kind of a good pub quiz question how old
is the universe people might know it
13.8
billion years
where does that number come from and uh
there's a little bit of a
interesting
difference emerging that might stand the
test of time and if it does stand the
test of time it either means we're not
understanding what we're doing or
there's perhaps some new physics out
there that we've not really accounted
for yet and that of course is the latter
would be very exciting so how do you
measure something as vast as the
universe like where do you start so we
think of our universe as evolving in
time and there are these special
sections and we're living in part of it
and for
at large distances their distant
galaxies will be actually moving apart
so if you can figure out you know how
far is that thing and how fast is it
moving away from me
and then
rewind time
then you can figure out how long the
universe has been expanding for so
essentially it boils down to sort of
speed equals distance over time
really i mean there's a lot more of
complications in there because we
actually know that the universe is
accelerating expanding and not just
expanding it's getting faster all the
time and then also the sort of effects
of general relativity when you
bring in really really distant objects
but basically
it's speed equals distance over time we
can either measure it by looking at
galaxies that are not that far away from
us and determining how they're evolving
with respect to one another and we these
we do these music well when i say not
that far away from us i have to be a
little bit careful some of these
galaxies uh we're looking at about seven
billion years old so
they are a hell of a long way away from
us so to measure this you obviously
can't just use one object you need a lot
of them you need your statistics to be
good
so let's start with distance okay so how
do you measure the distance
of an object again it uses a concept
that people might be quite familiar with
so it uses this idea of apparent
brightness so if you're crossing the
street at night you might look left
right green cross code all that but
you'll see a car headlights coming
towards you and you'll know whether it's
safe to cross because you know how
bright car headlights normally are when
they're right near you and you'll say
okay well it's not that bright and so i
can figure out that it's far enough away
that it's safe for me to cross so
astronomers basically use the same
concept and they say okay we need to
find something that we know what
brightness it should be
and how bright it appears can tell us
how far away it is by looking for what
we might call standard candles in them
and these are the supernovae they're
typical ones type 1a supernovae and in
type 1a you have a binary star system
one of them will have reached the end of
its life and have gone normal supernova
okay and what it'll left behind is its
core
which we call a white dwarf it'll
probably still be glowing somewhat what
that white dwarf is sort of held up by
is this thing called electron degeneracy
pressure that the whole thing is held up
by the fact that the electrons don't
want to be pushed any closer together
the other star though meanwhile is sort
of still orbiting around it okay still
in its sort of normal life cycle
but eventually it will start to run out
of hydrogen and it will swell to what we
call a red giant and then that's
probably going to come into sort of the
sphere of influence of this little white
dwarf and that white dwarf is going to
start sort of accreting
that material from that other star and
then it's going to keep building up mass
and building up its mass until it
reaches the point where those electrons
being pushed together can't support it
anymore
and then it will go supernova in this
type 1a again again
yeah supernova twice wow this is pretty
cool the thing is because of that
electron generacy pressure because
electrons are the same across the entire
universe that mass that it reaches
1.4 times the mass of the sun is the
same everywhere therefore it's always
going to explode with the same energy so
from that you can calculate their
distance quite easily knowing that their
brightness is dimmed so much over that
distance how do we know the supernova
we're looking at is a type 1a yeah good
question so
all the type 1as have a very um
like distinctive
signature in their spectra so not just
the all of the light we receive if we
take that light and split it through a
prism and we look at its constituent
parts it'll have a very specific
signature in its elements so first of
all you have to be able to recognize
what type it is and then you can say
okay so how bright does that appear now
so it's all about because these things
happen really quickly and start to fade
really quickly you have to have sort of
like uh really quick response times with
telescopes to be able to figure out what
these things are so you'll see a type 1a
go off in a galaxy you know how bright
it should be you know how bright it
looked and that tells you how far away
that whole galaxy is exactly yeah it's
really difficult to tell the distance to
galaxies themselves because you have no
idea what size the galaxy could be and
you know it could be the same size as
the milky way it might not be it might
be bigger it might be smaller and so
when you have a supernova that's then
