[MUSIC PLAYING]
- Hi.
Welcome everybody to our
second installation of Next
In Science.
The idea here is
to bring together
early career scientists
who are working
in somewhat allied, but not
too closely allied fields,
to present their work.
The idea being that a lot of
times the most exciting science
is being done by early
career scientists, who
are in a sense-- I wouldn't
say putting their chips
on the roulette wheel, but
finding interesting new fields
that look like they're
going to be promising.
And so as a result,
you get treated
to things that are
more interdisciplinary,
and you can expect them
to grow in the future.
So it's very much a frontier
set of talks that we have today.
My name's John Huth, and
I'm a Physics professor
in the university here, and I'm
also a Venture Faculty member
with the Radcliffe
Institute-- which means
that I get to do programming.
I'm hosting a science
symposium on October 28th,
which some of you may be
interested in attending.
It's on oceans--
we're going to talk
about life in the very early
oceans here on the planet,
then talk about the
role of the ocean
in driving climate and the
feedbacks between climate
and the ocean, and then
finally, marine life
in the ocean with a particular
emphasis on and New England.
Prior to that, on
October 24th, we
have a talk by Kerry Emanuel who
is a climate researcher at MIT.
And his particular
field of interest
is extreme climate events,
in particular, driven
by rising sea temperatures.
And he is the author
of a paper describing
the emergence of something
called a hypercane, which is
a super hurricane that forms.
So the idea of being
with rising sea
surface temperatures, we'll
have fewer hurricanes,
but the ones we do are
more severe, as a result.
So you may want to
tune in for that.
All of these are
open to the public.
So today's Next
In Science, we're
bringing together
four scientists
who are going to talk about
their work broadly based
in astrophysics and astronomy.
And I've chosen speakers
in part because they
will talk about astrophysics and
astronomy on different scales.
And so we're going to start
with the largest scale--
the universe as
a whole, and work
our way into smaller territory.
I always bring a little bit
of my own interests into this.
I'm interested in
astrophysics, because I'm
a particle physicist I do work
at CERN at the Large Hadron
Collider.
And one of the
intersections there
is, we're looking for forms
of matter that might explain
the structure of the
universe, and in particular,
some of the work
that Cora is doing
informs us on what kind of
particles may be out there.
So that's one reason
that I'm interested,
but I have a more immediate and
somewhat parochial interest.
I'm teaching a freshman
seminar and we're
talking about early
models of the universe
and then we'll get
into modern cosmology
at the end of the seminar.
And one of the things that
we're reading in two weeks
is the Divine Comedy
by Dante Alighieri.
And one thing that struck
me in the Divine Comedy
was that hell is a lot more
interesting than paradise.
If you read it, you know
there's all these details
and you can really get
your teeth into it,
but then you get paradise and
it's all this ethereal kind
of floaty stuff
that says, oh you
can't understand it because
there are mysteries out here,
and you know about 25
cantos of mysteries
that you can't understand.
It's a little tedious, right?
So I thought about it a bit.
And wondered, why was this?
Why was this?
And when we had
this panel together,
I suddenly realized
that in Dante's era,
he knew a lot about geography.
He walks all around Tuscany
and sees all the waterfalls
and all sorts of things, but
the knowledge of paradise
is very limited.
You just have this
Aristotelian model
and these ideas which are
kind of vague and ill-formed.
But seven centuries later, we
have these amazing instruments
that allow us to give us
windows deep into the universe
and back in time.
And you can put flesh
on the bones now.
I mean, you can imagine
a trip to Jupiter
or you can imagine
a trip to Mars
or you can imagine what it's
like in the very early universe
and even the sound of the cosmic
microwave background radiation.
So you could almost imagine
that if Dante was born now,
and he went to write
the Divine Comedy,
Paradiso would be this
amazingly-detailed
and would be every bit
as good as the inferno.
So this is one of
the lessons that I
hope to impart to my
freshman in my seminar.
So having said that, let me
introduce our first speaker
Cora Dvorkin.
She is-- let's see
if I can do this
without looking at my notes--
she is from Argentina.
She got her doctorate at the
University of Chicago in 2011.
And she recently joined the
faculty in the Harvard Physics
Department last year.
Prior to that, she
was a Hubble Fellow
in the Institute for
Theory and Computation
at the Center for Astrophysics.
Did I get it?
- Yes.
- Great.
So Cora's work is in the
universe, the structure
of the universe,
at large, drawing
on a number of different
data sets and windows.
And it's proving to be
a remarkable picture
we're able to understand a lot
of details of the structure
and how this reflects on the
matter content of the universe.
So without further ado,
let me introduce-- well,
I am introducing her.
Oh, she's also the Shutzer
Assistant Professor
at Radcliffe.
Sorry about that.
And the title over talk is
Deciphering the Early Universe,
Connecting Theory
with Observations.
Let me also tell you that
talks are back-to-back--
Cora and then Salvatore And
then at the end of those
to two talks, we'll
take questions up front.
So keep your questions.
Write them down, hold them,
and then we'll take them
and then we'll have a break.
OK?
[APPLAUSE]
- Thank you very much
for the introduction
and thank you very much
for the invitation.
It's really a
pleasure to be here.
So today, As John
said, I will try
to make heaven more interesting.
I completely agree with your
opinion of the Divine Comedy.
So hopefully we can
make it all better.
So I will talk about deciphering
the early universe-- connecting
theory with observations.
So for this talk I chose
one sort of direction
in which my research has
been going, which is,
trying to understand
the physics that
seeded the first structures
in the universe-- trying
to understand what gave
rise to the structures
that we can observe today.
And for this, I will be
talking today about how
we can probe the physics
of the very early universe
by using observations
of something
known as the cosmic
microwave background.
And I will go in detail
into what this is,
and the large scale
structure of the universe.
So let me start by
showing this picture here.
This picture here represents a
very thin-- this line of time
represent 13.8 billion
years of the universe.
So the universe began as
a hot, condensed plasma
of particles in
thermal equilibrium
at very high energies.
The universe expanded and cooled
and many physical processes
happened along the way.
Some physical processes
we know very well.
Some of them we
don't know what's
the physics that gave rise to
this process and some of us
are interested in trying to
shed light on these physics.
So interesting processes
that happened along the way
are recombination-- about 0.4
mega years after the Big Bang.
Protons and electrons combine
into hydrogen. These periods
known as recombination.
And since then, the universe
became mainly transparent
to cosmic micro
background or CMB photons.
So since then, photons--
mainly freestream toward us,
until the time in which the
radiation from the first stars
and quasars reionizes the
universe-- about 500 megayears
after the Big Bang.
And about 6% these
photons we scatter-- 6%
comes from the latest data
set coming from the Planck
satellite.
OK?
So mainly they
freestream toward us,
until this period in which a
very small fraction of them
rescattered.
And so today, we
observe these photons
at microwave frequencies, and
at a temperature of about 2.7
Kelvin.
So I thought they would
give to the general audience
a little history, just
this slide of history
of how the cosmic microwave
background was actually found.
So in 1964, two astronomers--
Arnold Penzias Bob Wilson--
they were radio
astronomers looking
for sources of radio
waves, and they
were looking at radio waves with
their antenna in New Jersey.
And they were finding an
excess isotropic noise
in this antenna built at Bell
Labs if this story they meet
says actually this
is true because I've
heard Bob Wilson telling the
same story that they even
thought he was dropping from
germs in their telescope
Klein the telescope that
cleaned the telescope
and the excess noise
would not go away.
