until the 1920s we thought our universe
had just one Galaxy and that was the
entire universe and of course we are at
the center of it I mean why wouldn't we
be but then came along henria levit a
young underpay astronomer working at the
Harvard she made a groundbreaking
discovery that changed everything within
just a few years we realized not only is
the universe teaming with billions of
galaxies but that the universe itself is
expanding and apparently made Einstein
say that he had made the big blunder of
his life but wait a second what exactly
was that blunder and wasn't it Edwin
Hubble who discovered that the Universe
was expanding what exactly did she do
and most importantly why didn't I ever
hear about her well it turns out that we
are always told an oversimplified story
about how Edwin Hubble discovered that
the universe is expanding but when you
look into the details oh man the details
are a Marvel of scientific thinking full
of paradig shifts all centered around
her groundbreaking discovery and so I
thought I should make this video to set
the record got straight so if you're
ready to ReDiscover the real story of
how we figured out the universe is
expanding let's begin so let's back up a
little bit because it all starts when we
ask a fundamental question to the man
who started it all Newton so Newton
according to you everything attracts
everything else in the universe if that
is the case and if the universe has been
there forever that was a dominant we
back then that the Universe was eternal
then the question was why hasn't
everything just clumped together and
Newton's answer to this is I don't know
he really had no answer for this
question so we move on and we talk to
Einstein who has a much better theory of
gravity Einstein same question and
Einstein says I do have an answer for
you what if at large distances gravity
is also having some kind of a repulsive
force that could counteract the
attractive forces and the universe could
stay balanced in fact you can even see
this in his f fi equations see the
original field equation had the space
time curvature on the left side a
constant and matter and energy on the
right side so mat and energy tells
SpaceTime how to curve that produces the
attractive gravity but then he added in
another term the negative sign
represents that it has the opposite
effect on the SpaceTime curvature as of
matter energy so this produces the
anti-gravity repulsive force and it does
not depend on matter and energy it is
purely a function of space and he
tweaked this constant has to get the
universe balanced and he called it the
infamous cosmological constant so look
the universe was balanced and at this
point I'm like Einstein did you just
pull out this term out of thin air just
to make the universe stable and I sense
like yeah but do you have anything
better yes I do said Alexander fredman a
Russian physicist fredman says he solved
the Einstein's field equation for the
entire universe by making some
assumptions and realize you don't really
need the repulsive term in fact he
figured out according to Einstein's
equations the universe itself can be
dynamic it can be contracting or it can
be expanding so look he says Einstein
the universe doesn't even have to be
static so we don't need this term at all
and Einstein's response to this was wow
that makes no sense and this is probably
how the conversation went Freedman give
me your notes let me check hey there's a
mathematical error and fredman says no
there isn't and Einstein's like yeah no
there isn't but still it doesn't make
sense what are you talking about space
stretching and expanding that doesn't
make sense space has to be static
everything needs to be balanced and the
cosmological constant stays that was a
dominant whe back then and and fredman's
idea was just that a wild idea to make
any kind of progress we needed to start
making some concrete observations and
that's exactly where our first unsung
hero comes in vesto slier
slier was an American astronomer and he
was tasked with I kid you not finding
martians why slier because he was really
good at spectral analysis you know where
you look at the light decompose it and
figure out what elements are present so
the Hope was he could Point telescopes
at Mars and find evidence for lifegiving
stuff like water or breathable air Etc
okay so slyer what happened slier says
he was doing that but in spare time he
was really interested in figuring out
what this spiral looking things in sky
in the SkyWars and I'm like slier what
are you talking about these are gal oh
yeah back then we didn't know that there
were other galaxies we thought that
there was only one Galaxy I find it a
little weird that there was a time not
too long ago I mean relativity was
discoed already but we believed that
there was only one Galaxy but if you
think about it we live in a similar time
today where we believe there's only one
universe but anyways coming at coming
back to this slier was super interested
in what the spiral looking things were
they called them the spiral nebula back
then so slier what did you do slier says
I did what I do best spectral analysis
so I looked at the Spectrum and he found
something really really interesting he
found that the light from all the spiral
looking things were mostly red shifted
what does that mean well you probably
already know this but remember whenever
a source of light is moving then the
light in the direction of the motion
gets squeezed making smaller wavelengths
and the light in the direction opposite
to it gets stretched out giving you
longer wavelengths and shorter
wavelengths look to us bluish and so we
call that the blue shift and the longer
wavelength look to us as reddish towards
the red end of the spectrum we call that
the red shift if something is red
shifted we know it's going away from us
and if something is blue shifted we know
it's coming towards us okay he found
there were some galaxies like Andromeda
for example where blue shifted but most
other galaxies that he noticed about 30
to 40 were all red shifted and so this
meant that a few galaxies which again
remember back then we didn't know right
so we thought there were spiral nebulas
within the Milky Way but anyways this
meant that some of these spiral things
were hurling towards us and he could
estimate the speeds it was speed was
close to about 200 300 km/ second and
rest of the others were just moving away
at staggering speeds in fact he found
some of them were moving at about 2,000
km/ second purely from the red shift in
fact apparently he got a standing
ovation when he represented this because
this was the fastest thing ever recorded
