Axions: Ghost Riders in the Sky

At a first glance, our galaxy appears to be made of stars separated by vast and empty space. However, we now know that this space is filled with things that are more difficult to see – gas, photons and the elusive dark matter. We do not know what kind of particle dark matter is made of. In most theories it is heavier than the proton, present in space as a few particles per liter. However, there is another possibility – the axion, a particle originally imagined here at SLAC to explain a mysterious property of the neutron. If they exist, axions would be light and ghostly, streaming almost invisibly through the galaxy, and a single liter of space would contain a billion billion of them. This lecture describes the properties of the axion and explain how physicists are trying to observe this particle. The discovery of axions would not only revolutionize fundamental physics but would also open up a new, rapid and encrypted communication technology.

good evening and welcome to the current

installment of the slack public lectures I'm very glad to see you all here

tonight today we're going to talk about a hypothetical particle called the acción

many of you may have gotten the impression from previous public lectures that the dark matter of the universe is

some heavy invisible particle and we wanted to break that impression to a

certain extent by telling you about another candidate which is very different which could make up the dark

matter and to do this we're very lucky to have tonight as our speaker Hendrik

Vogel who's a postdoctoral fellow in the slack theoretical physics group Hendrik

got his PhD in Munich at the Max Planck Institute that hassan berg founded he

wrote his thesis on the effect of neutrinos and other invisible particles on stars and then he came here to study

more kinds of invisible particles and so today he'll tell you about the acción

proverbially the ghost riders in the sky so Hendrik thank you very much


thank you very much for the introduction Michael and thanks to everyone in the audience for being here I'm very excited

to tell you about the acción today and as Michael said the accident is a hypothetical particle but if we

discovered it it might mean that we find a new way how to how to understand

fundamental physics it might solve the problem of dark matter and it can also lead to innovation in everyday

technology most importantly however if the action exists it might mean that we

are basically surrounded by ghosts but before I continue that story let me

start with a picture of the night sky so this is similar to what you would see if

you went outside in an hour from now and what you can identify here is a couple

of stars you see Orion spelt but if you take a step back then you will see that

most of this picture is actually black most of this picture is actually the

space as in between the stars and if you're like me then in school you were

basically told that this space is empty that there is nothing inside there but I

want to take a closer look at that it's not actually true and if there is something what can it tell us about our

universe about how it came to be this is

a picture which was taken with a much better camera by the European Southern Observatory and this is a 360-degree

panoramic view of the sky as you see it from Earth so here this is the Milky Way

this is the disk of our galaxy and if you could look on top of our galaxy then

it probably looks something like this so here in the middle you have the galactic

center this is where a supermassive black hole resides and attracts all the

stars the circle around it and the stars they form this spiral pattern our Sun

our solar system is here so that means if we look towards the galactic center

and in this picture that would be a slice like this if we turn by 90 degrees

that would correspond to a slice which looks like this and we can look away

from the galactic center and then they will correspond to this of course you

can also look above or below the disc and that would correspond to these regions here okay so what do we see here

so first of all you see these light dots again so there are lots of stars in this picture however you also see this kind

of structure here which seems to obscure something behind it and these are dust

clouds so this is similar to what you might find if you vacuum your living

room but now this flies in space and this substance this dust is really good

at absorbing light so if there are stars behind it we are not able to see them so

luckily this visible light is not the only thing that we can use to look at

the sky so visible light with a certain frequencies here you can go to two

longer wavelengths in this direction so for example there's radio frequencies so this is what you might know from your

car radio this is what uses to play music there's microwave radiation which has a

slightly shorter wavelengths and yeah this is named or this tool you know from

your kitchen is named after after this there is infrared radiation and that is

the radiation that the body emits which has a certain kind of temperature at

room temperature midst this heat radiation infrared radiation so if you look at a person in the infrared then

you can see where this person this person is rather hot and where he's rather cold on the other side of the

visible spectrum there's UV radiation this is what gives you sunburns if you spend too much time

out in the Sun if you've broken a bone then you might have seen pictures like

this which are taken in the x-ray which is that even higher energies

shorter wavelength and the most energetic electromagnetic radiation that

we know that is gamma rays and those are usually produced in for example black

holes so some Astrophysical objects and this black hole here that's a blazer it

creates matter and converts that matter into energy which is then emitted in terms of gamma rays okay let us see what

happens if we look in the radio and for this you cannot use just a simple camera

you need something much bigger for example something like this this is the dish antenna very close by and with this