At the same time, in parallel,
Peebles, who was here yesterday
giving historical lecture at
the CFA, Jim Peebles, Dicke,
Wilkinson, the and
Roll, they were
doing theoretical
calculations and they
predicted that there should be
a signal of about 3 Kelvin being
coming from the Big Bang.
There were two
disconnected groups.
One of them had no idea
about the CMB expectations.
And an astrophysicist
from MIT, Burke,
talked to Penzias and Wilson.
Penzias and Wilson
repeatedly were
trying to find
people to tell them
about their source of noise
that couldn't go away.
They talked to him.
They talked to Burke.
Burke told them about their
work from Peebles, et al.,
and they told them that
they were very likely
finding something very
big-- something that
was very sought for.
And so the story ended
with them announcing
the discovery of the cosmic
microwave background.
And years later, they won
the Nobel Prize for this.
So this is a neat story
of a discovery that
came accidentally,
in some sense,
because they were not
looking exactly for it OK,
so in 1992, fluctuations around
the temperature of the CMB
were found.
So in 1992, a satellite known
as COBE, the Cosmic Background
Explorer, found
that the CMB mainly
follows a blackbody
distribution,
with a temperature of about
2.7 Kelvin and fluctuations
around these temperature
of 1 part in 10 to the 5.
And this is a map of how
COBE sees these temperature
fluctuations.
This part over
here is the galaxy
that has to be masked because
it's so bright that if we don't
mask it, we would not be
able to see these temperature
fluctuations.
And because of this
discovery, this team
also won the Nobel Prize.
OK, so this is a
more current picture
coming from data taken
by the Planck satellite.
The Planck satellite
was launched in 2009.
It took data until
very recently.
Papers are still coming out
from the analysis of these data.
This is a snapshot of
the very early universe.
And in this talk, I will
explain why I make this claim.
So what is observed here is that
temperature fluctuations have
different positions in the sky.
And the idea is
that by measuring
the statistical properties of
these temperature fluctuations
we can infer the physics
from the very early universe.
These temperature
fluctuations have
been measured to have a
[INAUDIBLE] temperature
of about 100 microKelvin
and their distribution
is Gaussian at first order.
So the CMB power spectrum,
these temperature fluctuations,
when I say power spectrum,
I mean the square amplitude
of these temperature
fluctuations.
The CMB power spectrum has
been predicted, as I already
told you, and measured
with great precision
over the last few decades.
And I'm showing you hear a
plot of the temperature power
spectrum of the
CMB as a function
of angular scale in the sky.
So scales here correspond
to smaller angular scales
and scales here correspond
to larger angular scales.
OK?
In red, I'm showing you
the data that has been
taken by the Planck satellite.
Notice the arrow bars
in these data sets.
This is a really
remarkable measurement.
In green and shining is the
best fit model for these data.
This best fit model
works very well.
I will be talking about,
in the next slides,
about the standard model that
we have in cosmology today.
And this structure of
these acoustic peaks
carry a lot of physics inside.
The structure of
these acoustic peaks
carry the information
of acoustic oscillations
in the photon-baryon plasma
at the time of recombination.
OK?
So the photons were strongly
scattering with the electrons,
via Thomson scattering.
The electrons were
interacting with the protons,
via Coulomb scattering.
And all of this was
going together--
baryons were trying to fall
into gravitational potentials,
coming from the dark matter.
And they were going out due to
the pressure of the photons.
So they were oscillating
inside and outside
this potential wells, and this
is why we see acoustic peaks.
The first peek is the baryons
and the photons going in
with gravitation potential.
The second peak actually
corresponds to a trough,
but because I'm
squaring it, it's
seen as a peak-- corresponds
to the photons going out.
And this is what we observe.
That's why we see the peaks
in the temperature power
spectrum of the CMB.
OK so here is where we
stand in cosmology today.
We have a standard model of
cosmology known as Lamda-CDM.
We have a homogeneous
background,
and we measure the parameters
of this background very well.
We know by CMB and large
constructual observations
that we have approximately
5% of baryonic matter.
When I say baryonic
matter I mean the matter
that we can observe-- you,
me, the stars the planets,
this building-- the
matter that we are used to
and that is visible to us.
We have 27% of
color earth matter,
and it's matter that only
interacts gravitationally
but it's not visible.
And we have approximately--
we know from observations
that we have approximately
68% of dark energy, which
is a source of energy
that we attribute
to-- that we attribute the
expansion, the accelerated
expansion of the universe that
is observed in recent times.
On top of this
homogeneous background,
we have perturbations
that are seeded
in their very early universe.
We measure these perturbations
to be nearly scale-invariant.
They almost do not depend on
the scales see considered.
And as I said before, they
are approximately Gaussian.
So we made a lot of
progress in measuring
all of these parameters
with very high precision.
However, big questions remain.
What is the Lamda of our
Lamda-CDM standard model
of cosmology?
What is dark energy?
We don't know.
What if the CDM of our
Lamda-CDM-- standard model
of cosmology-- what is this cold
dark matter that is observed?
And what gave rise to the first
structures in the universe?
So I do researching in
these different lines,
but today, I thought, given
that I have 30 minutes,
I will focus my talk on some
questions we are currently
trying to answer as
a community related
to the physics that seeded
the first structures
in the universe.
So this is what I will
base my talk today
in this question-- what
is the physics that
seeded the first
structures in the universe?
OK So inflation was a theory
for the very early universe
proposed by Alan Guth
and others In 1981.
Alan Guth is a professor at MIT.
And this theory, that goes
under the name by inflation,
is the main paradigm
that explains
the observed inhomogeneities
in the universe today.
Inflation is a period and
of accelerated expansion
in the first fraction of the
second after the Big Bang.
And it explains why the universe
is approximately homogeneous
and spatially flat.
We had several problems
before the theory of Inflation
was proposed.
The problems were related
to why the universe is
observed to be so flat, why the
universe is observed to be so
homogeneous at very
different scales
so this theory came into place.
It was proposed as a
solution for these problems.
So in the simplest
model of inflation,
there is a single scaler
field-- and by scaler field
I mean a field that
takes different values
in different positions-- and
more common-- and more common
scaler field that we are
very used to it this,
for example, the temperature.
OK?
That's a scalar field.
So the field takes different
values at different positions.
So in that simplest
models of inflation,
there is single scaler field
slowly rolling down a potential
This scaler field is known
as the inflect on field.
So while it rolls
down the potential--
while it rolls of down the
potential-- different modes,
different perturbations
in this field
stop being in constant contact.
When they stop being
in constant contact,
and by this I mean that they
cannot exchange information
with each other--
a perturbation--
an initial seed is frozen.
OK?
And this initial seed is
what then gives rise to CMB
fluctuations that
we observe today.
OK so the square amplitude
of this initial seed,
I will call it the primordial
power spectrum-- which
is the square amplitude of this
initial seed that gives rise
to the observed CMB fluctuations
and the observed large-scale
structure of the universe.
So the idea is that the
field during inflation
takes different values, and
these different values are
associated with the
CMB fluctuations
at different angular
scales in the sky.
So the idea is that by
looking at different angular
scales in this guy
the CMB power spectrum
may be able to different regions
of inflationary potential
during inflation.
So this is the link--
this is an inverse problem
that we have here.
We have the observations
of the CMB power
spectrum, either
temperature polarization.
