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.
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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.