A black hole is a very dense object in space
so dense it has almost infinite density
infinite mass in a very very small area
to the point where even light can’t escape from it.
So if you’re within the event horizon
you can’t escape from a black hole.
We know nowadays that black holes
are not only mathematical abstractions of the theory
but they are astrophysical objects
and we know actually in the Universe
there are at least a couple of tens of black holes in our Galaxy
with a mass comparable to the mass of our Sun
or slightly larger, 10 times, 20 times larger
And what we call supermassive black holes
probably at the centre of every galaxy
with a mass comprised between
one million and one billion times the mass of the Sun
If we are able actually to pinpoint, discover and identify black holes
very far away in the Universe
we have a very powerful tool
to study the evolution of galaxies
across the whole history of the Universe
from the earliest stage when galaxies formed
to the local Universe, to nowadays
So if you have two of these black holes
and they happen to come close to each other
they’re eventually going to get caught in each other’s gravity
At that point they start spiral
closer and closer and closer
and eventually they merge together
forming a single object.
But the gravity of a black hole is so strong
it actually warps space
Well, all mass warps space
but black holes warp it to a very high degree
So when two of these objects are very close together
they end up having ripples form in spacetime
and those ripples propagate out throughout the whole Universe
These are what we call gravitational waves
So, up until today
we’ve measured gravitational waves
coming from small objects
When I say small it’s anywhere between
one to a hundred times the mass of the Sun
so very, very big astrophysical objects
But in the future
we want to be measuring gravitational waves
coming from supermassive black holes
so not two small black holes smashing together
but two galaxies coming together
When two galaxies come together
you get black holes of the order of
a million solar masses
smashing into each other
And we want to do that using the LISA mission
which is a European Space Agency
future mission for gravitational wave detection
So LISA would be looking for primarily supermassive black holes smashing together
One of the things we should say with LISA
is that you can’t point it
it’s more like a microphone than a telescope
so it measures everything at the same time
So if two supermassive black holes merge anywhere in the Universe
LISA will see it
When two small black holes
like the ones measured by LIGO
when they are formed
they’re just formed as individual objects
and those objects then get caught with each other
and eventually merge
but we don’t expect much gas to be in the vicinity
However, if you get two galaxies merging
then you have the entire galactic disc coming together
so you have two black holes
surrounded by an enormous amount of gas
At that point
as the black holes spiral together
they’re also pulling the gas
so as well as getting the gravitational wave signature
coming from the black holes merging
we may also get electromagnetic signature
Athena is the next large X-ray observatory of the European Space Agency
It will be the largest X-ray mission
and probably the largest mission that ESA is going to build
The goal of Athena is
investigating with unprecedented accuracy and depth
the X-ray Universe
In particular,
Athena is going to measure a very high number of black holes
several hundred thousands of black holes
from very nearby to very far away in the Universe
so in the early phase of the evolution of the Universe
In particular with Athena
we are interested at those black holes
that are the oldest
so the ones that formed first in the history of the Universe
a few hundred million years after the big bang
When two galaxies merge
eventually
many things happen
we know the gas and the dust in the galaxy merge
there are lots of shock
lots of turbulence
but as far as the black holes are concerned
eventually and inevitably they will coalesce
they will merge and one bigger black hole will be formed
from the two black holes at the centre of the galaxies
At that point
many interesting things will happen
one of the most interesting is the fact that the merging
the process of spiraling
and then merging created gravitational waves
which is what we hope to be able to detect with LISA
And also the matter
so the gas and dust surrounding these black holes
will start moving
depending on the warps and the changes of spacetime
induced by the spiraling black holes
So we will have a unique opportunity
of making another test of how matter behaves
due to the general relativity prediction
due to the force of gravity
in a unique environment
that has never been tested before
where the spacetime is particularly strongly stirred
due to the fact that we have two black holes spiraling around each other
The X-ray signature will be measured by Athena
and LISA measures the gravitational wave
and together
we get everything
we get the environment of the black holes
we get the black hole mass
the environment
the location
It really just gives a whole wealth of information
you can’t get with one or the other instrument
So these are unique observation opportunities
that will be possible only if we have both Athena and LISA
operating together at least for a few years
LISA will be able to measure two black holes
coming towards each other
maybe up to about a month before they actually merge together
At that point
we won’t really know where they are
we would just know somewhere in the Universe these two objects exist
As the LISA detectors go around the Sun
we can start to pinpoint where it is
We don’t have to move very far
so that we can start to narrow down where the object actually is
Just before the actual merger
we can get it down to an area of about maybe one square degree
maybe ten square degrees
which is still huge compared to other instruments
But at that point
we can then point Athena to the same area
and Athena can start to scan the sky where we think this object is
and we have a very characteristic signature we’re looking for
As the two black holes are orbiting
they stream matter from the surrounding gas
and that modulates the X-rays
So we can look for the gravitational wave signature
and we can look for the corresponding X-ray signature
and with that we can try to nail down exactly which object
which galaxy is producing the signal
Eventually the localization
at least for some of the best signal-to-noise events
will be so good
that we will be able to stare with Athena in the exact position of the merging
see the merging and see what happens afterwards.
And at that point
magic happens
we hope at least
We will be able to see how an active galactic nucleus
so an accreting black hole, is born.
So we will see how X-ray emissions from accreting black holes start
actually emerging through the Athena observations
and this will be a unique opportunity
to know how actually the accreting black holes are born
start emit light
and this can tell us a lot for instance
about how relativistic jets
which we know are a very powerful component in the Universe
that we see in different kinds of astrophysical environments
are born
I think this is one of the most interesting measurements we can ever make
At the moment we know gravitational waves exist, from the ground based
we know supermassive black holes exist, from galaxies
and we know that some galaxies are very active
in X-rays
called active galactic nuclei
with jets, very high energy jets
coming from off the central black hole
What we don’t know is how do these jets actually turn on.
So by measuring the two black holes as they merge together
called the inspiral phase
we really start to understand the dynamics of the centre of the galaxy during this
After they merge
we may actually see the jet turn on
If we could see an active galactic nucleus turn on in real time
that would just be amazing.