And we want to
understand the physics
of the very early universe
a fraction of a second
after the Big Bang.
So our goal as a community,
and my goal in one
of my line of research,
is to shed light
on the physics of inflation
by using CMB observations.
This is just an
example of work that I
have been doing
on reconstruction
on this inverse
problem, on taking
data measured by
the Planck satellite
of the temperature of the photos
that come from the Big Bang.
These temperature data has been
measured by Planck satellite.
I'm trying to reconstruct this
primordial power spectrum that
is this seed for
these fluctuations
that we observe today.
So this is a reconstruction
that wed did using--
with Vinıcius Miranda and
Wayne Hu using temperature
fluctuations and an ongoing
work with a graduate student
at Harvard, [INAUDIBLE], is
to make a full reconstruction
of the primordial power
spectrum by using not only that
temperature fluctuations
of these photons,
but another property
of these photons known
as the polarization.
And I will talk about
these in a few slides.
We can also infer the shape
of the inflationary potential.
And here, I'm
showing you here-- we
can put constrains using
data from the Planck
satellite or the WMAP satellite
or many other telescopes that
are all over the world, we
can try to put constraints
on the shape of the
inflationary potential
to see how all these
[? inflect ?] on field
is moving along the potential--
if it's moving slowly,
if it has abrupt in the way.
So this is another example.
And we can make predictions.
I talked to you about the
temperature fluctuations
of the CMB photons, but the
CMB photons are also polarized.
They oscillate in
particular directions--
this is what is the polarization
of the CMB fluctuations.
And so with temperature
measurements of the temperature
fluctuations.
We can make predictions as
to what we should observe
for the CMB polarization.
And we can learn about
the physics of inflation
looking at these predictions.
Now another question
that we can ask
is-- I talked to you about
one field or a particle,
if you want, during
inflation-- can
we probe other primordial
particles during inflation?
Can we probe physical
properties of these particles?
And so I am just showing
you this slide just for you
to get an idea of
what you can do
with the large-scale
structure of the universe.
Primordial particles affect
the large-scale structure
of the universe in
very distinctive ways.
So for example, work
that I have been
doing over the past
few years is related
to probing primordial
particles spin 2.
When I say "spin," I mean a
quantum mechanical inherent
angular momentum
of the particle.
So we can program primordial
particles of spin 2
by looking at the statistics
of the ellipticities
of the galaxies-- and this
is another talk per se,
but I just want to give
you a feeling of some work
that I have been doing and
it's being done in this field.
You can also look at the
galaxy velocity field
and try to probe primordial
particles with spin 1--
this is a work in progress
with my post-doc Azadeh
Moradinezhad.
And in principle,
higher spin particles
which are studied-- which have
been studied for a long time
by string theories
and predicted,
in principle, they should
also leave an imprint
on the large construction
of the universe.
And this is more subtle and
it's part of work in progress.
OK let me tell you in my last 10
minutes about the polarization
that I already
mentioned of the CMB
and how you can hope to
learn about the energy
scale at which
inflation happen using
the polarization of
the CMB fluctuations.
So the CMB polarization
is linearly polarized.
And by this, I mean
that it oscillates
in distinctive planes.
OK?
The polarization is generated
by a process known as Thomson
scattering-- scattering of
photons off free electrons.
So the photons scatter
off free electrons,
and each electron,
along the line of sight,
sees a local temperature
quadruple-- a local temperature
distribution-- that is the
source of the polarization
that we observe today.
We can decompose the
CMB polarization.
When you show the composition--
this is just the basics--
but when you show the
composition is to decomposition
to E and B modes in the
same way electromagnetism
is decomposing to E and B modes.
Here, just to give
you a picture of how
the E and B modes of the
CMB polarization look like.
So that E-modes have their
direction either radially--
they are oriented
radially around cold spots
or tangentially
around hot spots.
And the B-mode has the same
orientation as the E-modes,
rotated by 45
degrees OK so that's
how that how we decompose
the polarization of the CMB.
So different processes give
rise to different types
of polarization.
Density and velocity
fluctuations
plus linear evolution--
just nothing
happened to these photons--
this gets rise to E-modes.
Now in order to
have B-modes, you
can either have B-modes
by a process known
as gravitational lensing.
Gravitational
lensing is the bend--
it's how the photons bend their
path when they are traveling
toward Earth because of massive
structures in the universe.
Massive structures
in the universe
curve the space
time and the photons
bend their back
toward us, and this
is known as
gravitational lensing
and this affect gives
rise to B-modes.
And another source
of B-modes comes
from primordial
gravitational waves.
I believe Salvatore will talk
about gravitational waves,
but perhaps not primordial
gravitational waves.
So bear in mind that
these will be two
types of gravitational waves.
So gravitational waves
arise from inflation,
because we expect to
have quantum fluctuations
in the space-time fabric
of the universe that
get expand in this accelerated--
in this period of accelerated
expansion-- a fraction of a
second after the Big Bang.
OK?
Those are the
gravitational waves.
And that's why they're exciting,
because a measure-- well,
that's one part of
why they're exciting,
but a measurement of
these gravitational waves
would provide us with
information about the quantum
properties of the metric.
So it will tell us something
about quantum gravity.
So gravitational waves--
also, so as I told you,
the stored space time.
And this creates a quadruple
that each electron sees as
well, along the line of sight.
And this source is polarization.
And they leave an imprint into
their B-mode polarization.
Here I'm showing
you just so that you
have a picture of
what these B-modes
look like as a function.
When I say multiple moment, I
mean angular scale in the sky.
So remember that these
are large angular scales
and these are small
angular scales.
This is the B-mode
on power spectrum.
In black, I'm showing
the B-mode power spectrum
coming from
gravitational lensing,
from the way the
photos bend their path
due to massive structure
in the universe
when they're coming toward us.
And in blue, I'm
showing you a signature
of gravitational waves
for a certain amplitude.
I just picked an
arbitrary amplitude here.
These two peaks correspond
to scattering of photos
off free electrons happening
either at the period
of reionization at
these very large scales,
or at the period
of recombination
at these much shorter scales--
smaller scales, sorry.
And the goal here of
this line of research
is to try to find out if these
gravitational waves are present
or not.
And so this line of
research puts limits
into the possible amplitude that
these gradation waves can have.
So they're
interesting because we
can learn this amplitude of
these gravitational waves,
we will be able, or we
could, potentially-- we
don't know if we will
ever measure them,
but hopefully we will.
We could potentially learn about
the energy scale of inflation.
So a measurement of
gravitational waves
would not only provide a direct
measurement of the expansion
rate of the universe
during inflation,
during the extent of the minus
35 seconds after the Big Bang,
but also, it would provide
a measurement of the energy
scale of inflation.
OK?
So this quantity r
over here is a measure
of the amplitude of the
gravitational waves.
So is this quantity is greater
than 0.1 this would imply--
or if it's the
order of 0.1, this
would imply that the energy
scale of the inflation
is about 10 to the 16GV.
Think that the LHC goes
up to perhaps 14 TV
so this is a TV's 10 to the 3GV.
This is orders of
magnitude larger
than what any possible
accelerator ever
built-- not only now, but ever
built in Earth can ever reach.
So this is a window
to the highest energy
scales in the universe.
These energy scales
are way larger
than any energy scale
that could be probed
that was proven-- that
was probed in the past
and that could be ever probed
in earth by an accelerator.
These are current limits on
primordial gravitational waves.
