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Cosmic Explosions and Accelerators

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>>Jennifer Harbster:
Good afternoon! I’m Jennifer Harbster. I’m the science Section Head
for the Science, Technology, and Business Division here
at the Library of Congress. I’d like to welcome
you to today’s program: Cosmic Explosions and
Cosmic Accelerators. This program is part of
the 2019 series of lectures that is presented through a
partnership between our division and NASA Space, Goddard
Space Flight Center. And it’s my pleasure today to also introduce our
speaker, Dr. Regina Caputo. She’s a research astrophysicist
at Goddard, and she works in the highly energetic world
of gamma ray astrophysics. And before joining Goddard,
she was a research scientist at the University of Maryland. Dr. Caputo received her
undergraduate degree, degree in physics at
Colorado School of Minds. And a doctorate degree in
experimental particle physics from Stony Brook University. During her undergraduate,
during her graduate years, she spent time at CERN, working
onsite with the atlas detector, which is very, very cool. [Laughter] Onsite, like not
using the data remotely, but actually onsite. She joined the Atlas
group as a postdoc, postdoctoral researcher, and
she continued her postdoc work at UC Santa Cruz. When she became a member of the Fermi Large Area
Telescope Collaboration. So today, Dr. Caputo’s going to
discuss her work in the field of multi-messenger astronomy,
and the contributions of Fermi to our understanding of
the extreme universe. And so please join me in
welcoming Dr. Regina Caputo to the Library of Congress. [ Applause ]>>Dr. Regina Caputo: Okay, so
thank you, thank you, Stephanie. Thank you, Jennifer. And thank-I’d like to thank
everybody at the Library of Congress for giving me the
opportunity to talk to you about something that I
find extremely exciting, and that’s this dawn of the new,
of a new era of astrophysics that we’re calling the
multi-messenger era. And so before I get into
what exactly it means by multi-messenger
astrophysics, I’d like to start by taking a step back. And so for a very long
time, for hundreds of years, our understanding
of the universe came from observing with our eyes. So you would see
optical photons and, and that is how we understood
the universe around us. And so this photo
is of the galaxy. And so this is the
galaxy in optical light, and what we can see is that a
lot of it is obscured by dust, so there’s, you can’t
quite see to the center. But we did learn that our
galaxy is quite a dusty place, and you could see lots
of stars, of course, and different clusters. Now, when we were able to
utilize different wavelengths of light, infrared,
optical, radio, what we learned was this is
the same picture of our galaxy, as you can see the optical
light is towards the bottom. But there’s other wavelengths
of light that go straight through the dust that
don’t get obscured. And we learn that there
are different regions that have different
temperatures, and that there’s different
objects that produce this kind of light in our own galaxy. Things we had never
been able to see before. And so having all of these
different wavelengths from radio to infrared to optical
to X-ray which pierces through very dense things,
all the way up to gamma rays, we were able to get a
really complete picture of what our galaxy and what
the universe is made of. And so this is an
illustration of the universe in multi-wavelength light. And so I’m sure you’re
familiar with the parable of the blind people
and the elephant, where each one can observe
a different piece of it, and together, they
can get an idea of what the full picture is. And so this is what we get
when we observe the universe in different wavelengths
of light. Now, I like this image
because it goes through all of the different wavelengths
that I talked about. Radio, all the way to gamma ray, and it gives you a different
idea of the different sizes of the wavelengths of light. So radio can be the
size of human beings. Think, imagine waves that are
the size of people or buildings. All the way to gamma rays, which
are the size of atomic nuclei, so they’re very,
very, very tiny. And because of this, we
need many different ways to observe them. You’ll also notice the
top bar, that not all of these penetrate
the earth atmosphere. Now, for the case of gamma
rays, that’s really good because it’s very high energy, and our atmosphere
protects us from gamma rays. But what that means is to
observe all of the wavelengths of light, you need to put
some telescopes in space. And so a lot of these
observations were not possible until we were able to launch
telescopes into space, and get above our
protective atmosphere. Now most wavelengths of light,
optical, infrared, you know, things you can see
with your eyes, are produced naturally in stars. So like our sun emits light
in lots of different waves. It peaks right in the visible,
which is beneficial to us because that’s where we see. And we call this beam produced
thermally, so it’s a process that stars naturally make. However, gamma rays
are different. The sun and normal stars don’t
naturally produce particles at such high energies, and
so how do we get gamma rays? Things that are really,
really, really energetic? Nine orders of magnitude bigger than what we can
see with our eyes. And the way that you do this
is you need an energy source. So you need something
that’s really energetic, and you need particle
acceleration, and then you need a way
to produce the gamma rays, and then you want to
