A phenomenological dipole anisotropy in primordial perturbations induces dipolar and quadrupolar anisotropies in SIGW energy density spectra, producing frequency-dependent PTA overlap reduction functions that depend on pulsar sky distribution, but NANOGrav 15-year data yields no significant evidence
Imprints of a Primordial Preferred Direction on the Microwave Background
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abstract
Rotational invariance is a well-established feature of low-energy physics. Violations of this symmetry must be extremely small today, but could have been larger in earlier epochs. In this paper we examine the consequences of a small breaking of rotational invariance during the inflationary era when the primordial density fluctuations were generated. Assuming that a fixed-norm vector picked out a preferred direction during the inflationary era, we explore the imprint it would leave on the cosmic microwave background anisotropy, and provide explicit formulas for the expected amplitudes $<a_{lm}a_{l'm'}^*>$ of the spherical-harmonic coefficients. We suggest that it is natural to expect that the imprint on the primordial power spectrum of a preferred spatial direction is approximately scale-invariant, and examine a simple model in which this is true.
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Forecasts show that future 21 cm surveys can deliver moderate constraints on the scale-dependent growth index and HI bias in viable f(R) models.
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Exploring the statistical anisotropy of primordial curvature perturbations with pulsar timing arrays
A phenomenological dipole anisotropy in primordial perturbations induces dipolar and quadrupolar anisotropies in SIGW energy density spectra, producing frequency-dependent PTA overlap reduction functions that depend on pulsar sky distribution, but NANOGrav 15-year data yields no significant evidence
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Constraining Scale-Dependent Growth in $f(R)$ Gravity with Future 21 cm Surveys
Forecasts show that future 21 cm surveys can deliver moderate constraints on the scale-dependent growth index and HI bias in viable f(R) models.