Zusammenfassung
We consider the simplest anisotropic generalization, as a correction, to the
standard $Łambda$CDM model, by replacing the spatially flat Robertson-Walker
metric by the Bianchi type-I metric, which brings in a new term $Ømega_\sigma
0a^-6$ (mimicking the stiff fluid) in the average expansion rate $H(a)$ of
the Universe. From Hubble and Pantheon data, relevant to the late Universe
($z2.4$), we obtain the constraint $Ømega_\sigma0łesssim10^-3$,
in line with the model independent constraints. When the baryonic acoustic
oscillations and cosmic microwave background (CMB) data are included, the
constraint improves by 12 orders of magnitude, i.e.,
$Ømega_\sigma0łesssim10^-15$. We find that this constraint could alter
neither the matter-radiation equality redshift nor the peak of the matter
perturbations. Demanding that the expansion anisotropy has no significant
effect on the standard Big Bang Nucleosynthesis (BBN), we find the constraint
$Ømega_\sigma0łesssim10^-23$. We show explicitly that the constraint from
BBN renders the expansion anisotropy irrelevant to make a detectable change in
the CMB quadrupole temperature, whereas the constraint from the cosmological
data provides the temperature change up to $\sim11\;K$.
Nutzer