[ Music ]
>> Okay if I could have
your attention please,
we shall get started.
Our speaker today comes to
us from Italy which places me
in the position of perhaps
having only the second-best
accent in the room today.
After initially studying in Pisa
and Trieste he undertook
his postdoctoral research
at Cal Tech before assuming
professorship at UC Santa Cruz
where he is now the
director of graduate studies.
When I saw the title of his
talk I was very pleased,
it really ties in
very well to a lot
of the curriculum we have here
at Sonoma State in astronomy
and is certainly a topic I know
many people are very interested
in just simply for the sake
of it being very interesting.
So with that said let's
give a warm welcome
to our speaker today.
[ Applause ]
>> Thank you very much Tom for
the flattering introduction.
So I'm really happy
to be here today.
I know I'll be a little less
happy the next time I drive
up here on the occasion of
the May 12 [inaudible] that I
for unknown reason signed up to.
So today I will talk about the
connection between the smallest
and the largest things
that we measure
and that we think we
understand in the universe.
So scientists tend
to be pretty arrogant
about what the they know
about the physical world,
I think there's very little
reason to be arrogant.
For example, we actually have no
idea why the universe is filled
up with stuff and not
with a bunch of photons.
And so another way to
phrase this statement is
that we don't know why the
universe contains matter
and essentially no antimatter.
And yet another way to phrase
this is what produced this
symmetry between
matter and antimatter
when essentially all laws
of physics are identical
for matter and antimatter.
We don't know, there's
no fundamental physics
that explains why the
universe contains matter.
Secondly, we know that the
universe contains a lot
of matter that fundamentally we
don't understand the nature of.
And what we call this matter
is dark matter and as I'm going
to explain, the way we try
and understand the formation
of structures in the universe
depends on the very existence
of this dark matter and
we don't know what it is.
We are certain that it's there,
we are essentially certain
in the sense of a scientific
statement that it's there,
but we don't know what it is.
So let me start out by saying
that cosmology is a
[inaudible] word that comes
from two Greek words,
cosmos and logos.
Logos is you might
know means word,
but it also means knowledge of.
And cosmos means the cosmos.
He it also at its
root means the order.
So cosmology really is
the knowledge of the order
that governs the universe and
that's something that you know
as human beings we've
always felt a need to have.
You know we always felt the need
to have some knowledge
of what surrounds us.
So in ancient times
Egyptians had this particular
cosmological model.
Later on in western
civilizations other cosmological
models were developed
because we really wanted
to make sense of it all.
Today we have yet another
cosmological model called the
big bang theory and this
cosmological model is based sort
of in lay times on observations
and direct measurements
that we make of the universe.
And in this other half, this
is a [inaudible] scale in time,
it goes all the way back to
10 to the minus 44 seconds
after the big bang we
make extrapolations.
And this is the realm
where folks like me,
particle physicists, use
particle physics to model
and to understand what
happened in the very,
very early stages
of the universe.
Now this is all stuff that
we've learned by looking
at the universe,
literally looking
at the universe using
photons, using light at a bunch
of different frequencies.
For example, measurements
of the cosmic [inaudible]
radial measurements.
We look at stars and
galaxies with optical photos,
with infrared and ultraviolet
photons, gamma ray photons.
But the new kid on the block
is a completely new way to look
at the universe that we
just have put our hands on,
which is gravity waves.
Gravity waves that have been
first directly conclusively
detected just in 2015
for which of course
as you know the Nobel
prize was awarded.
This measurement is
absolutely astonishing,
it's absolutely mindboggling
if you think about it.
So they're measuring
how the very fabric
of space-time changes
by distances of 10
to the minus 19 meters.
So 10 to the minus 19 is
a billionth times smaller
than an atom, billionth
time smaller than that.
So you would say you're crazy
because quantum effects
certainly do not allow you
to measure things a
billionth times smaller
than the quantum scale.
But in fact you can using these
very large interferometers
and lasers and very, very,
very carefully controlling
backgrounds of all sorts.
I'm not going to get into this,
but this is a magnificent
enterprise
that at the moment
involves two observatories
in the United States
and one in Italy.
And the type of observations
that these observatories
have delivered to us
so far are things like mergers
of black holes, you know tens
of times larger than the
sun that have produced
and delivered an
incredible amount of energy
that we got [inaudible].
So the important thing
for somebody who cares
about understanding the
universe when it comes
to gravity waves is the fact
that gravity waves are
intrinsically much better probes
of the universe than light.
The reason is that light
you know it's very easy
to block light okay,
my hand blocks light.
It has a harder time
blocking higher frequencies,
but ultimately light
of any frequency cannot
penetrate the [inaudible]
universe, again it
gets absorbed.
Gravity waves don't
get absorbed.
So gravity waves
give us a handle
on essentially a whole
new view of the universe
across essentially
all cosmic times.
And we just now have opened our
eyes to this new entirely very,
very promising way of
looking into the universe.
So let me go back to this
and once again emphasize how
we have now this completely new
and formidable tool.
So this model that
I'm going to talk
about today it's called the
standard cosmological model.
And so why am I here
talking about this model
and not you know a model
based on Egyptian goddesses.
Well the reason why I
do that and the reason
that this is different
from that is
that the standard
cosmological model is,
I'm going to make the claim,
that it is a scientific theory,
it's a scientific theory.
So what do I mean
by the statement
that the standard
cosmological model is as opposed
to other formulations of the
cosmos is a scientific theory?
Well I'm not a philosopher
of science
but I know what some smart
philosophers of science have had
to say about scientific
theories.
And one important statement
about scientific theories is
that for a statement
to be considered
scientific there's got
to be ways to falsify it.
So you've got to make
predictions that you can go out
and make a measurement
and falsify,
you can never verify
a scientific theory.
But for a theory to be
scientific you've got to be able
to come up with a prediction
that then your observer
friend can go out and test.
So another important
quote in the way I think
about scientific theories comes
from this Franciscan monk,
William of Ockham a
long time ago said
that [foreign language].
You should not multiply
entities beyond necessity.
