Shedding light on dark matter Patricia Burchat
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as a particle physicist I study the
elementary particles and how they
interact on the most fundamental level
for most of my research career I’ve been
using accelerators such as the electron
accelerator at Stanford University just
up the road to study things on the
smallest scale but more recently I’ve
been turning my attention to the
universe on the largest scale because as
I’ll explain to you the questions on the
smallest and the largest scale are
actually very connected so I’m going to
tell you about our 21st century view of
the universe what it’s made of and what
the big questions in the physical
sciences are at least some of the big
questions so recently we have realized
that the ordinary matter in the universe
and by ordinary matter
I mean you okay me the planets the Stars
the galaxies the ordinary matter makes
up only a few percent of the content of
the universe almost a quarter or
approximately a quarter of the matter in
the universe is stuff that’s invisible
by invisible I mean it doesn’t absorb in
the electromagnetic spectrum it doesn’t
emit in the electromagnetic spectrum it
doesn’t reflect it doesn’t interact with
the electromagnetic spectrum which is
what we use to detect things doesn’t
interact at all so how do we know it’s
there
we know it’s there by its gravitational
effects in fact this dark matter
dominates the gravitational effects in
the universe on a large scale and I’ll
be telling you about the evidence for
that what about the rest of the pie the
rest of the pie is a very mysterious
substance called dark energy more about
that later okay so for now let’s turn to
the evidence for dark matter in these
galaxies especially in a spiral galaxy
like this most of the mass of the stars
is concentrated in the middle of the
galaxy this huge mass of all these stars
keeps stars in circular orbits in the
galaxies so we have these stars going
around in circles like this as you can
imagine even if you know though physics
this should be intuitive okay that stars
that are closer to the mass in the
middle will be rotating at a higher
speed than those that are fahrt further
out here okay so what you would expect
is that if you measured the orbital
speed of the star
ours that they should be slower on the
edges than on the inside in other words
if we measured speed as a function of
distance this is the only time I’m going
to show a graph okay we would expect
that it goes down as the distance
increases from the center of the galaxy
when those measurements are made instead
what we find is that the speed is
basically constant as a function of
distance if it’s constant that means
that the stars out here are feeling the
gravitational effects of matter that we
do not see in fact this galaxy and every
other galaxy appears to be embedded in a
cloud of this invisible dark matter and
this cloud of matter is much more
spherical than the galaxies themselves
and it extends over much what wider
range than the galaxies so we see the
galaxy and fixate on that but it’s
actually a cloud of dark matter that’s
dominating the structure and the
dynamics of this galaxy galaxies
themselves are not strewn randomly in
space they tend to cluster and this is
an example of a very actually famous
cluster the Coma Cluster and there are
thousands of galaxies in this cluster
there the white fuzzy elliptical things
here so these galaxies clusters we take
a snapshot now we take a snapshot in a
decade it’ll look identical but these
galaxies are actually moving at
extremely high speeds they’re moving
around in in this gravitational
potential well of this cluster okay so
all these galaxies are moving we can
measure the speeds of these galaxies
their orbital velocities and figure out
how much mass is in this cluster and
again what we find is that there is much
more mass there than can be accounted
for by the galaxies that we see or if we
look in other parts of the
electromagnetic spectrum we see that
there’s a lot of gas in this cluster as
well but that cannot account for the
mass either in fact there appears to be
about ten times as much mass here in the
form of this invisible or dark matter as
there is in the ordinary matter okay it
would be nice if we could see this dark
matter a little bit more directly I’m
just putting this big blue blob on there
okay to try to remind you that it’s
there can we see it more visually yes we
can and so let me lead you through how
we can do this so here is an observer it
could be an eye could be a telescope and
suppose there’s a galaxy out here in the
universe
how do we see that galaxy a ray of light
leaves the galaxy and travels through
the universe for perhaps billions of
years before it enters the telescope or
your eye now how do we deduce where the
galaxy is well we deduce it by the
direction that the Ray is traveling as
it enters our eye right we say the ray
of light came this way the galaxy must
be there okay now suppose I put in the
middle a cluster of galaxies and don’t
forget the dark matter okay now if we
consider a different ray of light one
going off like this
we now need to take into account what
Einstein predicted when he developed
