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invisible to the naked eye our night sky
is scattered with the hundreds of
billions of galaxies that fill the known
universe like the Stars these galaxies
form constellations hidden patterns that
echo the reverberations of matter and
light from an epoch long before galaxies
ever formed these are the baryon
acoustic oscillations and they may hold
the key to understanding the nature of
dark energy the field of cosmology the
study of the universe on its larger
scales was once the least precise in all
of astrophysics the impossibly vast
distances made accurate measurements
near impossible but as our telescopes
and our techniques improved over the
last few decades things are now very
different we live in the era of
precision cosmology in which we know -
stunning detail the properties that
govern the very birth evolution and the
end of our universe you talk about one
of those properties in a recent episode
the Hubble constant and about a growing
conflict in its measured value which
hints at strange new physics today we'll
talk about another measurement that may
help resolve this crisis the baryon
acoustic oscillations they have the
fossils of the first sound waves in the
universe imprinted on the distribution
of galaxies on the sky and in these
patterns we could read the expansion
history of the universe let's start at
the beginning for the first few hundred
thousand years in the life of our
universe all of the space was filled
with hydrogen and helium in plasma form
protons and the lightest of nuclei
forged in the first minutes after the
Big Bang and still so hot that no atoms
could form and electrons buzzed free of
their nuclei these particles of matter
are our baryons there's also light in
fact around a billion photons for every
electron but no photon is safe from a
free electrons unbound electrons present
a huge
target to scatter any wavelength of
light and scatter they did constantly my
photon could barely travel any distance
before colliding with an electron the
electrons in turn exerted their
electromagnetic pool on the nuclei we
say that in this state light was coupled
with matter and baryons and photons
formed a single strange fluid a baryon
photon plasma there are three profound
differences between the behavior of
matter in this state compared to the
gentle gas nebula of the modern universe
first the baryon photon plasma was
opaque there exists no lines of sight to
anything during that time and we'll come
back to that later second light was able
to exert an enormous pressure on this
plasma as well see that will lead to the
production of colossal sound waves and
third those sound waves traveled fast
rapid interaction between the charged
particles of the plasma via the trapped
photons meant that ripples in the plasma
traveled at over half the speed of light
mix in this soup of baryons and photons
was dark matter in fact dark matter
outweighs baryons by a factor of five
which means it was by far the dominant
gravitational influence in the early
universe as it still is but unlike
baryons dark matter does not interact
with light at all light exerts no
pressure on dark matter okay so the
universe is filled with this hot ocean
of baryons photons and dark matter now
in order for it to do anything
interesting we need one final ingredient
density fluctuations a teensy bit more
matter here or a teensy bit less there
these fluctuations probably were the
remnants of random quantum fluctuations
from when the universe was subatomic in
size and now with an expanded of North
Lee by cosmic inflation in the beginning
instant of the Big Bang
immediately after that two competing
forces began to work each evidence
region pulled gravitationally on its
surroundings drawing matter towards it
in particular the dark matter flowed
inwards towards this density peak but
also at that density peak the imprisoned
photons exerted an enormous outward
pressure to equalize this pressure
radiation pushed outwards and carried
the baryons with it this resulted in an
acoustic wave a true sound wave in the
form of an expanding shell of increased
density and remember sound travelled at
over half the speed of light back then
so the shell expanded fast but as it
expanded so did the universe as matter
became more diffuse and the photons
themselves were stretched red shifted to
lower energies the universe cooled at
380,000 years the plasma hit a critical
temperature of 3000 Kelvin around the
surface temperature of the coolest red
dwarf stars at this temperature
electrons could finally be captured by
nuclei and the first true atoms formed
the barians transitioned in phase from a
plasma to a gas we call this phase
transition events recombination well
free electrons are able to interact with
any frequency of lights electrons bound
into atoms ah restricted to only those
specific frequencies corresponding to
the energy level transitions of that
atom as a result light and matter would
no longer coupled the universe went from
obey to transparent over a period of
several thousand years as the wave of
