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on may 1st 2015 a group of scientists
predicted that the following november we
would see a star go supernova billions
of light years away in a spiral galaxy
designated sp1149
this was the first time anyone had ever
tried to predict a supernova
and for good reason they are incredibly
rare and unpredictable
for a star larger than eight times the
mass of our sun a supernova marks the
end of its life cycle running out of
fuel in its core the star collapses in
on itself and then in the ensuing crush
of matter it violently explodes
a supernova can be as bright as a whole
galaxy and importantly the light emitted
follows a predictable pattern it glows
brightly for weeks and then fades down
over a period of months
but supernovae are rare in any galaxy of
a hundred billion stars or so you can
expect on average only two per century
so just try picking the star that's
going to explode
now we can say how long a given star
will live based on its mass luminosity
and color temperature these data
pinpoint its stage of life in a
predictable life cycle
but the estimate of exactly when a large
star will go supernova has big error
bars
take the red supergiant beetlejuice for
example in our own milky way galaxy it
is a great candidate to go supernova
scientists think betelgeuse will explode
any day now in the next few hundred
thousand years
when it does it'll be so bright you can
see it in the daytime
and it will rival the full moon's
brightness at night compared to the
lifespan of a star a hundred thousand
years is a brief window of time but for
us short-lived humans it may as well be
forever
so you might think it would have been a
tough sell when the scientists who
predicted we'd see a supernova in
november 2015 asked for time on the
hubble space telescope to take pictures
of galaxy sp1149
but their request was granted
they could image this part of the sky
roughly once a month starting on october
30th
before this the galaxy was too close to
the sun to point hubble at it
in this first image taken at the end of
october there is no supernova
the next image was taken on november
14th
again
no supernova
but in the third image taken on december
11th jackpot there's a supernova right
where they predicted it would be and
almost exactly when they said it would
happen
so how did they manage to predict a
supernova almost to the month
well the truth is they had seen this
same exact supernova before
not once not twice but four additional
times
a year earlier and five months before
the predictions hubble took this image
see those four bright dots those are
multiple images of that same supernova
the reason we see the same supernova in
four different locations is because
there is a lens between us and the
exploding star
not a lens made of glass of course but a
gravitational lens made of a huge amount
of ordinary matter and dark matter
gravitational lensing tends to magnify
distant sources and increase their
apparent brightness as rays of light
become concentrated this smears the
image of distant galaxies into arcs
strands and all kinds of weird shapes
and of course to some distant observer
in another galaxy the light from our own
sun and milky way galaxy may be
similarly warped
in gravitational lensing there are three
essential components the source the lens
and the telescope if the lens and the
source are spherically symmetric and if
the source lens and telescope are
perfectly aligned you get what's known
as an einstein ring
the light from the source is bent around
the lens equally in all directions
leading to the image of a smeared out
ring
if the source and the lens are
spherically symmetric but are not
perfectly aligned then what we end up
seeing is a break in the einstein ring
it splits into two semi-circles
and if the source lens and telescope are
aligned but the lens is not axially
symmetric for example it could have an
elliptical shape
well then you get four images in the
shape of a cross
an einstein cross
so what happened with our supernova is
9.3 billion years ago a dying star in a
galaxy far far away went out with a bang
as a supernova
the explosion sent out a blast of light
in all directions about five billion
years ago before the earth even existed
that light encountered a very massive
object that warped space-time it was a
cluster of galaxies called macs
j1149.5 plus 2223
yeah i know that's a mouthful but the
name just tells us where in the sky it
was discovered by the massive cluster
survey
this galaxy cluster is made up of lots
of massive substructures like individual
galaxies and halos of dark matter
at some point as the light traveled
through this region it encountered an
elliptical galaxy almost perfectly lined
up with where the earth would eventually
be
and the gravitational deflection focused
light rays that were initially diverging
onto paths that converged at the earth
this is why we saw the same supernova in
four different locations
not only did the supernova appear at
four different places the images also
appeared at different times
relative to the first image the others
were delayed by periods ranging from
five days to over three weeks and we
could measure this time delay because of
the distinctive light curve of the
supernova
some of the images of the explosion were
further along in their light curve than
others
this was a particularly lucky discovery
it's the first time a multiply lensed
