Lecture 7

Question 1

The CMBR originated at a point when the universe was in

Answer 1

thermal equilibrium

Question 2

photons interact very strongly with electrons and so have a

Answer 2

very short mean free path

Question 3

photons mean free path increases until

Answer 3

it matches the size of the universe

Question 4

a consequence of the photons mean free path matching the size of the universe is that the

Answer 4

photons can now travel unimpeded

Question 5

decoupling

Answer 5

photons travelling unimpeded : the photons decouple from matter

Question 6

CMBR is formed during

Answer 6

decoupling

Question 7

At what temperature T was the CMBR formed

Answer 7

we can make a estimate using the Saha equation

Tdec = 3000K at z = 1000

Question 8

Recombination refers to the period

Answer 8

during which electrons and protons first become bound as hydrogen nuclei

Question 9

Decoupling refers to the point

Answer 9

where photons fall out of thermal equilibrium with matter

Question 10

The surface of Last Scattering

Answer 10

the universe as it was just after it become cool enough for photons to travel without being scattered by electrons

Question 11

CMBR forms at

Answer 11

lower temperature than that defined by the epoch of matter-radiation equality

Question 12

the universe was already

Answer 12

matter-dominated when the CMBR formed

Question 13

the epochs of matter-radiation equality and decoupling occurred

Answer 13

very early on in the evolution of the universe and within a few hundred thousand years of each other

Question 14

For T > Teq the universe is

Answer 14

radiation dominated with

a(t) ∝ t^1/2

Question 15

formula for T

Answer 15

T = 1.4x10^10 K / √t

Question 16

formula for E(typical)

Answer 16

E(typical)(t) = 3 MeV/√t

Question 17

Nucleosynthesis

Answer 17

the typical nuclear binding energy is 1MeV so at this time the lightest nuclei can begin to form

Question 18

Strongly interacting plasma

Answer 18

The universe comprised of strongly interacting plasma of free electrons, protons, neutrons, photons and neutrinos

the neutrinos and other dark matter particles would have already decoupled from the photons at this time

Question 19

Quark-Hadron Phase Transition

Answer 19

the universe comprised a strongly interacting soup of quarks and leptons

during this era the quarks became confined which is why we refer to a quark-hadron phase transition

Question 20

We only have indirect evidence

Answer 20

of the behaviour of fundamental particles

Question 21

the Planck era

Answer 21

need quantum theory of gravity

we believe that quantum fluctuations are the likely origin of the CMBR temperature variations