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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