This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Every black hole hides a question at its center. Einstein's theory predicts that whatever falls in is crushed into a singularity, a point of infinite density where the theory itself breaks down.
Most physicists expect quantum gravity to replace that point with something finite. But the center lies hidden behind the horizon. How could we ever learn what is there?
A disturbed black hole, for instance, one just born from a merger, rings like a struck bell, shedding gravitational waves in a few quickly fading tones that physicists call quasinormal modes. Each tone has a pitch and a fading rate, set by the shape of spacetime around the black hole. Build the center differently, and the black hole should ring differently.
Over the past year, my colleague Davide Batic and I, with Fabio Scardigli for the first two papers, computed these tones for three black holes whose centers are reshaped by ideas from quantum gravity. Our third paper, now published in Physics of the Dark Universe, completes the series, and the answers fall into a simple pattern: where gravity weakens at short distances, the black hole rings higher and longer; where it grows stronger, it rings lower and dies away sooner. In Newton's and Einstein's theories, one number, Newton's constant, fixes the strength of gravity.
Several approaches to quantum gravity suggest that it depends on distance. In asymptotic safety, an idea due to Steven Weinberg, gravity weakens at the very shortest distances. Put such a running constant into Einstein's black hole, and its center changes.
In our first paper, in The European Physical Journal C, the constant follows the quantum correction to Newton's law, and gravity grows stronger near the center. The singularity survives, but it is spread over a sphere, the edge of a core of Planck density, still hidden behind the horizon. In the second paper, in Physical Review D, we studied the black hole that Alfio Bonanno and Martin Reuter built from asymptotic safety in 2000.
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