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)