back then so the big question was what
did this mean well for some people this
didn't mean anything this was just an
unusual phenomena maybe something is
happening maybe the red shifts mean
something else I don't know but this did
not have any broader significance for
but for some other people including
slier this was maybe beginning to tell
us that the these things are not part of
the Milky Way at all and maybe they're
questioning just just maybe they're
separate galaxies Al together but
remember thinking of other galaxies was
crazy talk back then it's kind of like
today thinking about other universes you
better have solid evidence if you want
to talk about that in science and this
just wouldn't do we needed something
more but I'm like wait a second why
couldn't we just measure distances to
these spiral looking things I mean if we
knew how how far they are we would know
but we didn't have the tools to measure
distances back then right I mean the
best tool to measure distances back then
was by using Parallax you know Parallax
where the usual demonstration is you
close one of your eyes align two fingers
and then switch the eyes and you see the
finger
shifting and what you notice is the
finger that's closer to you shifts more
compared to the one that's farther away
so that means whichever shifts more is
closer and if the shifts is lesser it's
farther away and we did the same thing
with the Stars we call this stellar
parallax we measure from two different
Vantage points and we figure out how
much the shift is and in other words and
measure this angle by knowing this angle
we could estimate how far the star or
anything else was but the problem is the
farther the star is the smaller this
angle becomes and after a point this
angle becomes just too small to measure
anything and therefore the parallx had a
range and the range was roughly back
then about 100 light years now if you
think about it 100 light years we could
measure distances that took light 100
years that is pretty incredible but in
the grand scheme of things we believe
that the Milky Way was about 30,000
light years it's an underestimate today
we know it's about 100,000 light years
but because the Milky Way is so big even
with Parallax of you know range of 100
light years it is nothing and so
everything was pretty much out of range
so if you tried to measure distances to
these spiral looking things you wouldn't
get anything so you don't know whether
it's at 200 Li years or 2 million Li
years there was just no way of knowing
things we needed some serious upgrades
on our Cosmic measurement systems kind
physics the link is also in the description now back to the video so we
needed some serious upgrades in our
distant measurement systems right and
that's exactly where Henrietta levit
comes in she was hired as a human
computer at the Harvard Observatory what
does that mean well back then we didn't
have a lot of computing power so we had
to manually analyze the data and women
were hired for that mostly because you
could pay them less that's just how it
worked back then and when levit was
meticulously analyzing thousands of you
know data of thousands of stars she came
across a startling discovery that
upgraded our measurement systems so lit
what exactly did you do so leit says
mahes if you have a light bulb and you
know exactly how bright it is and then
let's say you see a similar bright bub
bub but much dimmer then that means that
that bulb must be farther away that's
why it's appearing dim to you right so
this means that if you know the apparent
brightness of the bulb you can compare
it with the true brightness and you can
estimate how far it is because of the
inverse Square law right and this also
works the other way around if you know
how far it is and if you know the
apparent brightness well you can find
out the true brightness you got that
this is going to be important and I'm
like yeah that's makes sense you're
using brightness as a proxy to measure
distances so what you're saying levit is
that we can use the bulbs in the sky the
stars that we have in the sky their
brightness as proxy for measuring
distances but wait this would only work
provided we had some kind of a standard
star whose true brightness we knew and
we could identify that we we're looking
at those particular kinds of stars but
stars shine with all kinds of brightness
right so how does it work out well L it
says normal star cannot be used for this
but there are certain particular kind of
stars called cied variables and I'm like
cied what so variable stars are the ones
whose brightness changes over time it
pulsates and I'm like that's just
twinkling of stars isn't it and she says
no mahes twinkling of stars happen
because of the atmospheric refraction
here the stars are truly changing their
brightness and this happens over days
not few seconds and of those variable
Stars SEI variables are are the ones
whose brightness changes in a very
particular way it has a signature
pattern which means by analyzing how the
Stars brightness changes you can
identify a cied variable and I'm like
that is awesome now I have some kind of
a standard star the cifi variable but
wait a second this would only work if
all cides had the same brightness is
that the case and levit says no it's not
but and this is where the story gets
really interesting most of the times
this gets oversimplified because there's
some subtle signs involved but for you
and I this is the perfect moment this is
the climax because this is where the
groundbreaking discovery happens are you
ready okay while analyzing data she
found about 25 CI variables in all in
one spot the small megalin Cloud what's
interesting is that she found that some
had a much higher Peak brightness
compared to the other and since they are
in the same spot which means they're
pretty much at the same distance from
the earth this variation is not due to
the distance this variation is truly
because some cides are brighter than the
other and she could now study them to
see if there's any pattern and she did
find a pattern she found that if you
plot a graph of period of the CI
variables the pulsation period on the
x-axis actually the log of the pulsation
periods okay and the apparent Peak
brightness it's apparent because it's
the brightness that appears to you when
looking from the earth if you plot this
graph this is what she found that is a
straight line what she had figured out
is a relationship between the period and
the peak brightness the shorter ones the
ones that have shorter periods which
means they fluctuate much more quickly
they have a you know lower Peak
brightness and the ones that have much
longer periods they have a much higher
