The cores of black holes may not hold points of infinite density as
currently thought, but portals to elsewhere in the universe, theoretical
physicists say.
Image credit: NASA/JPLCaltech

A maddening enigma called a singularity  a region of infinite
density  lies at the heart of each black hole, according to general
relativity, the modern theory of gravity. The infinite nature of
singularities means that space and time as we know them cease to exist
there.
Scientists have long sought ways to avoid the complete breakdown of
all the known laws of physics brought on by singularities. Now
researchers suggest the centers of black holes may not hold
singularities after all.
These new findings are based on loop quantum gravity, one of the
leading theories seeking to unite quantum mechanics and general
relativity into a single theory that can explain all the forces of the
universe. In loop quantum gravity, the four dimensions of spacetime are
composed of networks of intersecting loops — ripples of the
gravitational field.
The researchers applied loop quantum gravity theory to the simplest
model of black hole — a spherical, uncharged, nonrotating body known
as a Schwarzschild black hole.
"We have been looking at various aspects of spherical models for several years," said researcher Jorge Pullin,
a theoretical physicist at the Louisiana State University in Baton
Rouge. "We like them because they are at the frontier of what is
possible in loop quantum gravity today — a bit more complicated than the
cosmologies that have been studied over the last decade, but not so
complicated as to become intractable. An 'aha' moment was when we
realized we can carry out an important simplification of the equations
of the model."
Instead of a singularity, they found the center of this black hole only held a region of highly curved spacetime.
"This is a clean treatment of what happens inside a black hole,
using a quantum theory of gravity," said theoretical physicist Carlo
Rovelli at AixMarseille University in Marseille, France, who did not
take part in this study. "It has long been expected that the
singularities in the centers of black holes are cured by quantum
gravity, and this is the conclusion that this work supports."
Theoretical physicists had previously shown that with loop quantum gravity, they could eliminate the singularity that past research suggested existed at the Big Bang.
Instead of emerging from a point of infinite density, their work
proposed the cosmos was born from a "Big Bounce," expanding outward
after a prior universe collapsed.
"Perhaps in the future it can be shown that all singularities are removed by the theory," Pullin said.
Just as loop quantum gravity replaced the singularity at the Big
Bang with a bridge to another universe, these new findings replace each
singularity in black holes with "a bridge to another region in the
future of our universe," Pullin said. Although prior studies also
suggested black holes harbored such bridges, researchers had believed
the singularities in black holes prevented any way of crossing those
bridges.
"I think that this shows that loop quantum gravity is very vital
and bubbling, and continues to produce exciting new results and new
ideas," Rovelli said.
Pullin emphasized that they used a very simple model in this study,
consisting of only highly curved spacetime without representing the
actual matter found inside real black holes. The models for the study
were also exactly spherically symmetrical, unlike many black holes,
which spin and thus differ across their surfaces. Finally, in their
model the black hole was there forever and will be there forever — in
reality, black holes generally form after the collapse of stars and
should one day evaporate away if they no longer have matter or energy to
devour.
"Adding matter and having a black hole that evolves is what we are aiming for next," Pullin said.
Pullin and his colleague Rodolfo Gambini detailed their findings online May 23 in the journal Physical Review Letters.
Charles Q. Choi, Inside Science News Service
Charles Q. Choi is a freelance science writer based in New York City who has written for The New York Times, Scientific American, Wired, Science, Nature, and many other news outlets.
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