My Ph.D
The main aim of my Ph.D thesis is to measure the scattering cross-section of dark matter.
What is a cross-section?!
The cross-section of something is the probability of object doing something. For example the annihilation cross-section of a particle is the probability that a particle (e.g. an electron, neutron, proton, quarks, positrons etc.) when coming close to another particle will hit and destroy one another. The cross section I am looking at is the scattering cross section. In other words it is the probability that the dark matter particle will hit another dark matter particle and bounce off. This probability is related to the size of the particle, and also other things such as the velocity of the particle and the mass of the particle. Using the interaction cross-section of dark matter we can infer what the particle actually is!!
What is dark matter?!
Black holes are NOT dark matter ( although some claim they can explain the astronomical observations that lead to the idea of dark matter ). Dark matter is basically a theoretical particle, like a proton or an electron, that exists in our Universe interacting mainly with Gravity and but also very weakly with the weak force.
We know that protons, electrons and other particles exist because they interact with photons, in that they can absorb and re-emit them. This means that we can see them, however dark matter does not interact with photons and therefore we can not see it.! !
If we cant see it how do we know it exists?!
We think we understand gravity pretty well. It has survived some of our toughest tests, yet on larger scales we find funny things. If we look at galaxies we notice that the gas and stars are orbiting around the centre faster than they should be (according to our laws of
gravity). The only way we can explain this is if we either change our laws ( and Einstein was wrong) or there is more matter than we can see, i.e some dark matter.
Explanations of the strange behaviour using modifications to gravity have all but been discounted since a group of astronomers discovered the 'Bullet Cluster'. A cluster, is a huge structure containing thousands of galaxies all orbiting a huge central galaxy, a little like electrons orbiting a central nucleus. The clusters can be up to 10,000 times the size of our galaxy. These clusters not only contain thousands of galaxies, but the space in between the orbiting galaxies is a sea of super hot gas and dark matter.
The 'Bullet Cluster', is not one, but two of these clusters that have collided and smashed into one another. We are fortunate enough to have observed this cluster car crash just after it has happened.
Interestingly enough, as the gas of the two clusters hit each, one passed through the other, on of the gas 'halos' slowed down. Its bit like, if you jumped in into a swimming pool and someone threw a lead weight in next to you, you would slow down whereas the lead weight would drop to the bottom of the pool only slowing a little bit.
Now imagine this swimming pool is sligthly raised, so its more like a very large bath tub. I jump in the swimming pool, someone throws in the lead weight and some drops a stone next to the bath tub so its falling through air. If we take a photo and measure the distance we all are above the ground, the stone outside the bath tub will be closest to the ground, the lead weight second then i will be closest to the top of the bath tub. This is analagous to what happened with the bullet cluster.
When the clusters hit, the gas (me in this scenario) of one cluster slowed down dramatically. The dark matter (the lead weight), slowed a little bit, but not much, and the galaxies of the cluster continued on as if nothing happened (the stone outside the tub). This meant that the gas, dark matter and galaxies of one of the clusters all separated, due to their cross-section.
The properties of the lead weight and me will change the separation of us when we hit the water, thus if we measure the separations we can infer properties of the lead weight, or in this case the dark matter!
The main aim of my Ph.D thesis is to measure the scattering cross-section of dark matter.
What is a cross-section?!
The cross-section of something is the probability of object doing something. For example the annihilation cross-section of a particle is the probability that a particle (e.g. an electron, neutron, proton, quarks, positrons etc.) when coming close to another particle will hit and destroy one another. The cross section I am looking at is the scattering cross section. In other words it is the probability that the dark matter particle will hit another dark matter particle and bounce off. This probability is related to the size of the particle, and also other things such as the velocity of the particle and the mass of the particle. Using the interaction cross-section of dark matter we can infer what the particle actually is!!
What is dark matter?!
Black holes are NOT dark matter ( although some claim they can explain the astronomical observations that lead to the idea of dark matter ). Dark matter is basically a theoretical particle, like a proton or an electron, that exists in our Universe interacting mainly with Gravity and but also very weakly with the weak force.
We know that protons, electrons and other particles exist because they interact with photons, in that they can absorb and re-emit them. This means that we can see them, however dark matter does not interact with photons and therefore we can not see it.! !
If we cant see it how do we know it exists?!
We think we understand gravity pretty well. It has survived some of our toughest tests, yet on larger scales we find funny things. If we look at galaxies we notice that the gas and stars are orbiting around the centre faster than they should be (according to our laws of
gravity). The only way we can explain this is if we either change our laws ( and Einstein was wrong) or there is more matter than we can see, i.e some dark matter.
Explanations of the strange behaviour using modifications to gravity have all but been discounted since a group of astronomers discovered the 'Bullet Cluster'. A cluster, is a huge structure containing thousands of galaxies all orbiting a huge central galaxy, a little like electrons orbiting a central nucleus. The clusters can be up to 10,000 times the size of our galaxy. These clusters not only contain thousands of galaxies, but the space in between the orbiting galaxies is a sea of super hot gas and dark matter.
The 'Bullet Cluster', is not one, but two of these clusters that have collided and smashed into one another. We are fortunate enough to have observed this cluster car crash just after it has happened.
Interestingly enough, as the gas of the two clusters hit each, one passed through the other, on of the gas 'halos' slowed down. Its bit like, if you jumped in into a swimming pool and someone threw a lead weight in next to you, you would slow down whereas the lead weight would drop to the bottom of the pool only slowing a little bit.
Now imagine this swimming pool is sligthly raised, so its more like a very large bath tub. I jump in the swimming pool, someone throws in the lead weight and some drops a stone next to the bath tub so its falling through air. If we take a photo and measure the distance we all are above the ground, the stone outside the bath tub will be closest to the ground, the lead weight second then i will be closest to the top of the bath tub. This is analagous to what happened with the bullet cluster.
When the clusters hit, the gas (me in this scenario) of one cluster slowed down dramatically. The dark matter (the lead weight), slowed a little bit, but not much, and the galaxies of the cluster continued on as if nothing happened (the stone outside the tub). This meant that the gas, dark matter and galaxies of one of the clusters all separated, due to their cross-section.
The properties of the lead weight and me will change the separation of us when we hit the water, thus if we measure the separations we can infer properties of the lead weight, or in this case the dark matter!