A scientific theory is
a theory that does away
with any unnecessary ingredient,
that it's bare bone
construction,
it doesn't have any
flourishing around it.
And finally, my countryman
Galileo explained also a long
time ago that there is a
very, very important feature
to a scientific theory, which
is the language it uses.
The language of a scientific
theory is [foreign language],
it's mathematical language.
Okay that's a key
feature of anything
that we could call
a scientific theory.
And in these beautiful Italian
words Galileo you know says
that without speaking a
mathematical language we just
wander in this obscure
labyrinth without any guidance.
So mathematics is you know one
of the most important pillars
in what a scientific theory is.
So a scientific theory
is something
that makes quantitative
predictions using the language
of mathematics.
That makes predictions that in
addition to being quantitative,
so with a number attached
to it is also falsifiable.
So it's got to be at least
in principle, an experiment
that can test the predictions,
the mathematical
predictions of this theory.
Also a theory has to consistent
with pre-existing experimental
results and at least as accurate
as any theory that preceded it.
Think for example
of generativity
as a theory of gravity.
Well generativity made sense
at the beginning even
before making predictions
that you know were compatible
with it because as a subset,
as a limit of the theory it
contained Newtonian gravity.
Okay Newtonian gravity
was a well-defined limit
of a broader theory
that included you know
experimental evidences
and experimental predictions
of these pre-existing theories.
And finally and I think also
importantly it's parsimonious,
so it doesn't have any
unnecessary ingredient to it,
it sticks to the bare meaning.
So the scientific nature of the
standard cosmological model has
to do with the fact that as I'm
going to show you in one slide,
this model makes quantitative
predictions which are testable
with observation and it
employs a bare bone minimum
of ingredients.
So what I'm going to talk
about today in the first part
of my talk is what the nature
of these ingredients is.
Okay so what are these bare
bone ingredients that we need.
The predictions that the
standard cosmological model
makes are just amazing,
are just incredible.
So without essentially fixing
any parameters the standard
cosmological model predicts
the variations of temperature
in the sky of the cosmic
microwave background
on angular scales that vary
across orders of magnitude.
So from very, very small
scales, very, very large scales.
These are data points,
you cannot even see
the [inaudible] bars
of these data points here.
And this is a prediction,
a sonic [inaudible],
that's a prediction
of this theory.
Similarly, this is another
prediction of the theory
that tells you as a function
of the size of structures
in the universe going
from very small
to very large structures
in these directions.
It predicts how many for example
galaxies [inaudible] of size
or clusters of galaxies or
[inaudible] galaxies there are
on average in the sky.
And then you know you
can count these galaxies
and compare your predictions to
your observations that result.
This is another sort of miracle
of the standard cosmological
model,
this is the abundance
of light atoms.
So heavier elements
of synthesizing stars
in complicated processes that
you know have little to do
with the very early universe.
But light elements like
deuterium, helium-3, helium-7,
helium-4 are synthesized in the
very early universe by a network
of nuclear reactions
that is very well-known.
That we know from direct
nuclear physics experiments.
That we can place in
the context of a hot,
dense and expanding universe.
And we can make predictions
for the abundances of all
of these light elements
as a function
of just one single parameter,
which is the final density
or the number of leftover matter
of the matter antimatter
asymmetry okay.
As I explained in the beginning
we don't know what the value
of this parameter is,
but we can measure it.
And then it's going to
be the job of theorists
to figure out why we have here.
But the important point of
this plot is that observations
of the abundance of
light elements all agree
with one number, the
[inaudible] photo ratio
or how many relic protons
survive from the annihilation
of protons and antiprotons
in the early universe.
And it's one number
that explains some
of these abundances.
Also for example the height
of this peak with respect
to this other peak is
another indirect measurement
of the same number
and they match.
So the standard model
of cosmology is a theory
that makes a lot of independent
predictions as a function
of certain number of parameters
that once they're
fixed they're fixed.
And any one measurement could
falsify these predictions.
You know if for example
you took this point here
and you measure it here
you would kill the standard
cosmological model okay
or the height of this peak
which by the way right now
is actually much better
than what [inaudible] here.
It's another sharp prediction
if the data doesn't agree
with that the model is excluded.
So I want you guys to
take seriously this model,
it works beautifully.
There's only one problem and the
problem is that the ingredients
of this model are pretty
weird okay, they're minimal
but they're pretty weird.
The first ingredient
that I'm going
to tell you guys
about is dark matter.
So the picture we
have of galaxies
like the milky way is one
where the visible stuff,
gas and stars, is embedded
in this huge, much,
much larger [inaudible]
of dark matter,
very diluted type matter,
much, much less dense
than ordinary matter in the
form that we experience,
even you know the gas in the air
is a lot denser than dark matter
in the galaxy at our position.
We think that the same is true
for essentially any galaxy,
very, very small galaxies
to very, very large clusters
of thousands of galaxies
like the milky way.
And we really need
to have dark matter.
So the way which we think
galaxies from has to do
with the existence of tiny
little density perturbations
in the very early universe
that grew under gravity.
See the problem is
that normal matter has,
[inaudible] gravity
has other attractions
for example [inaudible]
attraction.
And so structures that
would like to collapse
under gravity cannot because
of the pressure provided
by electromagnetic interactions.
So this [inaudible]
perturbations don't really
affect normal matter up
until very, very late times.
In fact, the measurement of
the size of those perturbations
that remain with the cosmic
microwave background tells us
that if you only had ordinary
matter you could not form
galaxies in time.
It's a very simple
argument that has to do
with the [inaudible] growth
of density perturbations.
Without dark matter
galaxies would not form,
there just wouldn't
be enough time
between cosmic microwave
background decoupling
and today, so that's a fact.
So we do need dark matter
because dark matter being
dark doesn't have this
outwards pressure.
And so structures clumps of
dark matter are free to collapse
under gravity with nothing
else pushing stuff out.
Does that make sense guys?
So dark matter is a
fundamental pillar
of how we think structures
formed in the early years okay.
