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arXiv · 2509.24712

CMB Hemispherical Power Asymmetry from Early Phase of Inflation

Akash Gandhi·Mohit Panwar·Pankaj Jain
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Abstract

We investigate the hemispherical power asymmetry observed in the CMBR by attributing it to an early inhomogeneous phase of cosmic expansion. Unlike the conventional assumption of a perfectly isotropic and homogeneous pre-inflationary Universe, we introduce a small inhomogeneous perturbation, treated within a perturbative framework. Our analysis builds on previously developed empirical models of inhomogeneous primordial power spectrum models based on dipole modulation. Using in-in formalism, we compute two-point correlations directly from the metric and demonstrate that, at leading order, this introduces a direction-dependent power spectrum that breaks rotational symmetry and naturally selects a preferred direction, relating observed violation of the cosmological principle to inflationary power spectra arising from scalar field fluctuations. Additionally, we find that this framework produces correlations between multipoles separated by $Δl=1$, leading to distinctive signatures in the multipole space. Furthermore, we constrain the parameters of the inhomogeneous perturbation using observed PR4 \texttt{Commander} CMB data.

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Akash Gandhi, Mohit Panwar, Pankaj Jain. 2026-08-04. CMB Hemispherical Power Asymmetry from Early Phase of Inflation. https://doi.org/10.1088/1475-7516%2F2026%2F08%2F012

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