[Music]
you
good afternoon the universe is very
large it's been around for thirteen
point five billion years it's about
ninety point 95 point five billion years
large it consists of stars and galaxies
clusters of galaxies and quasars so I'm
going to tell you today about how we
know that the universe is large and how
do we measure distances to all these
objects and for that we need to climb
the cosmic distance ladder which I wish
to do today right so this is a ladder
and each rung of the ladder represents
one step towards the edge of the
universe so the first step is to find
distance Lemuria stars which we do using
parallax so which is the theme of this
afternoon's session
the second step is to start which are
further away using seafood and which are
known as seafood variables then we come
to the galaxies and then we come to
galaxies which are much further away and
then what are the distances to these
objects now that is measured in parsecs
so what's a parsec a parsec is 3.26
light-years and how much of the light
here it is the distance traveled by
light in one year which is about 9
trillion kilometers right so we start
with starts which are just 100 parsec
away just our backyard 300 light-years
away and then when you go up to the top
of the ladder we are at 4000 mega parsec
that is an unimaginably large distance
but there is much further to get which I
will show you that we can't right so
then what is this this is our own Milky
Way galaxy taken from the inside it
consists of about a hundred billion
stars then you tend to the level stars
so if I looked at it from the talk
the first view or the edge on view but
if we could look at it from the top
which we can't then you would see it at
a nice circle with at least kind of
bulge or a bar and spiral arms now in
this picture which are taken with the
Hubble Deep Field
it's called the ultra-deep field every
object that you see every dot that you
see is a galaxy so each dot contains 10
to the 11 - 10 to the 12 stars you can
imagine how many planets are there and
how many civilizations
I mean Star Wars is nothing compared to
this right so now this is a microscopic
part of the sky and when you take all
the galaxies all over then you know how
many there are and that is what we have
to deal with right so now what is
battle-ax now you'll hear about that for
many speakers this afternoon but as
parallax means that apparent motion for
me of an object when I look at it from
different directions so supposing I'm
sitting in a train I look out of the
window and I see the trees going by very
fast but objects which are further away
the towns and the villages and the
mountains very distant they don't seem
to move at all and why is that
because I first look at a tree from one
side then very quickly from the other
side so it seems to me passing my now
astronomers make use of this to measure
distances to the stars how do we do that
now imagine the sky the very faint stars
then there are not so faint stars and
then there's a bright star which is
nearby
supposing I draw an immense triangle
then first time at Point a and then I
look at the star with my telescope then
I go to point B and look at the star
with my telescope and then I get the
battle ax angle which is drawn there and
the distance is from the middle of the
line a B to the star and what is that
distance given by it is given by the
length of a a divided by the angle as
simple of that but how do I draw this
triangle in the sky and for that I
depend upon my own platform mother earth
which is moving in space so here you see
a picture of the earth at one point I
look at a star using my telescope then I
just go to sleep for six months get up
and then the earth is at the other side
and then I look at it again so I got
this triangle I got the parallax I got
the distance so who did this first it
was done up for a star called 61 Cygni
and then the distance or 3.8 parsec and
if I've done by Bessel in 1838 quite
long time ago but why were the ancients
not able to do it
the ancients said because we don't
measure parallax
there's no parallax the earth is not
moving but that's not true it is just
that they did not have the instruments
to make the accurate measurements which
are necessary so now we have got the pad
lights for a thousand stars so up to
about 100 parsec which is again our
backyard how do we move further the
problem with going further is the fact
that the atmosphere blurs the images of
stars so I can't measure accurately the
distances add up their positions in the
sky and therefore I can't get the
parallax so how do we beat that you just
get onto a satellite so here's the
satellite which is called Gaia
these are launched in 2013 and which is
going round going around what not the
earth like our communication satellites
do but it is going around a very special
point which is known as l2 the second
lagrangian point which you see I'm sorry
the second lagrangian point which you
see there and this satellite goes around
it and then it observes stars the star
feels very very accurately and get bad
Laxus very accurately and that makes us
much further to about 30,000 light years
but you see that the further you go the
smaller is angle and not even something
which is as technologically advanced as