a picture of this guy might look like this so again you see the Galactic disc

here in the middle here would be galactic center and the red radiation

here means that there's lots of lots of radiation lots of photons very intense radiation is coming from that region if

you look above the disc then in these blue areas there's a very weak radiation

coming okay so there seems to be some substance rather diffuse substance which

emits this now luckily at this particular frequency here we have an understanding of what this means what

amidst this kind of radiation is hydrogen so to remind you hydrogen that

is a proton and an electron and protons and electrons they have a property

called spin spin is similar to just a magnet and similarly these two either

want to repulse each other or they want to attract each other depending if they are aligned or anti aligned so what this

hydrogen here wants to do it wants to because these two be positive in this

one around to go in this to this state so this is similar to magnets if you let go of them one wants to turn around and

attract itself to the other one now when it does it it's gained some energy and

this energy has to be radiated off with a wave Paquette or what we call the photons so

some electromagnetic radiation and this photon here has exactly this frequencies

1420 megahertz so this picture actually shows you where there is lots of

hydrogen in in space in in our galaxy so

this means there's lots of hydrogen here flying around in our galaxy how much

exactly well let's assume you could go to your supermarket and buy a gallon of

space then it turns out you would find roughly 4000 of these hydrogen atoms

inside your gallon of space so it's not empty there's something inside there well it's not that much either if you

compare to air for example air has this times more atoms per gallon so okay

close to empty but we only look at the radio right now let's see what happens

if we for example look in the microwave

in the microwave the picture looks completely different from before you see

some structure here in the middle but apart from that the sky seems

surprisingly uniform so in whatever direction you look into there seems to

be the same intensity coming from everywhere it's as if there's some object surrounding us which emits this

very particular intensity of microwave radiation and from this intensity you

can infer that the temperature of this object is roughly 2.7 Kelvin that means

minus 455 Fahrenheit so this is very very close to the absolute zero it's a

very cold object that's surrounding us there now the discovery of this

radiation was even surprising to the discoverers Arno Penzias and Robert

Wilson they set out with this kind of machine here they tried to measure some

radiation coming off from some satellites so they had to have a really sensitive measurement to actually pick

up that radiation so what they had to do they had to calibrate their instrument

here really well really finely they I really had to understand it and then

they said oh they did some test measurements and they saw some mysterious noise coming from it from

every direction okay they didn't understand that so they went back they recalibrated everything they found some

pigeon droppings inside the camera which they had to clean up but this mysterious

noise remained now luckily not far away

from them a couple of miles in Princeton there were a couple of theorists thinking about this kind of radiation

that it should actually exist and the reason for this is the creation of

universe the Big Bang the Big Bang that happened thirteen point five billion

years ago should have emitted some some photons there should be some afterglow to this event and these photons were

created they traveled through time and space for thirteen point five billion years for us to be detected today so

that means that this picture is not just some radiation this is actually a picture of the early universe of how it

was created so this object that's surrounding us with this 2.7 Kelvin that's actually the Big Bang that's the

beginning of the universe so that means the temperature of the universe today

that is related is 2.7 Kelvin so really really cold okay coming back to a gallon

of space that means that there are a couple of photons flying around as well and you can infer from this temperature

that they are two million of these in your gallon of space now these are not

ordinary photons these are really photons coming from the beginning of time from the Big Bang now it turns out

because this is a picture of the beginning of the universe it tells us a

lot about how it got created and what is the universe is made of it's basically a

fingerprint of our universe and this is why

science agencies send up three satellites to measure this radiation much more finely so the first mission

was Kobe in 1989 and so basically these

three instruments they're very fine thermometers they can measure very slight temperature differences in one

part in 10,000 in different directions so Kobe went up and they saw this 360

degree panoramic view of the sky they saw that there are some coolest spots in

some directions so not perfect 2.7 Kelvin but slightly cooler and then

there's some hotter spots here in the Galactic disk and also away from it so

this was so great that we measured these slight temperature differences that Kobe gotta know the price and there was

another mission send up by NASA which was which is called W map and you can see that it could measure these

temperature differences much more finally who has it but much better resolution and then there was a European

mission called plunk which was even more fine as a much better instrument on

board and could measure really all the differences very well so if we would do

min here then Kobe would see a structure like this with W map you can already

resolve a bit more of the structure you see some cool spots you see some hot spots here and plunk yeah you see it's

