A Hemispherical Power Asymmetry from Inflation

TL;DR

The paper investigates the hemispherical power asymmetry observed in the CMB and tests whether it can originate from inflationary physics. It shows that a single-field slow-roll inflaton cannot generate the asymmetry without violating homogeneity, with an upper limit $A_{\mathrm{max}} \approx 9.5\times 10^{-3}$; a curvaton model with a subdominant field and a large-scale superhorizon perturbation can generate $A \approx 0.2$ while respecting quadrupole and non-Gaussianity constraints via $f_{\mathrm{NL}} \simeq \frac{5\xi^2}{4R}$. The curvaton mechanism yields a viable region in $(R,\xi)$ space and predicts a potentially measurable non-Gaussianity ($f_{\mathrm{NL}} \gtrsim 50$ for $A \simeq 0.2$) and a modified inflationary consistency relation $r \to r(1-\xi)$ with $n_s = 1 - 2\epsilon - (1-\xi)(4\epsilon - 2\eta)$, as well as possible isocurvature signatures. If verified, this would imply a richer multi-field inflationary dynamics and a modulated primordial power across the sky.

Abstract

Measurements of CMB temperature fluctuations by the Wilkinson Microwave Anisotropy Probe (WMAP) indicate that the fluctuation amplitude in one half of the sky differs from the amplitude in the other half. We show that such an asymmetry cannot be generated during single-field slow-roll inflation without violating constraints to the homogeneity of the Universe. In contrast, a multi-field inflationary theory, the curvaton model, can produce this power asymmetry without violating the homogeneity constraint. The mechanism requires the introduction of a large-amplitude superhorizon perturbation to the curvaton field, possibly a pre-inflationary remnant or a superhorizon curvaton-web structure. The model makes several predictions, including non-Gaussianity and modifications to the inflationary consistency relation, that will be tested with forthcoming CMB experiments.

Figures (2)

• Figure 1: Measurements of temperature fluctuations in the cosmic microwave background (CMB) show that the rms temperature-fluctuation amplitude is larger in one side of the sky than in the other. We investigate here whether this may arise as a consequence of a large-scale mode of an inflaton or curvaton.
• Figure 2: The $R$-$\xi$ parameter space for the curvaton model that produces a power asymmetry $A=0.2$ (top) and $A=0.05$ (bottom). Here $R$ is the fraction of the cosmological density due to curvaton decay, and $\xi$ is the fraction of the power due to the curvaton. The upper limit to $R$ comes from the CMB-quadrupole constraint. The lower bound comes from $f_{\mathrm NL}\leq100$. The lower limit to $\xi$ comes from the requirement that the fractional change in the curvaton field across the observable Universe be less than one. If $A$ is lowered, the lower bound to $R$ remains unchanged, but the upper bound increases, proportional to $A^{-2}$. The lower limit to $\xi$ also decreases as $A$ decreases, proportional to $A$.