Without it there's no known
mechanism that will lead
to the formation of structures.
You know leave alone the fact
that if you have a cosmology
with dark matter you exactly
reproduce many details
of how structures form.
Without dark matter the
structure wouldn't form
at all okay.
so again, the picture is these
teeny initial [inaudible]
perturbations are clumped
in their dark matter content
while variants float around.
And eventually this
big gravitation
of [inaudible] capture
all the variants
that see these very deep
potential wells, they fall in,
start forming stars, start
forming galaxies, and bigger
and bigger structures form okay.
So again, the universe is too
young for structures to form
without the seed of dark
matter density perturbations.
So you can go further and in
this simulation which was run
by my colleague Joel
Primack and collaborators
at the University of New Mexico.
You can create on your computer,
on your supercomputer a
simulation of the universe
that you populated
with particles
that behave gravitationally
like dark matter
and you just let it go, you
just let it evolve in time.
And what you do is you
directly observe the formation
of these clumps of dark matter.
Okay so what we're looking
at here is how the
universe would look
like if dark matter were not
dark and we could actually see
where the dark matter is.
In fact, what we do see
are things like this
when they are populated
with galaxies.
So galaxies form because
the dark matter has these
high-density [inaudible] regions
where [inaudible]
and start falling.
In fact, our galaxy
is more similar to one
of these [inaudible] ones, this
is more akin to large clusters
of thousands of galaxies.
But the general structure is one
where the cosmos is [inaudible]
in sort of a similar way.
So clumps that are increasingly
smaller in size, in fact many
such clumps don't [inaudible]
to any galaxies at all.
If the clumps are too small they
cannot attract enough variance
to start star formation.
So we think that in fact
the universe is filled
up with these smaller clumps
that are yet to be detected.
So you can say okay what
do you make of this.
Well what you make are pictures
where you take those simulations
and you're putting
galaxies according
to how big dark matter
halos are okay,
with very well-defined
prescriptions
and you compare observations
and simulations
where the observations are
nothing but big, big pictures
of large swaths of this kind.
And you count things
like you know two
or three-point correlation
functions and density
of structure is a function
of scale and blah blah blah.
And you make a lot
of predictions
and you compare again
the right and the left
and they look amazingly
similar in a very,
very quantitatively
testable way.
But there's more, dark matter
is also essential to the way
in which we observe
the late time dynamics
of cosmic structures to evolve.
So for example here in this
little picture that I'm going
to -- well I can let
play is a simulation
of dark matter plus variance.
And the simulation that
shows how the merger
to these galaxies happens in
and [inaudible] simulations.
So in these computer-generated
universes
versus real pictures
of galaxies.
So again, that was a simulation,
that's a real picture
and you know simulation
goes on and gets
to a different snapshot.
And then you compare, you rotate
because of course you don't get
to choose where you
look galaxies from,
but you can rotate a
cosmological simulation,
you compare with pictures
of the actual universe.
And again, you can turn
this beautiful picture
into quantitative predictions.
Do your simulations reproduce
the pattern of galaxy mergers
that is actually
observed and can you do
that without dark matter halos
encompassing these galaxies?
Well the answer is no,
you need dark matter
to reproduce the final details
of these merger processes.
Now that's all good even
if it's a little bit weird,
but there's more.
So you know about 20
years ago people realized
that there's more to the
universe than dark matter
and this was awarded a
Nobel prize back in 2011.
And this has to do
with what happens
to the expansion
of the universe.
So if the universe
were filled only
with matter you have
this big explosion,
you will have this
big explosion,
but you know the universe would
be filled with matter and so
at some point you run out
of steam and matter starts
to re-collapse gravitationally
okay under its own weight.
However, the universe
doesn't look that way.
The expansion of the
universe is not slowing down,
in fact it's accelerating.
So the rate at which the
universe is expanding is
measured to be growing
with time okay,
the rate at which
the universe expands.
So the acceleration
itself instead of slowing
down as it should, if
the universe were filled
with matter, contains
something that we don't know
of that makes the acceleration
of the universe a
positive quantity.
So it makes the expansion
of the universe increasingly
fast with time.
So you can imagine that if
you measure as a function
of time the scale of the
universe and the size
of the universe in a
moment we arrive here.
And you know if there's nothing
that pumps the acceleration
of the universe up we're
going to end up here.
If there's a lot of this weird
stuff that pushes the expansion
of the universe
to be accelerated we're
going to be up there.
And if [inaudible] kind of
in between we arrive here.
So scientists figure with the
observation about essentially
where these curves are from
where we can observe them is
that we are on the red line.
So as far as we can
tell today we're going
to have this constant expansion
driven by a mysterious substance
that for lack of good
names we call dark energy.
Yes.
>> What is the big crunch?
>> So the big crunch would be
a re-collapse of the universe
under its own gravitational
effects.
So you've got a universe
filled with matter and you know
as you run out of steam from the
initial big bang you re-collapse
under the weight
of the [inaudible],
the universe itself.
And so given certain
energy content
of the universe you can predict
whether or not you re-collapse
in the singularity
and that's not going
to be the case data say.
Okay so to sort of
visualize what I'm conveying
of this standard
cosmological model,
you got two competing effects
in how structures
form in the universe.
First you have the dark
matter and [inaudible]
that makes galaxies
form, that makes things
to collapse, and merge together.
On the other hand,
you've got dark energy.
So dark energy played
essentially no role
in the very early
universe we know
that from direct measurement.
But it's becoming a bigger
and bigger deal as
the universe ages.
So later and later
times, in fact starting
at about five [inaudible] years
dark energy was [inaudible]
energy component
in the universe.
So as my friend Rocky Kolb of
the University of Chicago likes
to put it, don't let the bright
lights fool you the dark side
controls the universe.
Dark matter holds it together,
dark energy determines
its destiny.
Another friend of mine who's now
at Cal Tech Shawn Carroll-Coles
[assumed spelling],
this is a preposterous universe.
I'm going to explain to you how
we measure how much dark matter
and how much dark energy
there is in the universe.
