so dark energy is uh the name we give
for the thing that we don't know
that's causing the expansion of the
universe to accelerate it it's something
that's that's pushing everything apart
from everything else
is it like something between all the
particles kind of like like samson
pushing on pillars like pushing them all
or like yeah i think it's like the yeast
in your bread that causes it to rise
it's kind of pushing the structures in
the universe apart from one another when
you're lying in bed thinking about it
what does it look like to you do you do
sort of visualize it in any way or is it
do you just see equations and numbers
bouncing around your head um
yeah i don't i don't think we know
enough about it to kind of say oh it
definitely looks like this yeah to me
it's um
it's it's equations and numbers on the
one hand and observations we have of the
universe on the other hand well we have
some ideas of what it is so one of those
is a thing called the chameleon
so these are particles that change their
mass depending on where they're living
and changing the mass of the particle
allows it to hide from some of our
experiments
so the problem with dark energy is that
as well as pushing the expansion of the
universe apart it ought to come along
with an extra force as well so we ought
to see kind of here between you and me
there'd be an extra kind of interac
extra attraction and we don't see that
and so trying to explain
why we don't see those forces has been
one of the big problems of um of
understanding dark energy surely that
means
it doesn't exist like i can accept
chameleons and masses changing and
things fluctuating but there still would
be something it would just be the
quantity of it would be changing but
you're saying we see nothing so
chameleons help us explain why we see
nothing chameleon becomes very heavy
inside me and so um
the the matter inside me
uh any chameleons that that are produced
there they can't they can't get out and
so they can't transmit a signal to you
because because they're so massive they
get trapped these aren't protons they
aren't electrons they aren't neutrons
they're not photons they're
they're just some they're just
just a thing another another new thing
that that we we think helps us solve
some problems that we don't understand
are chameleons abundant yeah
um so dark energy makes up 70 of the
universe so
uh at the moment so yeah they're
everywhere so 70 of the stuff in this
room
are these particles which are changing
mass and doing their thing and you're
telling me
we just can't see them we can't see them
yeah so maybe maybe not 70 of the stuff
in this room because so 70 of the stuff
in the universe but obviously mata is
more densely clumps
here than on average in the universe but
but yeah there's still a lot of them
around and we don't see them because
they're really good at camouflaging
themselves this feels a bit like you're
telling me there's a room full of
elephants and the elephant can change
its mass from one ton to two tons to
half a ton to 500 grams to 90 tons
but i'd still be seeing elephants
everywhere
right so but the fact that they the fact
that they change their masses is is what
makes them hard hard to see there are
force carrying particles so in the same
way that photons carry electromagnetic
interactions
so actually so
the reason that that we see
electromagnetic interactions um in our
daily lives but we don't see weak
interactions in our daily lives even
though the theories look very similar is
because the the particles that transmit
weak forces are very heavy
so they can only transmit forces over
very short ranges whereas the photons
massless can transmit information over
you know as long a distance as we want
and so that's why the the fact that the
mass of the chameleon changes uh
makes it hard to see because it's it's
only transmitting interactions over very
very short distance scales that we don't
experience but that doesn't sound like a
problem of its changeability that sounds
like a problem just if it's massiveness
right is the problem that these
particles are massive or that they are
changing so the problem is that um if
they're going to to help you explain why
the expansion of the universe is
accelerating they need to be really
light because they need to be having
effects over sort of distance scales the
whole size of the universe today so that
means that needs a light particle
but then we expect to see that in our
daily lives and we don't so we need to
explain
how it's changed from from one behavior
on very very very long distance scales
to to a different behavior here in the
room so is the suggestion that these
chameleon particles
become
low mass to do their business and then
go back to hiding as big mass exactly
yeah so they quickly come out of their
cave do their naughty work and then hide
again yeah
okay
that seems
convenient for scientists who can't find
it right so so then we have to
uh yeah we that would be too convenient
we have to
to look at what you could do to try and
see them because they don't hide
everywhere what you're looking for are
extra forces in the universe and you can
do that in in all sorts of ways you can
you can have two
uh masses and a laboratory experiment
and see if you can measure the forces
between them so people have like
cavendish did that for the first time to
test test gravity people have been doing
that for a long long time
you can measure how planets orbit in the
solar system that tells you about the
forces between the sun and and the
planets and the moon and the earth
and you can look at how
stars clump together to form galaxies
how galaxies clump together to form
clusters if you can measure that
precisely that those will tell you about
extra forces in the universe the problem
is those are all the experiments from
which the comedian hides i thought we've
already found it because we we see this
expansion this increasing acceleration
so it's not hiding in fact it's showing
itself on the biggest stage possible on
the universal stage we see one
consequence of dark energy but one
one observation isn't enough to build a
whole theory on we want to understand
all the ways in which um dark energy
behaves and in particular we would we
would want to understand how it
interacts with the matter fields that
we're made of
um and that's very difficult to
understand just from observing how the
expansion of the universe
has evolved what we need to try and find
the comedian is to measure forces on
things that are really tiny because if
we're looking on really tiny scales with
really tiny particles then there isn't
space for the comedian to hide more tiny
than kevin scale yeah so we're talking
atomic distance scales we now have the
ability to do this to measure forces on