plasma and photons decoupled
light began to stream freely through the
universe as the cosmic background
radiation but the plasma now hydrogen
and helium gas stalled the speed of
sound dropped from half the speed of
light
to only hundreds of meters per second
the wave of plasma turned gas
essentially froze in its current state
the radius of that shell became fixed to
the rate of expansion of the
verse and its size well the exact
distance that sound could travel over
the age of the universe at that time we
call this the sound horizon and at
recombination it was around five hundred
thousand light-years while all of this
was happening dark matter was doing its
own thing immune to the radiation
pressure the central dark matter over
density had continued to grow it pulled
on the expanding shell and was pulled by
it when the expanding wave froze both
dark matter and baryons flowed together
and consolidated the new structure once
more in the gravitational grip of dark
matter hydrogen and helium could begin
the long work of collapsing into stars
and galaxies as the universe continued
to expand and now over thirteen and a
half billion years later the universe is
expanded by a factor of 1,100 so those
rings should be a hundred fifty mega
parsecs across five hundred million
light years so how does this primordial
history lesson help us understand the
subsequent expansion of the universe
well crazily we can still see those
rings not made of plasma or gas but made
of galaxies before I show you what they
look like a note of caution the story I
just told you about the early
development of these density
fluctuations is far from the whole
picture in reality the density waves
sloshed inwards and outwards gravity
pitted against radiation pressure
density everywhere oscillated with a
rate of oscillation depending on the
size of the initial fluctuation those
are the oscillations in the baryon
acoustic oscillations and the complex
signature of that sloshing is imprinted
in detail on the temperature map of the
cosmic microwave background radiation
that something will explore in a
follow-up episode because it's perhaps
the most powerful tool we have in
cosmology but today we're gonna focus on
this first outbound density pulse then
what it looks like now in the
arrangement of galaxies so the acoustic
shells at recombination overlapped in a
complex web
those rings were further Smit out over
the thousands of years it took for the
universe to fully transition from plasma
to gas collapse that web into galaxies
over the age of the universe and at
first glance it looks like a random
smattering of galaxies on the sky but
the pattern is there detecting it
required the most detailed surveys of
the heavens ever conducted
it required galaxy redshift surveys
redshift is just the amount by which a
galaxy's light has been stretched as it
traveled through the expanding universe
the more stretching the longer that
light has traveled and so the more
distant that galaxy must be redshifted
gives distance and with careful
measurements of galaxies positions on
the sky a redshift survey can produce a
three-dimensional atlas of the universe
with a galaxy redshift survey in hand
how do you find patterns in what looks
like a random splattering of tens to
hundreds of thousands of galaxies
well you expect that galaxies should
mostly form in the Centers of those
primordial density fluctuations where
the dark matter was the most
concentrated but you should also have a
slight overabundance of galaxies at
exactly 150 mega parsecs from those
clusters corresponding to the acoustic
rings everything is so hopelessly
overlapping but here's what you do in
your atlas of the universe take slices
of the universe each slice a certain
distance from us within that slice tally
up the distances on the sky between
every pair of galaxies you should see a
lot of galaxies that are close together
that's the clustering from the giant
Dark Matter density Peaks they should
also see a slight excess of galaxies
pairs with separations of 150 megaparsec
these are the Galaxy pairs where one is
at the center of the Dark Matter peak
and one is on the surrounding ring that
bump seems small but it's statistically
very significant but also exactly where
we expected it to be this baryon
acoustic oscillations signal was first
spotted in 2005 by the Sloan Digital Sky
Survey in the northern hemisphere
galaxies and the 2df survey in the South
since then the Wiggles II boss and 60 F
surveys have improved the measurements
now there was a powerful driving
motivation to make this measurement and
to improve it it was to confirm dark
energy dark energy was first discovered
by using distant supernovae as distance
measurements to track the rate of
expansion of the universe those
observations revealed that the expansion
rate is accelerating due to an unknown
influence but we call dark energy but
such an incredible claim requires
independent checking it requires an
independent test of the expansion
history and to the baryon acoustic
oscillations see we know exactly how big
those acoustic rings should be our