bright enough to outshine that entire
galaxy you can then put a distance on
that so now we need the speed okay so we
need to know the speed that this galaxy
that this supernova is residing in is
moving away from us again we use a handy
little phenomenon called redshift okay
so people might have heard of this
before it's this idea that because the
universe is expanding
the light that's given off by that
supernova will be stretched as it
travels across that space okay and it
will be shifted and lengthened to longer
wavelengths so to read a wavelength
since redshift because it is being
stretched we can then say okay we can
convert that into a speed at which that
is moving away from us similar to a
doppler shift so we can turn a redshift
of say 0.5
into a recession velocity and again
you're going to need the spectra of that
supernova not just to identify that it
is a type 1a but actually look at the
light see how much it's been shifted by
and therefore calculate the speed that
it's moving away from us so basically
what we're going to end up with i'm
going to draw this is very exciting and
you're going to end up with a nice plot
of your distance going to be in a really
weird unit that astronomers like
called a mega parsec which is a distance
not a time thanks star wars and then
you're going to have it against
basically redshift which will be
in uh
be translated into a speed in kilometers
per second and what you end up with
you're going to measure this for as many
supernovas as you can and you can end up
with them all sort of dotted
like this
you'll be able to draw a nice best fit
line through them okay because what
you'll see is that your velocity
that you're
receding at is going to be proportional
to your distance it's a nice correlation
there so actually what we end up calling
this proportional constant is the
hubble's constant or h naught
here like that and this h naught is
going to basically be in
kilometers
per second
per megaparsec okay so you'll notice
this is a distance and this is the
measure of distance so really if we put
those in the same units and we canceled
them out it'd really be in one over
seconds
and so if we do
one over h naught
we're going to end up with something
that's in seconds
which will give us
the age of the universe a time in
seconds in seconds which would be a big
number i don't know
there is a lot of uncertainty in this
method how do you know also that you're
getting the right value
what you ideally need is another method
to be able to confirm whether that value
you're getting is right or not with
supernova
the current supernova camp is saying
that the hubble's constant
is 73.24
kilometers per second per megaparsec and
then there's another way there are
a number of ways but the second way
which i think is particularly
interesting is by looking at the
very early universe
and looking at the fluctuations in the
temperature of the universe due to
events that occurred at the very
beginning of the universe maybe due to
inflationary perturbations and and you
can look at the hot and cold spots and
and look at the the radiation that's
emitted in the turn and it reaches us in
the microwave background microwave range
so it's called the cosmic microwave
background radiation now you can fit
the hot and cold spots the distribution
of these temperature fluctuations with a
given cosmological model you pick your
cosmological model and you see how well
it fits the data and the best fitting
cosmological model is called the lambda
cdm model lambda as in the cosmological
constant it gives you a value of h
naught today which is about 66
kilometers per second per megaparsec so
you think oh come on they're close
enough
one's measuring the universe
as a result of physics that we're seeing
that occurred 300 000 years 380 000
years after the big bang one is you know
much more recent
seven billion years or so after the big
bang and you're within nine percent the
issue is this that the each of the sets
of people that have been doing these
measurements are working very hard to
understand the uncertainties in their
measurement and the uncertainties
both in their experiments their
systematic uncertainties
and they've both now reached the stage
where they error bars of each don't
overlap with one another anymore so
there's been a lot of contention for a
very long time about
what value
is actually right for hubble's constant
and ideally what you want is a
completely independent measure we didn't
have that for a very long time until
2017. so in 2017 there was a neutron
star neutron star collision detected by
the ligo and virgo collaboration and the
really cool thing about it was that
there was also
an optical detection
of the same event okay so not only did
you check the gravitational waves we
detected it in optical and gamma and
x-ray and all of these different
electromagnetic spectrum
uh radiation which is really exciting
but it also meant that it became what's
known as a standard siren so notice that
a candle or a standard ruler a standard
siren so something that you know how
loud it should be
and then you can compare it to how loud
you observe it to be so from the
gravitational waves we got how loud is
it supposed to be compared to what i
model it to be and then from the light
that we detected we again got a red
shift and therefore we could know its
