Lecture 10: Cosmology & Particle Physics

Question 1

We’re looking at high energies and temperatures things which are relevant on

Answer 1

the sub-atomic scale

Question 2

Electromagnetism, the weak, the strong and gravity are distinct today but the strength of each

Answer 2

scales differently with increasing energies

Question 3

Increase the energies enough and the forces will

Answer 3

unify

this means they become of comparable strength

Question 4

electroweak theory (EW)

Answer 4

the EM and weak forces unify at around 300GeV or 10^15K

Question 5

Quantum chromodynamics (QCD)

Answer 5

a description of strong nuclear interactions

Question 6

standard model

Answer 6

includes both EW and QCD

Question 7

grand unified theory (GUT)

Answer 7

would require 10^15 GeV but current particle accelerators are not powerful enough to test this

Question 8

Theory of everything (TOE)

Answer 8

a theory of quantum gravity would require 10^19 GeV but we would also need testable predictions to validate it

Question 9

The basic difficulty in cosmology is to do with

Answer 9

mass

Question 10

In order to explain cosmological observations (i.e.: the universe appearing flat) we need

Answer 10

more mass than we currently observe

Question 11

the only classes of elementary particle known experimentally

Answer 11

1. matter fermions
2. bosons

Question 12

matter fermions

Answer 12

leptons and quarks
both of which are spin 1/2 particles

Question 13

bosons

Answer 13

including gauge bosons, which are spin 1 particles and are the force carriers in the standard model

Question 14

elementary particles

Answer 14

something effectively point-like with no size or structure

Question 15

EM radiation is quantised and can be polarised so the photon has

Answer 15

two polarisation or spin states

Question 16

effective number of degrees of freedom for particles

Answer 16

geff = nspin x nanti x npauli

Question 17

nspin

Answer 17

either spin up or spin down for photons

Question 18

nanti

Answer 18

tells you if the particle has an anti particle

if yes set it to two
else set it to one

Question 19

npauli

Answer 19

tells you if the particle obeys the Pauli exclusion principle, integrate over the Fermi-Dirac distribution to get it if true.

Question 20

spin essentially refers to the

Answer 20

quanta of angular momentum

Question 21

for photons, nanti=

Answer 21

1
light is its own anti-particle

Question 22

for electrons, nanti=

Answer 22

2 (electron has the positron)

Question 23

photons are bosons so have npauli=

Answer 23

1

Question 24

fermions have npauli=

Answer 24

7/8

Question 25

s=1/2 and ms=+/- 1/2 for

Answer 25

electrons and fermions

Question 26

s=0,1 for

Answer 26

bosons

Question 27

* It turned out that the reason we were not detecting enough neutrinos was

Answer 27

that we were essentially digging in the wrong place

Question 28

we did not see neutrinos until the discovery that

Answer 28

neutrinos oscillated between type with a small mass difference of 10^-2eV

Question 29

If neutrinos were highly relativistic at decoupling they would be

Answer 29

hot dark matter

Question 30

Massive neutrinos would therefore not clump together on small scales (galaxies) and structure would

Answer 30

become washed out **does not match observations of the clustering of galaxies**

Question 31

we believe that non-baryonic matter is

Answer 31

cold and does not consist of neutrinos

Question 32

Sakharov Conditions for Baryogenesis

Answer 32

baryon number violation cp violation departure from thermal equilibrium

Question 33

baryon number violation

Answer 33

requires the proton to decay

Question 34

cp violation

Answer 34

requires reaction rates for particles and anti particles to be different

Question 35

departure from thermal equilibrium

Answer 35

departure from thermal equilibrium permits reactions to proceed preferentially in one direction.

Question 36

There has been some limited evidence of both baryon number and CP violation at the LHCb experiment

Answer 36

but observations of baryon density in the universe cannot currently be explained: we need a new theory.