Kind: captions
Language: en
this episode was sponsored by kiwiko
more about them at the end of the show
until recently half the universe was
missing or hidden or just undetected and
no i'm not talking about dark matter or
dark energy which make up 27 and 68 of
our universe respectively no i'm talking
about normal ordinary matter which makes
up you and me and the planets and stars
and nebulae and basically everything you
can see
and since most of this stuff is made of
protons and neutrons which are forms of
baryons this has been known as the
missing baryon problem we expect the
universe to be made up of five percent
baryonic matter
but when we go looking we only find two
and a half percent
now the first question you're probably
asking is why should we expect the
universe to be five percent ordinary
baryonic matter in the first place
the answer is because with that density
we can explain the relative abundances
of different elements that we observe in
the universe specifically the ratio of
deuterium to hydrogen to helium
in the beginning like right after the
big bang there were all of these
neutrons and protons whizzing around it
was incredibly hot and there was tons of
radiation the universe was radiation
dominated but as the universe expanded
it cooled to the point where protons and
neutrons could start fusing together a
particularly stable nucleus to form
would be helium-4 made of two neutrons
and two protons the problem was to form
helium-4 you first have to form
deuterium one proton and one neutron and
this is a less stable nucleus and as
quickly as it formed it would get
smashed apart
but by about 10 seconds after the big
bang the universe had cooled
sufficiently that deuterium could form
and as soon as it did it would rapidly
fuse into helium
the rate at which this happened depended
on the density of matter in the early
universe the higher the density the
faster this fusion could occur
then by 20 minutes after the big bang
the temperature had dropped low enough
that fusion could no longer occur so at
this point the elemental abundances were
locked in like a snapshot of this moment
there was 75 hydrogen and 25 helium by
mass which is basically still what we
observe in the universe today
of the hydrogen nuclei 26 out of every
million were deuterium
what's amazing about deuterium is that
it's stable it doesn't decay and there
are no known processes that can produce
it in significant quantities since the
big bang and that means virtually all
the deuterium in the universe today
including the one out of every 6 000
hydrogen atoms in tap water was created
not in stars but in the first 20 minutes
after the big bang
when we look deep into space the oldest
light we can see is the cosmic microwave
background radiation the afterglow of
the big bang which has been traveling
through the universe unimpeded since
about 400 000 years after the big bang
and so we can literally count up those
photons and work out the density of
radiation right after the big bang and
using the value of 26 deuterium nuclei
per million hydrogen nuclei well we can
work out the ratio of baryonic matter to
photons and that is how we work out that
there should be about five percent
baryonic matter in the universe
so in the late 1990s a scientist went
looking for all this baryonic matter it
was a census of sorts
they added up all of the planets and
stars and black holes galaxies dust
clouds gas basically
everything you can see or infer exists
using a telescope and what they found is
that everything that i normally think of
as the actual stuff in our universe
it only makes up barely 20 percent of
all the baryonic matter so where is the
rest
well not all ordinary matter is glowing
brightly or is illuminated by nearby
stars it's not dark matter but it is
ordinary matter that is just in darkness
and so if you want to find those baryons
well one way is to use a backlight
a bright source of light very far away
and that also means in the very early
universe and quasars are the perfect
backlight
their luminosity can be thousands of
times that of whole galaxies
the light comes from the accretion disk
of a supermassive black hole at the
center of an early galaxy as it engulfs
all this matter
and since it is so distant the light we
receive from quasars is heavily
red-shifted
for example the light emitted when a
hydrogen atom goes from its first
excited state to its ground state the
lyman alpha transition it produces
ultraviolet light of around
121.6 nanometers in a lab
but from a quasar it can be observed as
a peak in their spectrum at over 560
nanometers that is yellow light
what's fascinating is if you look to the
left of this peak you see many little
dips
these are absorption lines created by
neutral hydrogen atoms that lie along
our line of sight with the quasar
when light from the quasar reaches
neutral hydrogen the photons that can
excite the electrons from the ground
state to the first excited state are
absorbed this is the same lyman alpha
transition but since these patches of
hydrogen gas are closer to us they are
less red-shifted so the notches they
make in the spectrum are at shorter and
shorter wavelengths the closer the gas
is to us this has been described as the
lyman alpha forest
it's like a one-dimensional map that
shows us where and how much neutral
hydrogen gas lies along the line
connecting us to the quasar
adding all of that neutral hydrogen gas
into our baryon budget brings us almost
to 50 percent
so where is the other half of the
baryons
well computer simulations of the entire
universe suggested they are out there
just in between the galaxies in these
sheets or filaments and they're very
spread out just one to ten particles per
cubic meter plus these particles are
ionized so they don't absorb the light
like the neutral hydrogen gas and
they're in a temperature range between
about 100 000 and 10 million kelvin a
range astronomers like to refer to as
warm hot so this is known as the warm
hot intergalactic medium or whim for
short but finding the whim has been a
real challenge because they're ionized
because of their temperature they only
emit or absorb in the high energy uv or
low energy x-rays now some people have