The take out message
from this plot
is that we are making
progress in putting limits
on these amplitude of the
primordial irritation waves,
but they have not
been detected yet.
This is work that I did on
collaboration with the BICEP
team and their Planck satellite
in which we combined their data
with other data sets coming
from the Planck satellite that
could be a source of confusion
with their data sets.
And these sorts of confusion
comes from something
that we cosmologies call
foregrounds-- because they
are a source of noise for us.
Some other people study
these-- they're not
foregrounds for other
people, but for us, they
are foregrounds.
These other sources
of confusion come
from emissions from
polarized dust particles
in the galaxy that
could be confused
with primordial B-modes.
So the quotes for primordial
gravitational waves
is continuing and will continue.
I'm showing you here an
approximate raw sensitivity
as a plot that's a
function of years.
WMAP was the satellite
in the early 2000s.
Planck, I talked about Planck,
it was launched in 2009,
and it was another satellite.
And now as a community,
as a big community effort,
we have recently
proposed an experiment
that we call CMB Stage 4.
Its stage-- the different
stages really grow from Stage 1
to Stage 2, 3, and
4, they grow in order
of magnitude of the number
of detectors that we need.
The CMB Stage 4
experiment proposes
building different
telescopes all over the world
to map the sky and to be
able to map these temperature
and polarization
fluctuations of the CMB.
This is my final slide.
This is what we
expect to have as
a constraint on
these quantity r that
was measuring the amplitude of
these primordial gravitational
waves.
So here, we around
this number-- 0.3.
With CMB Stage 4 in the
fourth year of CMB Stage 4,
this is just a projection
that we made as a community,
we expect to reduce
the [INAUDIBLE]
by about a factor
between 10 to 100.
This came out very
recently, actually.
This came out this week in a
science paper that we wrote.
And in this book, we not only
talk about possible science
that you can do with
the CMB Stage 4 proposed
experiment to learn about
the physics of the very
early universe,
but for example, I
was leading a dark matter
chapter in which we
also proposed different
physics that you
will be able to learn
with the CMB Stage 4
experiment about dark matter.
There were other--
you can also learn
about neutrinos, dark energy,
et cetera, et cetera, et cetera.
OK so I will end here
with my conclusions.
So I show you that we
can probe the shape
of the inflationary potential
by using CMB observations,
so by mapping observations
that we make today
with the very, very early
period of the universe.
We talked about how large-scale
structure of the universe
offers a unique way of
probing physical properties
of other possible
primordial particles.
And I showed you
the current state
of the art for constraints,
at that moment.
We are constraining, we
didn't measure anything,
at the moment.
But here are the constraints on
gravitational wave amplitudes.
And you should
really stay tuned,
because in the next
decade, we will expect
to tighten these limits
and on the amplitude
of primordial gravitational
waves by a lot,
and perhaps, even measure.
There are many experiments
that are built.
That are currently
proposed, that are currently
taking data-- such as BICEP3
and Keck, different frequencies,
EBEX, POLARBEAR, SPIDER,
Advanced ActPol, SPTP3G,
Simons Observatory, CMB-S4,
LiteBIRD, PIXIE, CORE, et
cetera, et cetera, et cetera.
In blue, I just put
the ones that I been
involved in writing proposals.
But these are lots
of experiments
that are being proposed and
this is really a golden era,
hopefully, that will come for
primordial gravitational waves.
OK, thank you.
[APPLAUSE]
No questions, right?
- Yeah, we're going to
hold our own questions.
So Salvatore is
originally from Italy.
He did his undergraduate work
at the University of Bologna.
And he did his doctoral work
at the Pierre and Marie Curie
University in Paris, got
his doctorate in 2012.
And then he joined MIT
as a postdoctoral fellow
and then became a
research scientist.
And he is now just about to
become a member of the faculty
at MIT.
- Yeah.
- And he has been working
on gravitational waves,
direct observation of
gravitational waves--
not produced primordially,
as Cora was talking about,
but produced in very violent
collisions in the universe,
at large.
And in fact, most
of you, many of you,
probably saw the
news of the discovery
of gravitational waves.
- No spoilers.
- What?
- No spoilers.
- No spoilers.
OK, sorry about that.
Even hear the sound of it?
Are you going to play
us the sound of it?
- No, I can make the sound but--
- Well, that would
be even better.
OK, so the talk
is, everything you
want to know about
gravitational waves
but were afraid to ask,
by Salvatore Vitale.
[APPLAUSE]
- Thank you.
And I hope you can hear me well.
And so the way
this talk is made,
it basically
anticipates the Q&A.
And it's a long
series of Q&A. I've
been asked to talk about
what I do for a living
and this is gravitational
waves, basically.
You can see here two black
holes rotating and emitting
gravitational waves.
Well, you cannot see them,
they're black, but OK.
OK, if you have questions,
we have the Q&A later
or I have coffee break.
So let me with the very
first basic question
that you may ask if you
don't spend your whole day
thinking about this thing is,
what are gravitational waves?
If you open a physics
textbook-- a good one--
you'll find typical
sentences like this.
They say the
gravitational waves are
ripples in the space-time
continuum, emitted
by any system with a
non-constant quadrupole moment.
OK which is not
particularly enlightening.
You can look at the
Einstein equation, OK.
And that is [INAUDIBLE]
you start from the Einstein
equation, you put
[? out your ?] your metric,
crank the machinery, and you
obtain gravitational waves.
Now, I could spend the next
25 minutes talking about this,
but I care about you, so
I'd rather use images, OK?
So this will give the idea,
well, at the direct order,
if you take a small
stone and you throw it
out in a pond which is at rest,
you will create a perturbation
at the center where
something happened
and this perturbation
will propagate outward.
OK?
So in this example,
the waves are just
a perturbation on the water
and the continuum is the pond.
In our case, the continuum
is just the space-time,
and is a very stiff material.
It's very hard to [? de-form ?]
this is why our stones needs
to be much, much bigger.
And in particular,
in what I do, I
focus on compact objects,
which is a fancy way of calling
neutron stars and black holes.
If you take two of
these objects and you
make them spiraling
around each other,
they lose energy and this energy
goes into gravitational waves.
And the image is very similar
to what you've seen before.
OK, so you may
think, oh, this is
pretty similar to
electromagnetic waves.
You have charges that move and
they make electromagnetic wave.
And there are points in
common, there are similarities.
There are also
important differences.
The one that I like to think
about is that in Cora's talk
you have seen that the electron
and all the photon interact
and they interact
with everything.
The gravitational field
this very different.
It's much more shy.
It doesn't interact with
anything, basically, OK?
This means that while light that
you receive from star pulsars,
quasars, whatever-- can be
easily absorbed, obscured,
bent, deflected, reflected
whatever-- gravitational waves
do not have this issue.
They basically can
go from one side
to the other of the
universe without being
disturbed by anyone.
And I hope some of you at
least will have recognized
my quote from Neil Young.
But if not, it's OK.
And I should have
used Bob Dillon,
but I didn't have time
to change the slides.
OK so how do we detect
these gravitational waves?
To answer this question,
we should first
look at what they do when
they go through something.
And what they do is that they--
well, they basically change
this space-time itself.
And the way it is it gets
manifest is in the fact
that if you have an observer,
a free-floating observer which
means, [INAUDIBLE] for
example, the distance
between these objects
will change with time.
So that if you
start, for example,
with 4 particles
within a ring, four
masses, and the
gravitational wave passes by,
the ring will become an ellipse.