supernova has ever been observed
there are other objects that due to
gravitational lensing appear multiple
times on the sky like multiple images of
galaxies but these objects don't change
predictably with time so there's no way
to use their images to work out the
relative time delay between them
now one reason for the time delay
between the supernova images is because
the four paths the light took were
different lengths so it just took the
light longer to travel further
but there is another reason which is
that light passing through curved
space-time appears to travel more slowly
relative to an external observer
this is much less intuitive but it's a
well-established and well-tested part of
general relativity back in 1964 irwin
shapiro suggested it would be possible
to test this gravitational time delay by
sending radar signals to venus and
measuring how long it takes for the echo
to come back
he calculated that due to the
gravitational influence of the sun the
signals would take an extra 200
microseconds when venus was on the other
side of the sun compared to when it was
close to us
this is solely a gravitational time
delay not related to the extra distance
the light has to travel and within a
couple years experimental data revealed
the gravitational time delay for light
traveling past the sun was exactly as
predicted today in order to accurately
determine the distance to the voyager
and pioneer spacecraft this shapiro time
delay must be taken into account
now i want you to have a look at the
four supernova images again
do you notice how the same galaxy
appears three times in this image
that is the supernova's host galaxy
it is itself lensed by the massive
galaxy cluster nacs
j1149.5 plus
two three
in fact this cluster lenses tens of
galaxies so scientists had been studying
and modeling the distribution of matter
in the cluster long before the supernova
they asked if we see these four images
of the supernova in one image of its
host galaxy when would the supernova
appear in these other two images of the
host galaxy and using the models of mass
distribution and general relativity they
calculated that in this image the
supernova would have appeared 20 years
earlier
in 1995.
now there are no close-up pictures of
this part of the sky from 1995 so
there is no way to check
but in the other image of the galaxy
they predicted the supernova would
appear again in about one year's time
almost exactly when it showed up in that
hubble image
this successful prediction is a
fantastic confirmation of our
understanding of light and gravity on
the scale of the whole universe but it
has even bigger implications one of the
hottest debates in astronomy right now
is how fast is our universe actually
expanding this is measured by the hubble
constant the rate at which distant
galaxies are receding away from each
other depending on their distance apart
now there are two main ways this
measurement has traditionally been made
one is to look for stars in the nearby
universe whose absolute luminosity we
know
then we can use how bright they appear
to us to determine how far away they are
if you combine this distance information
with how red-shifted their light is you
can work out how fast the universe is
expanding this is known as the distance
ladder method and the value of the
hubble constant it produces is around 74
kilometers per second per megaparsec
meaning for every megaparsec that
separates two galaxies they will on
average be moving apart at 74 kilometers
per second
the other way to measure the hubble
constant is to study the features in the
cosmic microwave background radiation
which is essentially just a picture of
the early universe
using the standard model of cosmology
which is called lambda cdm we can work
out how this early universe picture
would expand over time
the result obtained from this method
comes out to 67 kilometers per second
per megaparsec substantially slower than
the 74 found via the other method
now over the years both of these
techniques have been refined reducing
their uncertainties but the values have
not gotten closer together so at this
moment the two values really seem
different they are on the cusp of being
a five sigma result
astrophysicist joseph silk has called it
a possible crisis for cosmology but
there are independent ways to measure
the hubble constant and one of them is
to look at a multiply lensed supernova
and use the time delay between their
appearances to work it out
this was first proposed by a norwegian
astronomer sierra refstall in 1964.
since this is the first observed
multiply lensed supernova it has become
known as supernova refstal
calculations of the hubble constant from
this data yield a value of 64 kilometers
per second per megaparsec now although
that result has large error bars it is
more in line with the measurements of
the cosmic microwave background
radiation than with the distance ladder
method
the thing i keep thinking about is how
strange space is
i mean i used to think of it essentially
like glass fundamentally transparent
with some foggy regions or some slight
distortions but here we have space
warping light on multiple curved
trajectories making the same event
appear in six different places on the
sky separated by days weeks a year and
20 years
and what is contained in those
distortions is information about the
workings of our entire universe
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