Updated Constraints on Infrared Cutoff Models and Implications for Large-Scale CMB Anomalies

TL;DR

This study revisits various IR cut-off models in the primordial power spectrum, constrains them with current cosmological data, and finds no significant evidence favoring these models over the standard spectrum, while exploring their impact on large-scale anomalies.

Contribution

The paper provides updated constraints on IR cut-off models using recent cosmological data and analyzes their implications for large-scale CMB anomalies and the reionisation optical depth.

Findings

IR cut-off models do not significantly improve fit over standard spectrum

Current data show no strong evidence for IR cut-offs

Large-scale suppression models are insufficient to increase reionisation optical depth

Abstract

The nearly scale-invariant primordial power spectrum provides the standard initial conditions for cosmological perturbations. However, the largest scales remain only weakly constrained by CMB observations, leaving room for deviations such as an infrared (IR) cut-off. This possibility is further motivated by the persistence of large-scale CMB anomalies, most notably the low quadrupole power. In this work, we revisit several broad classes of phenomenologically motivated IR cut-off scenarios using parametrised functional forms of the primordial power spectrum. We confront these models with the latest CMB, BAO, and supernova data and derive updated constraints on the cut-off scale and associated features. Our results remain consistent with earlier studies, showing that although such models suppress power at low multipoles, the improvement in fit is marginal and does not overcome the associated parameter penalties. We therefore find no statistically significant evidence favouring IR cut-off models over the standard power-law spectrum with current data. We further explore the interplay between IR cut-off features and a possible increase in the reionisation optical depth, motivated by the recent CMB-BAO tension highlighted by DESI DR2 within the $\Lambda$CDM framework. We show that the additional freedom introduced by large-scale suppression is generally insufficient to support a substantial increase in optical depth, owing to the weak statistical preference for suppressed large-scale temperature power. Finally, we examine the implications of IR cut-off models for large-scale CMB anomalies by analysing the corresponding anomaly statistics within a Bayesian framework.