But we know pretty precisely
that dark energy makes
up 70% today of the energy
that's in the universe
and dark matter makes
up about 25%.
So the rest of the stuff
is really minor details,
in fact it's mostly it's
[inaudible] hydrogen and helium.
And [inaudible] our friends
in the chemistry department
to make a big deal out of, but
actually 0.03% of the universe.
So let me tell you a little
bit about dark energy.
So dark energy clearly is a
big open problem, you know 70%
of the universe is in that form.
But we know of a very,
very trivial solution
to the dark energy problem,
that's called the
cosmological concept.
I'm not going to belabor
on what these are,
but these are Einstein's
equations of generativity.
In a very compact form it looks
like one equation is
actually 16 equations,
mu and nu can be zero,
one, two or three.
And what [inaudible] is
essentially it's called the
metric tensor is the way
in which the geometry
of the universe looks like okay
at a given point
in time and space.
So you've got the 16
nonlinear second order
of [inaudible] equations.
The right-hand side is a
so-called tensor, team mu nu,
which is called the matter
energy tensor and it's a kind
of component that
truly has to do
with what the universe obtains.
So you know matter,
radiation, and anything else.
But there's also piece
which notice comes
with [inaudible] sign, which
is called the vacuum engine.
So that's a piece that
Einstein was very well aware of.
In fact, Einstein himself
postulated this piece
because when he formulated
generativity folks didn't know
about the expansion
of the universe.
And Einstein actually postulated
this term to curate the fact
that in this theory the
university would expand unless
there was [inaudible].
Of course the universe does
expand and so Einstein went back
to saying well this term must
be zero, but he was wrong again.
He was wrong because
the universe expands
at an accelerated rate and
that has to do with the fact
that this term here actually
does exist very, very small
and it has kind of
the [inaudible] sign
if you think about it as fluid.
And you can think of
fluids as simple objects
where you relate the
pressure to the density okay.
So for example matter
is pressureless fluid,
radiation is a fluid with
[inaudible] radiation pressure.
Well the vacuum energy is
a negative pressure fluid,
so it's fluid that drives sort
of the accelerated expansion
of the universe because it
acts as antigravity okay.
Its presence repels matter away.
Okay so [inaudible]
that theoretically was long
known to possibly exist.
And here's a rare picture
of Einstein working
on dark energy back in the day.
So dark energy in my opinion is
definitely worth thinking about,
but we do know of a very,
very well-justified
compelling candidate,
the cosmological constant.
Now my theory colleagues work
very hard at thinking about why
that constant has that value.
But to me that line of
investigation is [inaudible],
you know you can ask the
same question about the mass
of the electron, why is the
mass of the electron 511 kV?
I don't know, I don't have a
good answer, it's a constant.
It would be great to have
a theory that explained
in detail the value of the mass
of the electron, the newton,
and blah blah blah, all of
the [inaudible] particles,
as well as the electromagnet
constant and so on.
But I think the fact that
there is a natural theoretical
candidate made this type
of inquiry not as profound
as you know other questions are.
So let me get to dark matter,
so dark matter is something
that we can measure very well
for essentially the
following reasons.
Because we can do
subtractions [inaudible].
So we can measure the total
matter contents of the universe
which is on this right axis
quite accurately from a variety
of different pieces
of information.
This is the so-called
[inaudible], I'm not going
to explain what these are.
But as you can see
they are very sensitive
to how much matter there
is in the universe.
This is the cosmic [inaudible].
These are supernovae.
On the vertical axis you've
got how much dark energy is
there okay.
So from this plain and the
fact that these three pieces
of information intersect
only here allow us to say
that you know the
total matter that's
in the universe has
a very narrow range.
Now we also know that baryonic
matter density quite well,
in fact I've already told
you how to measure it
for light elements and the
cosmic microwave background.
So by taking the 30% or so
total matter density in terms
of the total energy density
in the universe minus the 4%
of baryonic matter density
gives you a leftover amount 26%.
That is how we measure
the [inaudible]
of dark matter in the universe.
So when it comes to the matter
and density itself the dark
matter is really the huge
submergent part of the ice
[inaudible] right, it's 85%
or so of the global matter
content of the universe.
What do I call matter?
Where stuff that interacts
gravitationally okay.
The dark energy obeys the
laws of general relativity,
but it doesn't behave
as matter okay,
it's not a pressureless fluid.
So why is it so important
to think about dark matter?
Well I think it's so important
for the following simple reason,
that when you look at the known
particles of the standard model
of particle physics we've got no
candidates for the dark matter.
So let me walk you
through why that is.
So certainly dark
matter must be stable,
so things like the Higgs Boson
or Gauge Bosons [inaudible]
interactions don't work,
these guys decay
you know a millionth
of a second timescales.
Galaxies you know live
for 10 to the 17th second
so that's not going to work.
You've got force here
and the blue one,
all these guys interact
strongly,
that cannot be the dark matter,
otherwise we would have seen it
you know dark matter would have
interacted with nuclei
and we would have detected
it so that doesn't work.
Also dark matter is dark, it
cannot have an electric charge
so it cannot be photons created.
But they also [inaudible]
leptons,
these guys are unstable,
dark matter is not made
of electrons for sure.
So you've got this
last little piece
of hope [inaudible] particle
physics called the neutrinos.
It turns out the neutrinos
also cannot be the dark matter
and the reason is it's
slightly more [inaudible].
So neutrinos are neutral,
we are now they are massive
from their [inaudible],
but the problem is
that neutrinos are
very, very light.
They're so light that
they move relativistically
in the universe as
structured for.
And so that trick
that I was telling you
about of collapsing
structures to form the seeds
for the formation of
galaxies doesn't work.
Okay because neutrinos
when structures form
are relativistic,
they move essentially
at the speed of light.
And so they just don't
collapse gravitationally not
because they have
electromagnetic pressure
but because they have
very large speed.
That's why the standard
cosmological model is often
called the cold dark
matter cosmology.
You want dark matter particles
to be cold, to be moving slowly
in such a way that collapse
of early [inaudible]
perturbations can happen.
