Researchers have detected the signature of massive merger vibrations hidden in 2019 data from LIGO and Virgo machines.
Introduction
Black hole mergers produce gravitational waves that can be detected on Earth (computer simulations). The largest black hole merger ever discovered appears to have produced a black hole with a mass 150 times that of the sun, challenging some accepted theories. Researchers now say they have found evidence for the required vibrations produced by the black hole resulting from the merger as it settles into a spherical shape.
Discovery of the required vibrations
The results provide a new and rigorous test of Albert Einstein’s general theory of relativity – the theory of gravity that offers detailed predictions about black holes and gravitational waves – according to Stephen Giddings, a theoretical physicist at the University of California, Santa Barbara. “We are really exploring new territory here,” says Giddings.
Physicist Padri Krishnan, one of the study’s authors, states that he had been working on this type of analysis as a theoretical possibility earlier in his career. “At that time, I never thought I would see such a measurement in my lifetime,” says Krishnan, who now works at Radboud University in the Netherlands. The results were published this week in the journal Physical Review Letters.
Detection of gravitational waves
Since the dawn of gravitational wave science in 2015, detecting black hole mergers has become routine. The dual sensors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Washington state and Louisiana detect such mergers more than once a week on average.
Data from LIGO and from the smaller Virgo observatory near Pisa in Italy reveal gravitational wave signatures from two massive bodies orbiting each other until they merge. The frequency of these orbits and how that frequency increases over time until the moment of merger reveals the masses of the two bodies and the single black hole resulting from their merger. Generally, the greater the masses of the two bodies, the longer their orbits at the moment of merging, and the lower the frequencies of their gravitational waves.
However, among the many events detected so far, GW190521 – named for the date it was discovered on May 21, 2019 – was distinguished by its low merger frequency such that the system only entered the sensitivity range of LIGO and Virgo during its final orbits.
Searching for evidence of resonance
Krishnan and his colleagues, who are not affiliated with the LIGO and Virgo collaboration, wanted to see if the gravitational waves from that event could carry information not only from the time before the collision but also from the moments immediately after. At the moment two black holes merge, the resulting black hole is in an asymmetric shape. But black holes are only stable when they are spherical (or nearly spherical if they are spinning rapidly). Within a fraction of a second, they settle into a symmetric, low-energy shape.
Just as a bell rings at specific frequencies determined by its shape, the stable black hole rings and emits gravitational waves at frequencies determined by its mass and spin, according to Krishnan. Measuring the resonant frequencies provides a way to estimate the black hole’s properties as an alternative to the rotated frequency characteristics.
Krishnan and his colleagues reanalyzed the data from the GW190521 event to search for evidence of resonance. They found two distinct resonant frequencies, placing the resulting black hole at around 250 solar masses – a much heavier mass range than the original analysis conducted by the LIGO-Virgo team.
References: Capano, C. D. et al. Phys. Rev. Lett. 131, 221402 (2023). Abbott, R. et al. Phys. Rev. Lett. https://doi.org/10.1103/PhysRevLett.125.101102 (2020). Abbott, R. et al. Astrophys. J. Lett. https://doi.org/10.3847/2041-8213/aba493 (2020).
Source:
https://www.nature.com/articles/d41586-023-03813-w
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