Translator: Tijana Mihajlović Reviewer: Peter van de Ven
I spend most of my waking hours worrying about exotica in the Universe.
I work on dark matter, dark energy, and black holes.
But while I'm thinking about these very abstract mathematical things,
once in a while I think back
to the little girl that I was growing up in India.
And how did I get here?
Of course, we all know, growing up, having won the birth lottery,
that we did some right things too:
we at least worked hard, we went to the right colleges, and so on.
But then, there are these serendipities in life,
these little moments that, when they happen,
you don't understand the import of them,
but they fundamentally transform your life,
and you realize that later on.
For me, that moment, that little twist of fate
actually happened when I was a graduate student in Cambridge, England,
studying at Trinity College.
And I had been profiled
in a BBC documentary called "Stephen Hawkin's Universe"
for my work on mapping dark matter.
And that was going to be telecast.
It was shown; there were six episodes,
and it was shown every Sunday night.
And the episode that I was in
was going to be shown on 31st August, 1997,
and that is a special day.
I don't know how many of you probably remember
that is the day there was a tragic event: Princess Diana died.
And of course, in Cambridge my friends had assembled a big party,
and they were all going to sit there
and make fun of what I looked like on camera.
And that whole thing got scuppered because the TV programs went out of whack,
and this episode was moved to the following Sunday.
It turns out that that little delay
played a very insidious role
in launching me into the orbit that enabled a lot of other things
in terms of following my passion for my work.
So, this episode was shown on Sunday,
and the following Monday, there was a fellowship election of Trinity College,
and this is a very prestigious thing for a young scholar;
it's what gets you to be a don in Cambridge,
and I was in the running.
And I had had the advantage the previous night
of having being able to explicate on national television
exactly what I did, and why I loved what I did.
And I'm sure that played a role in launching me into the right orbit.
So what orbit is that, and what does it enable me to do?
It turns out that most of the matter in the Universe
is not the ordinary stuff that you and I are made of -
hydrogen atoms and so on; no atoms that we know about.
Most of the stuff in the Universe is made of dark matter.
Dark matter, in fact, is about 90% of the mass repository in the Universe.
What does dark matter do?
Well, you can't see it; that's why it's called dark.
(Laughter)
But it does get revealed by this beautiful phenomenon,
and this phenomenon is called gravitational lensing;
essentially, it's the bending of light.
So, what dark matter -
in fact, what any matter does in the Universe
is cause a little pothole in the fabric of space-time.
The presence of any kind of mass - so even our Sun -
causes a little pothole,
and depending on how much mass there is,
the pothole is deeper and more virulent.
And what light travel in the Universe -
abundance of light traveling to us from distant objects -
allows you to do is map these potholes,
and that's how you discover dark matter.
So light beams from objects -
galaxies, typically, or quasars -
very early in the Universe travel in these tubes -
these tubes show you light traveling -
and we end up seeing an image that is no longer a true image.
And the reason it's not the true image
is the shapes of these galaxies get distorted dramatically
because of traversing these very many potholes
that have been created by matter that we do see
as well as the vast amounts of matter that we don't see.
So this is how dark matter reveals itself to us:
by producing these potholes that we can then track.
So, when people ask me, "What do you do?"
the simplest was to explain what I do is, "I look at the potholes."
It's like looking at the potholes on a road
and figuring out what the road might be made of;
the distribution of potholes.
And, of course, the way you do it
is by looking at what it does to the suspension of your vehicle:
how does it distort?
So in this case, how light gets distorted.
It turns out that in the Universe, although there's a lot of dark matter,
it is distributed in very particular ways.
There are regions of the Universe that are heaped with dark matter.
There are regions that are very lightly smeared.
And so, this is one such heaped region of dark matter;
it's called a cluster of galaxies.
And what you see here in fuzzy yellow are galaxies -
about a thousand galaxies -
that are actually held together by the gravity of the unseen dark matter.
You don't see it here, but what you do see by eye already
are the really distorted shapes -
these sort of our key pulled-out shapes -
of distant galaxies that lie behind the screen.
So, in fact, what we're really seeing
is we're looking back in time
and looking at the imprint of every pothole,
every piece of mass light encounters on its way to us.
And it encounters a lot of stuff from very early on in the Universe.
And you may all know, when we look at the Sun during the daytime,
we're seeing the Sun as it was eight minutes ago;
it takes eight minutes for light to travel from the Sun to us.
If something happens to the Sun, we won’t know for eight minutes.
Now just push that boundary,
and think about the Sun as our nearest star.
As we go further and further out,
it takes light longer and longer to reach us.
So we're actually looking out into the Universe
and looking back in time.
And this is precisely what allows you
to do this mapping that I'm talking about, of dark matter.
So what is done in the dark matter mapping
is retracing these paths of light
and figuring out how the potholes are made,
how deep they are.
And what is shown here, in this green diagram,
is a reconstruction from the paths of light rays
coming from distant galaxies,
distorted by this foreground lump of dark matter
that is a cluster that we just saw.
And what we see there are actually lumps of dark matter.
Yeah, it doesn't look like potholes because I flipped it.
What you see as the peaks are aggregates of dark matter
that are sitting around the inner regions of that object.
And why is this important?
This is important
because our understanding of how structure forms in the Universe
hinges on dark matter;
dark matter is the key player.
Dark matter is the scaffolding on which all the structure -
including our galaxy and other distant galaxies -
actually form.
So, what I'm showing you here
is sort of the Brad Pitt and Angelina Jolie
of clusters of galaxies.
(Laughter)
This is the most photographed,
this is the most impressive galaxy cluster.
This is one of the regions in the Universe
that has the most dark matter accumulated, okay?
And what I am sure what you would see is the stuff without this blue haze.
And what this blue haze is is the derived map of dark matter
put on the visible Hubble Space Telescope image
of this cluster.
So there is all this mass that is lurking here
that you don't actually see.
Now, you may wonder why you should care.
Well, human beings have always been interested in mapping and figuring out -
Ever since we started looking up at the night sky,
we first wanted to know the lay of the land.
So there were these navigational explorations and expeditions;
we mapped the world; we mapped the solar system;
and now we map the cosmos.
So, when I was growing up, I wanted to be an explorer;
that's really all I wanted to be.
I wanted to be this adventurous explorer.
But, you know, I didn't dig getting scurvy,
so I didn't really want to go on the sea voyages.
I wanted to be a nice armchair explorer.
Well, to put it more appropriately,
an explorer sitting in front of her computer screen,
making new maps.
So I see what I do as mapmaking.
And the reason this mapmaking is so fantastic
is because it allows all of us now
to look up at the night sky, not just in awe,
but with some understanding,
with the profound understanding of what is unfolding in the Universe,
allowing us to unravel things that are unseen,
like dark matter.
Thank you very much.
(Applause)