So neutrinos sadly also
cannot be the dark matter.
So for particle physicists like
me this is extremely interesting
because probably
it's the cleanest
and most compelling
evidence we have so far
for new physics beyond
the standard model.
We need a new ingredient,
we need something new
besides the particle content
of the standard model.
So that's why you know
literally hundreds of thousands
of papers have been written
and hundreds of thousands
of millions have been spent
searching for dark matter.
So let me summarize what
I've talked about so far.
So modern cosmology I claimed
is a scientific theory.
It's a scientific theory because
it makes quantitative testable
predictions that are
mathematically consistent
and that are economical.
Okay so modern cosmology has
two key ingredients in addition
to what we know and
love in the universe.
These two ingredients are dark
matter which seeds the formation
of the structure which is
responsible for the formation
of large clumps of matter
where the stars form
in the early universe.
And dark energy which
is responsible
for the accelerated
expansion of the universe.
In addition of force to
all of this stuff we know,
such as particles and
generativity and the way
in which these interact
in the early universe.
So dark energy is the big
guy in the energy [inaudible]
of the universe today,
but it has a very natural,
very compelling explanation
that to me is completely fine
and observation is
completely fine too,
the cosmological constant okay.
So from a [inaudible]
theoretical standpoint it's
absolutely trivial to
explain the existence
of the cosmological constant.
It's absolutely nontrivial
to explain its value okay,
to explain why the cosmological
constant has the value it has.
Some people say well
if it didn't have the value it
has the universe would not form,
there wouldn't be observers,
and therefore it's completely
meaningless and we got
to take those values
of the cosmological
constant out of the picture.
That's a so-called landscape
theory that also kind of meshes
with the notion of
anthropic principle.
Okay you need observers and
observers only can exist
if the cosmological
constant has that value.
Now is that a good explanation?
I don't think so, I don't think
that is a compelling
explanation to the value it has.
But again, it's just as
interesting a question
as asking what the
mass of the electron is
or what the charge
of the electron is.
Much more interesting
to me is the question
of what is the dark matter
and that's what I'm going
to tell you guys about.
So in principle dark matter
can be completely dark,
it can easily not have
any meaningful interaction
with us whatsoever and the
universe will be just fine.
But we tend to be optimists
and if you are an optimist
like me you think that
maybe at some level,
to some degree the dark matter
does have some interactions
with our world, with
the visible world.
We don't know what
that interaction is,
but in particle physics
language you can think
of the dark matter model as
being defined by what the weight
of the dark matter as a particle
is by its mass number one.
And number two, by how strongly
this particle here chi interacts
with particles in the visible
sector in the standard model,
such as quarks or electrons
or photons and so on.
So you've got on the one hand
the mass of the particle,
on the other hand how strongly
that particle interacts with us.
Okay so that's your
parameter space.
Now theorists like me are
very creative, so if you look
at this plain where
again you've got the mass
of [inaudible] interaction
strength
of the dark matter particle
with physical matter just
look at these numbers.
Okay it goes from
10 to the minus 33,
the GeV is roughly the mass
of the proton so we're talking
about 10 to the minus 33 the
mass of the proton all the way
up to 10 to the 19 times
the mass of the proton.
So basically, we have
no idea what the mass
of the dark matter
[inaudible], no idea okay.
[Inaudible] is even worse, it
goes from 10 to the minus 39
in these units [inaudible]
to 10 to the 24 okay,
roughly [inaudible] is the
strength of proton [inaudible].
So again, we have no idea.
All we can do is come up
with models which are more
or less justifiable on the
basis of theoretical arguments
or maybe on the basis
of aesthetics
and populate this
parameter space.
So a lot of people
think, including me
that there's a reason
of parameter space
that is particularly
interesting.
And the reason is the following,
so there's one thing
we know very well
about the dark matter,
it's the [inaudible].
So if you live in this purple
region here where the strength
of interaction and the mass
are such that the abundance
of the dark matter from
the early universe comes
out to be just right okay.
So in just the same way as
light elements have an abundance
and you can calculate
[inaudible] parameter,
here you can think of exactly
the same thing happening.
So from the thermal history
of the early universe
if you have a weakly
interacting massive particle,
a particle with a mass at
the weak interaction scale.
And weak interactions it turns
out its abundance is exactly
the abundance of the dark matter
that we measure in the universe.
So that's an interesting
argument, it's one possibility.
Okay and people have
gone off and assumed
that then [inaudible] is
weakly interactions okay,
the interaction separates
[inaudible]
and mediate the decay
of the neutron.
Things we know very well
from you know [inaudible].
And since that's the case then
you have some room to play
in looking for the dark
matter is a particle
and that's the business that
I've been in for many years now.
So for example you can take
two particles of dark matter
and these particles can
self-annihilate producing things
you can measure, such as photons
or antimatter or neutrinos,
things you can actually
go out and measure.
So that's called
indirect detection
because you indirectly
detect the degree
of this annihilation
processes of [inaudible].
Then there's this
other line of business
that I've also been
very involved
in called direct
detection what you want
to measure is the scattering
of the dark matter off
of ordinary matter.
And so you have dark
matter particles,
imagine you have a proton and
the dark matter keeps the proton
and gives the proton a little
bit of energy and momentum okay.
And so I understand
next week you're going
to have a speaker who's
going to tell you everything
about how various
[inaudible] are trying
to measure this teensy-weensy
energy of depositions
where the dark matter
is predicted
to impart toward ordinary
matter as it shoots
through our position
in the galaxy.
Then finally, you have
another direction [inaudible]
at this diagram which is well
maybe I can take two protons
and smash them together and
then produce the dark matter
in the lab okay.
So that's actually
completely uninteresting
because if you do
produce dark matter
in the lab you're not going
to see it, it's hard to see.
But there is some silver lining,
the silver lining
is the following
that you can radiate stuff
from the initial state,
such as photons or jets.
