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5 Cosmology and General Relativity

The interested reader can find a detailed derivation of the above formula and a fullfledged discussion of its consequences and applications in Chap. 8 of the recent book [9] on the Physical Foundations of Cosmology by Mukhanov, who is one of the main actors in the new season of Theoretical Cosmology opened by the observation of the CMB anisotropies. Here we confine ourselves to sketch the logical connection between the power spectrum of primordial fluctuations and the correlation function of observed anisotropies.

As we already stressed, the main point is the Sachs Wolfe effect (5.10.26) which implies:

δT

0, x0, n) =

 

δT

ηr , xr ), n

+ Φ ηr

, xr ) Φ ηr , xr )

(5.10.31)

 

 

 

 

 

 

 

T0

 

T0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

grav. pot. at recom

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

fluc. today

 

 

 

 

fluc. at ricom

 

 

Here x(η) denotes the geodesic followed by a photon that arrives today into our measuring instrument from a direction n and was emitted at conformal time ηr from the Last scattering Surface. Such a geodesic is the straight line:

x(η) = x0 + nη0)

(5.10.32)

An elaboration of formula (5.10.31), which we skip, allows to encode into two functions δ and δ the contribution of the primordial temperature fluctuations

δT

 

ηr , xr ), n

T0

leading to the following Fourier decomposition of the temperature fluctuations at the present time:

δT

0, x0, n) =

d3k

Φ +

 

δ

 

3δk

 

 

exp ik ·

x0

+ nr η0)

T

0

(2π )3/2

4

4κ2 ∂η

0

 

 

 

 

 

 

k

 

 

ηr

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(5.10.33)

where the suffix k means that of the corresponding expression one has taken the kth Fourier component. Inserted into the correlation function (5.10.27), the above decomposition of the temperature fluctuation field yields the result (5.10.30) upon use of the standard multipole expansion formula:

 

sin|n1η1 n2η2|)

(2

 

1)j

(κη

 

)j

(κη

 

)P (cos θ )

(5.10.34)

 

=

+

1

2

 

κ

n1η1

n2

η2

|

 

 

 

 

 

 

 

|

 

 

 

=0

 

 

 

 

 

 

 

 

 

 

5.10.3 Conclusive Remarks on CMB Anisotropies

Without the ambition of presenting too much detailed derivations, which would be out of the scope of the present Course in General Relativity, we have tried to outline

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