get rid of anything that absorbs the gamma rays. And so to make it simple, we
need some kind of extreme event. Think cosmic explosions. We need some kind
of extreme fields. Think, you know, cosmic acceleration,
big magnetic fields. And then we need
particles to produce them, and so cosmic rays are charged
particles and when they interact with light, they
will make gamma rays, and that’s how you
get the gamma ray sky. And so I mentioned that in
order to observe gamma rays, you need to go into space. And so that is exactly–
oh, there’s sound, too. So you’ll see the launch
of Fermi ten years ago.>>Nine, eight, seven,
six, five, four, three, engines start.>>And that’s the PI, and one of the main scientific
collaborators the day before.>>Gamma ray telescope,
searching for unseen physics in the stars of the galaxies.>>And so that was the launch of the Fermi Gamma-ray
Space Telescope. It was originally called GLAST,
but then once it was launched, it was renamed after
Enrico Fermi, in his honor, because he studied cosmic
rays and gamma rays. So this happened 11 years
ago, and ever since then, Fermi has been surveying
the gamma ray sky. It’s made up of two
different telescopes. The first telescope is called
the Large Area Telescope, and it is the silver
box that’s located at the top of the instrument. That has a very wide
field of view. It observes 20% of the
sky at any given time. That’s about the field
of view of your eyes. And it observes the full
sky every three hours. The second instrument,
the second telescope, is called the Gamma-ray
Burst Monitor. And I’ll be talking
a little bit more about what gamma-ray bursts are,
but these are special detectors that observe the
full un-occulted sky. So everything that isn’t blocked by the earth the Gamma-ray
Burst Monitor observes. So I’d shown you at the
beginning what the galaxy looks like in several different
wavelengths. And this image is the
gamma ray sky from Fermi. And the main yellow band that
you can see through the middle, that is the galaxy, so
if you had special eyes that could only observe
gamma rays, this is what the galaxy
would look like to you, the night sky would
look like to you. And so what we can see
is that just like I said in the beginning, where
you needed extreme events and targets and extreme fields
in order to get gamma rays, you see a lot of that is
happening in our galaxy, because you can see the galaxy
is very bright in gamma rays. You’ll also notice that there’s
a lot of speckles on top. These are all gamma ray sources. And so just to orient
you a little bit, we’re out in the spiral
arms of the galaxy, and so when we’re looking into
the center, so the very center of this map is the very
center of our galaxy. And I’ll note, that’s
not to scale [laughter]. And so, those spots that you see that are throughout
the gamma ray sky, those are mostly
outside of the galaxy, and we call those
extragalactic sources. And those are made up
of, of active galaxies where the central black hole of the galaxy is actively
accreting material and shooting it out in jets. There’s also gamma ray bursts. Globular clusters are
galaxies that are nearby that orbit the Milky Way,
and also starburst galaxies. These are just galaxies that
are very actively having star formation. Now coming a little closer in
our own galaxy, we have lots of these extreme events. We have supernova
remnants, pulsars. Pulsars are neutron stars, and
neutron stars are what happens when a massive star explodes, but isn’t quite massive
enough to make a black hole. And so these make the densest
material in the universe, which is a ball of neutrons, which is why we call
them a neutron star. But there’s things that
are even closer to us that produce gamma
rays, so the sun, when it has big solar
flares, will emit gamma rays, and we detect those with Fermi. And even more interestingly are
terrestrial gamma ray flashes. That’s lightning. Sometimes lightning can make
gamma rays that we detect, and so these are very
interesting things that we see. Now, with our ten years of
observations, we’ve been able to identify over 5000
sources of gamma rays. And that’s more than what you
can see with your naked eye on a clear night, and so
there’s lots and lots of things that make gamma rays
in our universe. But what you don’t see in
this lovely sky map is any transient astrophysics. So something that flashes on really bright
and then turns off. And so I also have another map for you that’s a
transient sky map, and so just to give you an idea,
these are gamma ray bursts, so I said I would
talk a little bit more about those, and I will. And this is over about
a two-year period. And you could see
as the time goes on, the number of gamma ray
bursts that we’re detecting, and you could see these little
spots that just come up and pop. And so the sky, the gamma
ray sky is not something that’s static. It’s very dynamic. There’s always something
that’s changing and going on or bursting, and
it’s very exciting. And so then what exactly
are gamma ray bursts? So gamma ray bursts were
actually discovered by accident in the sixties when we had
satellites that were looking to see for gamma ray
activity on the ground, and they observed these really
bright flashes of gamma rays. And they’re the brightest
electromagnetic events in the universe. And it’s an extremely
energetic explosion, and so far, we’ve only observed them
in distant galaxies. We haven’t seen any in our own. Now, gamma ray bursts
can last anywhere from a few milliseconds, so shorter than a
second, to several hours. And because we’re very
creative with naming things, we’ve broken these into two
categories: short and long. [ Laughter ] The long gamma ray bursts,
