Kind: captions
Language: en
i am at a gold mine a couple hours
outside of melbourne because one
kilometer underground they're putting in
a detector to look for dark matter let's
go
it's gonna take 30 minutes to go down a
kilometer underground
dark matter is thought to make up 85
of all the matter in existence it could
form a shadow universe five times more
massive than everything we can see
over the past several decades over 50
experiments have tried to make a direct
detection of dark matter but none of
them has found anything
except one
under a mountain in the italian alps
there is a dark matter detector called
damalibra it's been collecting data for
around 20 years and every year it sees
the same peculiar results
the rate of detections increases to a
peak in june and then decreases to a
minimum in november
some scientists think this could be the
first direct evidence of dark matter
but why would dark matter create a
periodic annual signal
well this is our galaxy or at least what
it looks like with visible light
astronomers suspect it is surrounded and
permeated by a huge sphere of dark
matter
invisible particles that are zipping
around all in random directions
according to most theories dark matter
doesn't interact with anything including
itself except through gravity
we think there should be five times as
much dark matter as there is ordinary
matter
[Music]
now our solar system is moving around
the galaxy at 220 kilometers per second
that means we're also moving through
dark matter at this rate
except earth orbits the sun at 30
kilometers a second so for half the year
we're moving with the sun going faster
through dark matter and the other half
the year we're moving in the opposite
direction so going slower through dark
matter
and the idea is we encounter more dark
matter when we're moving through it
fastest which happens to be in june and
less of it when we're moving slowest
which happens in november
the actual geometry is a little more
complicated the solar system is tilted
at 60 degrees relative to the plane of
the galaxy but the idea still works
so the signal observed at dhamma libra
may be due to this motion through dark
matter
or
it might not be due to dark matter at
all it could just be something mundane
like the temperature humidity moisture
in the soil the snow on the mountain or
the number of tourists in italy all of
these things fluctuate with a period of
one year
and that is why they're going to build
an almost identical experiment in the
southern hemisphere down the bottom of
this gold mine outside of melbourne
because there the seasons are reversed
but our motion through dark matter is
still the same so if we see the same
signal it's pretty strong evidence for
the existence of dark matter one of the
big problems that dharma libra has is
that there are other very similar
experiments that don't see anything
and this has led to you know a lot of
uncertainty about is the dharma libra
signal really dark matter so yeah you
know we don't know right the favorite
thing in science
but why do we think dark matter exists
in the first place
in 1933 swiss astronomer fritz zwicky
was studying the coma cluster a
collection of more than a thousand
galaxies
these galaxies are gravitationally bound
together so they all orbit around their
collective center of mass
zoiki measured the orbital speeds of
these galaxies and found that some were
moving way faster than he expected
it was as if there was a lot more matter
in the cluster than he could see
pulling everything inwards
so he proposed the existence of
invisible matter which he called duncal
materi the origin of the term dark
matter
no one really took this idea seriously
but 40 years later dark matter turned up
again
vera rubin and kent ford observed the
motion of stars in the andromeda galaxy
and they expected that the further out
from the center you go the slower the
stars would be orbiting
but this is not what they found
the rotational velocity stays almost
constant with increasing distance from
the center without additional mass in
the galaxy to pull those stars in they
should be flung off into space
the result was the same in other
galaxies using radio telescopes albert
bosma and others measured hydrogen gas
even further out from a galaxy's center
but the rotational velocity still stayed
constant
one way to explain this is to posit the
existence of matter we can't see
dark matter which holds all these
galaxies together
so let's say you have a star and this
represents the mass of everything in the
center of the galaxy that's pulling the
star in the star can maintain a stable
orbit if its centripetal force is equal
to the gravitational attraction to all
the mass
in the rest of the galaxy and so you can
see that at about a distance of one
meter this is the speed of the orbit
okay but what happens if we add some
dark matter so this water bottle
represents the matter we can't see now
there is more
mass
pulling this star
into the middle which means at the same
orbit
it can now go much faster and in fact it
must go faster to maintain that orbit
and this explains the observation this
is what we see
by looking at the rotation speeds of
stars scientists estimate that about 85
percent of the mass of a galaxy is dark
matter
but there's another way to explain these
observations without invoking dark
matter and that is to modify our theory
of gravity what's the supporting
evidence for thinking that the particle
idea is totally misguided and we should
actually be looking at a revised theory
of gravity you can either invoke
something we can't see or you just say
well the universe is what we can see
and we need a way to explain what's
going on out there and the only way we
can do that is by modifying the laws of
physics so when you look at the
outskirts of galaxies you know they've
got a lot of centripetal acceleration
you know dark matter says oh well that
centripetal acceleration is due to the
gravitational effect of dark matter
whereas the people like come on will say
no that's centripetal acceleration
that's just the fact that it's now
reached this floor and can't get any
lower so they're saying that there's not
additional
force due to dark matter but there's a
limit to how low the acceleration could
go i think the consensus is hugely in
favor of it being a physical substance
in that it just seems
reasonable that is that there could be
other particles out there that we
haven't seen yet
and there's more evidence
this is the bullet cluster a site where
two clusters of galaxies collided
most of the ordinary mass of these
clusters is in the interstellar gas
and when the collision occurred the
interstellar gas interacted heated up
and slowed down