it's even better it's even more resolved here cool spots hot spots now why is

this important well it's important

because it tells us something about the beginning of the universe and it tells us that there's this substance called

dark matter and also dark energy so how do we infer this well to know this we

have to compare real data to simulations we have a we have a hunch about how the

universe works during the Big Bang so we can do computer simulations of this early universe and then we can compare

the outcome of these simulations to the real data and see if they match so for

example if we do a simulation and we put in all the visible meta that means protons neutrons electrons then the CMB

the Cosmic Microwave Background here will look like this so this looks completely different from

what you see here you see many more of these hotspots you see many more of these cool spots and they are more

extreme as well and the structure here looks much more smooth than in a

scenario where you only have visible matter however if we put in for example

this dark matter which means some slowly moving particles which can only interact gravitationally and cannot emit any

light then we can make make these simulations agree with a real data so

here you see yeah same amount of hot spots and cool spots and the structure looks very similar

now you can't compare these two pictures point by point because there's some random initial condition that we don't

know so the only thing you can compare is really the structure which looks very similar now if you like this kind of

pictures and you want to play around with different universes I encourage you

to go to this website after this and you can you can play around a bit and see how different website universes would

look like okay so from this from this agreement here we can infer that there's

only 5% of visible matter in the universe there's 26% of dark matter so

these slowly moving particles only interacting gravitationally and then there's this mysterious component which

I'm not going to talk about much dark energy which is 69% and dark energy is

some some energy in space which is fill space and it tries to force the universe

apart it tries to accelerate the universe now these components here

especially dark matter they don't stone there's no theorist you're thinking okay let's add this kind of substance we

actually have indication that this dark matter exists and we have had that indication for a hundred years

and the reason is that we've measured the velocity of objects flying around

galaxies and observations don't match your expectations let me tell you what

the what the expectations are here so what you see here is some galaxies here

in the middle and we want to follow these three objects here which going to fly around it now the velocity with

which these objects fly around the galactic center will depend on its mass but also inversely on the distance so

these objects will fly like this and velocity is determined by the mass

inside their orbits divided by the distance here all right so now we can

compare this to the observation and the observation that will be the white dots here and well they are much faster than

what we expected here in the inner orbit this is pink so it means these two seem

to agree but the further you go outside the larger discrepancy becomes between

the expected and observed rotation curves and the reason for this is so we

believe now that there is dark matter in our galaxy which extends all the way and that increases the mass that is there

and it's larger than the mass you can infer from just looking at how many stars there are inside this galaxy so

there seems to be this dark dogmatic component accelerating objects outside in our galaxy okay great so now coming

back to a gallon of space can we now say how much dark matter there is unfortunately not we have a problem that

we don't know what this dark matter is so only then I can tell you how much dark matter there will be in the gallon

of space now that might seem disappointing to you but if you now ask

your local particle physicist about this they'd be really happy to assist you with this question and the reason is

that particle physicists always try to push the boundary of our understanding of nature they have a huge list with

candidates for how to extend nature and dark matter is an opportunity for them

to see if their their ideas somehow fit with with nature and with dark matter

for example but it's not that simple not every idea just fits there is a long

list of properties that your candidate for Dark Matter must fulfill so one

thing it has to be a slowly moving particle it has to be dark we're dark

means that it cannot emit any light and it also means they cannot interact in any any other way too strongly it's

really elusive it goes through meta easily and it's not you can't touch it

it has to be there during the Big Bang otherwise the picture of the Cosmic

Microwave Background wouldn't work out at it as it does right now it has to be there to influence the structure during

that period but it also has to be here today which means there has to be stable

it cannot decay and then it has to give us the right the correct rotation curves

okay there is still a long list which remains after crossing out all the

examples that don't work and I'm just gonna present a couple of them but the best one is over here but I'm going to

start with the whim so the whim used to be the prime candidate for dark

matter if you ask particle physicists about the whim they will say well for the last 30 years I would say they would

say yeah the wimp is the dark matter and it's the most interesting candidate and

the reason for this is that it was really easy to explain how the wimp a m-- to be in the early universe how it

was produced and also it was kind of a beautiful idea because it wasn't was a

rather heavy heavy particle so in the particle world there would be something

like a bowling ball flying around face and has a mass which is larger than the protons mass so if the WIMP was dark

