Infrared divergences during inflation

Takahiro Tanaka (YITP, Kyoto univ.)in collaboration with Yuko Urakawa （Barcelona univ.)

arXiv:1208.XXXX

PTP125:1067 arXiv:1009.2947,Phys.Rev.D82:121301 arXiv:1007.0468 PTP122: 779 arXiv:0902.3209 

PTP122:1207 arXiv:0904.4415 

Various IR issuesIR divergence coming from k-integral Secular growth in time (∝ Ht)n

Adiabatic perturbation, which can be locally absorbed by the choice of time slicing. Isocurvature perturbation ≈ field theory on a fixed curved backgroundTensor perturbation

Background trajectoryin field space

isocurvature

perturbation adiabatic

perturbation

f : a minimally coupled scalar field with a small mass (m2≪H2) in dS.

potential

summing up only long wavelength modes beyond the Horizon scale

distribution

2

432

0 3

232

2

2

mH

aHk

kHkd

Hm

aHreg

f

m2⇒0Large vacuum fluctuation

IR problem for isocurvature perturbation

If the field fluctuation is too large, it is easy to imagine that a naïve perturbative analysis will break down once interaction is introduced.

De Sitter inv. vac. state does not exist in the massless limit. Allen & Folacci(1987)

Kirsten & Garriga(1993)

Stochastic interpretation

aH ikekd

0

3 kxff

Let’s consider local average of f :

Equation of motion for f :

Hf

HV

dNd

23ff

More and more short wavelength modes participate in f as time

goes on.

in slow roll approximation

Newly participating modes act as random fluctuation

32 kHkk ff

NNHNfNf 4

In the case of massless lf4 : f 2 → l

2H

Namely, in the end, thermal equilibrium is realized : V ≈ T

4

(Starobinsky & Yokoyama (1994))

Wave function of the universe~parallel universes

Distant universe is quite different from ours.

Each small region in the above picture gives one representation of many parallel universes. However: wave function of the universe = “a superposition of all the possible parallel universes” Question is “simple expectation values are really observables

for us?”

Our observable universe

must be so to keep translational invariance of the wave fn. of the universe

Answer will be No!

“Are simple expectation values really observables for us?”

Decoherence of the wave function of the universe must be taken into

account

Decoherence

Cosmic expansion

Statistical ensemble

Various interactions

Superposition of wave packets

Correlated

f

Before After f Un-correlated

cbacba ccbbaa

Coarse grainingUnseen d.o.f.

Our classical observation picks up one of the

decohered wave packets. How can we evaluate the actual observables?

f

f

|a ＞ |b ＞ |c ＞

§IR divergence in single field inflation

min3 /log kaHkPkdyy

Factor coming from this loop:

scale invariant spectrum 31 k

curvature perturbation in co-

moving gauge. - no typical mass scale

0f Transverse traceless

ijij he exp22

Setup: 4D Einstein gravity + minimally coupled scalar fieldBroadening of averaged field can be absorbed by the proper choice of time coordinate.

Gauge issue in single field inflation In conventional cosmological perturbation

theory, gauge is not completely fixed.

Yuko Urakawa and T.T., PTP122: 779 arXiv:0902.3209

Time slicing can be uniquely specified: f =0 OK!but spatial coordinates are not.

jji

jj hh ,0

ijjiijgh ,, Residual gauge:

Elliptic-type differential

equation for i.

Not unique locally!

To solve the equation for i, by imposing boundary condition at infinity, we need information about un-observable region.

i

observable region time

direction

Basic idea why we expect IR finiteness in single field inflation

The local spatial average of can be set to 0 identically by an appropriate gauge choice. Time-dependent scale transformation.

Even if we choose such a local gauge, the evolution equation for stays hyperbolic. So, the interaction vertices are localized inside the past light cone.

Therefore, IR divergence does not appear as long as we compute in this local gauge. But here we assumed that the initial quantum state is free from IR divergence.

Complete gauge fixing vs.Genuine gauge-invariant quantities

Local gauge conditions. i Imposing boundary

conditions on the boundary of the

observable region

But unsatisfactory?

The results depend on the

choice of boundary

conditions.Translation

invariance is lost.

Genuine coordinate-independent quantities.

No influence from outsideComplete gauge fixing☺

Correlation functions for 3-d scalar curvature on f =constant slice. R(x1) R(x2) Coordinates do not have gauge invariant meaning.

x origin

x(XA, l=1) =XA + xAx

Specify the position by solving geodesic eq. 022 ldxD i

ii XdDx 0l

l with initial condition

XA

gR(XA) := R(x(XA, l=1)) = R(XA) + xA R(XA) + …gR(X1) gR(X2) should be truly coordinate independent.

(Giddings & Sloth 1005.1056)(Byrnes et al. 1005.33307)Use of geodesic coordinates:

02 21 Ie x

In f =0 gauge, EOM is very simple

IR regularity may require

Non-linearity is concentrated on this term.

03 22

2 ett

Formal solution in IR limit can be obtained as

III e 212

22

2 3 ett with L-1 being the formal inverse of .

IIIg eeR

xx212 24

Only relevant terms in the IR limit were kept.

Hdd log2

2

2

121

1212

21 22 xxxxxx xx IIIgg eeRR

IR divergent factor∋

Extra requirement for IR regularity

Then, is impossible,

However, L-1 should be defined for each Fourier component. tfekdtf k

-k

i ~, 131 xkx

Because for I ≡ d

3k (eikx vk(t) ak + h.c.),

for arbitrary function f (t,x)

kxkxkx aei i

I while

k

xk aee iI

21

with 222

2 3 kettk

02 21 IIe x

02 21 Ie x

IR regularity may require

With this choice, IR divergence disappears.

..2

212 23221 chtveDakde i

kI kkx

kxx

・ extension to the higher order:

)(272log

24

2121log XXkXX i

kkIgg evkkdRR

IR divergent factor total derivative

..2 321 chtveDakde ikI k

kxkx

kkkk vDvek 2122

Instead, one can impose

, , which reduces to conditions on the mode functions.

kk ff iik ek

kddekD 2/32/3

logwith

What is the physical meaning of these conditions?

2222 xdedtds Background gauge:

int0 HHH •Quadratic part in and s is identical to s = 0 case. •Interaction Hamiltonian is obtained just by replacing 　　 the argument with s.

kkkk vDvek 2122 In addition to considering gR, we need additional conditions

Physical meaning of IR regularity condition

and its higher order extension.

22222 ~~ xdedtsd s xx se~

sHHH ~~~int0

xx ~~

~

~

Therefore, one can use 1) common mode functions for I and I

~

II sII ~~~

I ≡ d

3k (eikx vk(t) ak + h.c.) I ≡ d

3k (eikx vk(t) ak + h.c.)

~ ~

2) common iteration scheme.

However, the Euclidean vacuum state (h0 →±i ∞ ) (should) satisfies this condition. (Proof will be given in our new paper (still incomplete!))

It looks quite non-trivial to find consistent IR regular states.

kkkk vDvek 2122

Dkvk(h0)=0 : incompatible with the normalization condition.

We may require 0~~~~~~~0~00 2121 nn xxxxxx

the previous condition compatible with Fourier decomposition

Retarded integral with (h0)=I(h0) guarantees the commutation relation of

Euclidean vacuum and its excited states (should) satisfy the IR regular condition.

We obtained the conditions for the absence of IR divergences.

“Wave function must be homogeneous in the direction of background scale transformation”

Summary

It requires further investigation whether there are other (non-trivial and natural) quantum states compatible with the IR regularity.