Peak brightness and I'm like this is all
we needed right now if I know the period
of a CI variable I know exactly how
bright it is right and levit says no my
almost because this is the apparent
brightness this is not the true
brightness but that's okay now all we
have to do is find a cied variable
within our parallx range then we can we
know exactly how far it is and then we
can find its true brightness and once we
find its true brightness we can use the
Levits relationship the period
Luminosity relationship to find the true
brightness of any other SEI long story
short because of levit law as we call it
today we knew we understood what was the
true brightness of any SE fi you give me
the period and you will now figure out
the true brightness of that cide and you
can use that as a standard for measuring
distances wasn't that incredible so the
big question is how much of an upgrade
was this well remember back then um the
the range of the paralax measurements
was slow up to a few hundred light years
with CI variables that range increased
to about a million liers that's right
this gave was a serious upgrade on our
distance measuring distance and now we
had everything needed for discovering
Paradigm shifts in astronomy so let's
calm down what happens next well first
of all her boss named harow shapley who
was the director of The Howard observ
obviously got access to her you know her
publication Levitz law and he
immediately started started identifying
CI variables within the Milky Way and he
found that the Milky Way was actually
much bigger than previously expected
roughly about 100,000 light years but
more importantly he also found some CI
variables in the center of the Milky Way
and by calculating how far that was he
realized that it was about 30,000 light
years away which meant again we realized
that we are not at the center of the
Milky Way and now we finally get to
Edwin Hubble a lawyer turned astronomer
he found a cified variable in the
Andromeda which means he could Now find
the distance to it and what did he find
well he found for the very first time
the distance to the Andromeda to be
close to a million light years this
means for the very first time we realize
that the Andromeda is not within the
Milky Way but it's much outside and by
looking at its apparent size we now
figured out that it must be much bigger
like a Galaxy this is how Humanity
discovered a new Galaxy this was huge
remember this is as huge as today
discovering new universe altoe now of
course we know that this was an
underestimate the actual value is close
to about 2 and A2 million light years
but who cares what matters is that
pretty soon we realize that our universe
is teaming with billions of galaxies
this now brings us to the most important
Discovery H will ever made remember how
slier had already calculated the red
shap values and he had figured out how
fast these things were already moving
but we didn't know what that meant now
Hubble said hey let me just go ahead and
find some CI variables then I can figure
out how far these things are and he did
that in a few years he figured out that
all of these galaxies are millions of
light years away but most importantly he
found a beautiful relationship between
the distance and the recession velocity
he found that the ones the galaxies that
are closer are moving away slower and
the ones that are much farther are
moving away much faster in fact there
was a beautiful relationship when he
plotted distance versus relation uh
recession velocity he found that they
all lie pretty much on a straight line
This is called the hubbles law but what
does that mean well one interpretation
was the most straightforward one that
the galaxies are indeed just moving
through space but there are two problems
with that well first of all this means
that you know you our Milky Way comes at
the center of all of this motion and
Humanity has finally learned his lesson
we know that we are not at the center of
anything so this must be true from all
perspectives that everything must be
moving away from everything else um as
seen from any perspective but more
importantly it doesn't also explain why
things that are farther away are moving
faster so what model can fit this
observation fredman's model remember how
fredman had suggested by theoretically
just by solving Einstein's uh field
equation that the universe itself could
be stretching or Contracting but now
let's see what happens if the universe
itself stretches and expands well see
properly what we find f look the ones
that are closer move away at a shorter
you know at a shorter velocity and the
ones that are farther they move away a
lot more exactly aligning with Hubble's
Law and notice there's nothing special
about Milky Way if I change the Vantage
Point let's say I look at the same thing
you know this expansion from this
perspective I get the same result now
boom you see that everything is moving
away from this one according to the
Hubble's Law that is beautiful isn't it
this is how we realize that the Universe
must be expanding the fredman's model
perfectly fits the observations and
pretty soon we realize that if the
universe is expanding then if you wind
back the clock it must have had the
beginning and so the Big Bang hypothesis
was was put forward actually not really
there is a strong counterargument put
against the Big Bang there's a way to
imagine the universe expanding without
ever having a beginning and that's
homework for you look up something
called the steady state universe and you
can also see how we eventually disproved
that because at the end of the day it's
about scientific thinking so don't just
don't just take my word for it do some
analysis yourself but anyways the point
of the story was that we made some
groundbreaking discoveries like shapely
discovering that Earth you know the sun
is no longer at the center of the
milkyway or Hubble figuring out that the
universe is expanding but they would
they couldn't have done it without
levet's law without her meticulous you
know analysis and discovery of the
Levits law this wouldn't have been
possible and they were openly crediting
her for their work in fact Hubble even
mentioned that she deserved a Nobel
Prize and so when someone actually tried
to nominate her for a Nobel Prize
unfortunately she had suddenly passed
away because of a stomach
cancer anyways Einstein finally realized
that fredman was right we did not need
to force fit this particular term
because the universe not static in the
first place so he got rid of the
cosmological constant stopped working on
it and apparently called it the biggest
blunder he had made in his life or was
it