And so there's a lot of
very smart experimentalists
[inaudible] with a large
hadron collider who look
for stuff they can't see,
they call it missing energy
or missing transverse energy
[inaudible], plus molar jet
or molar photons so [inaudible]
essentially have nothing plus
one single photon
very high energy
or one single jet [inaudible].
Okay so you've got different
ways if the dark matter is
in that particular
corner of prime space
to look for dark matter.
So let me give you a little bit
of an overview of where we stand
in searching for this
type of dark matter.
So here is the [inaudible] I'm
sure you've heard of in Geneva,
Switzerland, CMS and Atlas,
but also [inaudible] we have
LHCb are actively looking
for this missing energy
plus [inaudible] events,
nothing has been found so far.
But the parameter space is still
pretty open and not constrained.
So let me tell you just in
one slide direct detection.
So direct detection again
you have dark matter shooting
through at a pretty high rate
our position in the galaxy.
Occasionally you can [inaudible]
the nuclei or electrons
of ordinary matter and maybe
you have such a good experience
that you can distinguish
that tiny energy deposition,
we're talking about
[inaudible] energies.
From anything else it
could produce such things,
for example [inaudible], natural
radioactive, and other things.
So if you're pretty smart,
you control your [inaudible]
well you can send links
or discover dark matter.
And in fact this is an old plot
and it shows where theorists
like me think dark matter could
exist in its parameter space,
again the same as before,
the mass versus the
interaction strength.
There's some experiments
actually in Italy
that has claimed for many
years a positive signal
so the [inaudible]
detected dark matter.
The problem is that they're
being very mysterious
with the experiment,
so they don't
like for other experiments use
the same crystals that they use,
the same [inaudible]
that they use.
So that you know is very
exciting because science works
by being very visible okay.
You want experiments that if I
didn't believe what you find I'm
going to set up my experiment
and check your results.
So that does seem to be possible
with this Italian experiment.
And also these are
the exclusion limits
from a bunch of other
experiments.
They are therefore
ruling out the results
of this Italian experiment.
So the [inaudible], they
might have something
but they're being really unfair
to the scientific
community being secretive
with their experiment.
Of course you know their
thinking is well we're seeing a
Nobel prize thing here
and so we're not going
to let anybody else work on it.
But that's [inaudible].
So anyways I've been in this
business of indirect detection
for a long time and the reason
is that there's a lot of things
that can come out from this
self-destruction or annihilation
of dark matter particles.
So one of the things is
antimatter, antimatter exists
in the cosmic radiation,
it's measured,
and it's actually
pretty abundant,
maybe one part 10 to the 4.
But it's rare enough
that if dark matter
by annihilating produces any
antimatter we should be able
to measure it, we should be able
to see those little bumps on top
of normal cosmic
[inaudible] processes.
Gamma rays, the Fermi telescope
has been used extensively
to search for dark matter.
But also, Fermi has been
used to search for positrons
and electrons from dark matter
and I've been very
involved in that.
Dark matter can also produce
other frequency photos,
I'm lucky enough that my
wife is an x-ray astronomer
and so we've collaborated a
lot looking for dark matter
with x-ray telescopes,
that's been a lot of fun.
Radial telescopes also can be
used to look for axions or even
for WIMPs weakly
interactive massive particles.
And then you've got neutrino
telescopes, huge telescopes,
kilometer size detectors that
exist under the sea or even
under the South Pole and that's
a whole different colloquium.
So all of these messengers are
out there and they're ready
to be used by people working
on searching for dark matter.
So the question to
a particle physicist
like me is can we use
these messengers in ways
that inform you know particle
physics, in ways that are clear
and crisp experimentally enough
to learn about new physics
from a particle standpoint.
And that's been a fun area to
be in, so I've worked a lot
for example on this particular
signal which is an excess
of high energy positrons, the
cosmic radiation that you see
up here, the [inaudible]
cosmic for example down here.
So you get a signal, about
10 years ago I was one
of those claiming that that's
exactly what you expect
from pulsars, so you've
got astrophysical objects
that could fake the
signal very well.
This has been in the
news again recently,
there's been new detections
that inform that signal.
Hundreds of papers have
been written on an excess
of gamma rays from the center of
the galaxy which looks precisely
as what you would expect from
dark matter annihilation.
So there's been a
lot of excitement.
My group has worked a lot
on understanding what you
know ordinary processes
from cosmic rays or even from
the supermassive black hole
at the center of the galaxy
could obfuscate the signal
and could fake the signal.
And we think that that's
not dark matter it's
probably astrophysics.
But again, it's been very
interesting possible signal
of dark matter.
Recently again, x-rays there's
been, well if you [inaudible]
and well you know people claim
that there's a [inaudible]
here at 3.5 kV.
We've looked at that
very closely,
you know people were certain
to have discovered dark matter
in the form of [inaudible]
neutrinos, of 7 kV mass
and we think it's just
potassium atoms being excited.
So even the fact that the mass
[inaudible] and even the fact
that we think it's potassium.
We wrote a paper called dark
matter search is going bananas,
which actually made fun
of work done at Harvard.
So you know we send the
Harvard folks, not make fun,
but you know referred
[inaudible].
So we got, you know, we sent
them the paper before putting it
on the archive and you
know they get back to us be
like you don't dare do
that, of course we did it.
But it was fun, you know, the
social aspects of science.
Anyways, we don't think
that's dark matter it's
probably bananas.
So we've got excesses
of antihelium-3
that also have caught
my attention.
But I'm running out of time
and so more recently we've
actually kind of switched away
from this [inaudible] story and
you know what LIGO detected,
this black hole [inaudible]
we think could actually have
to do with dark matter.
In fact, we've calculated
what the maximum amount
of black hole dark
matter could be
in the universe given everything
we know about black holes
and everything we know
about dark matter.
And our conclusion is that
yes, it is a possibility
that primordial black holes,
some black holes that don't come
from the end cycle evolution of
stars, but they're primordial
and produced in the
very early universe.
Those black holes could
be the dark matter.
And they could be what
we are seeing with LIGO,
we were seeing gravity
wave mergers.