general relativity and that was that the
gravitational field due to mass will
deflect not only the trajectory of
particles but will deflect light itself
so this light ray will not continue in a
straight line but would rather band and
could end up going into our eye where
will this observer see the galaxy you
can respond up right we extrapolate
backwards and say the galaxy is up here
is there any other ray of light that
could make it into the observers eye
from that galaxy yes
great I see people going down like this
so a ray of light could go down be bent
up into the observers eye and the
observer sees a ray of light here now
take into account the fact that we live
in a three-dimensional universe okay
three-dimensional space are there any
other rays of light that could make it
into the eye yes the Rays would lie on a
I like to say yeah on a a cone so
there’s a whole array of light rays of
light on a cone that will all be bent by
that cluster and make it into the
observers eye if there’s a cone of light
coming into my eye what do I see a
circle a ring it’s called an Einstein
ring Einstein predicted that okay now
it’ll only be a perfect ring if the
source the deflector and the eyeball in
this case are all in a perfectly
straight line if they’re slightly skewed
we’ll see a different image now you can
do an experiment tonight over the
reception okay
to figure out what that image will look
like because it turns out that there is
a kind of lens that we can devise that
has the right shape to produce the
kind of effect we call this
gravitational lensing and so this is
your instrument okay but ignore the top
part it’s the base that I want you to
concentrate okay so actually at home
whenever we break a wineglass I save the
bottom take it over to the machine shop
we shave it off and I have a little
gravitational lens okay so it’s got the
right shape to produce the lensing and
so the next thing you need to do in your
experiment is grab a napkin I grabbed a
piece of graph paper I’m a physicist so
a napkin draw a little model galaxy in
the middle and now put the lens over the
galaxy and what you’ll find is that
you’ll see a ring an Einstein ring now
move the base off to the side and the
ring will split up into arcs okay and
you can put it on top of any image on
the graph paper you can see how lovely
the lines on the graph paper have been
distorted and again this is a kind of an
accurate model of what happens with the
gravitational lensing okay so the
question is do we see this in the sky
do we see arcs in the sky when we look
at say a cluster of galaxies and the
answer is yes and so here’s an image
from the Hubble Space Telescope many of
the images you are seeing earlier from
the Hubble Space Telescope well first of
all for the golden shaped galaxies those
are the galaxies in the cluster they’re
the ones that are embedded in that sea
of dark matter that are causing the
bending of the light to cause these
optical illusions or mirages practically
of the background galaxies so the
streaks that you see all these streaks
are actually distorted images of
galaxies that are much further away so
what we can do then is based on how much
distortion we see in those images we can
calculate how much mass there must be in
this cluster and it’s an enormous amount
of mass and also you can tell by eye by
looking at this that these arcs are not
centered on individual galaxies they are
centered on some more spread-out
structure and that is the dark matter
that is being that is in which the
cluster is embedded okay so this is the
closest you can get to kind of seeing at
least the effects of the dark matter
with your naked eye okay so a quick
review then okay to say that you’re
following so the evidence that we have
that a quarter of the universe is dark
matter this graph
patiently attracting stuff is that
galaxies the speeds with which stars
orbit and galaxies is much too large it
must be embedded in dark matter the
speed with which galaxies within
clusters are orbiting is much too large
it must be embedded in dark matter and
we see these gravitational lensing
effects these distortions that say that
again clusters are embedded in dark
matter ok so now let’s turn to dark
energy so to understand the evidence for
dark energy we need to discuss something
that Stephen Hawking referred to in the
previous session and that is the fact
that space itself is expanding so if we
imagine a section of our infinite
universe ok and so I’ve put down four
spiral galaxies ok and imagine that you
put down a set of tape measures okay so
every line on here corresponds to a tape
measure horizontal or vertical for
measuring where things are if you could
do this what you would find that with
each passing day each passing year each
passing billions of years okay the
distance between galaxies is getting
greater and it’s not because galaxies
are moving away from each other through
space they’re not necessarily moving
through space they are moving away from
each other because space itself is
getting bigger okay that’s what the
expansion of the universe or space means
so they’re moving further apart now and
what Stephen Hawking mentioned as well
is that after the Big Bang space
expanded at a very rapid rate
but because gravitationally attracting