we were able to identify that they occurred
with supernovas. So they’re associated with the
core collapse of a massive star. And in 2003, we observed this
long GRB with a supernova. However, short gamma ray bursts, we weren’t quite
sure what made them. What makes this really short
flash of intense light. And so this was an
outstanding question. And so, remember I talked a
little bit about neutron stars? Sometimes, they’re in binaries, and when they’re
orbiting each other, what they’ll do is they’ll lose
energy from gravitational waves. And when they collapse on each
other, when they merge together, what will happen is a short
gamma ray burst is produced, and this intense gamma rays will
occur along the polar access of this merger. Not if that jet of gamma
rays is pointed at us, then we see it as
a gamma ray burst. And this is something that the
Gamma-ray Burst Monitor finds all the time. And so as you saw in that
previous slide that I show with the little spots
happening all the time, and so this is an example of
the data that we would see with the constant
background, and then all of a sudden a “bloop!” There’s a gamma ray burst. Now this was a theory that this
is what produced short gamma ray bursts until August 17, 2017. When Fermi was observing,
like it normally does, and it detected a short gamma
ray burst, and at the same time, or I should say,
1.7 seconds later, the LIGO gravitational wave
detectors found a gravitational wave that was associated with
the merging of neutron stars. And so what this told
us was that yes, indeed, the thing that makes,
the progenitor of short gamma ray
bursts are neutron stars. And so that was very exciting
because every telescope, and I’m not exaggerating when I
say every telescope in the world that possibly could, once had
that information from LIGO and from Fermi, looked at
that point in the sky to try to figure out what was going on. And what we were able to observe
is that neutron star material, when it merged, the
material underwent rapid process nucleosynthesis. Which that just means
that really, really heavy elements
were able to form, and it expanded and decayed. And that radiation is
seen, is called a kilonova. So you guys have
heard of supernova. This is a kilonova, and
this was the first time that a kilonova was observed. And remember how I
said it’s important to have many wavelengths
of light. We observed it in
ultraviolet optical, infrared, radio, gamma, everything. And what we were able to determine was the
ejective velocity, the mass, the composition, and
even more excitingly, what this told us has
revolutionized this new era of multi-messenger
astrophysics because now instead of just having light, we
have gravitational waves. And that’s why we call it,
it’s a different messenger. Photon’s light is one messenger, and gravitational waves
is another messenger. And so from this one, single
event, from August 17, 2017, what did we learn? Well, our understanding of how elements formed
completely changed. If I showed you this
picture for, in 2015 or 2016, we would have thought that most
heavy elements would have been produced in supernova. But the problem was is that
there weren’t enough supernova, and we really weren’t
able to observe enough of the heavy element formation to match what we actually
observe as abundances. But all of those yellow
boxes, all of that is, is produced from
kilonova emissions, from merging neutron stars. And I always like to point out
that two people’s favorites, so anybody that has any gold
or platinum in the audience, that’s stuff from
merging neutron stars. That’s where it came from. I always think that’s
super cool. [ Laughter ] In addition, we learned that the
speed of gravity was the same as the speed of light because
we were able to observe both of these events happening
at the same time. And what was exciting about
learning that the speed of gravity was the same
as the speed of light, is that we learned
that they couple to space time in the same way. And what that means is
that it rules out a lot of alternative theories
of dark, of gravity to try to explain dark matter. So we learned something
fundamental about the universe. The other really cool
thing that we were able to do was measure
the Hubble constant in a completely new way. And as we get more of these
merging neutron stars, we can measure even better,
and that will give us a handle on the expansion
of the universe. And so this was just this
one event that has given us so much information and has
really led us into this new era where we can look at the
universe, not just with light, but through gravitational waves
through these cosmic explosions. So the other thing that
I planned to talk to you about today is a
few more messengers. So cosmic rays, for the most
part, are made up of protons, and they’re very
high-energy particles. They go from very low
energy to very high energy. And one of the outstanding
questions is what is creating these very high-energy protons? Now thankfully our
atmosphere, as you can see, protects us from cosmic rays,
but this is just them entering into the atmosphere
and showering into particles that are safe. So one of the challenges
with cosmic rays is because they’re charged
particles, they bend in magnetic fields. So just like how this
cartoon is illustrating, if you have a proton, and
it’s traveling from a source like this big cartoon star,
whenever it interacts, it interacts in a magnetic
field, it will bend away. And so we don’t actually
know where cosmic rays come from because they don’t point
back to where they came from. Unlike light. Light points back to