So I don't have time to
talk about the other mystery
that we have which is where
ordinary matter comes from.
And I think you know
the key problem here is
that we've got a very small
asymmetry, it's 1 in 10
to the 10, we don't know where
this asymmetry comes from.
There's a famous
saying in mathematics
that there aren't
enough small numbers
to meet the many
demands made of them.
But I would like to
understand the small number
in terms of new dynamics.
And again, I think going
back to this the key aspect,
there's many theories
of baryogenesis,
a very high scale
[inaudible], all untestable.
The theory that I've
been working on operates
at very low scale, at a
scale that has been tested
with the large hadron collider
and that's why I think it's an
interesting theory to work on.
It's called electro-weak
baryogenesis.
And I'm going to
leave you with this,
electro-weak baryogenesis
actually predicts gravity waves
from the very early universe,
from times that are a tenth
of a billionth of a second.
And for those gravity waves
unfortunately they'd be here
in the millihertz frequencies
where current detectors
don't work.
But as you might or might not
know the next generation gravity
wave experiments are
going to be sent in space.
The Europeans are
on top of this,
NASA was like well first
detect gravity waves
and then we're going
to be on board,
so NASA is on board
now eventually.
And so is collaborating
with [inaudible] on this.
This is an amazing, amazing
detector, so look at this scale,
there's the sun, mercury,
Venus, earth and this is
where the [inaudible],
way out in space.
And these are going to
be interferometry legs
of the detectors shining
lasers against each other.
With no seismic noise at all.
So I'm going to leave you with
this, cosmic puzzle is far
from being solved, but
I think and especially
for the young folks in the room
and for the old folks there
are exciting opportunities
to write new chapters
in the book of science.
And I'm going to
leave you with this.
[ Applause ]
>> Well thank you very
much for that presentation,
it was a wonderful talk to have.
I know we are approaching
5 o'clock and many
of our extra credit students
have other commitments
at that time.
So at this point those students
who have extra credit
are welcome to depart,
please do so as quickly
and quietly as you can.
And we will take
questions from the rest
of the audience at that point.
Our speaker will be staying
for dinner with us tonight,
so if you're interested in
joining us for that, please come
and see me at the end.
Otherwise just 30
seconds we'll start
up questions from the audience.
[ Background Conversations ]
Scott first question.
>> Yeah, [inaudible]
the black hole.
Is there something
important about the timing
of their formation [inaudible],
you know in baryogenesis
in these very remarkable,
you know, the deuterium,
the lithium requirements right.
I mean they're ordinary
matter that's going
to become a black hole
that's going to change that.
So what are the thoughts
about primordial black holes?
>> Yeah, so first of all
since as you say the [inaudible]
synthesis provides a very crisp
measurement of the
amount on variance
in the universe you definitely
cannot have any substantial
fraction of dark
matter be in the form
of astrophysical black matter.
So these objects form typically,
you know standard
formation scenarios.
At the end of inflation
we have some period
where for whatever
reason probably has
to do with inflation ends.
So during reheating
you have some amount
of high density fluctuations
that are so large
that they usually
collapse in black holes.
At very, very early
times we're talking
about you know temperatures
on the order
of maybe 10 to the 15 GvE or so.
So these guys depending
on their mass,
depending on their mass
they can evaporate.
So they evaporate
always in any case
through [inaudible] radiation.
But they can evaporate
early enough
to disrupt the [inaudible]
synthesis.
So once again, if you have
light enough black holes their
lifetime is inversely
related to their mass,
so the lighter they are
the quicker they evaporate,
the more energy they inject.
So if that's the case, so for
example for you know black holes
of 10 to the 17 grams
[inaudible]
or lighter these guys
evaporate during the [inaudible]
of the universe and you
perturb all sorts of things,
so they're very strongly
constrained.
The black holes that are
primordial and have masses
in the tens of solar mass range
definitely you know do not
create any problems from the
standpoint of evaporation,
they evaporate very little,
they don't perturb anything.
But they're so massive that
they do create problems
in the stability of structures,
such as for example dwarf
galaxies or [inaudible].
So there are some constraints
of very massive primordial
black holes from that.
So there's a variety
of constraints
and as you know very well those
constraints were [inaudible]
on these objects as well across
a wide variety of masses.
But what we found was
that you know depending
on the mass distribution
of black holes you can have
entirely of the dark matter be
of primordial black hole origin.
>> You had your diagram
of a parameter space
and potential different
explanations of dark matter,
you said the WIMP box sits,
I mean convenient was the
word you used, in the middle.
I didn't catch though
if there was any sort
of meaningful indications that
it should be there or just
that it fits nicely into
what we have so far.
>> Yeah, so that's
a great question.
So from a theoretical standpoint
you know WIMPs have been sold
and marketed based upon the fact
that you know the
standard [inaudible]
of particle physics does require
to some extent new physics
at the electro-weak scale
for a variety of reasons,
one of them is the
so-called hierarchic problem,
another reason is gauge
coupling unification.
And so having new physics at the
electro-weak scale always seemed
like something very
compelling and very plausible.
The LHC has not found
any evidence
of strongly interacting
particles
at the electro-weak scale.
It really doesn't have much
to say about you know WIMPs
at the electro-weak scale other
than the searches
I've mentioned.
In addition to this you know
where WIMPs are, you know,
where WIMPs are from
a standpoint
of cosmology are
perfect thermal relics.
So in just the same
way as we think
that you know deuterium
is mostly a thermal relic.
So if you have a particle with a
mass of about 100 times the mass
of the proton and
weak interactions,
then its density today
is pretty much right,
it's pretty much the
dark matter abundance.
You know it's exactly
the same way as you know
if you have a barium density
that is [inaudible] you produce
the right amount of deuterium.
So that is sort of independent,
theoretically independent
motivation for WIMPs.
You know if it comes
to primordial black
holes why should there be
that many primordial black
holes, there's no good reason.
You can cook up an inflation
model that produces them
in the right amount, but there's
no you know compelling reason
for there to be that many
primordial black holes.
Yes.