matter there would be around 1200 of these per gallon of space because the

wimp was so attractive to many physicists there were a lot so and are

currently lots of experiments trying to search for the wimp unfortunately so far

in our vehicle spare experiments have found a positive indication for the wimp to exist and this is why in the last 10

years there has been a shift the paradigm shift a bit away from the wimp and towards other ideas that might

explain Dark Matter one of them is for example the macho the macho that's a

massive compact halo objects what it means is that it's basically black holes which were created in the early universe

and they fly around in our galaxy today as well now because black holes are

rather massive object that means that if you bought a gallon of space you probably wouldn't find any black hole in

there and if a black hole made it into this gallon well you wouldn't be able to

buy it because they've been swallowed by a black hole another candidate is the

sterile neutrino so you might have heard of the three visible neutrinos though

these three brothers here we know of their existence and they like to play each other play with each other a lot

but we think that there is a fourth brother and this is this is this one and

it doesn't really like to talk to any of his brothers so this is why it's called sterile it takes distance and because it

doesn't want to talk to his brothers or anything actually it's a good document Metta candidate and there would be

roughly 10 million of these sterile neutrinos in each gallon of space my

favorite candidate is the acción and the reason for for this is that the accident

solves another problem with nature the so called strong CP problem but I'm gonna call the

neutrons electric spin problem here and I'm gonna explain you what this problem

is and how the action solves it and hopefully after that you like the Xen as much as I do

so the neutron here so we have protons and neutrons they make up matter the

neutron has a property called a magnetic spin so this is similar again to this

magnet here I showed you before with the hydrogen atom and this magnetic spin is

such that if you put this Neutron inside the magnetic field that means that the

spin starts to rotate so this is our expectation the spin rotates around its

axis and this is also what we observe so this is great this works

however the neutron is also supposed to have an electric spin that is similar

but this one tries to rotate inside of an electric field so now if you put the

neutron inside the electric field the following happens we would expect it to spin but unfortunately it doesn't so

this doesn't work and this is a puzzle and this puzzle is the neutrons electric

spin problem now turns out this is

electric spin depends on the fundamental constant of nature called theta at the

constants of nature you might know are for example the electric charge or the gravitational constant now this

particular constant here seems to be almost zero very close to zero because

we do not observe this electric spin here and that's somewhat puzzling it's

summer means that we live in a very special universe let me let me show you

why so we know that theta lies in between minus PI and PI and any value on

this axis would give us an electric spin everywhere except for a

very tiny region around zero and by tiny I mean one in ten billion so the puzzle

is why nature somehow chose to lie somewhere in this very tiny region or

put in a different way if I asked you to give me a random number between minus PI

and pi the probability that you gave me a number where we would observe an electric spin is almost 1 and the

probability that you gave me a number which is in this small area here would

be vanishingly small or in a different picture let's say we drop marbles on

this and just read out the value where the marble Falls we just drop marbles randomly they might fall here they might

fall there but the probability that they fall exactly in this very very tiny

region here is very very small and we physicists we somehow train to see these

these odd features in in nature because they might tell us something more about

nature there might be a reason why this is and one reason for this was found by

Roberto PJ and Helen Quinn in 97 1977 when they were both at stand for it and

Helen Quinn later went on to be his deaf scientists see its lack as well and they

wrote this paper which was very influential and still is and they they

found a method how to get the right value of the theatre parameter in the

following way so they noticed that actually this line here is not straight

like this it's actually more Bend more like a bathtub so now we should drop a

marble it falls somewhere but it doesn't roll down because the friction is really

high so it wants to stay here so p'chenk win they envisioned that

there is some substance some oil in the universe which overcomes this friction

and lets this marble roll down to zero so that means that wherever the marble

folds it will roll down to zero so the probability that there's no electric spin is actually one so that means theta

today will be zero that will remove this electric spin and this means our

expectation now is that the neutron does not rotate inside an electric field and that matches with the observation so

that's great now the substance itself this oil is

really hard to detect but fortunately shortly after this Frank will check in

Steven Weinberg who was it slack for a while as well starting to think about this they at the same time but

independently found that there's another observable consequence and this observable consequence of this paycheck

way solution to the Neutron spin problem this consequence is the acción and they

called it the acción after a famous laundry detergent because the axion

removes a stain from understanding of nature okay so where what does the

oxygen how does it come into this game now if you look at this see if you have

water then you know that if you shake it up a lot or you have lots of wind then you get a wave like this the action is