used very sophisticated techniques to
try to find the whim
but then recently a naturally occurring
physical phenomenon allowed us to find
all of the missing baryons
let's find out how
first we need to talk lightning
and i promise this is related okay so
did you know that it's possible to
detect lightning from the other side of
the earth
this is because lightning produces a
flash of electromagnetic radiation in
all parts of the spectrum i mean we see
the white light but there's also broad
spectrum radio waves which are released
and if you were nearby you could detect
those as a pulse
but the very low frequency radio waves
can actually travel up and out of the
atmosphere and they get guided along the
earth's magnetic field lines out several
radii from the earth and then back down
where they can be detected in the other
hemisphere
except if they're detected there they
don't come in as a single pulse instead
they are spread out as a whistler now if
you play these radio waves through a
speaker we can actually hear them so
listen to this
[Music]
you hear that descending tone that
sounds like a sci-fi laser gun yeah that
is lightning on the other side of the
earth
so what's happening here well as the
radio waves travel through the earth's
magnetosphere they encounter free
electrons which slows them down and more
for the lower frequency waves this is
dispersion just as a prism separates
white light into its component colors
the plasma in the magnetosphere
separates the radio waves into its
component frequencies low frequencies
are slowed down more than high
frequencies so what started as a pulse
ends up as a whistler and the amount of
dispersion tells you how many free
electrons that radio wave had to pass
through to reach the detector now just
imagine we could do something very
similar to find all the ionized baryons
in the universe all we would need is a
bright flash of radio waves somewhere in
the distant universe
and as if on cue in 2007 astronomers
found the first fast radio burst which
is just what it sounds like a very short
duration
pulse of intense radio waves and it came
from
the deep universe from other galaxies
now these pulses can be incredibly
powerful i'm talking
billions or trillions of times as
powerful as the sun but they last for an
order of a millisecond we don't really
know what creates them though some
people suspect that it's magnetars or
neutron stars or some sort of collision
between these very powerful massive
objects like black holes and neutron
stars
but for our purposes
all we need to know is that those
flashes exist and that we can use them
to look at their dispersion and figure
out how many ionized baryons are between
us and the source and this is exactly
what one recent paper did in nature they
plotted out the dispersion measure of
several of these fast radio bursts
versus the redshift of their host galaxy
and what they found was sure enough the
further out these fast radio bursts were
the more dispersed their signal when it
reached the earth and in fact using
their measurements they were able to
estimate the total baryonic matter that
is out there and that includes all the
ionized particles in the whim and they
found that it was five percent
they found the missing baryons roughly
50 of them are in that warm hot
intergalactic medium
and so this validates what we had been
thinking the whole time
you know what surprised me in making
this video was realizing just how little
of the ordinary matter from the big bang
ended up in things like stars and
galaxies what i normally consider as the
stuff of the universe no that's only
like 10 or 20 percent of all the
baryonic matter so it turns out the
formation of these interesting
structures is a really inefficient
process
but this finding is yet another triumph
for science those computer simulations
run decades ago turned out largely to be
correct and so everyone involved should
be congratulated
but this also highlights for me the
difference between scientists and
non-scientists i feel like
non-scientists like being right they
like when things turn out the way they
were expecting but scientists on the
other hand
they want things to work out not the way
they expected because that's the way we
get clues into what new physics is still
out there to be discovered
i guess for now we'll have to be content
with being right
hey this video was sponsored by kiwiko
which i'm really excited about because
i've been using their monthly crates
since the beginning of the year with my
four-year-old he got it as a christmas
present and i've got to say they have
been a great resource for learning at
home each crate has super cool hands-on
projects that are fun to make and they
expose kids to steam concepts that's
science technology engineering art and
math now each box includes all the
supplies needed meaning no extra trips
to the store and kiwi co offers eight
subscription lines for different age
groups and topics they sent me some of
the older age group crates to try out
and i had a lot of fun uh making this
this is a planetarium and something you
might not know is kiwico is not only a
subscription service you can browse
their store and purchase individual
projects or value packs for different
ages no subscription required so let's
talk about why kiwi co is so useful
right now i mean anyone who has been at
home with kids over the past few months
knows just how important it is to get
them engaged in something other than a
screen
the summer brain drain happens every
year but it is especially challenging
right now with disruptions to school and
online learning just not being as good
as you know in-person hands-on learning
now what i know from my own experience
is my son loves making kiwico projects
with me
he has a lot of fun and doesn't even
realize he's learning what do you see
you know one day we will talk about the
missing baryons but for right now these
projects are a great way for us to
connect and learn together so if you
want to try it out you can get 20 off
everything in the kiwico store using
code veritasium or go to kiwico.com
veritasium i'll put that link in the
description so i really want to thank
kiwiko for supporting me and i want to
thank you for watching