where it came from. So photons travel and don’t
interact with magnetic fields. Now the other thing,
the other particle that we have is a neutrino. Now neutrinos are very hard
to detect because they, we think of them sometimes
as the ghost particle because they don’t
interact with much. They mostly like to
travel through the universe and not interact with anything. However, neutrinos, because
they don’t interact with much, also point back to the source. So why, why am I
talking about neutrinos? Why are neutrinos interesting? Well, neutrinos will
give us a hint as to where cosmic rays come from. And the reason is because
when you have protons, so that’s the little “p” on the
chalkboard, and they interact with other photons–
that’s the little gamma. They’ll produce pions
which is the little Pi with the zero by it. And those pions will
decay into gamma rays, and so we can see
directly from protons that gamma rays are produced
with proton interactions. But, you know, like I said,
lots of things make gamma rays. Electrons can make gamma rays. Explosions make gamma rays. So that’s not a unique
identifier of protons. However, protons can
also make charged pions. And when they do, they’ll
decay into a neutrino. And this is unique to
protons, and so we know if we can observe a
source that made neutrinos, we know that neutrinos, that– . We know that if we had a
source that has protons, and that we can observe
neutrinos from it, we know that the
protons were there. Now, we have a telescope
that can observe gamma rays. And there’s also a telescope in
the South Pole called IceCube. And it detects neutrinos,
and so for a long time, they’ve been trying to
find the sources for these, for high-energy neutrinos. And so I mention that these
neutrinos are the smoking-gun signature for protons. And so what we’d really like to
do, what the overall goal is, is that we’d like to find
neutrinos coming from, something from outside
the galaxy, to try to understand
what is making these very high-energy neutrinos. Now of course just like how
Fermi has a gamma ray sky map, IceCube also has a
neutrino sky map. And so that’s what
this image is of. And what you can tell is that,
well first of all, like I said, it’s much harder
to detect neutrinos than it is to detect gamma rays. And so they have
many fewer neutrinos than we have gamma rays. However, just looking at
this map, it doesn’t appear that there is really any
significant source that’s calling out. And so you say, “Okay,
they don’t, they can’t find a
source themselves.” And so why don’t we go back
to look at the photon data, so use all of these messengers
that we have at our disposal? And we know that there are
lots of different things that make gamma rays,
and a lot of things that produce really
high-energy processes, and so what of these
could also make neutrinos? And so we have all of these
ingredients for gamma rays. We have the active
galaxies where the core is, is accreting material and
shooting out jets of particles. That’s a good though;
however, we, looking at the active
galaxies that we know of, we see that there’s
only an upper limit. We weren’t able to identify that there was proton
acceleration there. Gamma ray bursts. The source I just
talked to you about. That seems like a
reasonable place. However, we weren’t able
to find any evidence of neutrinos associated
with gamma ray bursts. And so contributes
less than one percent. The other option is maybe
star-forming regions. Star-forming regions, there’s
lots of active things happening, and young start exploding. Maybe this is another
source, but we weren’t able to find any neutrinos
coming preferentially from star-forming
regions either. And so this was a long
puzzle from the time that IceCube had started
taking data in 2010 or so, all the way up until,
until recently. So I talked a little bit about
active galaxies; however, there’s more to the story
because in addition to the cores of these galaxies accreting,
sometimes some things change in that accretion region, and
it causes the galaxy to flare. And so what you’re looking
at is an illustration of the very dense
core of a galaxy. No, yeah. A very dense
core of a galaxy. And we’re going to zoom
in and at the center where there’s a super
massive black hole, there will be material that’s
getting accreted, and sometimes, when this really hot, dense
material is accreting, and something changes, it’ll
cause an increase in the amount of gamma rays that
come out of it. And so these are
not static objects. They’re very dynamic objects,
and sometimes they flare, and sometimes they’re quiescent. And to give you an illustration
of this, so Fermi observes over 2000 of these kinds
of active galaxies. We call them blazars. And over a three-month period, you could see just how
dynamic the gamma ray flux is, and so you’ll see as the, these
are circling the 11 most active. Sometimes they flare up bright, and sometimes they’re
quite, quite quiescent. Now because IceCube, there
might be some correlation with these flares, started
issuing alerts in April of 2016. And so what you’re looking at
is the dynamic gamma ray sky, and then pretty soon, once we
hit April 2016, you’ll start to see little neutrino alerts
pop up, and that’s the location of where neutrino was observed. And what we were hoping for was
that there was a correlation between one of these
flaring gamma ray sources and a high-energy
neutrino event. And in September 22, 2017. So 2017 was a really
busy year [laughter]. We were able to observe a blazar that had never been
flaring before. It was totally quiescent from
all the Fermi observations. And it was at that
time that we were able to observe a really
