Gravitational echoes may confirm the late Stephen Hawking’s quantum black hole hypothesis
Eons ago, a cataclysmic crash between two neutron stars caused a convulsion that spread ripples of gravitational waves across the cosmos.
Niayesh Afshordi
Professor, Faculty of Science
> Waterloo Centre for Astrophysics
> Perimeter Institute for Theoretical Physics
About 130 million years later, on Aug. 17, 2017, the Laser Interferometer Gravitational Wave Observatory (LIGO) detectors on Earth picked up the chirping of those gravitational waves.
There is evidence the colliding neutron stars — which are incredibly dense stellar corpses that are only the size of a city but heavier than the sun — merged into a rapidly spinning small black hole.
For Afshordi, who is also an associate faculty member at the Perimeter Institute for Theoretical Physics, that is an exciting laboratory to test new ideas. He and colleague Jahed Abedi from the Albert Einstein Institute in Germany found evidence of a low- frequency signal that could revise our understanding of black holes and open up a new era for physics.
Although it hasn’t yet been conclusively proven, the signal could be “gravitational wave echoes” reflecting off of a “fuzz” of quantum particles near the horizon of a black hole, he says.
Afshordi and Abedi were awarded the prestigious Buchalter Cosmology Prize in 2020 for the find, described in their paper, Echoes from the Abyss in the Journal of Cosmology and Astroparticle Physics.
A gravitational wave echo is something like what you might hear if you shout into a mountainous area and hear your voice reverberating back, Afshordi says. “In this case, we are not talking about a voice, but you can imagine a gravitational wave bouncing off this quantum fuzz near the horizon and then a smaller wave gets out and reflects back, again and again,” Afshordi says. This would show up as low-frequency signals.
Afshordi is hoping more convincing data might come from future similar detections and more sensitive instruments that are on the horizon. But if gravitational wave echoes prove out, it could revise our picture of black holes.
The late Stephen Hawking showed, in the 1980s, that by the laws of quantum physics, black holes should emit what is now known as “Hawking radiation.” Yet black holes are supposed to be simple, bald, “beasts with no hair,” meaning they contain nothing but mass, spin and charge. Yet if Hawking radiation comes out with nothing but mass, spin and charge, what happened to the rest of the information about the particles that went in? That’s the information paradox.
One possibility is that black holes are not bald but are in fact much more complex. Afshordi’s latest work to find evidence of a quantum “fuzz” on the black hole horizon that reflects gravitational wave echoes could show that.
It could also help reconcile general relativity, which describes gravity in the larger cosmos, with quantum mechanics that describes the interactions of tiny particles and other forces. Finding this holy grail of a unified theory of quantum gravity has been a decades-long quest that has led Afshordi into numerous academic partnerships in research that tackles everything from how the universe began to why it seems to be expanding at an accelerated rate now.
Lately, Afshordi and his colleagues have been applying their skills to the global challenge of the COVID-19 pandemic as well.
Modern-day telescopes and detectors are generating massive amounts of data that astrophysicists are now able to mine, analyze and pull together as they never could before.
When the COVID-19 pandemic hit, Afshordi and colleagues at Waterloo and Perimeter Institute started applying data to help predict how COVID-19 spreads and what happens when different types of lockdown measures are applied. “It’s all about how we can combine large data sets, understand what's happening and make predictions.”
This is still a “work in progress,” but Afshordi says it demonstrates how something practical can come from science that may seem abstract.
As a boy, Afshordi loved detective stories that involved piecing together different clues to solve a mystery. Science is much the same, he says. It is all about piecing together clues to discover the deeper underlying principles in nature. “The universe is a detective story,” Afshordi says.
Niayesh Afshordi discusses his research concerning with the deepest, darkest secrets of the universe.