So the space-- the distance
will increase in one direction
and decrease in the other, and
so on and so forth. [INAUDIBLE]
So you may think, OK, it's easy.
The only thing I
need to do to check
if there are gravitational
waves is put a few things
around, monitor their distance.
If I see that the
distances is varying
with a characteristic
pattern-- there
you go-- gravitational waves.
It's not that easy, though.
So don't believe the smiley face
because if you do the numbers,
you will find out that the
typical gravitation wave will
introduce a relative
distance which
is a variation, which we
constrain of roughly one
part on 10 to the 21.
And this is a very,
very large number.
So you have to be able to
measure very small variation.
When we give talk around
with my colleague,
we typically use the
example of an atom.
This is a hydrogen atom.
You go inside the nucleus
which is a proton and go
on [? inside, ?] [? inside, ?]
[? inside. ?] And this is
the proton and the
[? radiational ?] way to be
able to measure is the
one you would see here.
It's pretty small.
Now not many of us have
seen protons on their life,
so I found another
image, which I like best.
It's not about length,
it's more visual.
If you go to the seaside
and count the grain of sand,
you'll get a very large number.
If you do the same as
the size everywhere
in the coastline of the
whole planet, if you believe
the questionable web
page I found where
they do this
calculation, they found
there is a few 10 to the 21.
So the measurement
we need to do,
it's comparable to being able
to remove one grain of sand
from the whole planet and
see the difference, OK?
This is what we are doing.
And for us, desperate and crazy
as it may be, luckily for us,
there were people
in the past who
were not scared by this task.
In particular, I have a
picture of Ray Vice who
was a professor here at MIT who
in the 70s, got pen and paper
and showed that you can
actually build an apparatus that
can measure this kind
of variation in length.
And this original
idea was developed.
And several or tens
of years later,
it became what we
call LIGO, which
stands for a Laser
Interferometer
Gravitational-Wave Observatory.
Now we are not going through
the math or the data,
but I want to give you
an idea about the works.
LIGO is basically-- you can two
pictures, there two of them.
It's an interferometer,
for those of you
who know what I mean, of
arms four kilometers along.
The way it works
is the following.
You have laser
light-- like this one
just a bit better-- entering
an apparatus, hits a mirror.
Half of the light goes up,
half of the lights continue.
And there are
mirrors at the ends,
so the light bounces back.
Now this is not a
typical interferometer,
the light bounces back and
forth a few hundred times.
Then it goes out and
is recombined here.
Now light has a nice property
that if you combine it
in the right way, it can
interfere in a way that
cancels out, basically.
So you have darkness.
And this is the
condition in which we
keep our instruments normally.
Now if a gravitational
wave passes through,
because of what I
told you before,
this length will
change in a way,
and this direction will
change in another way.
So the interference condition
at the end is not met anymore
and we see some
light coming out.
And so this is the
basic idea of how
we use laser interferometry to
measure a very tiny variation
in distance.
We have two of these
instruments in the US.
One is in Louisiana, the
other is in Washington state.
And there are a few
others which are either
being built or planned
around the world,
and all together they
work as a network, OK,
to increase our sensitivity.
So what I do for
a living is more
about learning something
about the sources
of these gravitational waves.
So what can we learn?
Well.
If we go back to our pond,
you can imagine pretty easily,
I think, that if you
are sitting on the side,
you don't see what has
been thrown to the pond,
you only receive these
waves coming to you.
If you're very good at
physics or mathematics,
you can think of using
the shape and maybe
the relative distance
between this wave front
and so on and so forth
to learn something
about what happened
in the middle that
caused this perturbation
to start with.
And this is exactly what
we-- what I do, at least.
And so my objects of interest,
as I mentioned before,
are compact objects.
And these are leftovers
of very massive stars
after they end
their nuclear fuel
and they supernovae and explode.
What is left in the
middle, it's either
a neutron star which is
somewhere in this [INAUDIBLE]
nebula or a black hole
which you're seeing here,
hitting its companion.
Now both of these categories of
objects come with pretty many
open questions, and they're
very interesting objects,
otherwise , we wouldn't be here.
So the first one-- neutron star.
And neutron star-- you
probably know these.
We're talking of the objects
which are roughly 1.5 times
the mass of our own
star, but packed together
in a radius of 10 kilometers.
Now these conditions are
so extreme that, in fact,
pretty much like in
Cora's CMB example,
we can only produce this
condition in the lab so to be
frank, we have no idea
how matter behaves in this
condition, because we
cannot make them at earth.
So one of the
things I want to do
is study these neutron
stars is learn something
about their composition and
their [INAUDIBLE] state.
Another interesting
thing is verify
whether are many
astronomers seem to think,
neutron stars mashing
one and the other
are responsible for what we call
a GRBs, which are very bright
and energetic flashes of
light that we sometimes see,
or if they produce most of
the metal in the universe.
"Metal" means everything--
[INAUDIBLE] an idiom,
in this case.
And another related question
is, what is the maximum mass
of neutron star?
We don't really know and
[? there are ?] consequences
nuclear physics.
Things get even weirder when
you move to a black hole
because you have even more
mass, and even more compact.
And now these objects--
and then we'll
come back to this in
a minute-- produce
extreme gravitational field.
Now a priori, there is no reason
why Einstein General relativity
should work for those skies.
So this is the first probably
question comes to mind,
Einstein, inventor,
discovered GR
to explain what was happening
in the solar system, which
is a very quiet place, as
compared to black holes.
It's several order
magnitude extrapolation.
Of course, Einstein
was Einstein,
so it seems to be working,
which is very annoying.
Come back to this later.
Anyway, so there are several
other questions which
we want to answer, like how
fast can this black hole
rotate around their axis?
We don't know.
There are conjectures,
for example,
by Hawking, that
says that there is
a limit on how fast these
black holes can spin.
But we would like to verify
whether that's the case.
We don't know how
big they can get,
or rather, we know that they can
be either a few times our sun
or millions of times as massive.
We don't know if they can
have any value in the middle.
And we don't know
when they first
formed is of the universe.
Some people may
even think, suggest
that may be dark
matter, or whatever.
For us crazy-- something nice
about physics, and for us
crazy, as crazy
as your theory is,
there is someone who's
already thought about it.
OK, to tell you what I mean
by extreme with any image,
Let me use Saturn,
which I think all of you
have seen once in
your life here.
It has this beautiful ring,
and so on and so forth.
Now if Saturn were
a black hole, you
would see something
like this which
you know, if you're
seen Interstellar,
the movie Interstellar.
This is a black
hole with a ring.
But you see a pretty striking
difference between Saturn
and the black hole is that
the ring in the black hole
also goes up and down,
which seems very weird.
What is going on is not that
the ring is going up and down,
it's that the black holes are
pretty aggressive objects.
So if my eye here on
the right and looking
at the black hole
this way, what happens
is that the photons emitted
by the side of the disk, which
normally I should not be
able to see it because it's
on the other side,
try to escape--
for example, vertically or
with some angle but they get
attracted by the black hole,
by the gravitational pull
of the black hole, so they are
deformed and they go this way.
Which means that I
can see basically,
facing the black hole, I can
see the rare of the ring--
both the upper side
and the other side.
So this is something
which is pretty far
from what we typically
think and imagine
or experience in our life.
OK so this is about the sources.
How do the waves look like?