high-energy neutrino coming from that source. And so this is, again,
the smoking gun of finding really high-energy
protons, where they come from. And so the IceCube
detector is kind of unlike any telescope
you can imagine. What it’s made of
is these strings of photomultiplier tubes. And so what they do is just
detect flashes of light, and it’s deep in the
ice in Antarctica. And so these little balls
are the little detectors that detect the flashes
of light. And when a neutrino
zips through, these little balls flash,
and that data gets recorded. And so what you’re looking at
is the actual event from the, from the little balls that light
up, kilometers under the ice, and you see where it started, and it’s tracked
through the ice. So where did this
neutrino come from? Where is this blazar that
just started flaring? And so it’s call TXS0506. Again, not super
creative with the names. And it was in the
Orion constellation. And so when you look up at the
sky, this is what you would see, and to help you, there’s Orion. And if you could
see with gamma rays, this is what you would see. And that is before the flare,
and this is after the flare. And this particular blazar is
3.7 billion light years away. And so that means that the
light we see today came from 2.7 billion years
ago, and this was one of the 50 brightest active
galaxies that we’ve observed. Now, to give you a little
handle because, you know, it’s a little hard to
see sometimes on that, just the intensity maps, we’ve made this illustration
that’s called raindrop animation. And so what you’ll see is the
different circles are different gamma rays that came
from the source. And the size and the
color is associated with the energy of
the gamma ray. So the smaller circles
are lower energy. The bigger circles, the darker
circles are higher energy. And this is what it looked like
during most of the mission. And so you could see, it’s
a pretty bright source. We’re getting lots of
gamma rays from it. However, when it
started to flare, this is what it was doing. So you could see something
clearly changed in that source between when we would
observe it normally, and when we were observing it
when this neutrino happened. And this is what it looked
like on September 22, 2017. So just like before,
astronomers got excited, and so all the telescopes
started to point at this source so that we could try to
understand what is going on. And so IceCube sent
around a notice. We call them GCNs, so
gamma-ray coordinate network. It said, “Hey, look! We found a neutrino! Somebody, somebody tell
us what it could be.” And so Swift, which is another
space-based observatory, started looking at it,
and then Fermi said, “Hey, we’ve noticed this
flaring blazar. It’s been flaring
for a few months. Maybe this is the source.” And then MAGIC, a very high
energy gamma ray detector, started observing it. And to give you a little idea, when Swift was observing
it initially, that big, red circle was the region that IceCube said this
neutrino came from. And so each of those little
circles is the field of view of Swift, and so it looked in
all of these little regions to try to find what
source it could be from. So it’s challenging. But then Fermi, when
it said, “Hey! We see a flaring blazar,”
this is what it was based on. So this is a light curve, and the light curve is
just the flux plotted as a function of time. And so you could see the
flux was relatively constant, but then all of a sudden in
April of 2017, the flux went up by a factor of six,
so something changed. Something started
accelerating protons. And then MAGIC, this very
high-energy telescope, also started observing the
source, and for the first time, they were able to
observe this source. It had never been seen
before at such high energies. And so this was very exciting,
and really gave us confidence that this was the
source that was, that produced this
particular neutrino. And so remember on the previous
slide, I showed you the circle where IceCube said to
look at the neutrino. So this is the circle,
but with Fermi data. And so the circle is where
IceCube said to look, and then the very center of
the circle, so right spot-on where IceCube said to look, that is where Fermi
found that active galaxy. And then this is where
MAGIC was able to look, and you can see they all
just line up perfectly. So IceCube data, the neutrino,
the gamma ray data from Fermi, and the gamma ray
data from MAGIC. All of them just lined
up really nicely. And so what, what
were we able to learn? So you know, we learned
a lot from the previous, you know, cosmic explosion. So what were we able to learn
from this cosmic accelerator? And so we know that there
is this accretion disk right at the center of the
super massive black hole. And something changed
within this accretion disk to start this particular flare. And we know that this
accretion disk has to be close to the super massive
black hole because this is where the protons are
going to be coming from. We also know that these jets
of particles, both the protons and the electrons are traveling
at nearly the speed of light when they’re ejected
from this active galaxy, and that’s very exciting because
that gives us information about what is happening near
these cores of galaxies. Because it could be
also that at one point, our galaxy was an active
galaxy and that the center of our galaxy was also
accreting material. Although we don’t see evidence
that it’s doing it right now. And what was very
exciting is that this kind of source also emits
photons at all wavelengths. It’s very bright in radio,
it’s bright in X-ray, it’s bright in optical, and so
