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Unraveling the CMB lack-of-correlation anomaly with the cosmological gravitational wave background

29 May 2023arXiv:2305.18184links table onlyarchive 2025-07-28

Giacomo Galloni, Mario Ballardini, Nicola Bartolo, Alessandro Gruppuso, Luca Pagano, Angelo Ricciardone

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Since the very first observations, the Cosmic Microwave Background (CMB) has revealed on large-scales unexpected features known as anomalies, which challenge the standard Λ cold dark matter (ΛCDM) cosmological model. One such anomaly is the "lack-of-correlation", where the measured two-point angular correlation function of CMB temperature anisotropies is compatible with zero, differently from the predictions of the standard model. This anomaly could indicate a deviation from the standard model, unknown systematics, or simply a rare realization of the model itself. In this study, we explore the possibility that the lack-of-correlation anomaly is a consequence of living in a rare realization of the standard model, by leveraging the potential information provided by the cosmological gravitational wave background (CGWB) detectable by future gravitational wave (GW) interferometers. We analyze both constrained and unconstrained realizations of the CGWB to investigate the extent of information that GWs can offer. To quantify the impact of the CGWB on the lack-of-correlation anomaly, we employ established estimators and introduce a new estimator that addresses the "look-elsewhere" effect. Additionally, we consider three different maximum multipoles, denoted as ℓₘₐₓ, to account for the anticipated capabilities of future GW detectors (ℓₘₐₓ = 4, 6, 10). Summarizing our findings for the case of ℓₘₐₓ = 4, we identify the angular range [63° - 180°] as the region where future observations of the CGWB maximize the probability of rejecting the standard model. Furthermore, we calculate the expected significance of this observation, demonstrating that 98.81% (81.67%) of the GW realizations enhance the current significance of the anomaly when considering the full-sky (masked) Planck SMICA map as our CMB sky.

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