And for my work, I
focus, as I said,
on binary compact objects, so
I have two objects like two
black holes in this cartoon.
They started life what?
The part of their lives
which interesting for me,
around each other.
They are orbiting
faster and faster,
emitting gravitational waves.
When they are thought
about, we talk
of this spiral and these
gravitational waves
that they emit.
So these are the variation
on the space-time, basically.
And it's not the
same [INAUDIBLE],
it just gets slightly
louder and louder
and the frequency gets higher.
Then the two black holes,
or the two neutron stars
start touching each other.
We talk of merger, they merge.
And then what is left
is a single black hole,
because you have to measure
two of them together.
Since it was born in a violent
way, it's not spherical.
It's out of equilibrium.
So it has to release
the excess of energy
and we talk of
ringdown in this phase.
And this is the very last
bit of the wave from it.
So here's right
after the merger,
when the mess is happening.
And then basically
it goes down to zero,
because it's releasing all
the gravitational waves energy
left over.
I guess you're all
asking this question.
And now, this is all very
nice, but does it work?
We have these [? multi-denses ?]
and black holes are very weird
objects.
Maybe you made it all up.
Yes, it does.
And unless you have been
living in a different planet,
you have heard that
in the last year,
LIGO and Virgo
collaboration detected
two such binary black holes.
So the gravitational
wave is coming
from two of these mergers.
And it was nice, it was
everywhere [INAUDIBLE].
And here you can find-- we
actually added a third objects
which we cannot claim with
certainty that it was a black
hole-- I'm a personal
believer, OK,
so we claimed detection
of two of them.
And you can also see
these are the waveforms.
They look a lot like the
one I just showed you.
Now I should also say that
pretty much in the same way
if a Cora looks at
the CMB spectrum
from each of the feature-- like
the peak, [? the throat, ?]
whatever, she can learn
something about the universe--
if I look at one
of these waveform,
from each of these features like
their duration or the amplitude
or whatever, I can say
something about the source.
OK so this was
everywhere in the news.
We got the congratulation
from Obama, which is nice.
I put this because two things
are particularly funny to me.
The first one is that in the
Washington Post, although we
were pretty high
in importance, we
we're still below Meryl Streep,
Beyonce, Bloody Mess, whatever
that is, and Trumpism.
And the other thing
which I like is
that the Economist, given its
orientation, took our merger
and put us in the Merger
and Acquisition section
of their paper, which
I think is hilarious.
OK, moving forward.
So what do we learn?
We got these two
objects, I promise you
we would learn something
about black holes.
Did we?
Yes.
So we learned a few things.
First of all, we learned
that black holes can
be significantly more massive
than what was previously
found and discovered with
electromagnetic [? emission. ?]
So in this plot in this
cartoon, you see on the x-axis
the total mass of
the black hole.
This is 20, 40, 60, if you're
going to read it from there.
Now the blue objects
are the black holes
that they were previously
known through electromagnetic
observations, OK?
And you can see they all
live somewhere in between 5
and 20 star masses, maybe.
The red points
where there are bars
are the two and maybe
three black holes
that we discover with LIGO.
And you can see like in
some cases, like this guy
here, they are
significantly more massive
and scary than
what's found before.
You know, these are 35, and
these are 60 star masses.
Now in the next few
years, it's going
to be funny to discover
why we see this.
Are we targeting a
different population?
Or do the electromagnetic
metals have some selection bias
or a combination of both?
We don't know yet.
It's going to be interesting.
We also learned
something about the stars
that this black hole came from.
The fact that they
could get so massive
has implications on metallicity
of their progenital star,
in particular, it
puts an upper limit.
And we can talk about this if
you want at the coffee break.
But basically, if you
have too much metal,
you cool early so you
cannot become that big.
We also discovered, and got
to show that black holes can,
indeed, spin around their axis.
Now so for one of them, we could
say with pretty eye certainty
that it was spinning.
And for all of them, we
could say, basically,
that they are not
maximally spinning.
So they are far from
this theoretical limit
that some people think--
that most people think
should be there.
Now this may seem pretty vague.
I'm not telling
you that the spin
was 0.3 plus minus something.
However, the important
thing is that these
are the first direct measurement
of spins of black hole ever
made by humankind.
And let me tell you
what I mean here.
We already measured masses
and spins of black holes, OK?
But the way we have done it,
it's indirect measurement.
So first of all, what
you need is a black hold
in a binary system.
We call it x-ray binary.
So if you want to
measure, for example,
the spin of black hole in this
way, what you're measuring
is not the spin
of the black hole.
You're measuring properties
of the disk of gas
around the black hole.
OK?
I show above just to
impress you, [INAUDIBLE].
So you are measuring
something about the disk,
and from that you
infer the spin.
The same thing about the mass.
When you're
measuring, if you want
to know the mass
of the black hole,
is the mass of this
guy and his velocity.
So it's this different.
What we're seeing
instead here is
the direct imprint
of black hole mass
and spin on the space-time.
So it's a much
cleaner measurement.
OK, moving forward,
you may have read this.
And thank you for
the congratulation.
However, I showed this before.
This is wrong, OK?
We have not shown that
Einstein was right.
If I were the editor
of CNN, this CNN,
I would have titled this--
"It's not as sketchy,
but Einstein was not wrong."
That's what we have shown.
And actually, even better,
the gravitational waves signal
invented by lab are
compatible with what predicted
by Einstein, and by the way,
you cannot prove the theory is
right, only that it's wrong.
OK?
You can see also I have
a future in journalism
if physics doesn't work.
So I'll give you one example
of what I mean with this.
There we go.
I'll give you one
example of something
called massive graviton.
Now if you believe
in Einstein and GR,
the gravitational wave force--
forced [? and hence, ?]
gravitational waves, are carried
by-- well, the speed of light,
first of all.
So if you like to think in
a quantum point of view,
this means that
they are mediated
by a particle called
graviton which is massless,
like the photon.
And now if general
relativity is wrong,
you may think that the graviton
may have non-zero mass-- very
small but non-zero.
So one of the things we have
done with our discoveries
is trying to put an upper limit
on the mass of this graviton.
Now don't look at the
plot, we can focus just
on the question here.
We put an upper
limit on the mass
of this hypothetical graviton to
be something incredibly small.
It's 1.2 10 ti minus 22
in the span units which
is electron volt over c square.
Now John, you are--
a particle physicist
will tell you that
the neutranoids much,
much, much, much bigger.
And so it's a very small
number, but it's non-zero.
This is the point.
If we had decided
that this was zero,
we could have maybe said that
Einstein was right, it's zero.
By putting an upper
limit, we said
that 0 is compatible with
what we found-- which
is a very different thing, OK?
So what I mean here is that in
the next few months and years,
we can either prove
that GR is wrong,
if we find a violation
of it, or we can just
say that it keeps being
confirmed by the data.
Now in the next few
minutes, actually, I'm
early, which is good
for a coffee break,
I want to say something
about what happens next.
Let me start with what
I do, in particular,
which is, as I said, the
neutron star, black holes.
Now LIGO will restart
collecting data later in October
for another six months.
And then just a
small pause and we
continue for the
next three years.
Anyway, so in the
next month and years
we expect to detect way more
of this binary black holes.
And hopefully, since we
know they are out there,
we should also start detecting
binary neutron stars.
So part of my job in
the next few years
is going to be to
try to characterize
these objects-- the underlying
population, its properties.
And also, keep performing tests
of general [? activity. ?]
