We can discover new kinds of particles around black holes through gravitational waves

on February 11, 2016researchers of the Laser Interferometer Gravitational Wave Observatory (LIGO) announced the detection of gravitational waves (GW) for the first time. As predicted by Einstein’s General Theory of Relativity, these waves result from the merging of massive objects, creating ripples through spacetime that can be detected. Since then, astrophysicists have theorized numerous ways in which GWs could be used to study physics beyond the standard models of gravity and particle physics and advance our understanding of the universe.

To date, LWs have been proposed as a way to study dark matterthe interior of neutron stars and supernovasmergers between supermassive black holes, and more. In a recent researcha team of physicists from the University of Amsterdam and Harvard University has proposed a way in which GWs could be used to search for ultra-bright bosons around rotating black holes† This method could not only provide a new way to distinguish the properties of binary black holes, but also lead to the discovery of new particles outside the Standard Model.

The study was conducted by researchers from the Gravity Astroparticle Physics Amsterdam (GRAPPA), at the University of Amsterdam, with support from the Center for Theoretical Physics and the National Center for Theoretical Sciences at the University of Taipei (Taiwan) and Harvard University. The article describing their work, entitled “Sharp signals from boson clouds in binary black hole inspirations”, appeared recently in the Physical assessment letters.

It is a known fact that over time normal matter will fall towards black holes, which will form an accretion disk around the outer edge (also known as Event Horizon). This disk will accelerate to incredible speeds, causing the material inside to overheat and emit massive amounts of radiation as it slowly accelerates onto the face of the black hole. In recent decades, however, scientists have observed that black holes will lose some of their mass through a process called “superradiance.”

This phenomenon was studied by Stephen Hawking, who described how rotating black holes would give off radiation that would appear “real” to a close observer but “virtual” to a distant observer. While transferring this radiation from one frame of reference to another, the acceleration of the particle itself would cause it to transform from virtual to real. This exotic form of energy, known as “Hawking radiation”, clouds of low-mass particles form around a black hole. This leads to a “gravity atom”, so called because they resemble regular atoms (clouds of particles around a nucleus)

While scientists know that this phenomenon occurs, they also understand that it can only be explained by the existence of a new ultra-light particle that exists outside of the Standard Model. This was the focus of the new paper, in which lead author Daniel Baumann (GRAPPA and the University of Taipei) and colleagues investigated how superradiance spontaneously causes unstable clouds of ultralight bosons around black holes. Moreover, they suggest that the similarities between gravitational and regular atoms go deeper than their structure.

In short, they suggest that binary black holes can cause particles in their clouds to be ionized through the photoelectric effect† As described by Einstein, this happens when electromagnetic energy (such as light) makes contact with a material, causing it to emit excited electrons (photoelectrons). When applied to a binary black hole, Baumann and his colleagues show how clouds of ultralight bosons can absorb the “orbital energy” of a black hole companion. This would cause some of the bosons to be ejected and accelerated, as evidenced by the black hole’s associated GW signals.

Artist’s concept of a ‘gravity atom’, in which a nucleus (black hole) is surrounded by clouds of particles. Credit: UvA Institute of Physics

Finally, they showed how this process could drastically change the evolution of binary black holes by reducing the time it takes for the objects to merge. As they state:

“The orbital energy lost during this process can overwhelm the losses from GW emission, so that ionization drives the inspiration rather than just disrupting it. We show that the ionization power contains sharp features that lead to distinctiveness” kinks” in the evolution of the transmitted GW frequency.”

These “kinks,” they claim, will be observable by the next generation of GW interferometers such as the Laser Interferometer Space Antenna (LISA). This process could be used to discover a whole new class of ultralight particles and provide direct information about the mass and state of clouds with “gravity atoms”. In short, ongoing studies of GWs using more sensitive interferometers could reveal exotic physics that could advance our understanding of black holes and lead to new breakthroughs in particle physics.

This is one of the many possibilities opened up thanks to the revolution taking place with GW astronomy. In the coming years, astrophysicists hope to use them to study the most extreme environments in the universe, such as black holes and neutron stars. They hope so too primordial gravitational waves will reveal things about the early universe, the mystery of the matter/anti-matter imbalanceand lead to a quantum theory of gravity (aka a theory of everything).

Read further: Phys.orgPhysical Assessment Letters

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