the wave of this system here so if you shake up this bathtub here it will form

a wave it will be get excited and this excitation is the acción now it turns

out the acción is a good Dark Matter candidate if it ticks all the boxes it doesn't interact very strongly it gets

produced during the early universe so it is a dark matter Candida but it's an

unusual dogmatic candidate and as Michael said in the beginning it's

specially unusual because it's very light so this is the whim then the wind will be much heavier than

the acción and be more precise if the wind was as heavy as a tanker then the

axion would have the mass of an eyelash so really really small and the axon has

another interesting property it makes a ghost-like appearance in the early universe while wimps get created during

collisions of standards particles that we know of so neutrons electrons protons

collisions like this the action just appears out of nowhere it just comes to

be and the reason for this is that in the early universe this oil got created

at some point and when this oil got created the acción came to life okay

great so now that we refer that the action can be dark matter we can finally

say how many of these actions you would find in your gallon of space and it turns out it's 10 to the 19 now that

number doesn't or might not tell you very much but this is the amount of

insects you have on earth so in each gallon of space you would find as many

actions as you have insects on earth so

there are so many of these actions flying around why haven't we seen them

it turns out accents are really hard to detect they just fly through brick walls

without caring about it and that's a problem because our detectors for Wims

for example they rely on the fact that when dark matter particles fly through them these dark matter particles collide

with our detector and leave some trace axioms don't do that and the problem is

even worse even if a whole visible universe was made out of brick the axiom

would just fly through it without caring much and this is why this axiom was called

the invisible axiom for a while now luckily there is a different way and

here I have to credit Pierce akivi who's a professor in Florida right now and he was a postdoc here at slack for a while

as well and he came up with ideas how to actually detect this seemingly invisible

axiom now the detection technique relies on the fact that an action and the

photon so electromagnetic wave packet I actually the same thing when they when

they travel inside a magnetic field so that means that if you have an action

here you can convert it into a photon if you apply a magnetic field and the

probability for this to happen which I would call G gamma a is yeah is this

parameter G gamma now if you have trouble understanding this imagine a coffee machine a coffee machine converts

water into coffee so the water will be the X Y on the coffee is the photon and

then you have to put some beans on the top which will be the magnetic field but

this coffee machine is special it works in two directions there's sustained a coffee machine which

converts accents into photons but there's also an inverse coffee machine and this one filters out the coffee it

converts a photon into an axiom so this works both ways ok great

so now we have a method how to try to detect the action let's look at the map

on what we have to do so this is the parameter space of the acción you have a

mass here this is one electron the world bold which corresponds to one millionth

of the mass of the electron and you go orders of magnitude down here here on

this axis you have the probability to convert an action into a photon or in

the other way and up here the accidents basically chewing him if you throw it against the

wall just sticks to it down here it is really like a ghost it just goes through

everything it's really really hard to detect now this blue region here this is

where we want to get because this is where it's easiest to have the acción

and to have it solve both the neutrons spin problem neutrons Electric spin problem and also the dark metal problem

so this where we try to get but that doesn't mean that all the other white

areas here and it are uninteresting because the axion could also live somewhere here but that would mean that

we would require some more work to understand why the axion would be in these areas it might mean that the Dex

and it's not dark matter and something else or it does not solve the neutrons electric spin problem it's a bit more

complicated if we're somewhere here okay so the most advanced experiment is the

axiom document experiment admx and this one but started at Livermore Lab and

it's now being performed in Washington Washington University and they rely on

an electromagnetic cavity like this so cavity that is an instrument which is

very sensitive to a particular frequency of the electromagnetic spectrum and only to that frequency and this cavity they

put inside of this rack and you have very strong magnets inside here and you cool it down very close to the absolute

zero of temperature and then you try you wait and try to see the EXCI on so what

you hope for is that some of this dark matter action just flies inside your

cavity and then gets converted into a photon into electromagnetic power that you can pick up so this is basically

just a coffee machine you wait for the axon to come in and you try to get the coffee out now because the cavity is

very sensitive to a very particular frequency it means that it's very

sensitive to a particular action mass so what you have to do if you want to

scan all this parameter space you want to know if the accident lives at that mass or that mass you have to tune your

cavity to different masses and for this there are these rods inside here so when

you turn them the following happens the field in your cavity changes so the rods

are here and at the same time the frequency your cavity is sensitive to

changes as well now this is not as fast as shown here so usually it takes a

couple of months or a year to scan through all this and while you scan

through this what you try to look for is a peak like this so usually you have

just some random noise but what you try to see is that it actually some axons flying through it and then you see this

