Hello, I'm Mark Chen. I'm a Fellow in
CIFAR's Gravity & the Extreme Universe Program
and tonight I'm going to tell you
about how we are exploring the dark
universe from two kilometers underground.
So let me start with the composition of the universe on this first slide.
So since the dawn of civilization, mankind has sought to understand
what are the fundamental constituents of
the world around us: what are the elements? Fire air, water, earth - from antiquity,
the Greeks introduced the concept of the atom. Chemistry brings us the periodic table of the elements.
Particle physicists develop a standard model of the elementary particles,
and cosmology and astrophysicists
have observed the properties of the
galaxies and in the cosmos as well as
the cosmic microwave background, the
imprint of the radiation from the Big Bang,
looking at the universe's earliest
baby pictures in order to understand
that this is the composition of the
universe. That the universe is made up of
something called dark energy, which I
won't be able to describe tonight, and
that's the dominant composition -
component of the universe.
There's something called dark matter which I'll try to describe for you.  And the ordinary matter,
the particles that we understand, that make up this stage,
that make up the Telus Spark Centre, that make up you and I -  that's only 5% of the universe.
And so what is this dark matter? We have evidence for a dark matter from
this image, but also from many observations in in astrophysics.
This image is actually a composite of three different images. There are galaxies,
that are in the background and image, and
overlaid in red is an image in the
x-ray showing hot gas that resulted when
two galaxy clusters
billions of years ago, that  have passed
through each other, and the resulting
interaction of the hot gas gave rise to that signature that's observed.
But what's also shown is the gravitational image in blue, and that
gravitational image is produced by some
matter.  And we see that that matter is
what's responsible for clumping ordinary
matter, like that make up the galaxies
that are observed, and that this
this blue material, this matter, has
actually passed through each other
without interacting.
So this is - that image tells us something about what dark matter is.
What physicists have figured out, in terms of dark matter, is that we know that dark matter is
non-baryonic, that means it's made up of
particles that are not like ordinary matter,
that are like the protons and neutrons and electrons that you might be familiar with.
Physicists know that the dark matter is very weakly interacting,
because it appears to be collisionless
so that its interactions are
purely gravitational as far as the observations that we can make.
That dark matter is cold, it's made up of nonrelativistic particles, that it clumps together,
because of gravity, and as a result the
dark matter provides the seed that
allows galaxies, clusters of galaxies, the
large-scale structure of the universe,
to exist as we observe it. And as our
simulations can can predict, and be compared with observation.
But what physicists don't know is exactly what kind of particles
form the dark matter that pervades the universe.
And there are several ideas, and the leading ideas include something called supersymmetry,
that produces a "WIMP," a weakly
interacting massive particle,
extra space-time dimensions, the excitations could be a particle... could
manifest itself as a particle, dark matter, something called axions.
And so the question is, how do we detect dark matter, how do we determine what it is.
And on this slide, it's a simplified
overview of the ideas that particle theorists have about dark matter,
And it shows that there's many, many ideas.
And I'll point out, actually this slide comes from 2013, that's seven years ago.
And in the seven years, we haven't
simplified this any further.
So still, our picture, from theory, is
like this - but we need to go after this
to understand the nature of dark matter
with experiment, and see what what tells us.
So one of the types of experiments that is pursuing the detection of dark matter
is to build very large detectors deep underground,
so we seek shelter underground from cosmic radiation, so we're constantly bombarded
by radiation from space, high-energy
particles that rain down on us, and so
we go deep underground and so that these particles will be screened by the
material above, and so we when we locate our very sensitive detectors below,
we're hoping that these very weakly
interacting particles will pass through
and that we can observe them that way.
And so in Canada, we have one of the
world-leading facilities for this type of physics experiment, and what we have
is in the one of the mines one of the
very deep mines in Sudbury, we have a
underground lab that's called SNOLAB. And what we've done really over the
past 20 years is we've carved out of
rock and produced a very quiet location
in which we can place the most sensitive
detectors to search for dark matter.
And so SNOLAB is a follow-on to the Sudbury Neutrino Observatory.
The results from that original project garnered the Nobel Prize for Art MacDonald in 2015,
And so we're continuing with SNOLAB with new experiments, and I'll just show you a few of
the pictures of what we're doing, and
then afterwards if you're curious about
what these detectors are, then please
come and talk to me.
So here is an illustration - no, photos - not illustrations - photos of the
neutrino detector, as it's been refurbished, and you can see that here my colleague
is putting the finishing touches on the
upgraded neutrino detector
that's currently taking data now.
This is a detector that's filled with 3,000 kilograms of liquid argon. So liquid argon is at minus 190 degrees Celsius,
argon is a gas that's abundant in ordinary air, and when you
cool it down to those temperatures, when
you liquefy it, it becomes a detector
that then produces flashes of light that
can be very distinctive between
interactions of dark matter and ordinary
matter. And here, this detector is cooled
to below one degree above absolute zero
and when dark matter particles will
strike this detector it'll produce a
pulse of heat, heat up these detectors
a small degree and they can be measured with very sensitive detectors, and we're
also hoping with this installation being
installed in SNOLAB in the next few
years, that we'll try to detect dark matter that way. And so the quest to
understand the nature and composition of the universe continues, and I think
there's light at the end of the tunnel.
Thanks very much.