this, this gives us a handle on what exactly is happening
because we also are able to observe the neutrinos. Now we still don’t know exactly
what, what causes this change of flux in gamma rays, and that’s why we keep
studying them, obviously. Now what was also
really interesting was because now IceCube
had a source. They said, “Okay, we think that this source is
what’s producing neutrinos. Do we have any evidence? If we look back in time
at this particular region, do we see any change
in flux in neutrinos? And they actually found
evidence that there was a flare, a flare of neutrinos in 2015. But what was very interesting
is that it didn’t seem like there was any change in the
flux in the gamma ray emission. And so what causes
that neutrino flare? Now these neutrinos were
at much lower energies than the initial one, but
what caused that flare that wasn’t tied to gamma rays? And so this is one of those
mysteries that remains. But so to pull together
all of this picture, we now have for the first time, a source of neutrinos
that’s outside of our galaxy that’s really
accelerating particles to very high energies. We have these explosions that are producing
gravitational waves and tell us about how elements
are formed and tell us about different cosmology
aspects of the universe. In addition to all of the
other wavelengths of light that we had, and so this is
the, the multi-messenger age that we have entered
starting in about 2017. And so you’ve seen this picture
of the electromagnetic sky where you can look all the
way from radio to gamma rays to get a handle on the universe. We also have a gravitational
wave spectrum. We expect there to be
gravitational waves starting with the cosmic microwave
background. All the way up to
pulsars and supernovae. So things all the way from
the size of the universe in the Big Bang to neutron
stars which are about the size of the area inside the Beltway. And then we also have
the neutrino spectrum, so things that tell us all
about the cosmology and history of the universe going to solar
neutrinos and supernovae, all the way to active galaxies. And so this is the picture that
we now have of the universe and are going to
continue to explore with, with the instruments
that we have. And so yes, we’ve
entered the era of multi-messenger astrophysics. And what’s been very exciting is
as we’ve entered this new age, we’ve realized that Fermi
has really been this bridge between the electromagnetic
observations and the gravitational
waves and neutrinos, these two new messengers. Because this era was brought in by very extreme
astrophysical events happening. And so right now, we have Fermi,
but what’s happening next? What’s coming up in
the next generation? So like I said, right
now Fermi is flying. The Swift telescope
is also flying. They detect lots of
gamma-ray bursts. And we have LIGO which
just entered its third year of observing. However in the future, we’re going to get more
gravitational wave detectors, and NASA has plans to launch
small targeted missions. So one example of this
is a CubeSat satellite. So Fermi is about human-sized. You know, it’s like
a meter by a meter. This telescope is about
the size of a shoebox, and it will detect
gamma-ray bursts. And so that is a
small targeted mission that will launch in 2021. But even farther in the future
what we’d like is a follow-on to Fermi, and one that I am
working on is called AMEGO, and this is another large, a
large-scale gamma-ray mission that you know we’re working
on developing to follow up both active galaxies and
look for the transient sky, and also gamma-ray bursts. And in addition, there’s
plan to upgrade IceCube into a new generation
where it’s even larger, and it’s more sensitive
to detect more neutrinos. Because what would
really be the Holy Grail of multi-messenger astrophysics
is if we would be able to detect all of these
messengers at the same time. So not just gravitational
waves and light, and not just neutrinos and
light, but all three: neutrinos, gravitational waves, and the
electromagnetic spectrum, because that’s the way that
we would really get a complete picture of these
different sources and how the universe works. And so on that note, I’d like
to thank you for your attention, and I will leave up some
information about Fermi if you’d like to find more. [ Applause ]>>Jennifer Harbster: So we do
have time for some questions. And are we still repeating, if
you could repeat the questions?>>Dr. Regina Caputo:
Sure, sure! I could repeat the question. Yep.>>Jennifer Harbster:
Yeah, okay.>>Dr. Regina Caputo: Sure,
I saw your hand go up first.>>Audience Question: I thought
the gravitational wave detection stated that this was caused by
the merger of two black holes. But I thought you said
two neutron stars. So am I confused?>>Dr. Regina Caputo:
No, no, no. So the question was
is that you thought that the gravitational
wave event came from merging black holes,
not merging neutron stars. And that’s an excellent question
because they’ve observed both. The first gravitational
wave detection was from merging black holes. That was the, the biggest Nobel
Prize event, exactly, yes. However, LIGO is also sensitive
to merging neutron stars which are smaller
but harder to detect. And so this was the first and thus far only confirmed
merging of neutron stars. And what’s exciting is
when black holes merge, you don’t expect there to be
electromagnetic radiation. There was a tentative signal,
but we haven’t seen any with other merging black holes. But with neutron stars, that’s when we expect all
the fireworks: the kilonova, the production of
heavy elements, and so that’s what makes