>> It's possible you
covered this in your lecture
and it's also very possible
it went way over my head,
so allow me a fundamental
question.
If the dark energy is that great
a force that's accelerating the
universe constantly how did the
matter that was remaining manage
to [inaudible] into
all these galaxies
and the other phenomenon
of the universe?
>> Yeah, so that's really a
good question and the answer is
that you know if you look at the
energy density of dark energy.
I don't know if there's
anything I can write with.
But so the energy density
of the dark energy was,
well if it is a cosmological
concept was very, very,
very low at the time
when structures formed.
So a way to think about it,
I really wish I could draw
but you know if you
think about when photons
and electrons decouple
okay, CMB decoupling.
Well at that point in time
matter was a billion times
denser than dark
energy, a billion times
and radiation was pretty
much the same as matter.
Now you know today
the dark energy,
so the slope is kind
of like this.
Okay so as you go back
[inaudible] the universe this is
matter, this is a cosmological
constant, it's a constant.
So it's only in very, very
late times that the universe
and structures in the
universe have started
to feel the gravitational
effects of dark energy.
But your question
goes back to okay,
so maybe this is
not a constant okay,
so maybe this is not a constant.
So maybe it's something
dynamical, in fact I've worked
on scenarios where the dark
energy is not a constant,
but rather it's a
dynamical field
that has you know interactions
with other stuff
in the universe.
And so in that case you can
put very meaningful constraints
on you know preventing the dark
energy [inaudible] structure.
But the way to do
it is essentially
when structure formed the
dark energy was not behaving
as dark energy okay.
So you do know that you have
to have [inaudible] dark energy
when a structure forms
[inaudible] you were saying.
>> Thank you.
>> Yes.
>> I thought there was still
this [inaudible] 120th power
problem about the cosmological
constant being constant?
>> Oh good, good.
So believe it or not there
is an undergraduate student
from somewhere in northern
California who keeps writing me
that he solved the
cosmological constant problem
because you know he figured that
if you have a [inaudible] field
and he calculated the vacuum
energy of a [inaudible] field,
then you know you can set
what is technically known
as the cutoff scale.
So again, I don't
have anything to write
but if I did I would
write an expression
where you have a quantity with
the dimensions of momentum
or energy that you know
would go as the fourth power.
So you have essentially you
know some physics scale.
You mentioned this 120 orders
of magnitude that refers
to the fact that if you have
a vacuum energy associated
with a field and so this vacuum
energy depends essentially
on the fourth power of the
highest possible momentum
that the field can have.
So if this highest possible
momentum is at the scale
of quantum gravity, then
you have this 120 orders
of [inaudible], you should have
a huge cosmological constant.
Now you know particle physicists
are very brave individuals
who are never scared
about infinites,
we know how to take
care of infinities.
And the way to take care
of an infinity is your
[inaudible] infinity minus
infinity equals whatever
you want, which is true,
it's a mathematically
true statement.
And that's how we
do randomization.
So that's what you do.
So the point is how fine
you have to cancel infinity
against infinity okay.
And so the 120 orders of
magnitude just depends
on the fact that you think your
cancellation is not good enough
up until when quantum
gravity sets in.
So you know certainly
that 120 orders
of magnitude is cancelled
away okay by randomization.
But the problem is
that there is this left
over of infinity minus infinity.
The question is why that is
such a small quantity okay.
So you know technically there's
no problem that 120 orders
of magnitude, but
a big fine tuning.
So yeah, so some people are
very psychologically disturbed
by fine tuning.
Yeah, I just am not so what
can I tell you, you know,
I think it's more of a psycho,
you know, psychology kind
of question than a
physics question.
>> So it seems to me like
this is really a gravitational
mystery the whole
idea of [inaudible].
And without a quantum
theory of gravity,
do you need a quantum
theory of gravity you think
to solve this mystery, to put
that into the standard model?
>> Thanks.
So I was actually fearing your
question for a moment you know
because nobody has yet
brought up modified gravity,
which is yet a whole other.
See I can give you
a quote on that,
I'm not going to do it but.
So to answer your question,
no quantum gravity has probably
nothing to do with this.
These are very, very
low energy phenomenon.
So for example, dark energy is
something that you can think
of as having an energy scale,
again it's not a
[inaudible] scale,
it's 10 to the minus 4
electron volts and we know
that quantum gravity
starts to set in at 10
to the 19 giga-electron-volts.
So the remaining
orders for magnitude.
So it looks, you know, you
can think of this in terms
of distances, you can convert
energy scales you know one over.
And so that means that quantum
gravity affects distance scales
which are you know
maybe a billionth
of a billionth times
smaller than the size
of an atom while dark
energy affects scales
on you know scales the
order of galaxy clusters.
So it's just a different
realm of --
so there's probably nothing that
once my start colleagues figure
out quantum gravity that
I would have to say.
In fact you know it's kind of
disturbing that even if you talk
about black hole mergers and
you say well that's definitely,
it must have something to do
with you know quantum
gravity, in fact it doesn't.
But it doesn't even almost
have to do with generativity.
So the treatment of black hole
mergers is vastly Newtonian
up until the very, very last
moments newton gravity works
just fine.
Then you know there are some
GR effects that you have
to [inaudible] because you know
black holes are intrinsically
GR objects.
But quantum gravity just
you know operates on scales
that are vastly different
from what cosmology operates.
Now this is not true
of the very,
very early stage
of the universe.
So if you go down to 10 to
the minus 44 seconds then
for sure quantum gravity
was relevant back then.
But we think that those stages
are completely untestable
directly because there
was thing called inflation
that made us lose
memory of anything
that happened before inflation.
So yeah, so I mean I don't
want to sound too pessimistic,
but I think quantum gravity
and cosmology has a long ways
before thy talk to each other.
>> [Inaudible] fine
tuning and quantum gravity.
So as we have to turn over
the room to the next class
in just a few minutes we're
going to wrap at that point.
If you have any further
questions
for our speaker come
catch him at the end,
otherwise let's thank him again.
[ Applause ]
[ Music ]
