Now, we have a whole collection of galaxies to compare to the Milky Way like
the discovery excel planets only 80 years earlier.
We have this whole sampling of galaxies ready made and the astronomers launched
the study of these objects and comparision to the Milky Way thast is
still on going and we are still learning a lot.
as we'll see about how galaxies work and how they form and how they interact.
We had a leftover question from the Milky Way about this crazy rotation curve, and,
the person who pushed the study of rotation curve was an American astronomer
called Vera Rubin, who developed a fantastic technology that made it very
easy to measure the rotation curve of a galaxy with a single measurement,
basically measuring the Doppler shift as a function of position in the galaxy, and
produced large numbers of rotation curves of various spiral galaxies.
And here are the kinds of results. This is actually from a paper by Rubin
and what you see is basically the same thing we saw for the Milky Way.
The rotational velocity rises rapidly in the center and then oscillates a little
bit and then approaches a constant. What we do not see in any of these
galaxies is the decline outside the radius where the effective mass of the
galaxy is and so there's this large problem of missing mass.
It's not a problem confined to the Milky Way, there is a whole component of the
mass of a galaxy that we are missing, now.
Once you realize this, you also realize that, like the Milky Way, many, galaxies,
have orbiting satellites. So, Andromeda has several satellites that
orbit it, and other galaxies have, or-, orbital satellites that we can see.
And so, you can measure their orbital parameters to get a measure of the mass
inside their radius. So, in a larger radius And the farther
you look from a galaxy, the more the missing mass problem grows.
It seems that once you get farther out, 95% of the mass of a galaxy is not
accounted for by any of the components of a galaxy we have so far accounted for.
So what have we accounted for? We've accounted for all the stars that we can
see. Maybe off by some brown dwarfs.
We've accounted for gas and dust that we can see.
the Occam's, and, and, this, this extra matter is not clumped as tightly in the
center of a galaxy. Remember, it extends, and as you go
farther and farther away from the center, you're finding more and more missing
mass. So this is some broadly distributed mass,
a great big halo of mass around a galaxy. What's Occam's Razor? Well, the first
guess would be, maybe there's a collection of gas.
There's just a huge, we thought that maybe there were 60 billion solar masses
of gas out there. Maybe there are, it turns out you need a
factor of 100 more. It turns out that the visible mass of the
galaxy in the case of the Milky Way is about 5%.
We need 20 times more gas out there than we have, but if we have 20 times more gas
then we would start to see its extinction and absorption lines.
We don't see any absorption lines, whatever it is that's out there.
Is not only not producing light, it's also not absorbing light, it's optically
inactive. You can't hide, 100, 20 times the mass of
the Milky Way in gas surrounding us without being able to see it.
Something that, has no optical properties but is gravitationally active is an
interesting thing. the name that's given to this weird stuff
is Dark Matter. And, for example, for the Milky Way, our
measurements of the orbits of satellite galaxies and so on, predict that there is
a spherical, halo. The fact that it's spherical was
initially conjectured just because otherwise it would be concentrated in the
disk. But later measurements of orbiting
satelites and other thing, measurements we'll discuss.
Suggest a spherical halo, in fact, we know roughly the density profile of the
star matter halo, that extends between 2 and 300 kiloparsecs.
which means that if you give Andromeda a similar halo, the halo is essentially
touched. And a mass noticed that is as I said, 20
times larger than the mass of the entire, Milky Way galaxy, of all the components
that we've added up so far. So, most of the mass of the galaxy is in
this Dark Matter halo, that is what the sun is orbiting, certainly that is, since
it's a diffused halo The error that we found for the sun was small, but as you
get farther and farther out, the differences get bigger and bigger.
what is this stuff? Well, it's certainly an important, thing to figure out.
It dominates the galactic mass, therefore it is the thing that determines, things
like gravitational collapse and gravitational orbits and Gravitational
interactions between galaxies, and it's out there.
What is it? Well, again we've ruled out an envelope of gas.