individual atoms
um and
so and this relies on on some things uh
some principles of quantum mechanics
so it's a bit like doing a double slit
experiment
where you put the particle in a
superposition of states one of which
travels through the left hand slit one
of which travels through the right hand
slit and if you don't watch
which slit the particle goes through you
get an interference pattern so it's like
you had a wave instead of a particle so
what we do is is we take an atom and
what we what we'd like to do is is put
it into a superposition of two states
one of which feels a stronger force one
of which feels a weaker force so if you
wanted to measure gravity
so
the gravitational strength the
gravitational attraction of the earth
um what you'd do is you'd have one atom
moving horizontally and another one that
was that was doing some parabolic
trajectory okay so so they they take
different paths under gravity this is
the same atom or two different this is
the same atom
instead of
going through both the left slit and the
right slit of the double slit experiment
it takes both paths as long as we're not
watching it and then it's only at the
end when the two paths reconverge then
we do a measurement and what state we
observe the atom in
tells us about the strength of the force
that was acting on it while it was in
this superposition of states so you can
you can tell what it experienced on both
parts yeah it can tell you both stories
yeah exactly it tells you about the
differences between the two paths so the
chameleon is an extra force so we could
put some some lump of matter in in our
vacuum chamber
and there'd be an attraction between our
atoms and this an extra chameleon
attraction between between the atoms and
our
source mass
um but that why this is so good for
looking for comedians is that because
atoms are small
they
the chameleon can't hide from these
atoms it has to talk to them and that's
different from any other experiment that
we've done looking for these extra
forces before and that's just what
that's what makes it super sensitive to
to the existence of comedian fields why
does a chameleon have to talk to an atom
but doesn't have to talk to jupiter or a
bowl of lead or the sun right um so we
talked about um the fact that the
changing mass of the particle
um tells you about the the distance
scales over which it can transmit forces
unless the chameleon's mass can become
smaller than the size of the atom
it it can't hide from the atom
right whereas whereas jupiter is so big
easy for the chameleon kind of
interaction scale to become smaller than
jupiter and then it can't talk how do
you do this experiment though because
any chamber you build is going to be
made of stuff and it's going to be here
on earth and it's going to be
electromagnetic forces going on
presumably there's going to be so such a
mess of force
that this atom is experiencing anyway
how are you going to
isolate the chameleons force
right so actually um
yeah that's that's obviously the tricky
bit so you have your atoms in particular
it's hard to shield the gravitational
effects of the earth so what we do is we
arrange for the chameleon force to be
acting horizontally while the
gravitational force is acting vertically
and then you can you can set up your
measurement in which it's uh in a way so
that you're only sensitive to horizontal
forces and in that way you can you can
decouple the two effects electromagnetic
forces
have to exist within our experiment
because it's how we control
the the atoms how we move them around
and say they're they're a bit trickier
to deal with but if you're if you're
careful then then
they're a sort of subdominant thing that
you have to you have to control
presumably you don't go to the cupboard
in the lab and open up a jar of
chameleon particles how do you
you just bank on chameleons being in
there yeah they're they're everywhere so
they're they're going to be inside your
vacuum chamber we're planning to build
this experiment at imperial it's an
experiment you can do with with current
technology all you need is is a vacuum
chamber and a laser so the laser is what
helps you move the atoms around
um and then you just need a way of
cooling the atoms down
so that there isn't a lot of thermal
motion kind of happening that's
destroying the
the effects that you're looking for
those are all things we can do and this
is an experiment that lives on a table
you have one atom
you shine a late so and this atom is
living in its ground state okay so it's
not excited do you know what that
scenario
uh so normally rubidium would use um
and you shine the photon on the atom
uh and if you've chosen your laser beam
carefully
your atom absorbs that photon and it
gets excited but now you have to
conserve momentum you're in your atom
was standing still but the photon that
came in had some momentum now there's no
photon anymore so your atom had to pick
up the momentum of the incoming photon
so now you've got a way of moving atoms
around
but if you don't make a measurement at
that point you don't know if the atoms
absorb effect on or not so now it's it's
in this superposition of states one of
which is standing still one of which is
moving along you do that three times
basically you give it a kick in one
direction you give it a kick back
um and then there's a final uh kick from
the photon that brings our two paths
back together so one of which was just
standing still one of which was moving
the atom or the the part of the atom the
part of the state of the atom that that
moved that's been moving around has been
going closer to us a mass which is a
source of of our chameleons it's the
source of this extra comedian force
and so the two paths have experienced
kind of different
different chameleon effects so when you
bring them back together
the ones that have been traveling are in
their excited state the one that's been
staying still is in in in the ground
state
and so you can make a measurement of
your atom and the probability that you
see it in its excited state
is proportional to the strength of the
chameleon force that it's been feeling
as it's been moving around
so you're measuring how many atoms come
out in the excited state and that tells
you about where extra forces have been
have been acting on the atoms
this is quite exciting this is like this
is well this is this is really showing
science working the point is hook
reports this okay now did he make it up
did he have an aberration in his
telescope so he couldn't see it did he
have a spot in his eye okay
until somebody else has seen him has
reproduced it you can't be sure of that
but you see the principle it
by publishing here
you are stimulating debate stimulating
more observation and the science becomes
that bit more secure