additional power and a peak and from the position of this peak from the frequency

you can infer what the mass of the axon actually is but if you do not see a peak

like this you can also say that there's no mass no no axion living at that particular mass so a MX has searched for

the x-gen so far it hasn't found it but it can tell us already that the axon cannot live in any of these gray areas

so this is not the only admx there also other experiments which are similar

which also give us bounds like this now

because the last ten years the the action has gotten more and more track and there are more theorists of thinking

about it there were lots of proposals on how to detect it also in the other regions here which are currently not

being probed I just want to mention two of them which are somehow dear to me one

is the dark matter radio this is an experiment being done here it stand for it and it's slack at the moment they're

doing R&D to go for very low mass actions

the other one is Mad Max and the reason I like Mad Max so much is because my

former supervisor Javier had Ando actually came up with the idea and last

October they formally started the collaboration and do R&D to finally detect the acción and well Javier was

also the one who basically brought me into axiom physics and make me work on this that's pretty cool

so with these ideas in the future we will be able to test all this red area

here this is what we will be able to do

unfortunately these experiments have the assumption

that there's actually action documented flying around so that means they assume that our galaxy is filled with these

accion's and there are also some of them in our solar system and they just fly into a cavity and get converted into

photons and we can pick them up however it might be that the axiom doesn't look

like this but more looks like a clustered as a clustered structure so

there might be some accidents flying around here some here but there are none right now in our solar system and there

will be a problem with these kinds of searches because then there are no accidents around to detect so this

wouldn't work so there are other ideas how to then detect the action but

they're a bit harder because you have to first create an axiom and then measure it but a cool idea is the so called

light shining through the wall experiments so what these these experiments do is they take a laser and

they point it against the wall and then they put a detector behind the wall so

if there's no action what you would expect is that your laser shoots a photon it gets absorbed by the wall you

don't see anything behind the wall however if there is an accident we can

do the following thing we put a magnetic field before and after the wall

and so the left-hand side here that's the the inverse coffee machine so it can

create an accion and the other side is the normal coffee machine creating photon that means we send the photon it

gets converted to an action inside the magnetic field the X in just goes through the wall then it gets converted

back into a photon which we then can pick up so suddenly you have photons

where there shouldn't be any and that's a smoking gun signal for this action to

exist independent of if it's dark matter or flying around in our solar system or not so this kind of experiment is

currently being performed in Hamburg at Daisy in Germany and they use a laser

they have an cavity laser resonator here

with lots of magnets around so 400 meters in front of the wall and 100

meters behind the wall and these experiments are rather cheap because

they also reuse lots of equipment from other experiments we have done before so they use dipole magnets from the HERA

experiment Hera was a particle collider which ran in the 90s and they just

reused those magnets to do this kind of experiment and they are currently looking for for the action and they will

be able to probe this area here now this is actually stunning because I told you

it's really hard to do these experiments because you first have to create the acción which is hard and then you have

to convert it back into a photon to detect it which is harder again so it's

really impressive that they can reach so far down and okay they might not be able

to reach this favorite region here in blue but still it would be interesting

to see if there is something here and if we found something it would definitely be really really interesting so we can

take this technique and ramp it up so instead of a laser now we're going to

use the Sun the Sun emits photons and convert these photons into accion's

the accidents then fly towards earth where we wait with a magnet here and

some x-ray detectors and the wall is this shielding here in front of the

magnet so with this you look at the Sun for a couple of hours a day and try to

see if there are some photons somewhere we don't expect any and this is being done by cast at CERN at the moment so

this is saying LHC prototype magnet here they ramped it on the rack and with this

they follow the Sun for a couple of hours before a couple of hours a day and

try to see if there is an axiom now there's a ramped up version of this

which is currently being developed which is IXO the International axial Observatory and yeah this will be much

better than this in the cost okay so this is what the what light shine

through the world experiment could do the Alps so with costs we are able to probe this so cost was able to exclude

the existence of an accion in this gray area here so they even reach down to the blue region and IXO will be able to

reach you even further down and further into this favoured blue region here okay

now we use the Sun so why shouldn't we use black holes and lasers so the Blazer

is the black hole accreting matter converting into this very intense radiation and these photons might reach

earth as well where we are waiting with our Fermi satellite and we have lots of scientists here at slackers were working