And the nice thing
about most of what we do
is that we can start
stack detection.
So from 10 we learn
more than what
we learned with the first one.
So our tests will get
better and better with time
And now although my talk only
focused on compact binaries,
there are other
potential sources
of gravitational waves, for
example supernovae explosions,
OK?
And so hopefully
soon we'll start
to detect some of these
other interesting objects,
or what I personally
would prefer,
see something which we
have no idea why it is.
In science, typically it
is the most exciting thing
that can happen.
And something that
I also involve
with people that
are on my team, it's
thinking about
what happens next.
Now if you remember the first
or second slide of Cora's talk,
she had this 1.3
billion on the history
of the universe in one slide.
And now we with
LIGO are targeting
sources and black holes
which are in our backyard.
They are a redshift
of 0.5 maybe.
So they are pretty nearby.
And most-- well, a lot
of interesting stuff
happens earlier--
or farther away,
depends how you want to
think about distance.
So we are with
people we're working
on conceiving and thinking
about the next generation
of ground-based gravitational
wave observatories.
We even have names.
We have names before we
have money, which is nice,
personally.
And these guys, once
they get online--
we're talking 15 years to
be optimistic-- they would
be sensitive to black
holes, basically,
as far as you have
stars in the universe.
So a redshift of 6 of 10,
and actually even more.
We can see black holes
up to redshifts of 20
with these objects.
OK in the last one
or two minutes,
I would like to even expand
a bit more the horizon here
and stress the
fact that there are
several astrophysical phenomena
which are-- will be producing
detectable gravitational waves.
Now in my talk, I focused here
on terrestrial interferometers
and using the LIGO observatory
in Washington state.
Now as I said, this kind of
detector target supernovae,
compact binary, and
something like this.
And they are sensitive
to frequencies
on the order under their
[INAUDIBLE] to [INAUDIBLE].
This is not all of it, OK?
C mentioned, BICEP, and
the other experiments,
which on the other
side of the spectrum,
very early on in the
age of the universe
is zero-- whatever
that means-- and they
are targeting gravitational
waves from the very Big Bang.
Which we know will happen as
she said, in the next few years.
In the middle there is a lot
of other things happening.
For example, we are
already now taking data,
something called the
Pulsar Timing Array.
They target they call a
gravitational waves emitted
by supermassive black
holes, like the one
in the centers of the galaxies.
And the way it works
is pretty nice.
So around us, there
are all these pulsars
which emit flashes of light
in a very stable periodic way.
So you can use them
as clocks, basically.
So if a gravitational wave, a
train, passes in the universe,
it will change by a tiny amount
of the distance between us
and each of these pulsars,
in a different way.
So by timing it very
precisely, the arrival
time of these pulses, we can
measure gravitational waves.
And hopefully,they
will get something,
a positive results soon.
Something as that will happen
in the next five to 10 years
awfully it's
something called LISA.
LISA is basically
another interferometer.
So it is kind of like
LIGO, but it's in space.
OK?
So it's arms, instead
of being four kilometers
will be a few millions
or billions or billions
kilometers-- I
don't remember now.
And so because they're
not on the ground,
they're not limited
by seismic noise--
so by the earth shaking.
So they can go to lower
frequencies-- fraction
of the earth's [INAUDIBLE].
And if you are sensing to
these frequencies, what
you can look for,
it's again, well, A,
they are compact binaries, and
also extreme [? mass ?] ratios.
This is a very big black
hole with a very small one
going around it.
Or again, a
supermassive black hole
in the center of the galaxy.
So I think we can say
that in the next 10 years
or so, we'll have
a pretty good idea
and we'll have detected
gravitational waves
from everywhere in the spectrum.
So this is my last slide.
I updated my slides at the
end, in the spirit of Dante,
and I put this quote from
the Divine Comedy which
is "E quindi uscimmo a riveder
le stelle," which means,
"And then we went out
to see the stars again."
Now my stars are dead
and black-- thank you.
[APPLAUSE]
- So I was curious-- what
is, can you explain to me,
and perhaps the audience,
what the Hawking limit is,
how it arises for a
spinning black hole?
- Yeah, OK, so the
idea is the following.
As you may have
heard, black holes
are singularities in the
fabric of space-time, OK?
Now I think it was Hawking came
out with something which is
called the cosmic
censorship conjecture,
which says that you cannot
have naked singularities
in the space-time.
There are a few reasons-- if
the black hole is spinning,
you can violate causality
and other funny things.
So the way we protect ourselves
from this weird things
happening is that we put the
singularity around the black
hole-- where lights cannot
come out, as you know.
Well, [? direct ?] order.
So even if something very
weird happens inside,
we cannot see it, it's all OK.
Now it turns out that if
the spin of the black hole
is larger than some value,
the horizons disappear.
So you would be left with
a free, visible, naked
singularity.
And again, so this
is a conjecture--
if you don't want
naked singularity,
you need to have horizon
then the spin as a limit.
OK?
We want to prove that.
- OK.
First question.
- You said that the upper
limit of black holes as a mess
remains to be defined, what
about the lower limit of mass
of black hole in order for
it to have gravitational--
observable gravitational affect.
And secondly, how abundant
are the binary black holes as
opposed to single black holes?
- Can you repeat
the second question?
- How abundant are
binary black holes?
- Oh I see.
- --as opposed to
single back holes?
- So the first question is--
if I was on the high side
of the mass of black holes,
what about the low side?
Which is a great question.
right now, there
seem to be a gap
between the mass
of neutron stars
and the mass of black holes.
I said that neutron stars have
masses which are around 1.4,
1.5 solar masses.
Black holes, instead, seem to
start from 5 or 6 solar masses.
And the priori-- there is no
reason why it should be so.
OK, you may spectacles
are 2, 3, 4-- whatever.
Now this may be due
to observational bias,
or just we have been unlucky.
OK, it happens.
The samples of black
holes I show is 20, maybe.
We don't know that
many black holes yet.
And so that is one
of the things we
want to verify, in the
next few months and years--
whether we will detect
black holes which
are in the gap, basically, which
are masses lower than a 6 or 5.
And the second question is--
how many more binary black holes
there are as opposed to
individual black holes?
Now, if you had asked me
this question a year ago,
a year and a and
a couple of months
ago, I would tell you
maybe 0, because one
of the nice thing
about our discovery
is that it showed that you
can have binary black holes.
Because there were astrophysical
models which said--
not very many of them, but there
were astrophysical models which
said you cannot form a
binary black hole, basically.
OK?
And now we found one,
so they are out there.
More frequent-- well, I guess we
need some more time to decide.
We know that a lot of the stars
in the universe-- and actually,
my colleague astronomers maybe
there is a number which I
don't-- maybe 60%,
70% are in binaries?
OK, a lot of stars in the
universe are in binaries, OK?
So since our black holes
were stars to start with,
you can think that there is a
significant fraction of them.
The problem is that
to become black hole,
you need to become a
supernova, and supernova
is a pretty extreme
and violent phenomenon.
So some people think that when
both objects-- one at a time--
go supernova, they can
destroy the binary system.
They can basically
shoot the two objects
in different directions.
So all of these are things
that if you invite me to stay,
I will-- no, I don't know.
We'll see.
We have calculated already
a rate of how often this
happens in the universe.
And the rate is such that
in with our detectors,
we should see on the order
of already now, a few
of these objects per month.
OK?
So they're pretty common.