understanding of the behavior of the
original baryon photon plasma is
excellent so we know how far the
acoustic wave should have traveled
before being frozen by recombination and
we can confirm that prediction by
looking at the patterns in the cosmic
background radiation that means we know
how big those rings were when they
formed and we can see how big they are
at different points in the modern
universe from our redshift surveys
these rings give us a standard ruler on
the sky spanning all of cosmic time they
allow us to track the expansion rate of
the universe the baryon acoustic
oscillations agree with and confirm what
we measure using supernovae distances
the expansion of the universe is
accelerating they confirm the existence
of dark energy they also confirm the
dark energy behaves just as is predicted
by Einsteins cosmological constant it
appears unchanging over time just as
we'd expect if dark energy is
energy of the vacuum itself frankly dark
energy aside just being able to see
these patterns is cool enough for me I
mean think about it there are rings in
the sky inscribed in galaxies frozen
echoes of the very first sound waves to
reverberate across space-time thanks to
Google for supporting PBS Digital
Studios their mobile app science journal
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check out the link in the description
below ok so once again we're covering
comments from two episodes today it's
the crisis in cosmology and negative
mass perpetual motion episodes that
chess guy has a fun one to paraphrase as
the universe expands towards the
infinite future does that expansion
outpace the probability of collapsing
into a big crunch due to everything
quantum tunneling towards a single point
well the answer is it depends on what
size you consider for the entire
observable universe almost certainly
there could be no quantum tunneling Big
Crunch but there is some smaller size
that can eventually collapse that way
now you need to wait 10 to the power of
10 to the power of 25 years for the
first quantum tunneling to turn iron
stars into black holes and way way
longer than that for anything larger
than lettuce comments that the statement
it can't be real because it would break
our current understanding of physics is
hardly a good argument now while I agree
that this isn't a hard rule it is a good
guiding principle to be dubious of new
theories that contradict extremely well
tested old theories remember that things
like general relativity and much of
quantum field theory are verified to
stunning precision if the new theory
says nah that's all wrong
better be able to also explain how those
theories appear to be so right while
being wrong
Lewis elder miss recalls the appropriate
quote extraordinary claims require
extraordinary evidence
again this is a guiding principle and it
helps scientists sort through the
overwhelming flood of poorly thought-out
theories that they get bombarded with
all the time
seriously I should show you guys my
inbox one of these days mr. Miller
advises us to remember that the number
zero didn't exist for thousands of years
ergo time will prove negative masses
existence well first I'd say that the
number zero existed at least in a sense
before we discovered it it's a common
fallacy to draw analogies between
revolutionary ideas that took a long
time to prove or discover and fringe
ideas that may or may not ever be proved
what because for every revolutionary
idea that was proved right there are
countless of Fringe ideas that were
wrong you've never heard of those ones
not everything that we can imagine
exists in fact we can imagine infinitely
more things than a real still it's worth
looking into the wacky ideas for the
tiny fraction that do spark revolutions
but don't get too attached to any one of
them flux capacitor notes an alternate
model for how negative mass might behave
in so-called by metric gravity you can
have positive and negative masses but
each is described by its own set of
Einstein field equations that's kind of
like having parallel space times one
with positive and one with negative
masses which can still interact
gravitationally in these models like
masses attract and opposite masses repel
but you don't get the crazy runaway
motion that occurs if you put both
positive and negative masses in the same
space time so no perpetual motion
machines it can also be used to explain
dark energy and dark matter an example
is the Janus model of jean-pierre petit
this is a much more sophisticated model
than the one by Jamie Fonz
it is however just as speculative
okay this is my favorite comment ever
Adam who let asks whether these
explanations might be any more
understandable if the actor was actually
knowledgeable something-something
translation lost because the actor is
more a pretty face than an expert Adam
you made me blush normally it's he looks
like a regular-sized Tyrion Lannister
seriously every astrophysicists like me
craves being mistaken for an actor and I
feel like I've finally made it
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