distance so i bet you're dying to know
what value they found yeah yeah and if
they solved the whole mystery yeah
okay so here's the paper a gravitational
wave standard siren measurement of the
hubble constant and this is what people
had been waiting for in cosmology for
years
and i guess you want to see the plot
this blue line is what they got from the
baryonic acoustic constellations with
the standard rulers
this is what you got from standard
candles of supernova
and then this line here is the
likelihood of the hubble constant being
that value
uh given this this one neutron star
collision and can you see right where it
falls right between them right in the
middle
at about 70 kilometers per second per
megapasc
i know
it's so annoying because people were
waiting for so long as soon as people
heard that this happened they were like
oh we're constant we're finally gonna
know and it was just right down the
middle well so far it's one measurement
of course and so i'm sure i i assume
they think there's some big error bars
on it
for it to become a significant player
then you've got to have lots and lots of
neutron star mergers from which you can
then build up the statistics from what i
recall
for the next few years they're not
expecting to see very many voter one a
few
a year
well that means you're going to be
waiting 10 20 30 years to build up
significant statistics on it we're
basically in a waiting game now to see
what happens with this and whether we
can actually be sure of what the
hubble's constant is or not the equation
to work out when the time when it began
is trivial it's a really trivial
equation it only consists of
two terms in it and one but one of them
is the evolution of the hubble parameter
h naught is its value today
but it evolved in time and the way it
evolved in time depending on what's in
the universe and that's the thing people
are trying to pin down you know how much
of it is matter how much it is called
that matter how much it is dark energy
how much is in the curvature of the
space time and different combinations of
those three will give me a different
edge to the universe do you want to
calculate it yeah yeah okay let's do
this
yeah let's do it t what's your time of
your universe is going to be one over
your measurement for the hubble's
constant and so the hubble constant that
the ligo team found
was 70
plus 12 minus 8 because it wasn't very
precise
kilometers per second
per megapasc we're gonna have
one
over
70 basically so one megaparsec is
3.086 times 10 to the 22 meters
that value is burnt on your brain if
you've done an astronomy degree ever
before okay
so what we're actually going to do is
we're going to do 1
divided by 70
000 because we've made it into meters
okay so that's meters per second
and then divide that by 3.086
times 10 to the 22
meters okay
and then you'll be thankful to know that
i did that calculation before
and it was
4.41
times 10
to the 17
seconds so let's turn that into years
okay
so if we divide it by 60 we get it in
minutes if we divide that by 60 again we
get it in hours we did that by 24 we get
it in days and we divide that by
365.25 we get it in years
and you wonder what that comes out as
[Music]
13.97
billion
yes
so that means if we i mean that's so
exact that means if we like use a
calendar and go back we can find out if
the big bang happened on a wednesday
yeah
he really wanted to that would be really
cool actually we should do that
yeah
13.97 billion obviously you can't quote
it to that accuracy because uh ligo's
estimate was 70 plus 12-8 so really we
could only say 14 billion years
but
it's pretty close to sort of the 13.8
that people always quote that obviously
is from one of those two camps but
it obviously varies so people take a
nice average
okay so at brady's request okay
considering the fact that the detection
was made on the 17th of august 2017. and
using the numbers published in the paper
and using these numbers here we have
calculated that the big bang happened on
a tuesday
either a monday or a tuesday
and uh uh
what day the week would be and march the
6th well march the 6th it's my birthday
i mean it's pretty clear that that's
when it was yeah
if you enjoyed this video can i also
recommend our astronomy channel deep sky
videos which features a lot of the same
people you see on 60 symbols in addition
to some really cool videos about
telescopes the main thing we're doing is
making 110 films about the so-called
messier objects this is a list of 110
things in space that were listed by an
astronomer called charles messier he was
interested in comets and these 110
objects he knew weren't comets they were
smudgy on the sky like a comet but they
never moved so it was kind of like an
anti-list avoid these things
it was only many years later when
telescopes got better we realized
messier's list wasn't boring stuff you
should avoid but was in fact an
incredible bunch of things including
galaxies clusters of stars nebula things
messier probably hadn't even thought of
now we've done more than half of the 110
i think we must have done about 60.
we're not doing them in order we're sort
of jumping all over the place but if
you'd like to watch them i'll put a link
on the screen and in the video
description that's deep sky videos but
in the meantime thanks for watching 60
symbols and we'll see you here again
soon