-energy neutrino that KM3NeT saw.

Origins of a ghost particle

The researchers calculated the odds of this happening. There’s about an eight percent chance that a primordial black hole explosion happens close enough to Earth to explain the neutrino signal – once every 14 years.

“An eight percent chance is not terribly high, but it’s well within the range for which we should take such chances seriously,” said David Kaiser, a physicist at MIT and co-author of the study.

“So far, no other explanation has been found that can account for both the unexplained very-high-energy neutrinos and the even more surprising ultra-high-energy neutrino event.”

Chasing Hawking radiation

If this theory is right, then scientists may have just seen the first sign of Hawking radiation. It’s something that’s been predicted for decades but never observed directly.

“We don’t have any hope of detecting Hawking radiation from astrophysical black holes,” said lead author Alexandra Klipfel. “So if we ever want to see it, the smallest primordial black holes are our best chance.”

In the last fraction of a second before they vanish, these tiny black holes would blast out an enormous number of particles, including about 100 quintillion neutrinos. Some of them would be packed with the exact kind of energy KM3NeT detected.

“It turns out there’s this scenario where everything seems to line up, and not only can we show that most of the dark matter [in this scenario] is made of primordial black holes, but we can also produce these high-energy neutrinos from a fluke nearby primordial black hole explosion,” Klipfel said. “It’s something we can now try to look for and confirm with various experiments.”

Future of the theory

None of this proves the theory yet. Scientists will need more data – more high-energy neutrinos, more observations, and maybe one day, another explosion even closer to Earth.

But if this idea holds up, it could answer two of the biggest questions in physics: What is dark matter made of? And does Hawking radiation really exist?

“In that case, we could use all of our combined experience and instrumentation to try to measure still-hypothetical Hawking radiation,” Kaiser said.

“That would provide first-of-its-kind evidence for one of the pillars of our understanding of black holes – and could account for these otherwise anomalous high-energy neutrino events as well. That is a very exciting prospect.”

The full study was published in the journal Physical Review Letters.

Image Credit: Toby Gleason-Kaiser

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