And if you talk with
me at the coffee break,
I'd remember the number, I
can tell you how many of these
you have-- each
megaparsecs cube each year,
which is what astronomers
like to quote.
And remember, it's
like 30 or 40.
So they're not so uncommon.
Thank you.
- OK.
are there some other questions?
I have-- oh, over here.
OK.
I have one for Cora.
- I just wonder-- I get the
impression that by a microwave
background and by pulsar
arrays and by LIGO,
almost simultaneously
gravitational wave
or the effects of gravitational
wave seem to come into reach
of measurement.
And I wonder whether this
is a chance incidence,
or is it an intrinsic
reason since we're
talking about really, really
different orders of magnitude?
And you could imagine that
one effect is detectable
and the other is many,
many orders of magnitude
far from being measured.
- Do you want to?
- Please.
[INAUDIBLE]
- So your question, is
why we can measure--
let me rephrase your question.
Your question is, why we
seem to be able to measure?
- We detect gravitational
waves simultaneously
on very different
orders of magnitude.
- Yes.
So well, so first of all,
the gravitational waves
that, for example, we would be
able to measure with the CMB
come from a different
source, right?
So they come from the
quantum fluctuations
of the metric that get stretched
out in the period of 10
to the minus 35 seconds
or so after the Big Bang.
So these are primordial
gravitational waves.
Now the gravitational waves
that he was talking about,
that Salvatore
was talking about,
come from a different source.
These are coming from
emerging black holes.
So different effects--
different physical effects
produce gravitational
waves at different scales.
That's why we are looking
at different scales.
- No, I think the question--
and let me see if I can rephrase
it, is you find it surprising
that we have sensitivity across
different scales now-- of the
instrumentation seems to be
giving us all at the same
time the sensitivity.
And why is the
instrumentation at this level?
- So I think physicists tend
to be optimistic in general.
So we may not-- the fact that
I'm-- we're putting limits
and they're putting limits--
I don't know if these--
I think it's-- well, they
already have a detection.
we don't have a detection yet.
I think this is
completely coincidental
that we're still looking
for gravitational waves.
- Maybe I can add.
We don't-- shh-- there is
no [INAUDIBLE] in common
or anything.
There has not been
a breakthrough
that worked for both of
us or for the pulsar.
It's just accident.
- I think I can offer maybe
one thought, but I don't know.
- Please.
One issue is that
quantum mechanics was
sort of a playground
to understand physics
for a long time, but
now, quantum mechanics
is being applied to
devices and instrumentation
and it's starting to blossom.
And so you're starting
to see very sensitive
low-temperature
technologies that
are being brought to bear
that give us these windows.
So I mean, I can't go
into too much more detail,
but if you look at the
detectors for CMB polarization,
they've been evolving
over quite some time,
and the detector technologies
for LIGO, as well.
And they rely on an
understanding of quantum
mechanics and kind of
engineering of quantum
mechanics, if you like.
And I think that
what that has done
is it's gotten to a certain
level of maturity that gives us
new windows.
Now that doesn't
explain why you're
able to get gravitational
waves sensitives in both cases,
but it is the advent of these
super sensitive measurement
techniques that have
been evolving out
of quantum mechanical
technology, I guess.
Does that work for you?
- Yeah.
Let me add something.
So the limits-- I
talked about the limits
put by BICEP, because of things
that we know-- because BICEP
became very visible.
But the limits on gravitational,
on primordial gravitational
waves were there way before.
So BICEP maybe made an order
of magnitude improvement,
but they have been there
for years and years before.
So nothing is particular
special, I would say,
this period.
We are making progress,
but this progress
has been in a continuous
going on for years and years.
- Salvatore?
- I just want to add
that Einstein came out
with GR in 1916 and we made
this discovery in 2016.
We just waited for it.
Now, I'm joking.
It was accidental.
It was very nice,
but accidental.
- OK, other question?
- So my question is, one of
the things Salvatore touched on
is the way in which
science is reported,
and you know, I'm very
interested in the sort
of popular conception
of how things are
and the occasional
headlines that you read.
So a couple of headlines
I've seen very recently,
were one of them was-- "Cosmic
radiation may feed forms
of life they were we're
discovering"-- that in fact,
you don't need the sort
of standard formula that
we're using now to create life.
And the other one was just
over the last day or so
that there maybe something
like 10 times more galaxies
that some of the things
that have been coming back
from Hubble are
you know indicating
that we're under shooting
by an order of magnitude.
So I'm interested in
hearing a little bit more
about how you feel
science is reported,
and what you would like to
see in science reporting
that maybe you're not getting?
- Do you want to go first?
- OK, yeah, I think I
already expressed my opinion.
I understand that
if you had written,
"Einstein was not wrong," you
would not have sold the copies.
But I suggest you always
try to read the source.
Most often, if you read CNN,
the New York Times, or whatever,
they love their flashy
title, but then there's
also a link to the actual
article-- scientific article--
where things are discussed.
So I always try to go
this extra step even
for information which is not
pertinent to my own field.
And , yeah, I guess
that's my take.
- OK Yeah I saw I have
two things to say.
The first one is that
in general, in the news
we read things that are very
showy-- there is a tendency
to make a big title and
to put lots of lights
into certain breakthroughs
or to put those headlines.
But really, it is the
continuous progress
that is being done in
science, that should be per se
something really exciting.
And so I think that that
tendency to say-- in his case,
"Einstein was right,"
or well, in the case
of gravitational waves--
it's a different story.
But we all know
the story of BICEP.
People are willing to see some
breakthrough, some big titles,
and really the
excitement-- of course,
that is very excitement,
but sometimes the excitement
doesn't have to be like in
a Hollywood movie in which
everything is so dramatic.
The excitement is actually to
make these steps of progress
that we are continuously making.
And the second thing
is something curious
that happened after-- I don't
know if it's so much related
to your question, but I just
wanted to share my opinion.
Something curious that happened
after the BICEP announcement
was that-- well
the paper came out
the same day of
the announcement,
and because the announcement
in the newspapers was so big,
the community served as
sort of a peer review.
So the peer review
done by different parts
of the community was
happening at the same time
that these news were
appearing in the newspaper.
So I think sometimes it should
happen in the opposite way,
right It should appear in the
news after the peer review.
So this was a
little bit of chaos
that happened then was due to
everything coming out together
because there was such an urge
for showing such a big title,
when it was really impressive
what happened in any case,
they made that improvement--
the BICEP team made
an improvement of an
order of magnitude,
compared to the constraints
that existed before.
And that's already
for us, who work
in the field, something
incredible and worth
a title in the newspaper.
- And I can just
very quickly mention
that obviously I agree with
Cora and I can share maybe
the experience from our side.
What happened is that the press
conference and release was
after the paper had
been accepted by PRL,
so it had already been peer
reviewed by-- I cannot remember
or name our reviewers-- a lot.
And indeed, the reason
why was that the field
of gravitational waves
has a bumpy past.
There have been claims
in the 70s, maybe?
Something like this?
- 60s.
- 60s.
Of detection and they
have not being replicated,
so they're probably not true.
And obviously there's been
you know-- the BICEP papers.
So we wanted to be
extremely careful.
And you know, so over the
last-- we made the discovery
in September, and
we announced it
in February, which means
that for five months,
we have become
very good at lying
to people like family friends.
[LAUGHTER]
- Like a CIA agent
who comes home.
OK, so why don't
we take a break.
We'll resume at 3:00.
[APPLAUSE]
