DARK energy and dark matter, two of the greatest mysteries confronting
physicists, may be two sides of the same coin. A new and as yet
undiscovered kind of star could explain both phenomena and, in turn,
remove black holes from the lexicon of cosmology.
The audacious idea comes from George Chapline, a physicist at Lawrence
Livermore National Laboratory in California, and Nobel laureate Robert
Laughlin of Stanford University and their colleagues. Last week at the
22nd Pacific Coast Gravity Meeting in Santa Barbara, California, Chapline suggested that the objects that till now have been thought of
as black holes could in fact be dead stars that form as a result of an
obscure quantum phenomenon. These stars could explain both dark energy
and dark matter.
This radical suggestion would get round some fundamental problems posed
by the existence of black holes. One such problem arises from the idea
that once matter crosses a black hole's event horizon - the point beyond
which not even light can escape - it will be destroyed by the space-time
"singularity" at the centre of the black hole. Because information about
the matter is lost forever, this conflicts with the laws of quantum
mechanics, which state that information can never disappear from the
universe.
Another problem is that light from an object falling into a black hole
is stretched so dramatically by the immense gravity there that observers
outside will see time freeze: the object will appear to sit at the event
horizon for ever. This freezing of time also violates quantum mechanics.
"People have been vaguely uncomfortable about these problems for a
while, but they figured they'd get solved someday," says Chapline.
"But
that hasn't happened and I'm sure when historians look back, they'll
wonder why people didn't question these contradictions."
“People have been uneasy about these problems with
black holes, but
figured they'd get solved. That hasn't happened”
While looking for ways
to avoid these physical paradoxes, Chapline and Laughlin found some
answers in an unrelated phenomenon: the bizarre behaviour of
superconducting crystals as they go through something called "quantum
critical phase transition" (New Scientist, 28 January, p 40). During
this transition, the spin of the electrons in the crystals is predicted
to fluctuate wildly, but this prediction is not borne out by
observation. Instead, the fluctuations appear to slow down, and even
become still, as if time itself has slowed down.
"That was when we had our epiphany," Chapline says. He and
Laughlin realized that if a quantum critical phase transition happened on the
surface of a star, it would slow down time and the surface would behave
just like a black hole's event horizon. Quantum mechanics would not be
violated because in this scenario time would never freeze entirely.
"We
start with effects actually seen in the lab, which I think gives it more
credibility than black holes," says Chapline.
With this idea in mind, they - along with
Emil Mottola at the Los Alamos
National Laboratory in New Mexico, Pawel Mazur of the University of
South Carolina in Columbia and colleagues - analyzed the collapse of
massive stars in a way that did not allow any violation of quantum
mechanics. Sure enough, in place of black holes their analysis predicts
a phase transition that creates a thin quantum critical shell. The size
of this shell is determined by the star's mass and, crucially, does not
contain a space-time singularity. Instead, the shell contains a vacuum,
just like the energy-containing vacuum of free space. As the star's mass
collapses through the shell, it is converted to energy that contributes
to the energy of the vacuum.
The team's calculations show that the vacuum energy inside the shell has
a powerful anti-gravity effect, just like the dark energy that appears
to be causing the expansion of the universe to accelerate. Chapline has
dubbed the objects produced this way "dark energy stars".
Though this anti-gravity effect might be expected to blow the star's
shell apart, calculations by Francisco Lobo of the University of Lisbon
in Portugal have shown that stable dark energy stars can exist for a
number of different models of vacuum energy. What's more, these stable
stars would have shells that lie near the region where a black hole's
event horizon would form (Classical Quantum Gravity, vol 23, p 1525).
"Dark energy stars and black holes would have identical external
geometries, so it will be very difficult to tell them apart," Lobo says.
"All observations used as evidence for black holes - their gravitational
pull on objects and the formation of accretion discs of matter around
them - could also work as evidence for dark energy stars."
That does not mean they are completely indistinguishable. While
black
holes supposedly swallow anything that gets past the event horizon,
quantum critical shellsare a two-way street, Chapline says. Matter
crossing the shell decays, and the anti-gravity should spit some of the
remnants back out again. Also, quark particles crossing the shell should
decay by releasing positrons and gamma rays, which would pop out of the
surface. This could explain the excess positrons that are seen at the
centre of our galaxy, around the region that was hitherto thought to harbour a massive black hole. Conventional models cannot adequately
explain these positrons, Chapline says.
He and his colleagues have also calculated the energy spectrum of the
released gamma rays.
"It is very similar to the spectrum observed in
gamma-ray bursts," says Chapline.
The team also predicts that matter
falling into a
dark energy star will heat up the star, causing it to
emit infrared radiation.
"As telescopes improve over the next decade,
we'll be able to search for this light," says Chapline. "This is a
theory that should be proved one way or the other in five to ten years."
Black hole expert Marek Abramowicz at Gothenburg University in Sweden
agrees that the idea of dark energy stars is worth pursuing. "We really
don't have proof that black holes exist," he says. "This is a very
interesting alternative."
The most intriguing fallout from this idea has to do with the strength
of the vacuum energy inside the dark energy star. This energy is related
to the star's size, and for a star as big as our universe the calculated
vacuum energy inside its shell matches the value of dark energy seen in
the universe today.
"It's like we are living inside a
giant dark energy
star," Chapline says.
There is, of course, no explanation yet for how a
universe-sized star could come into being.
“The vacuum inside the star has a powerful anti-gravity effect, just
like the dark energy that is pulling the universe apart”
At the other end
of the size scale, small versions of these stars could explain dark
matter.
"The big bang would have created zillions of tiny dark energy
stars out of the vacuum," says Chapline, who worked on this idea with
Mazur. "Our universe is pervaded by dark energy, with tiny dark energy
stars peppered across it."
These small dark energy stars would behave
just like dark matter particles: their gravity would tug on the matter
around them, but they would otherwise be invisible.
Abramowicz says we know too little about dark energy and dark matter to
judge Chapline and Laughlin's idea, but he is not dismissing it out of
hand. "At the very least we can say the idea isn't impossible."
