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S =<br />

⇤<br />

d 4 x ⇥<br />

g<br />

⇥<br />

R<br />

16 G + X V (⇥)<br />

Scalar perturbations <br />

Tensor perturbations <br />

ds 2 =<br />

(1 + 2A)dt 2 +2 i Bdtdx i + a 2 (t) [(1 + 2R) ij + h ij ] dx i dx j<br />

Scalar <br />

S (2)<br />

s = dtd 3 xa 3 Q s Ṙ 2 c 2 s<br />

a 2 ( R)2<br />

For standard potential-­‐driven inflation <br />

Q s =<br />

˙2<br />

c 2<br />

2H 2 , s =1<br />

The amplitude of curvature perturbations <br />

can be estimated by solving <strong>the</strong> equation <br />

of motion which follows from <strong>the</strong> above <br />

action. <br />

<br />

<br />

<br />

<br />

The conditions for <strong>the</strong> avoidance <br />

of ghosts and Laplacian instabilities <br />

Q s > 0 c 2 s > 0<br />

,

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