those events very special and very exciting.>>Audience Question: Is a kilonova bigger
than a supernova?>>Dr. Regina Caputo:
So the question is, is a kilonova bigger than a
supernova, and the answer is no. [Laughter] It’s, it’s not
as energetic as a supernova.>>Audience Question: I’ve read
recently that there is an event, I think that they think
they’ll get the three things? Is that true that they’re going to possibly get the
three things all at once?>>Dr. Regina Caputo: So the question is can we get
all three messengers at once? Is there something coming up that would give us
all three messengers? And that’s a, I may not, I guess
we’re going to wait and see. I haven’t, I don’t know of any
event that’s going to happen, but that’s the fun of the transient sky is you
don’t know what’s coming until it hits you. Yes?>>Audience Question: You mentioned the
beginning you thought that gravitational waves are
mainly [inaudible] our idea about the rate of
expansion of the universe. I understand that they’re
really close now to getting within a range where they can
actually predict [inaudible]. Is this observation of
gravitational waves enabled them to really get to the point
where they’re going to be near to definitively saying what
rate of expansion there is? How this is going to affect the
[inaudible] of the universe?>>Dr. Regina Caputo:
Thanks, so the question is, and let’s make sure that
I get it right, is we, we now have another
measurement on the expansion of the universe, and is this
going to give us an indication of the actual fate
of the universe? And I think that right, right– so I’ll talk about
the expansion first. So right now, we have
two different ways of measuring the
expansion of the universe. And right now, they’re in
slight tension with each other. And what this could be
is because the expansion of the universe could have
changed as a function of time, so that is an outstanding
question. Because the different
measurements happened using different time periods. And so what this new measurement
will give us a better idea of is who was right, and when they’re
right, and who is more correct, I think, in that respect? It’s an independent measurement. But as far as the
fate of the universe, so we can learn what happened
with these kinds of events. Once we learn more
about say dark energy and how dark energy
interacts with the universe, that’ll give us a better handle on the eventual fate
of the universe. Because we’ll know if there’s
enough to collapse us back in to say like a big crunch,
or if we’re just going to continue to expand forever. So that it’s a slightly
different measurement that will give us
a handle on that.>>Audience Question: I hear that [inaudible] find
the measurement now to the point of which system they’re using. If it was in about
a two percent range of determining the
expansion of the universe, and when they get below
the two-percent range, they think that they can
actually provide a more definitive understanding.>>Dr. Regina Caputo: Yeah, so
the question was once we get to like a two-percent
uncertainty, we’ll have a more
definitive understanding. And certainly once we have
a more precise measurement, like you said, that will
inform other observations that will tell us about, more about the composition
of the universe. Because all these parameters go into understanding the
expansion of the universe. So I think that’s the
best I can answer. We need a cosmologist
up here, too. [Laughter] Go ahead.>>Audience Question:
[Inaudible] You mentioned that IceCube in 2015 saw a
non-gamma ray, neutron burst. You also mentioned
[inaudible] 1987. Has IceCube observed other
neutron events associated with supernova?>>Dr. Regina Caputo: So
it’s an excellent question. So the question is has IceCube
observed any other neutrino events associated
with supernova? Right, right, right, right. So yeah, unfortunately,
IceCube wasn’t running in 1987, [laughter] but so the, the thing
is is that all of the supernova that we’ve observed since
1987 have been too far away, and so we wouldn’t have, there weren’t enough
neutrinos produced for us to detect them here on earth. They were too far. And so if another supernova
happened within the galaxy or near in the galaxy, because
the one in 1987 happened in a neighboring galaxy. It was very close to us,
a large Magellanic Cloud. And so if one happened
that was that close, they would have been
able to detect it. But we haven’t had
one that close yet. I’d also say that
IceCube really focuses on the highest energy neutrinos
which supernova don’t produce. They produce lower
energy neutrinos, but lots of people
would be able, or lots of detectors would
be able to determine that. So in the back?>>Audience Question:
Listen, have you ever, so which way does the
gravitational waves– are they always [inaudible]
from the [inaudible]? Or do they ever go away from
earth or are the gravitational, are they stronger than the
earth’s gravitational field or what?>>Dr. Regina Caputo: Yeah, so the question was is do
the gravitational waves, are they stronger than the
earth’s gravitational field? Or would we, would they– ?>>Would they be
radiating away from, or towards the stars
coming towards earth? [ Inaudible ]>>Dr. Regina Caputo:
Ah, yeah, so, so looking for new sources
of, yeah, new source of energy or something like that. No that’s a, that’s
a great question. So as of right now,
so say for example, when you’ve got neutron
stars that are, that where their orbits are
getting closer and closer. Their orbits are decaying, they’re emitting
gravitational energy, and that’s how we’re able to detect those gravitational
waves. So they’re coming from
