MATT O'DOWD: This episode
is supported by 23andMe. If you study a map of the Cosmic
Microwave Background, or CMB, you may notice a large deep
blue splotch on the lower right. This is the cold spot. Is this feature a statistical
fluke, the signature of vast supervoids, or even the
imprint of another universe? Is that giant cold spot in the
cosmic microwave background really evidence of a collision
with another universe? That's what all the media
hype is saying, which means, it's time for another Space Time
Journal Club to sort it out. Today, we're going to talk about
a fascinating new publication by McKinsey et
al., 2017, titled, "Evidence Against a Super Void
Causing the CMB Cold Spot". First the name, evidence
against a supervoid. The leading explanation
for the cold spot was that it was imprinted on
the cosmic microwave background radiation as that
radiation passed through a giant empty
regions of the universe, so-called supervoids. The evidence against
part tells us that the authors think that
this hypothesis is wrong. This is partly what excited
everyone about the alternative, bubble-universe-collision
hypothesis. Before we go into detail about either
of these hypotheses, let's get the basics down. The Cosmic Microwave
Background is everywhere. It's the light that was
released at the moment that the first atoms formed
380,000 years after the Big Bang. For all of the gory details,
check out our previous episode. The CMB is amazingly uniform. When it formed in the
early hot universe, it was mostly infrared
light with a temperature of 3,000 Kelvin. 13 and 1/2 billion years of
cosmic expansion later, and it stretched to
microwave wavelengths, and to a temperature very
close to 2.725 Kelvin all across the sky. Although it's very
smooth, the CMB does show lots of very tiny
fluctuations in temperature. Those are all of those
smaller blotchy spots. We think that they came from
random quantum fluctuations from the very first
instant after the Big Bang. These were then
amplified by a period of exponential expansion
in the very early universe that we call inflation. The typical deviation from
the average temperature is around 20 microkelvin,
so the differences are one part in 100,000. The cold spot, which
is in the direction of the southern
constellation Eridanus is 150 microkelvin
cooler than the average. It's also a huge, 10 degrees
across for the coldest patch, with a less extreme halo and hot
rim that's 20 degrees across. Think 40 full moons. Now, it's possible to
explain the cold spot as just an unusually strong
random fluctuation in the CMB. Just like those small blotches,
but by chance, very large. Simulations show that you should
get a spot that size in the CMB in around 1 in 50 universes. So we might be in one of
those slightly rarer universes with a big CMB splotch. Really, in any given
universe, there should be a few
weird 1 in 50 things, but the odds are low enough
that it's worth investigating. Inoue and Silk in
2006 first proposed the cold spot could
be the imprint of a supervoid via the so-called
Integrated Sachs-Worlfe, or ISW, effect. The ISW effect is
dark energy in action. It's a cosmological tug-of-war. Gravity pulls things in while
dark energy pushes things out. A photon entering a
matter-rich galaxy cluster gets an energy boost as it
falls into the cluster's gravitational well. But by the time the
photon is on its way out, the expansion of the
universe has actually stretched out the
cluster, weakening its gravitational pull. There's a steep slope down
and a shallow slope up, just like a ski ramp. The photon exits
with a net energy gain, which would register as
a higher temperature on our CMB map. But the opposite happens when
the photon enters a void. It loses energy going
in, because it's being pulled backwards
by the higher density universe behind it. But the galaxies have
spread a little further out as it exits the
void, so it doesn't get pulled out as strongly. The ISW effect would be
tiny, in fact negligible, in a universe
without dark energy. The difference between the
going in and going out boosts would be so small that
they wouldn't be noticed. But around 4 billion
years ago, dark energy caused the expansion of our
universe to begin accelerating, whereas previously it had been
slowing down due to gravity. In an accelerating
universe, the difference in the ingoing and
outgoing boosts can be large enough
to be detected. So if there are giant voids in
the direction of the cold spot, then these could have sapped
energy from the CMB photons as they passed through. McKenzie et al. use the
Anglo-Australian Telescope in Outback New South Wales to
perform a spectroscopic survey of 7,000 galaxies in the
direction of the cold spot and out to a redshift of 0.4. Or in layman's terms, they split
the light from those galaxies into component wavelengths
and determined the shift in the wavelengths
of those spectra due to the expansion
of the universe, i.e., they measured redshifts. This gave distances to those
galaxies, which ultimately allowed them to build an
accurate 3D atlas of galaxies in the direction
of the cold spot all the way out to the point
where dark energy started to dominate the universe. They found three,
maybe four, supervoids. However, the combined ISW effect
they calculated from all four voids should only have produced
32 microkelvins of reduction in temperature in
the CMB, which is a mere 1/5 of the observed
150 microkelvin drop. McKenzie et al. also observed
a control region, G23, in the direction of
the star Fomalhaut. G23 has a similar void structure
to the cold spot's line of sight plus a couple
of overdensities. G23's ISW effect is calculated
to yield a 14 microkelvin drop in the CMB. And that's actually a good
match to the observed deviation of 15 microkelvins. So the control sample shows
that calculating ISW effect can lead to a number that
matches the true effect. Mackenzie et al.