Gaia can measure it and we need
something else and that's something well
so chanced upon and that's called a
seafood variable so what's that it's
completely different concept so you see
that here is the constellation of CPS
and their use here you know later called
Delta where the Delta C fears a star
there and it was discovered again very
long ago in the 18th century that the
star is variable which means that the
intensity of the star keeps increasing
and decreasing this happens to many
stars for many different reasons it
could be because one star eclipsing
the star or it could be just because the
style becomes bigger and smaller now to
think about seafood variables is that
they've got a very specific pattern of
variation so they go up and down this
particular variable it has a period of 5
days 8 hours and 48 minutes but you can
have a whole range of periods so I just
wanna my survey the sky they find many
objects which are variable and those
which show a pattern like this are the
seafood variables and why are there so
important because of the seafood
period-luminosity relation so don't
worry about the details of it but
plotted on the horizontal axis is the
period of a variable and plotted on the
vertical axis is the amount of energy
that it emits per second which is the
luminosity right so now you see that
each point represents a seafood variable
there and they lie along a straight line
so which means that if I find a new
seafood
sea field and I know the period which
can be measured then I immediately know
its luminosity and if I know its
luminosity I combine that with the
brightness that I measure from the earth
and with a simple relation get the
distance and so who did all this it was
Henrietta Leavitt who did it in 1900
between 1908 and 1912 she worked in
Harvard with a large group of women who
are called computers demo call computers
because they computed all day and then
they classified thousands and hundreds
of thousands of stars into the types and
did all kinds of investigations this
thing was incredibly important because
it has taken you from a short distance
that we could measure to a much bigger
distance I tell you in a moment how
important that was she was recommended
for the Nobel Prize but in those days
first of all Nobel Prizes were not given
to astronomy second women were not
particularly welcomed in astronomy
except that when they are doing as
computers so you see that today should I
definitely made it and now I will tell
you how she feed variable
take as much further distance to the
galaxies
so here the the nearest big galaxy to us
which is known as the Andromeda nebula
why nebula because it's a very diffuse
object during the indo-pak war I have
seen this object with very small
binoculars from the lawns of the eye
fire okay but give me the telescope you
get a magnificent view with the
photograph and then but until 1920 or so
people did not know whether this is an
object inside our galaxy or outside our
galaxy there are major debates about
that so how do we settle their question
like this you could go on philosophizing
and Lesley ok then how many angels dawn
so the tip of a needle right but
otherwise you take a practical point of
view and say that I want to make them
measure that distance to this object and
that was that stupendous task was
achieved by Edwin Hubble who is one of
the great astronomers of the previous
century so he worked for a very long
time on galaxies did very many important
things now look at the observation of
the Andromeda nebula that he did you see
this small little square here which has
been magnified and then there are
literally hundreds of thousands of stars
here and Hubble did the incredible think
of establishing that this little dot
that you see here is actually a variable
star and you see here the the original
photograph by Hubble where he says
variable with an exclamation mark right
so this turned out to be a see fit
variable therefore you know its period
and then it's given there and then that
you know its distance and that distance
the Hubble measured to be 1.5 million
light-years and what is the extent of
our galaxy a hundred thousand
light-years so which would mean that
this object is nowhere inside our galaxy
it is a plane outside ok which
established that the universe must be
filled with galaxies so you see that the
measurement of distance just getting up
of order further
of the cosmic distance ladder you see
how water important new thought it has
generated but that was not the end of
the story that are just the beginning
you see that these are all galaxies and
Slipher in 1912 before Hubble's time
actually measured looked at the
spectrums of the galaxies each spectrum
has got certain characteristic lines
when compared to the such similar lines
on the earth in our laboratories it is
found that the spectrum is redshifted
what do you mean by that that every
feature in the spectrum is a little bit
redder the wavelength of every feature
is increased and why does that happen
there's a very simple reason and it is
known as the Doppler effect so if you
see a locomotive coming to you from the
back with a horn or the whistle on and
then locomotive passes by you see a
significant change in the pitch that is