matter is embedded in this space it
tends to slow down the expansion of the
space okay
so the expansion slows down with time so
in the last century okay people debated
about whether this expansion of space
would continue forever whether it would
slow down you know will be slowing down
but continued forever
slowed down and stopped asymptotically
stopped or slowed down stop and then
reverse so it starts to contract again
so a little over a decade ago two groups
of business
astronomers set out to measure the rate
at which the expansion of space was
slowing down okay by how much less is
expanding today compared to say a couple
of billion years ago the startling
answer to this question okay from these
experiments was that space is expanding
at a faster rate today than it was a few
billion years ago okay so the expansion
of space is actually speeding up this
was a completely surprising result there
is no persuasive theoretical argument
for why this should happen okay no one
was predicting ahead of time this is
what’s going to be found it was the
opposite of what was expected so we need
something to be able to explain that now
it turns out in the mathematics you can
put it in as a term that’s an energy but
it’s a completely different type of
energy from anything we’ve ever seen
before we call it dark energy and it has
this effect of causing space to expand
but we don’t have a good motivation for
putting it in there at this point ok so
it’s really unexplained as to why we
need to put it in now so at this point
then what I want to really emphasize to
you is that first of all dark matter and
dark energy are completely different
things ok
there are really two mysteries out there
as to what makes up most of the universe
and they have very different effects
dark matter because it gravitationally
attracts it tends to encourage the
growth of structure okay so clusters of
galaxies will tend to form because of
all this gravitational attraction dark
energy on the other hand is putting more
and more space between the galaxies
makes it the gravitational attraction
between them decrease and so it impedes
the growth of structure so by looking at
things like clusters of galaxies and how
they they’re number density how many
there are as a function of time we can
learn about how dark matter and dark
energy compete against each other in
structure forming in terms of dark
matter I said that we don’t have any you
know really persuasive argument for dark
energy do we have anything for dark
matter and the answer is yes we have
well-motivated candidates for the dark
matter
now what do I mean by what well
motivated I mean that we have
mathematically consistent theories that
were actually introduced to explain a
completely different phenomenon okay
things that I haven’t even talked about
that each predict the existence of a
very weakly interacting new particle so
this is exactly what you want in physics
where a prediction comes out of a
mathematically consistent theory that
was actually developed for something
else but we don’t know if either of
those are actually the Dark Matter
candidate okay one or both who knows or
it could be something completely
different now we look for these dark
matter particles because after all they
are here in the room okay and they
didn’t come in the door they just pass
through anything they can come through
the building through the earth there so
non-interacting so one way to look for
them is to build detectors that are
extremely sensitive to a dark matter
particle coming through and bumping it
so a crystal that will ring if that
happens so one of my colleagues up the
road and his collaborators have built
such a detector and they’ve put it deep
down in an iron mine in Minnesota okay
deep under the ground in fact in the
last couple of days announced the most
sensitive results so far they haven’t
seen anything okay but it puts limits on
what the mass and the interaction
strength of these dark matter particles
are there’s going to be a satellite
telescope launched later this year and
it will look towards the middle of the
galaxy to see if we can see dark matter
particles annihilating and producing
gamma rays that could be detected with
this the Large Hadron Collider a
particle physics accelerator that will
be turning on later this year it is
possible that dark matter particles
might be produced at the Large Hadron
Collider now because they are so non
interactive they will actually escape
the detector so their signature will be
missing energy okay now unfortunately
there’s a lot of new physics whose
signature could be missing energy so
it’ll be hard to tell the difference and
finally for future endeavors there are
telescopes being designed specifically
to address the questions of dark matter
and dark energy ground-based telescope
and there are three space-based
telescopes that are in competition right
now to be launched to investigate dark
matter in darken
so in terms of the big questions what is
dark matter what is dark energy big
questions facing physics and I’m sure
you have lots of questions which I very
much look forward to addressing over the
next 72 hours while I’m here okay thank
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