the loss of energy in the neutron star
rotation system. And so they’re coming
from that system itself. And so when we detect them on
earth, they kind of pass right through us, and it, they’re
very, very, very, very tiny. So I mean you guys
were all on earth for, you know, what was it? August, August 17, 2017, and I don’t think you
noticed any big disturbances. Because it’s, it’s, you know, fractions of you
know an atom that, that they disrupt things
as they pass through. And so because they’re so
small, because gravity is such, such a weak force,
relatively speaking to the rest of the forces, it, there’s
just not a lot of energy. I mean, it’s a lot of
energy that gets deposited, but not a lot that
we would experience. But the earth does make
indentations in space time from the gravity that it has. So but we’re not losing energy
to gravitational radiation, and that’s why we don’t emit
gravitational waves like that. But boy, if somebody
figured out how to harness that energy [laughter]. We just observe it. Go ahead.>>Audience Question: How
are the neutrinos being detected [inaudible]? Indirect measurement
of direct measurement? I had thought it was, neutrinos
pass through everything.>>Dr. Regina Caputo: Yes, yes. So the, the question is how
does IceCube detect neutrinos, because as I said, they
don’t interact with much. They pass through everything. And the question is that, or the
answer is that most of the time, the neutrinos just pass
right through, but every once in a while, a neutrino interacts
with the ice in the detector, and when it interacts
with the ice, that’s when we see a shrink off
radiation in the form of a ring that emits, and that’s what
we detect is that interaction of that neutrino in the ice. And that’s– .>>Audience Question:
Does it have to be a particularly high
energy neutrino to do that? Or is it just, it’s random?>>Dr. Regina Caputo:
Yeah, so the, so the question is does it have to be a particularly high-energy
neutrino or is it random? And in order for the, for
the detector to detect it, it has to be high energy. It does detect much
lower-energy neutrinos, like it detects neutrinos from
the sun and from the atmosphere. But they call most of
that as a background. But yeah, they observe
neutrinos of all energies. Okay. You had a question?>>Audience Question: Are you
aware of the black hole event that caused a gravitational wave
but not the neutron star event? I’m curious, how many times have
we detected gravitational waves?>>Dr. Regina Caputo: That’s
an excellent question. So the question is how
many times have we detected gravitational waves,
and the answer is at this point, a handful. Like on the order of a dozen. Maybe a few dozen. It’s been mostly black hole
mergers with one exception. And so but in that time period,
since 2017, LIGO has also shut down to upgrade, and
so it hasn’t been running continuously. They, they have done, it’s a very complicated
engineering feat. They have interferometers
which are kilometers long, and they need them
all to be aligned, and everything to be working. And so in the past few years, there have been a
few dozen events. And one neutron star merger. All the rest have
been black hole. And actually, I could
advertise for LIGO a little bit. There is an app where if
you want to get an alert when there is a gravitational
wave detection. It’s not so many at this
point that you’d have to turn off your phone. But you know, I have
the app on my phone. You guys could download it and you could get
alerts whenever they, whenever there’s
one that’s spotted. [Inaudible] Exactly, exactly. It’s like whoa! Yeah?>>Audience Question: I’m
involved with cislunar and lunar missions, helping
scientists and engineers with upcoming missions. Is, what are your thoughts
about protecting future efforts in those areas to
protect humans, you know, and all that coming? Do you have thoughts about
what you learned or you know, how to protect humans
better from some of these high-energy particles?>>Dr. Regina Caputo: So
the question is how can we, I mean I guess what are my
thoughts about protecting humans from the high-energy particles? And so I would mostly say our
atmosphere does a pretty good job so far, so I’d say protect
the atmosphere [laughter]. That’s the main, the main thing,
because like I said, X-rays, gamma rays can’t penetrate,
and so that’s why we need to put these into space. And so I think that’s probably
the, the most we can do. I’d also say, you know,
having more gamma ray, so support more gamma
ray telescopes because then we could
keep observing [laughter].>>Audience Question:
Because when we go further into space though, how to
protect our astronauts?>>Dr. Regina Caputo:
Yeah, so the question is for future space travel,
how to protect astronauts, and that’s actually
a big challenge that, that we’re thinking about
as to how to do this because obviously,
if you leave earth, you don’t have the atmosphere. And I would, you know, I
don’t have any good ideas as to have to do it. But thick, thick walls
[laughter] protect you.>>Audience Question: They talk
about water and also regolith on the moon, underground,
and water maybe.>>Dr. Regina Caputo: Yeah,
no, I mean, I mean dense, dense material stops gamma rays. Like I said, obviously our
detector detects gamma rays, so it has to stop them. And so surrounding, you know,
a dense material would be, I guess I should say
a high atomic number, high-X material is the
way to stop gamma rays. So yep.>>Jennifer Harbster: We have
time for another question. No? Okay. Well thank you!>>Dr. Regina Caputo:
Well, thank you! [ Applause ]

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