Lecture 7 Flashcards
The CMBR originated at a point when the universe was in
thermal equilibrium
photons interact very strongly with electrons and so have a
very short mean free path
photons mean free path increases until
it matches the size of the universe
a consequence of the photons mean free path matching the size of the universe is that the
photons can now travel unimpeded
decoupling
photons travelling unimpeded : the photons decouple from matter
CMBR is formed during
decoupling
At what temperature T was the CMBR formed
we can make a estimate using the Saha equation
Tdec = 3000K at z = 1000
Recombination refers to the period
during which electrons and protons first become bound as hydrogen nuclei
Decoupling refers to the point
where photons fall out of thermal equilibrium with matter
The surface of Last Scattering
the universe as it was just after it become cool enough for photons to travel without being scattered by electrons
CMBR forms at
lower temperature than that defined by the epoch of matter-radiation equality
the universe was already
matter-dominated when the CMBR formed
the epochs of matter-radiation equality and decoupling occurred
very early on in the evolution of the universe and within a few hundred thousand years of each other
For T > Teq the universe is
radiation dominated with
a(t) ∝ t^1/2
formula for T
T = 1.4x10^10 K / √t
formula for E(typical)
E(typical)(t) = 3 MeV/√t
Nucleosynthesis
the typical nuclear binding energy is 1MeV so at this time the lightest nuclei can begin to form
Strongly interacting plasma
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
Quark-Hadron Phase Transition
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
We only have indirect evidence
of the behaviour of fundamental particles
the Planck era
need quantum theory of gravity
we believe that quantum fluctuations are the likely origin of the CMBR temperature variations
