Lecture 10: Cosmology & Particle Physics Flashcards
We’re looking at high energies and temperatures things which are relevant on
the sub-atomic scale
Electromagnetism, the weak, the strong and gravity are distinct today but the strength of each
scales differently with increasing energies
Increase the energies enough and the forces will
unify
this means they become of comparable strength
electroweak theory (EW)
the EM and weak forces unify at around 300GeV or 10^15K
Quantum chromodynamics (QCD)
a description of strong nuclear interactions
standard model
includes both EW and QCD
grand unified theory (GUT)
would require 10^15 GeV but current particle accelerators are not powerful enough to test this
Theory of everything (TOE)
a theory of quantum gravity would require 10^19 GeV but we would also need testable predictions to validate it
The basic difficulty in cosmology is to do with
mass
In order to explain cosmological observations (i.e.: the universe appearing flat) we need
more mass than we currently observe
the only classes of elementary particle known experimentally
- matter fermions
- bosons
matter fermions
leptons and quarks
both of which are spin 1/2 particles
bosons
including gauge bosons, which are spin 1 particles and are the force carriers in the standard model
elementary particles
something effectively point-like with no size or structure
EM radiation is quantised and can be polarised so the photon has
two polarisation or spin states
effective number of degrees of freedom for particles
geff = nspin x nanti x npauli
nspin
either spin up or spin down for photons
nanti
tells you if the particle has an anti particle
if yes set it to two
else set it to one
npauli
tells you if the particle obeys the Pauli exclusion principle, integrate over the Fermi-Dirac distribution to get it if true.
spin essentially refers to the
quanta of angular momentum
for photons, nanti=
1
light is its own anti-particle
for electrons, nanti=
2 (electron has the positron)
photons are bosons so have npauli=
1
fermions have npauli=
7/8
s=1/2 and ms=+/- 1/2 for
electrons and fermions
s=0,1 for
bosons
- It turned out that the reason we were not detecting enough neutrinos was
that we were essentially digging in the wrong place
we did not see neutrinos until the discovery that
neutrinos oscillated between type with a small mass difference of 10^-2eV
If neutrinos were highly relativistic at decoupling they would be
hot dark matter
Massive neutrinos would therefore not clump together on small scales (galaxies) and structure
would
become washed out
does not match observations of the clustering of galaxies
we believe that non-baryonic matter is
cold and does not consist of neutrinos
Sakharov Conditions for Baryogenesis
baryon number violation
cp violation
departure from thermal equilibrium
baryon number violation
requires the proton to decay
cp violation
requires reaction rates for particles and anti particles to be different
departure from thermal equilibrium
departure from thermal equilibrium permits reactions
to proceed preferentially in one direction.
There has been some limited evidence of both baryon number and CP violation at the LHCb
experiment
but observations of baryon density in the universe cannot currently be explained: we need a new theory.