because where it is approaching you the
wavelength has gone down goes down when
receding the wavelength goes up and the
same mystic story with light because
these galaxies are these galaxies that
they are rendered which means of the
wavelength has increased which means
that they are receding from us so that
was a tremendous discovery and then
Hubble went further and made a plot of
the velocity of recession against the
distance so you see that you can measure
the distance to only a small number of
galaxies but we can measure their
velocities to hundreds and thousands and
millions of galaxies so Hubble had only
a handful because you're working with
older technology and then he got this
straight line so what do they tell you
take a new galaxy and then measure the
redshift which is e very easy to do and
then you will get the velocity and
therefore you get the distance because
of this plot right so there's a side
line to the story this is known as the
Hubble law but last year the
International Astronomical Union decided
to call it the Hubble limit law and that
is because there was this very elegant
astronomer who was a Catholic priest who
worked in the Vatican Observatory who
had discovered all this independently
here discord what is now called the
hubble law independently so last year
conscious decision was taken to call it
the hubble lameta law now the galaxies
are all moving away from us but we are
not special so if you sit on any galaxy
you will see all galaxies are moving
away from you and what does this mean
that the universe is expanding so you
see that in the 1920s we had not
established whether a galaxy is a
diffused cloud inside our galaxy or
outside and in the 1930s we are
beginning to talk about the expansion of
the universe and that was the time
Einstein's general relativity had been
invented and so with the new field of
relativistic cosmology came into being
and it had a connection with
observations now look at this object it
is not like an elegant spiral it is not
an elliptical such a diffused gas but it
is a galaxy and how far away is this
galaxy the redshift is so far so high
then it should be moving at 11 times the
speed of light
is that possible no because Einstein has
told you that nothing can move faster
than light so how do we explain this
what I said long no he was doubly right
because first he said that nothing can
move faster than light and then in the
general theory of relativity he said
that space-time is curved and his
space-time is curved the simple
interpretation that Hubble and la
Mettrie put on it is no longer tenable
the maths becomes more complex but still
redshift remains a measure of the
distance and which takes us quite far
the last rung of the ladder which I'm
going to talk about today is a supernova
a supernova when you've got a very large
star many many times more massive than
the Sun when it finishes is evolution it
explodes because the interior of the
star collapses and forms what is known
as a neutron star it was made up of
neutrons it is incredibly dense it is
only ten kilometers in radius and then
the outer layers of the star are thrown
out in a supernova explosion many of you
I'm sure have seen an image of the Crab
Nebula which was first seen by chinese
astronomers in the year 1054 ad this is
a different kind of supernova
because when the stars are not
sufficiently massive they don't form a
neutron star at the end of the evolution
they form what is called a white dwarf
and a white dwarf he's made up of
electrons I mean the electrons which
provide the pressure for the Dodge and
then white dwarfs have a mass which is
less than 1.4 times the mass of the Sun
and in fact subramanyam Chandra Shekar
Drude that there can be no white dwarf
with a mass greater than 1.4 solar
masses because it can no longer be
supported by the pressure and it
collapses so here to start with a white
dwarf which is below 1.4 solar masses
and add a little bit of matter to it
this matter is taken from a companion it
attracted by the white dwarf also to the
white dwarf it could collapse and form a
neutron star but what was more likely to
happen that it ends his life in a great
configuration without leaving anything
inside and that is known as the
supernova type 1a and all supernova type
1a have exactly the same luminosity and
therefore the Act has distance
indicators and in fact it is using this
supernova this indicator that it has
been established that our universe is
not only expanding but the speed of
expansion is increasing it is becoming
accelerated accelerate all the time what
causes the acceleration dark energy so
if you have come to the end of the story
and we started with parallax went to see
Pete's fundamental plane which I didn't
explain supernovae and so and then
coming to the edge of the universe
we have climbed the cosmic distance
ladder and then but how much how we
reach the end of the observable universe
no we are at about four thousand twelve
thousand light years but the universe is
93 billion light years in size so are we
ever going to get there no we are never
ever going to get to the edge of the
universe and why is that I'm sure you
would like to know but it is a story for
a different time thank you very much
you