How else could you stick matter in a big halo around it in a way that we wouldn't
see it? Well, if you clump the matter into tight clumps, say, like brown dwarf
stars, or white dwarf stars, or neutron stars.
Well, a neutron star packs a lot of mass into a very tight region And so you would
not see absorption lines because by and large you wouldn't be looking right
through the neutron star. So packing matter into tight packages
would be one way to hide a lot of mass in a large halo of the density of neutron
stars doesn't need to be very high. Because there's a lot of volume there and
neutron stars are very dense. So this is one suggestion.
it goes under the name machos for massive compact halo objects.
so the search goes on. How would you find white dwarfs way out
there in the halo? Well we, we know that finding brown dwarfs is difficult even in
the local surrounding of the thin disk and their numbers have osculated back and
forth, over the past decade. How would you find your way out there?
Well gravitational lensing is the idea. So, what we see here, is the light curve
of a star, that is not a variable star. there is no periodisity.
But it is a Brightness suddenly intensified, quite significantly for a
period of about 30 days and then went back.
We have, measurements here in black, and a theoretical model in red that explains
what it is that was going on. What was happening to the star is that it
was, as the earth moved and the sun moved, it was for those 30 days lined up,
behind some dark object, like maybe a neutron star or a Brown Dwarf in between
us and this star and when you have Massive object in between the star and
the Earth, then what we see, we saw is that the deflection, the graviational
deflection of light, will cause a focusing effect so that all of the
starlight that would have gone in to some angle surrounding.
The line, the direct line between the star and earth will in fact be focused by
this gravitational lens and the star will appear brighter.
So this lensing effect is well understood.
It's a GR effect and we are I said we would apply relativity a little, so
gravitational lensing, this is called microlensing because the object that's
doing the lensing need not be very massive.
the whole point is that you have precise alignment.
And so we find these lensing events, very good we could use them to try to detect
how often they occur. We have a sense for where stars and
assume they're scattered randomly, scatter the neutron stars in the halo
randomly. And look and make predictions for the
frequency, with which, microlensing events should occur.
We do that calculation. We measure microlensing events and we
don't get nearly enough of them. In fact, no more than 10% of the missing
mass of the Milky Way, say, could possibly be explained by a Macho.
And so there may be some neutron stars, though it's not clear how they got out
there into the halo. But there may be some compact halo
objects, but certainly that is not the answer to the missing mass puzzle.So what
is it? Well, Occam's Razor tells you not to add new.
The ingredients to a theory until they're necessary.
But, it seems that at this point, we are compelled to add a completely new
ingredient to the theory, the new ingredient goes under the name WIMPS.
you know that this was, a naming, convention by physicists when there's
MACHOS and WIMPS and the WIMPS win. WIMPS stands for weakly interacting
massive particles. What are these objects? Well, these are
conjectured to be a completely new and yet undiscovered form of matter.
We know about quarks and leptons and protons and neutrons and theres a wimpon,
some new kind of particle that we have never detected.
We have never detected it because it interacts only very weakly with matter.
In fact, you can compute just as we did for neutrinos that hundreds of thousands
of them are passing through your body, at any while we're having this conversation,
and because they're weakly interacting nothing.
Happens. Oh, wait, so why not just say that
there's a halo full of neutrinos? Well, neutrinos are very light objects, and,
light particles, and they, at typical, energies that you'd expect them to have,
move relativistically, and would not clump.
Remember, these things interact to gravitationally clump, to be bound to the
Milky Way, or bound to themselves since their mass dominates, the mass of the
Milky Way, into this spherical halo and not go wandering off into space.
A halo made of neutrinos would not have the right properties.
this is why we're talking about Weakly Interacting Massive Particles, so massive
particles that interact with Other particles only weakly.
That's why we haven't seen them, that's why they pass through bodies, etc.
More over they have to interact with each other weakly, because if there were
strong interactions say electromagnetic interactions or electromagnetic To some
other electromagnetism interactions between these wimps, then they would rub
against each other. They would be able to lose energy into
efficiently convert gravitational potential energy into heat and then they
would collapse, we have black stars and then we would be back to the macho
scenario. We want them unlike.