so you'd expect that most of the mass of
the bullet cluster would be in the
middle where all of this gas is
but if you use gravitational lensing the
way that gravity bends light you can
actually measure where most of the mass
in this picture is and it isn't in the
middle it's actually on either side
so the best way to explain this is that
when the clusters collided all that gas
got stuck in the middle but the dark
matter passed right through
creating the most gravitational lensing
where we can see the least ordinary
matter
even more evidence for dark matter comes
from the oldest light in the universe
380 000 years after the big bang light
could finally travel through the
universe unimpeded
and this is what we see as the cosmic
microwave background or cmb
the red spots show where the early
universe was a little hotter and the
blue spots show where it was a little
cooler but these temperature differences
were tiny just .01 percent
but they are there
and you can turn this picture into a
graph by counting up how many blobs
there are of different sizes so there's
the most common size blob which results
in this peak but there are also other
common size blobs and so you get these
other peaks of decreasing size
now the height of these peaks depends on
how much dark matter there is in a
universe without dark matter the graph
looks like this
but as dark matter increases the
amplitudes of even-numbered peaks
decreases
to match the measurements of the cmb we
need about five times as much dark
matter as ordinary matter
this figure also agrees with the amount
of dark matter required to explain the
motion of stars in galaxies and the
motion of galaxies and clusters
so the dark matter hypothesis explains a
lot of different observations with a
simple theoretical framework that
there's some type of particle out there
that only interacts through gravity
but what is this particle exactly
well since we don't know scientists have
proposed a whole bunch of different
things that it could be and now we have
to try to go out and find them
the approach differs depending on what
you're trying to find
dama libra and the detector at the
bottom of the gold mine are looking for
wimps weakly interacting massive
particles these particles are expected
to weigh about as much as a proton but
interact with ordinary matter extremely
weakly
at the heart of the detector are seven
seven kilogram crystals of pure sodium
iodide so that's actually sodium iodide
in there yep i didn't expect it to be so
clear the idea is that very very rarely
a dark matter particle may hit a nucleus
in the crystal and transfer its energy
this creates a flash of light called a
scintillation which is detected by
photomultiplier tubes very sensitive
light detectors which are positioned
above and below each crystal
but there's a problem
even the purest sodium iodide crystal
contains radioactive potassium when a
potassium atom decays it emits an
electron and a gamma ray now the
electron can cause a scintillation in
the crystal just like the hypothesized
dark matter
so to eliminate these events the sodium
iodide crystals are submerged in a tank
full of 12 tons of linear alkyl benzene
this is a liquid scintillator that emits
light when exposed to a gamma ray and
that light can then be detected by
photomultiplier tubes in the tank
so if there's a simultaneous detection
in the crystal and in the tank it was
most likely a potassium decay
not a dark matter event
but there's another problem cosmic rays
energetic particles from the sun and
other galaxies hit the top of earth's
atmosphere creating muons essentially
heavy electrons which stream toward the
earth at close to the speed of light
muons can also create flashes of light
in the crystal
this is a muon detector and it's got
these three paddles of plastic here
separated by some pieces of steel if we
see a flash of light in all three
basically the same time then we know
that a muon has passed through them so
if i hit reset
we can see
counting up the muons
being seen so it's at least a few a
second
this is why all sensitive particle
detectors are located deep underground
here we have the muon detector now one
kilometer underground it's been running
for something like 15 minutes and there
have been no muon counts
yeah we'd have to leave this running for
a long time i think even if we wanted to
get a single hit we expect the number of
neons down here to be about a million
less and we didn't see a million up the
top so we're probably not going to see
any down here
and this is the whole point of putting a
dark matter detector underground you
want to get rid of all the background
that would create noise in the detector
but even this shielding is not enough
they'll have muon detectors immediately
above the tank so if a flash is seen in
a crystal at the same time a muon is
detected it can be ruled out
being underground brings its own
challenges the walls of the mine contain
trace amounts of radioactive elements
like uranium and thorium which decay
into radon gas the requirements here are
fairly serious for dark matter
experiments we have to completely
control the environment in particular
the radon level to counteract this the
walls of the cavern are coated with
special paint to contain radioactive
particles the crystals are immersed in a
continuous stream of pure nitrogen gas
and the entire detector is shielded by
120 tons of steel and plastic wow
look at the size of that cavern
there is a lot riding on this experiment
it will validate or disprove one of the
most contentious results in physics so
if we see nothing well this is the dead
of dharma libra but if we see something
well we are all happy
i actually like the idea that because
you know eighty percent of the mass of
the universe is dark matter or dark
stuff maybe there's more than just one
particle that dark matter is made of it
could be an entire dark standard model
if you like a dark version of everything
that we can see or maybe something more
complex because there's so much more of
it like i really hope it's that
do you think that dark matter interacts
with ordinary manner if we want to find
out what this stuff is we better hope
there's some level of interaction that
we can at least probe when it comes to
doing experiments if god gave me the
great book of physics and there were
two sections section a and section b one
for the luminous matter and one for dark
matter and they didn't talk to each
other i would say that was a very
peculiar universe but in science we have
to live with the possibility that you
know at some level we may never find the
answer it may elude us
but at least we tried
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