This week–

IT HAS long been accepted, at least

in theory, that other universes

might exist and might even

collide with ours. Yet the idea that

we would ever be able to see the

aftermath of such collisions, and so

find evidence of other universes,

has seemed beyond the scope of

science. That is set to change.

Anthony Aguirre of the

University of California, Santa

Cruz, thinks the proof of cosmic

collisions could be all around us,

as imprints in the cosmic

microwave background (CMB)

left over from the big bang.

According to the standard

model of cosmology, our universe

underwent a phase of exponential

expansion, known as inflation,

just after the big bang. In theory,

inflation could still be happening,

with bubbles of space-time

suddenly blowing up to create

new pocket universes. The

cosmological parameters, such as

the rate of expansion, and the

laws of physics could be different

in each new universe, potentially

giving rise to new types of matter.

The usual assumption is that

these other universes are

disconnected from us, and that

we can’t enter them and look

around, or observe them in any

way. “People often criticise

discussions of multiple universes

as meaningless because we can’t

detect whether they actually

exist,” says Aguirre. He doesn’t

accept this. Some of these

assumptions may be wrong, he

says, and other universes could

leave behind telltale signs of their

existence when they collide with

ours. “Frustratingly, not much

work had been done on the

observable consequences of

collisions,” he says.

One reason for this neglect

is the assumption that any such

collision would be fatal for our

universe, either because it would

be so violent or because the matter

and cosmological parameters

from the colliding universe, all

inhospitable to life, would bleed

into ours. “The bubble wall of the

other universe could start pushing

into ours, destroying everything

in its path,” Aguirre says.

Yet when Alex Vilenkin of

Tufts University in Medford,

Massachusetts, and his colleagues

recently calculated the probability

of such lethal collisions, they

found it to be very low. Aguirre

has now considered the chances

of benign collisions that leave our

universe more or less intact, such

as when another universe partially

infiltrates ours. If the cosmological

parameters in the second

universe are not too different

from our own, such a collision

would not necessarily destroy life,

Aguirre says. Another possibility

is that the wall of our universe

could expand into another,

destroying its inhabitants but

Danger, alien universe ahead

“There’s always a chance we’ll be

hit by a lethal bubble. It would

come without warning, and

we’ll evaporate in an instant”

ZEEYA MERALI

12 | NewScientist | 12 May 2007 www.newscientist.com

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070512_N_p12_13_Universes.indd Sec1:12070512_N_p12_13_Universes.indd Sec1:12 8/5/07 10:41:05 am8/5/07 10:41:05 am

leaving our universe unscathed.

Aguirre and his colleagues

Matthew Johnson and Assaf

Shomer calculated the probability

of such benign collisions and their

observable repercussions (www.

arxiv.org/abs/0704.3473). “What

was surprising to me is not only

that the signs would be there, but

that they should be huge enough

for us to observe,” says Aguirre.

What’s more, such collisions could

have occurred during the history

of the universe, they found.

Signs would be visible as

large-scale anomalies in the

distribution of hot and cold spots

in the CMB. “I probably shouldn’t

speculate, but observational

cosmologists have spotted signs

of a strange alignment in the CMB

that could be compatible with

this picture,” says Aguirre. This

large-scale pattern seems to be

aligned along what is often called

the “axis of evil”, though the

finding remains controversial

(New Scientist, 13 April, p 10).

Carlo Contaldi, an expert on

the CMB at Imperial College

London, likes Aguirre’s idea.

“It’s certainly one of the most

exciting explanations of the odd

CMB patterns,” he says. “Although

it may seem far-fetched, it is quite

plausible that if these collisions

did happen while our bubble was

inflating, they could leave such a

signature on the CMB.” More

detailed work needs to be done to

put this idea to the test, he says.

Vilenkin is also impressed.

“Until now we assumed that other

universes are not observable,” he

says. “We could be living in the

aftermath of such collisions,

potentially allowing us to see

evidence of other universes.”

If universes really are crashing

into us willy-nilly, should we be

worrying about a fatal collision?

“It’s true, there is always a chance

we will be hit by a lethal bubble,

which would come without

warning,” says Vilenkin. “But

since we’ll just evaporate in an

instant and there’s nothing

we can do to stop it, there’s really

no use in worrying.” ●

www.newscientist.com 12 May 2007 | NewScientist | 13

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COLLIDING UNIVERSESA more rapidly expanding universe nearby could collide with ours and eventually engulf it completely. Whether our universe survives or not depends on the cosmological parameters of the colliding universe

OUR UNIVERSE ALIEN UNIVERSE

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