Could black holes be portals to other universes?
http://space.newscientist.com/article/dn11745-could-black-holes-be-portals-to-other-universes.html

Could black holes be portals to other universes?


The objects scientists think are black holes could instead be wormholes
leading to other universes, a new study says. If so, it would help
resolve a quantum conundrum known as the black hole information paradox,
but critics say it would also raise new problems, such as how the
wormholes would form in the first place.
A black hole is an object with such a powerful gravitational field that
nothing, not even light, can escape it if it strays within a boundary
known as the event horizon. Einstein's theory of general relativity says
black holes should form whenever matter is squeezed into a small enough
space.

Though black holes are not seen directly, astronomers have identified
many objects that appear to be black holes based on observations of how
matter swirls around them.
But physicists Thibault Damour of the Institut des Hautes Etudes
Scientifiques in Bures-sur-Yvette, France, and Sergey Solodukhin of
International University Bremen in Germany now say that these objects
could be structures called wormholes instead.
Wormholes are warps in the fabric of space-time that connect one place
to another. If you imagine the universe as a two-dimensional sheet, you
can picture a wormhole as a "throat" connecting our sheet to another
one. In this scenario, the other sheet could be a universe of its own,
with its own stars, galaxies and planets.

Damour and Solodukhin studied what such a wormhole might look like, and
were surprised to discover that it would mimic a black hole so well that
it would be virtually impossible to tell the difference.

Hawking radiation
Matter would swirl around a wormhole in the same way as for a black
hole, since both objects distort the space around them in the same way.
One might hope to distinguish the two by something called Hawking
radiation, an emission of particles and light which should only come
from black holes and would have a characteristic energy spectrum. But
this radiation is so weak that it would be completely swamped by other
sources, such as the background glow of microwaves left over from the
big bang, making it unobservable in practice.

Another difference one might hope to exploit is that unlike black holes,
wormholes have no event horizon. This means that things could go in a
wormhole and come back out again. In fact, theorists say one variety of
wormhole wraps back onto itself, so that it leads not to another
universe, but back to its own entrance.
Daring plunge
But this does not provide a foolproof test either. Depending on the
detailed shape of the wormhole, it could take billions of years or more
for things to pop back out after falling in. With the right shape, even
the oldest wormholes in our universe would not have had time to spit
anything back out yet.

It seems the only way to decide the issue for sure with astronomical
black holes is to make a daring plunge inside. That would be a dangerous
gamble, because if it is a black hole, the incredibly strong
gravitational field inside would tear apart every atom in your body.
Even if it turns out to be a wormhole, the forces inside could still be
deadly.
Assuming you could survive, and the wormhole was not symmetric, you
might find yourself in another universe on the other side. Without
further intervention, the wormhole would tend to suck you back in and
carry you back to the opening in your universe.

Yo-yo motion
"The spaceship would do this yo-yo motion," Damour told New Scientist.
"[But] if you use your fuel, then you can escape from the attracting
power of the wormhole and explore" the space on the other side, he says.
But a friend in either universe might have to wait billions of years to
hear back from you, since the transit time could be excruciatingly long.
Such a delay would make meaningful communication with anyone on the
other side impossible. But the delay gets smaller with smaller
wormholes. If a microscopic wormhole could be found or constructed, the
delay across it could be as short as a few seconds, Solodukhin says,
potentially making two-way communication possible.

Stephen Hsu of the University of Oregon in Eugene, US, who has studied
the formation of black holes and the properties of wormholes, says he
agrees that distinguishing between the two types of object with
observations is practically impossible, at least with current
technology.

Exotic matter
"The most important property of a black hole ? that there is a 'point
of no return' for an object falling in ? is not something we can test
at the moment," he told New Scientist.
Still, he says the objects out there suspected to be black holes
probably really are black holes rather than wormholes. There are
plausible scenarios for forming black holes, he says, such as the
collapse of a massive star, but it is not clear how you would form a
wormhole.
"Wormholes that might be confused with a macroscopic black hole require
some kind of exotic matter to stabilise them, and it is not known
whether such exotic matter exists," he says.

Solodukhin says that a wormhole might form in much the same way that
black holes form, such as from a collapsing star. Physicists normally
expect in these situations that a black hole would be produced, but
Solodukhin says that quantum effects may stop the collapse just short of
producing a black hole, leading to a wormhole instead.

Microscopic black holes
He says this mechanism might be inevitable in a more complete picture of
physics that unites gravity and quantum mechanics ? a longstanding
goal of physics. If he is right, then wherever we used to expect black
holes to form, wormholes would form instead.
And there might be a way to test the conjecture. Some physicists say
that future particle accelerator experiments could produce microscopic
black holes (see Atom smasher may give birth to 'Black Saturns').

Such tiny black holes would emit measurable amounts of Hawking
radiation, proving that they are black holes rather than wormholes. But
if Solodukhin is right, and microscopic wormholes are formed instead, no
such radiation would be expected. "In that case, you would actually see
if it is a black hole or a wormhole," he says.
An added benefit of wormholes is that they could resolve the so-called
black hole information paradox. The only way anything can exit a black
hole is in the form of Hawking radiation, but it is not clear how the
radiation carries information about the original object that was
swallowed. This scrambling effect conflicts with quantum mechanics,
which forbids such erasing of information (see Black holes: The ultimate
quantum computers?).

"Theoretically, wormholes are much better than black holes because all
these problems with information loss don't exist in this case,"
Solodukhin says. Since wormholes have no event horizons, things are free
to leave without first being converted into Hawking radiation, so there
is no problem with lost information.