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

Ujjwal Upadhyay, Yashi Tiwari, Tarun Souradeep

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 $Λ$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.

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

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 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.
Paper Structure (13 sections, 4 equations, 15 figures, 6 tables)

This paper contains 13 sections, 4 equations, 15 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Infrared Cut-off Models
  3. Data and Methodology
  4. Datasets
  5. Methodology
  6. MCMC Analysis and Model Comparison
  7. Sampling CMB Anomaly Statistics
  8. Results
  9. Current Status of IR Cutoff Models
  10. IR Cutoff Models and CMB-BAO Discrepancy after DESI DR2
  11. IR Cutoff Models and CMB anomalies
  12. Conclusion and Discussion
  13. Constraints on the Full Parameter Space of IR cutoff models

Figures (15)

  • Figure 1: Primordial power spectra for the different IR cutoff scenarios discussed in Sec. \ref{['sec:second']}, shown alongside the reference power-law spectrum (black dashed) for comparison. The vertical line marks the scale $k_h = 2.25 \times 10^{-4}\,\mathrm{Mpc}^{-1}$, corresponding to the horizon scale of the best-fit $\Lambda$CDM model.
  • Figure 2: Constraints on IR-cutoff models using the full CMB dataset and the likelihood combinations listed in Table \ref{['tab:datasets']}. The standard $\Lambda$CDM model with a power-law primordial spectrum is also shown for comparison.
  • Figure 3: Constraints on IR cutoff parameter $k_c$ along with $A_s$ and $\tau_\text{reio}$ using the full CMB dataset and the likelihood combinations listed in Table 1. The standard $\Lambda$CDM model with a power-law primordial spectrum is also shown for comparison.
  • Figure 4:
  • Figure 5:
  • ...and 10 more figures