on on Fermi in trying to for example measure the photons coming from these places

now where's the wall in this game as I

told you before there's this cosmic microwave background photons flying around in space and it turns out that

these photons are somewhat a wall to these very high-energy photo gamma rays coming from the blazer but

there's a very good video from NASA which I will show you now which explains this much better so this was the blazer

and in the midst this high-energy particle and here you see the the

high-energy photons and they are flying

in a straight line and shortly after here you see these these blue photons coming in and those are the CMB photons

and they just fly randomly around and sometimes might happen that these

photons hit one of these high-energy photons and they create an electron-positron pair and these

electrons and positrons they could just goes in some other direction so you

can't see them later now not all all of the photons will be absorbed some of

them will make it to our galaxy and some of them will also make it to our solar

system so you see the Sun here and there

is the earth and we have the Fermi satellite waiting for those photons

there it is so when these photons come

in they again create an electron and the positron which we can then measure so that we can say we detected a photon

okay so we have a wall but the wall is kinda leaky not every photon gets

absorbed some of them make it through so but with this leaky wall we can still do

a light shining through the wall experiment here so we know that there is a magnetic field in space so what we can

do is there's a photon coming from this place it gets converted to intern and

accion the actually just flies through the Cosmic Microwave Background without

carrying much about it it gets converted back and we can pick it up with Fermi so

because the world is leaky measuring these photons does not necessarily mean that there is an action so here what we

have to do we have to compare our predictions for how leaky the wall is

with what we measure and from this we can infer that maybe there is we see

many more photons than we expect and that would mean that there is something like an acci on decreasing the the

absorption of this wall the leakiness of this wall okay so this is what we could

do before and with this method we are able to exclude this area here and this

is what we might be able to do in the future and this is actually a result I

wrote a paper about a couple of months ago so very proud to present it here so with

all of these searches and also documented searches we will be able to probe all of this at the moment okay so

what if we found it what if admx suddenly saw a peak like this in their

spectrum something like this and they can say with confidence we found the

acción it's dark matter has this in this mass well first of all this would be a

revolution for fundamental physics and we then understand why there is no

electric spin of the neutron it might tell us what dark matter is finally and it also has implications for

high energy physics and the reason is that the mere existence of the axiom

tells us that there is this oil substance I was talking about before there was this oil which made the marble

roll down and this might motivate more searches for this oil at high energies

that kaleidos for example but there's also more fun stuff you can do with an

action but beware what I'm gonna say now is rather speculative there's no

guarantee that this work will work ever but what you can do is you can use the

action as a communication channel so for example you're sending one side of earth

and you want to communicate with your friend who's on the other side or usually have to do you have to send a

photon around Earth with the axiom you can send a message right through earth

and that's much better you have 50% faster in communicating you can also use

it to do physical encryption so let's say you're here you have a you want to

deliver a message to your friend who's on a spaceship somewhere in orbit for example but there might be somebody is

sitting in between trying to read your messages and if you send the photon well

then they might pick it up and try to read what you were saying with the

absence it might be a bit harder because if you send the action then you have to

know at what frequency the axiom was sent to be able to really measure it and the reason is that it's really it takes

a lot of time to scan all the different frequencies to finally find a peak like this that means if you tell your friend

before he left Earth that I'm going to send my messengers at this particular frequency then he or she will be able to

tune the action detector exactly to that frequency and will be rather easy for them to detect your message somebody

else who does not know at what frequency you sent your message well he has to

scan through all of this to actually find that you were communicating

okay even more speculative is excellent space travel so if you're in your

spaceship and you have all these acts and documented flying around then you might as well use it right so for

example if you had a cavity like this then you might hope that some of these actions enter your spaceship enter your

cavity get convert into a photon and you can pick up this energy from the acción

and use it to power your lightbulbs or whatever you need in the spaceship you

can also do Xen sailing so there is a flex affections coming in you convert

them into a photon they bounce off a mirror and give a thrust to your spaceship so that now you have a way how

to do yeah sailing on this axiom wind alright so

we've come a long way from this picture of the night sky where we talked about

stars and about how empty space really is you've learned that there's hydrogen that there are photons inside there as

well and that there's also dark matter and this dark matter well that might be

axioms so the next time you go outside and look into the sky I hope that you do

not only see this but you might actually see the ghost riders in this guy thank

you [Applause]

we have time for some questions um the for those of you who haven't been

here before the process is the following you all have these microphones in front of you in the center at the bottom

