This document discusses non-Gaussianities in the cosmic microwave background from multifield inflation models. It summarizes that generating a large local form of non-Gaussianity (fNL) requires super-horizon evolution of the curvature perturbation after inflation, which can be achieved if the fields have isocurvature perturbations. However, multifield models are incomplete if they do not track the perturbations all the way to the final adiabatic post-inflationary state. The document outlines developing a toy model that captures the key physics of when adiabaticity is achieved to better understand the generation and freezing of fNL.

1. non-gaussianities
in multifield
inflation: the
approach to
adiabaticity and the
fate of fnl
navin sivanandam
[1011.4934 and 1104.5238 – joel meyers, ns]
a.i.m.s. – 21st november 2011

3. what is
inflation?

4. the universe is
the same in
every direction
but not quite

5. the same
exponential expansion
smooth
inhomogeneities
flatten universe
everything causally
connected

6. not quite
quantum fluctuations
stretched and frozen
seed structure growth

8. negative pressure
scalar field

9. slow roll
flat(ish) potential
ϕ

11. quantum fluctuations
2 + 1 degrees of
freedom
time dependent
background

12. gauge invariance
scalar ζ
fluctuations
constant outside
horizon

13. comoving
wavenumber
t

18. inflation is amazing
flatness and
horizons
monopoles
gaussian and scale
invariant

19. inflation is
(too?)
awesome

20. getting beyond
“too” awesome
experiments
tensor modes
non-gaussianities

21. gaussian?
2pt correlation function
is the whole story
temperature Ylm

22. non-gaussian?
anything else
model
dependent

23. bispectrum
fNL

24. experiment

25. shape = physics
triangles in k-space
fNL(local) (sleight of hand
warning)
small in single field models

26. models
curvaton
modulated
reheating
multifield mixing

27. superhorizon
evolution
adiabatic?

28. goal
completely
capture generating
fNL and freezing of
perturbations

29. outline
model
adiabaticity
extensions

30. model

31. 2 field = isocurvature
isocurvature = trouble

32. multifield
models are
incomplete!

33. track until adiabatic
after end
thermal eq’m
1 fluid
before end
1 field phase

34. multifield perturb
theory hard
time-dependent
pdes
cheat – delta N

36. delta N switches
hard pde
problem into
easier ode one

37. observables

39. delta N

44. local,
large fNL but
isocurvature
continual
superhorizon
evolution

45. make isocurvature
heavy

47. dynamic
quantity

48. summary

49. next?
higher pt
more fields ?
potentials?
adiabaticity?

50. during inflation
vs
after inflation

51. thermal eq’m
curvaton
modulated
reheating

52. find toy model
that captures
distinctions in
timing of
physics

53. summary
making perturbations
adiabatic kills fNL local
in several existing
models

54. how general?
local
fNL is tricky
multifield models
are incomplete