Constraints on Bianchi type-I spacetime extension of the standard $Lambda$CDM model. (arXiv:1905.06949v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Akarsu_O/0/1/0/all/0/1">Ozgur Akarsu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kumar_S/0/1/0/all/0/1">Suresh Kumar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sharma_S/0/1/0/all/0/1">Shivani Sharma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tedesco_L/0/1/0/all/0/1">Luigi Tedesco</a>
We consider the simplest anisotropic generalization, as a correction, to the
standard $Lambda$CDM model, by replacing the spatially flat Robertson-Walker
metric by the Bianchi type-I metric, which brings in a new term $Omega_{sigma
0}a^{-6}$ (mimicking the stiff fluid) in the average expansion rate $H(a)$ of
the Universe. From Hubble and Pantheon data, relevant to the late Universe
($zlesssim 2.4$), we obtain the constraint $Omega_{sigma0}lesssim10^{-3}$,
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.,
$Omega_{sigma0}lesssim10^{-15}$. 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
$Omega_{sigma0}lesssim10^{-23}$. 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 $sim11;mu rm K$.
We consider the simplest anisotropic generalization, as a correction, to the
standard $Lambda$CDM model, by replacing the spatially flat Robertson-Walker
metric by the Bianchi type-I metric, which brings in a new term $Omega_{sigma
0}a^{-6}$ (mimicking the stiff fluid) in the average expansion rate $H(a)$ of
the Universe. From Hubble and Pantheon data, relevant to the late Universe
($zlesssim 2.4$), we obtain the constraint $Omega_{sigma0}lesssim10^{-3}$,
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.,
$Omega_{sigma0}lesssim10^{-15}$. 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
$Omega_{sigma0}lesssim10^{-23}$. 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 $sim11;mu rm K$.
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