conclude that this means supervoids can't be the
sole explanation for the 150 microkelvin cold spot. What is it, then? Well, there's a good
chance it's actually just a statistical blip. The void hypothesis was
always the least crazy idea. But now, that seems
to be ruled out. It's at least worth talking
about the more crazy notions. First there's the ever recurring
idea that gravity is wrong. It's the same notion
that people have tried to use to explain dark matter. Basically, the idea is that the
calculation of the ISW effect, the effect of the
voids, is lower than expected because we
don't understand gravity on large scales. However, modified gravity
is on shaky ground because, well, dark matter
is looking more and more like real stuff, not
incorrect gravity. Also, the control field gave
roughly the right answer, which it shouldn't have if
our understanding of gravity was so far off. The other weird ideas
are about what happened in the inflationary era. They're ideas like
the amplification of topological defects
in the universe or an inhomogeneous
reheating at the end of a nonstandard inflation. But the one that gets
most people most excited is, of course,
that the cold spot is the mark left
due to a collision with another universe. So what's that all about? A popular version
of inflation theory is that of eternal inflation. The idea is that
the initial period of exponential expansion that
we call inflation actually lasts forever. It's a whole big topic, and
we'll do an episode on it at some point. But in an eternal
inflation scenario, a normal universe begins
when a small patch of the inflating
universe stabilizes. In particular, its vacuum
energy takes on a stable value. At that point, it
stops inflating and starts expanding normally. This can happen
spontaneously anywhere in the greater
inflating space time, resulting in bubble universes. And it could happen
frequently or rarely, depending on the completely
unknown details of the string theory parameter space. But regardless, in an
infinitely inflating space time, collisions between
bubble universes are eventually expected. So what happens when two
bubble universes collide? Well, they merge and exchange
an enormous amount of energy. Chang, Claiborne,
and Levi, 2009, figured out that this should
result in a temperature gradient across each universe. If that merger point
is distant from us, then this looks like
a hot or cold spot in the cosmic
microwave background. The colliding
multiverse explanation is still pretty fringe. McKenzie et al. have debunked
the more standard explanation of the cold spot, and
so multiverses are still in the running. If real, this would be the
first piece of evidence that a universe
beyond our own exists. However, more
detailed observations of the CMB in that
region are needed to rule out it being a
statistical fluke, which honestly it probably is. But if not, perhaps
once upon a time, we really did collide with
an entirely separate bubble of space time. Thanks to 23andMe for
sponsoring this episode. The name 23andMe
comes from the fact that human DNA is organized
into 23 pairs of chromosomes. 23andMe is a personal
genetic analysis company created to help people
understand their DNA. You'll be able to see which
regions around the world your ancestors come from,
understand how DNA impacts your health, and learn
how your DNA influences your facial features, hair,
sense of taste and smell, and sleep quality. I recently got my
DNA results back, and it turns out I have
a lot of Neanderthal DNA. For most of the
genes affected, I have one Homo sapiens and
one Neanderthal allele. But for one gene, I have a
double Neanderthal allele, which amounts to a
single nucleotide difference in the gene
in both chromosomes. What do we know about this gene? Well, get this, those with
double Neanderthal expressions are slightly less
likely to sneeze after eating dark chocolate. I kid you not. I had no idea that
was even a thing. As you can imagine, now
that I have something to fill in the, what is
your mutant super power box, I immediately submitted my
application to join the X-men. I haven't heard back
yet, but I think they're just trying to
figure out my super name. Go to 23andMe.com/spacetime to
support our show and learn more about your personal DNA story. Last week we talked about
the mysterious population three stars, the
very first generation of stars that appeared
soon after the Big Bang. We had tons of great questions
in the comment section. A few of you asked about looking
back into the old universe to find population three
stars, and that is indeed where we focus our search. The challenge is
that to look back over 13 billion
years in the past, we need to look to
insane distances. You can see galaxies forming
in the very early universe, but they're incredibly faint. And we need the
largest telescopes in the world to even
detect the entire galaxy, let alone any individual stars. To measure the metallicity
of a star or a galaxy, you need to be able to split
the light into a spectrum and look for emission lines,
light at the signature wavelengths of heavier elements. We can't collect nearly
enough light yet to do that. Right now, efforts are focused
on computational modeling of populations of stars to
predict the overall light that we expect to
come from a galaxy. Add population three stars
to those models and the light looks very different. Sobral et al.,
2015, found a galaxy in the old universe
whose light is very hard to explain without
a lot of pop three stars. This is really intriguing, but
still a little circumstantial. yeme asks why the
stellar populations are named backwards? Why not name the first
generation population one and go up from there? I know. Astronomers, right? We're stuck with all of
these weird old measures from the past, backwards
stellar populations, calling all heavy elements
metals, parsecs, stellar magnitudes. But hey, at least we
switched to metric.
He said 1/50 universes might have a cold spot like that. Does that mean the CMB pattern is the same no matter where you are in the universe?