Sky & Telescope Magazine December 2012 - TuLaTri.com

Cutting-Edge Cosmology
Surface of last scattering (CMB)
larger multiverse’s infl ation carries each bubble away
from every other one. So the violent-bubble-bath picture re
could be true, but we might never see a mark.
“It’s very much verifi able, but it’s not necessarily
falsifi able,” Johnson cautions. And if they never detect
anything? “Then we’re in murky waters,” he says.
“I think the primary skepticism surrounds how likely ely
we are to see” one of these collision marks, Kleban says. s.
“And I share that concern. I would never bet any signifi
cant amount of money that we’re going to detect it,
because we have to be a bit lucky.”
Nevertheless, bubble collisions off er the fi rst real possibility
of fi nding observational evidence for something
that Aguirre says his colleagues generally labeled as “a
relatively innocuous pastime that will probably turn up
nothing interesting.”
“Years ago,” he says, “when people were talking about
eternal infl ation and the multiverse, it was easy to dismiss
that as, ‘Well, that’s all fun but if we can’t observe it why
are we bothering to think about it? This is speculation,
fantasy — maybe your models predict it, but who cares?’”
With the chance to observe the aftermath of a cosmic
crash, bubble cosmology rises above the accusation of not
Rings Around the Rosies
COSMIC PUNCHES from other universes
could leave a second kind of bruise in the
cosmic microwave background. But this
second bruise wouldn’t be in the microwave
background’s temperature. It would
show up as a unique pattern in the way
the CMB’s light vibrates as it propagates
through space.
Ang Angle AAng AAn Angle An Angl A AAn Ang n ng n g ggl
gle g ggl g l lle l le l e of o of o of o of o f
separation se sse separation sse s se s sepa ep eepa e ep e eepa p ppa
p ppaar a aar aar a r rratio ra r ra at a aati
t tti tti
t tti
tio o on n
Earth
Ea EEarth a rt r t tthh
Reflected Reflected RRRRRRRRe Re Refl Reflected RRRefl RRe RRe Re Re Refl Refle Re Re Refl Refle Reefle
efle eflec eflflec fle flec fle fle flec ec ec ec ec ect ect ecte ect ected ct ct cte cted cte
te te tedd ted te ted ted te ed edddddwwwwwww ed ed edddw ed ed ed ed eddd dddwake dwwak d wake wake wak ww wa wake wwa wwa wwa wa wa wa wwak wakke
aak ak ake ak ak ake akk kke
ke ke kkkke ke k
Transmitted Transmitted TTTTTTTr TTra Transmitted TTTr Tr Tr TTr Tr TTTra Tra Tr Traa Tr Tra Tr Tran TTra raaaan raansssm an an an ans ans ansmit ann nsmmit nsm nnssmi
ns nsmit ssmmii sm sssm smitted smit mmiit mi mitte mmi mit mit mit it it itt itttt iit
ttte tt tted tted teed ted tted
te ted te te te ted ed ed ed edd
d dd dd
wake wake wak wwa
waakkke ww
wwak
wa wak wa ww
wake wake ww
ake aaaake akke
ke ke ke ke ke kk
ke ke ke kkkkke ke ke
CMB temperature bruise
Apparent
bruise edge
S&T: LEAH TISCIONE
26 December 2012 sky & telescope
If a wake from a universe collision traveled
through the universe, it would tweak
the universe’s matter density where it
passed. The odd thing about these density
blips is that they come in pairs. A sound
wave hitting a lake’s surface changes speed
as it passes from air to water, because
sound travels at diff erent speeds in diff erent
media. But not all the sound hitting the
water’s surface passes through. Some of it
refl ects. The result is two acoustic waves for
the price of one, with one going forward and
the other backward.
Two waves will also arise in an expanding
universe if the speed of sound changes
everywhere in space at roughly the same
time, says Matthew Kleban. That’s just
what happened when the universe thinned
and cooled from a fi reball soup to a near-
being real science. Uncovering that aftermath would be a
game-changing discovery.
There’s also the chance that, even if cosmologists
never fi nd a CMB bruise, they’ll fi nd something else they
weren’t looking for. They’d be in good company: science
often advances through serendipity as much as through
slogging along.
“The odds are high that I’ll be disappointed and we
won’t fi nd these things,” Kleban says. “But it’s always
worth pursuing these avenues, because you never really
know what’s there until you do.” ✦
Assistant editor Camille M. Carlisle is a fond denizen of our
cozy little cosmos and, frankly, plans to keep her sights within
its confi nes until evidence demands otherwise.
vacuum. The cosmic wake is a pressure
wave, like a sound wave. So when matter
took over in the universe, this wave split
into a refl ected part and a transmitted part,
leaving behind two density blips instead of
one. When the photons fl y soon after, they
scatter off both blips.
This scattering polarizes the CMB photons,
making their wavelengths wiggle in
certain directions in a specifi c way. If the
temperature bruise is about 12° across or
larger, the polarization signal will appear
as two rings, one inside the temperature
bruise, the other outside. If observers can
detect polarization signatures that match
up with a temperature disk in the CMB, that
would give cosmologists far more confi -
dence that they’ve actually found a bruise
from a colliding universe.
This cutaway shows the transmitted and refl ected wakes from a bubble collision. Sliding
the set of wake-bruise-wake slices nearer and farther from Earth corresponds to larger
and smaller temperature marks on the CMB, respectively. When the slices lie closer to the
sphere’s edge, the apparent angle of separation between the wakes grows larger, but it’s
never more than a couple of degrees.