Real matter, normal matter, will just collapse to form galaxies in which there
are stars and planets and people. We want these dark matter particles to
stay very smoothly distributed in this big halo and never collapse.
The way they never collapse is that they don't have a way to thermalize energy.
That is because they weakly interact not only with us but also with each other.
We do not know of a particle like this, so if they exist, they have not yet been
discovered, and in fact, various theoretical models, ask your favorite
theorist, they will have candidate, for what particle in some extension of
current theories might be uh,. The candidate for dark matter.
A popular candidate among many theorists, is, neutralinos that show up in super
symmetric extensions of the standard model of particle physics, but this is
not course on particle theory although you might be confused sometimes.
So theorists have all kinds of ideas; experimentalists, on the other hand, want
to actually detect this. Remember, 95% of the mass of the Milky
Way has never been seen, is in the form of a particle we haven't discovered.
Our standard model of particle physics is great for discovering the 5% we know
about. Interesting, and humbling, and so these
are 2 of the many ongoing experiments. These both work on a similar principle.
There are other, Ideas that are being conducted.
The idea here is that you have these very delicate detectors.
once in a long while, one of the dark matter particles traversing the detector
might interact with a proton in the detector, setting off a, sound wave
because it will have transferred some momentum to that proton, which will
exchange With the others in the crystal and that sound wave gets converted to an
electronic signal which will be detected, so any time that somebody claps their
hands within miles of this, the vibration is detected as a signal and of course,
lots of things might interact with the proton.
So you stick these things deep in mines underground, like neutrino detectors and
there are dark matter searches going on all over the world.
And at some point, hopefully, we will actually have dark matter detection and
we will begin to understand the properties of whatever it is that
constitutes most of the mass of our galaxy and, most of the mass of the
universe, as we shall see. And so, nice mystery here,
We don't know what it is. There are alternatives I should point out
to positing dark matter. They are continually being pushed into
less and less reasonable corners by increasing amounts of observational data.
But, one of the leading one is to say that Newtonian gravity is an
approximation, that there are corrections to it, not the Einsteinian corrections
that are valid at large gravitational fields and high velocities.
We're talking about corrections here that would become important at large
distances, where the Halos lie, so maybe at large distances, gravity, operates a
little differently, and so our Newtonian calculation of the mass enclosed was
wrong. this was a reasonable theory, certainly
no less reasonable than depositing a whole new kind of particle, but as we'll
see in a few clips We think we have evidence to exclude that reasonable
solution. There's still others in the creative
minds of theorists but most of us think that cold dark matter, as this stuff is
called, is what constitutes most of the mass of the universe.
One dark thing, all around This halo of, wierd particles,
We can look, into the core of other galaxies, just like we tried to peer into
the core of ours, and see stars orbiting very rapidly.
It's much easier to see the core of another galaxy, if you happen to be
seeing it, head on, there is less of a disc of dust and gas between you and the
core. And so, we can extend the rotation
curves. All the way, very close to the center and
we see that stars very close to the center in fact are moving at very high
velocities. This tells us very close to the center is
a very massive object, various other observations rule out most other
candidates and at the end of the day, were lead to the conclusion that like the
Milky Way. Most galaxies contain in their core, a
super massive blackhole in fact the milkyway's black-hole is somewhat
underwhelming masses of black-holes and other galaxies can be up to a billion
solar masses which is as much as the masses of say all the gas, disc gas in
the solar, in the milky way. it turns out that the properties of the
black hole in the center of a galaxy are correlated in interesting ways with
galactic parameters, the luminosity of the poles, the numbers of globular
clusters. This, of course, is a great hint as to
how to construct a theory of galactic evolution, where galaxies come from and
how they form. We'll move to what little I'm able to
confidently say about this in a bit. But, for now let's relish the fact that
in the center of every galaxy is a super massive black hole.
And at the outskirts of every galaxy is this huge halo of stuff and nobody has
any idea what it's made of.