there's a red button only one person can have their microphone on at the time at

this at a time so get recognized when you recognize press the red button then

you can be heard and then when you're done turn off your microphone and the

next person can have a question so who'd like to ask a question please so your

your hylia scope and other detector methods the earth has a magnetic field

so what do you do about that Sam the earth itself has a magnetic

fields are passing you're expecting the acción to pass through a magnetic field to be converted to a photon in it they

can 100 megahertz area and but the earth has a magnetic field so write them you

can estimate how big the conversion rate is of actions in the magnetic field of the earth it's not that large the the

magnets are much more powerful orders of magnitude and yeah this this magnitude

of the magnetic field is what dominates the conversion probability yeah

something like that yeah

this is black magazine symmetry announced that there was a galaxy found

a few skelux see that has perfectly no dark matter in it does that affect how

does that fit into the notion of the acción yeah I haven't read the paper yet

I guess it's rather puzzling that there's no document but in this case it might be that there are certain patches

in the sky or certain regions where there is no X Y undocumented for example so if you think about axioms it might be

that there are certain areas where there's more and where there's less of it so it might be that they these

galaxies formed without dogma - but yeah

I haven't haven't thought deeply about the paper yet

do you think that at the time of recombination or before the time of

recombination 380,000 years after the Big Bang these actions were contained

within the volume or had they escaped already well so recombination that means

that's the point in time when the Big Bang happened and the photons were emitted and so you're you're saying that

these these actually just fly well until that time the microwave background was

capped those captives yes now what I'm asking is is it can you imagine whether the actions were

also captive so the accents are so ghost-like that they they would not be

captured like the photon no they would just be there as a background inside Oh everywhere everywhere it it

depends a bit some people would say there might be certain areas where there's less dark matter so let's

accion's others there's a bit more but in general they would just just be there

at that time and on average you have to get the right amount of dark matter in

your universe but it might vary from area to area did you say during the

lecture that the the axion was one millionth the mass of the electron that's about the maximum of the maximum

in the mass scale which you can currently have yes so that would make them Mak the action

and even more ethereal than the in the neutrino so you say that's massive

industry no no them the mass of the neutrino I've heard quoted as being about a thousandth of the mass of an

electron yes right so I mean they have three of them three of these neutrinos one might still have a mass of zero but

one will have a mass of thousands of electronvolt but the accent can be

anywhere in that range you can be heavier than the Centrino can be lighter than that one I see okay thank you

[Music] if they're that light then at the time

they formed they'd be hot and relativistic and if they're relativistic and no way to slow down then how would

they clump around matter because that's normally why do you want to wimp you know a weakly interacting massive

particle because you know in order for it to clump if you don't have a way to

radiate you can't clump how does that work so that's a fun thing about the

axiom although it's so light it does not create it like the wimba does not get in

thermal contact so it does not get heated like the wimp does when it gets

created it's in its lowest energy possible and that means it's really really slow when it gets created it

moves yeah well for hours James there's

still a large velocity but for particle sense have really low velocity so it is

cold dark matter it is cold when it gets created and it stays so it has to be in

thermal contact with it the thing that creates it the reason why gets created

not because it not through these collisions so it's not a gallon getting them a contact it gets created because

this this oil gets created in the early universe there's a phase transition where we suddenly are in in a vacuum

where you have this oil and then you have very slight excitations of this oil

that's the axiom just because well you

can also think it in the following way when your model falls down on a certain value then when the oil comes in to

exists in the marble rolls down and starts oscillating around zero it actually doesn't stop at zero it starts

like moving this but this is a very slow rotation this is oscillation this very slow

oscillation that's the acción and this is not getting produced from anywhere

but really low energy oscillations it's complicated I know but we can talk about

it later as well if you want let's take two more questions first one is you please um what do you think about the

possibility that Dark Matter consists of more than one type of particle that

might very well be

okay yeah in once in your theories of dark matter or was particularly with

accion's a would you predict a single type of particle or do you are you

thinking that maybe a whole family of accion's could possibly exist so yeah

that's a good question so what you want is at least one action to solve the

nutrients electric spin problem but there are a couple of theories which predicts a whole family of these and

actually the white space I was showing you before on this map with a favorite region the white space would be probably

more filled with these family of vaccines instead of the accidents now you might debate about if these

families really exists but there's some theories like string theory which would predict that there's these kind of axons

and a whole family of them



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