i've been using accelerators such as the electron accelerator at stanford university just up the road to study things on the smallest scale
so recently we have realized that the ordinary matter in the universe and by ordinary matter i mean you ok me the planets the stars the galaxies the ordinary matter makes up only a few percent of the content of the universe
it doesn't interact with the electromagnetic spectrum which is what we use to detect things it doesn't interact at all so how do we know it's there we know it's there by its gravitational effects in fact
so for now let 's turn to the evidence for dark matter in these galaxies especially in a spiral galaxy like this
stars in circular orbits in the galaxy so we have these stars going around in circles like this as you can imagine even if you know physics this should be intuitive ok that
stars that are closer to the mass in the middle will be rotating at a higher speed than those that are further out here ok
so what you would expect is that if you measured the orbital speed of the stars 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 ok 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
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
fuzzy elliptical things here so these galaxy 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 this gravitational
can we see it more visually yes we can and so let me lead you through how we can do this so here 's an observer it could be an eye it 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
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 ok now
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 bend and could end up going into our
where will this observer see the galaxy
we extrapolate backwards and say the galaxy is up here is there any other ray of light that could make into the observer 's 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 observer 's eye and the observer sees a ray of light here
now take into account the fact that we live in a three dimensional universe ok a three dimensional space are there any other rays of light that could make it into the eye
like to see yeah on a cone so there's a whole ray of light rays of light on a cone that will all be bent by that cluster and make it into the observer 's eye if there is 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 ok now it will 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 ok to figure out what that image will look like
but ignore the top part it's the base that i want you to concentrate ok so actually at home whenever we break a wineglass i save the bottom
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 ok and you can put it on top of any image on the graph paper you can see how all the lines on the graph paper have been distorted and again this is a
model of what happens with the gravitational lensing ok 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 are earlier from the hubble space telescope well first of all for the golden shape galaxies those are the galaxies in the cluster
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 enormousamount 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 in which the cluster is embedded ok so this is the closest you can get to kind of seeing at least the effects of the dark matter with your naked eye
ok so a quick review then to see that you're following so the evidence that we have that a quarter of the universe is dark matter this gravitationally attracting stuff
is that galaxies the speeds with which stars orbiting 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 so every line on here corresponds to a tape measurehorizontal 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 ok
the distance between galaxies is getting greater and it's not because galaxies are moving away from each other through space
what stephen hawking mentioned as well is that after the big bang space expanded at a very rapid rate but because
so in the last century ok people debated about whether this expansion of space would continue forever
whether it would slow down you know will be slowing down but continue forever slow down
stop or slow down stop and then reverse so it starts to contract again so a little over a decade ago
groups of physicists and astronomers set out to measure the rate at which the expansion of space was slowing down
ok by how much less is it expanding today compared to say a couple of billion years ago the startling answer
to this question ok from these experiments was that space is expanding at
rate today than it was a few billion years ago
ok so the expansion of space is actually speeding up
there is no persuasive
now it turns out in the mathematics you can put it in as a term that's an energy but it's 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
matter because it gravitationally attracts it tends to encourage the growth of structure ok 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
their number density how many there are as a function of time we can learn about how dark matter and dark energycompete against each other
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 well motivated i mean that we have mathematically consistent theories
that were actually introduced to explain a completely different phenomena ok 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 ok 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 ok and they didn't come in the door they just pass through anything they can come through the building through the earth they're so non interacting
so one way to look for them is to build detectors that are extremelysensitive 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
ok deep under the ground and in fact in the last couple of days announced the most sensitive results so far they haven't seen anything ok but it puts limits on what the mass and the interaction strength of these dark matter particles are
the large hadron collider a particle physics accelerator that we'll 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 missingenergy ok now unfortunately there is a lot of new physics whose signature could be missingenergy so it will 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
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