Disentangling the Origins of the NANOGrav Signal: Early Universe Models and ΔN_eff Bounds

2025/08/21 by Ido Ben-Dayan, Utkarsh Kumar, Ben-Dayan, Ido +3 · 3 voices
Physics and Astronomy · #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #General Relativity and Quantum Cosmology (gr-qc) #High Energy Physics - Phenomenology (hep-ph) #High Energy Physics - Theory (hep-th) #astro-ph.CO #gr-qc #hep-ph #hep-th

paper · pdf · doi:10.48550/arxiv.2508.15134

arxiv published 2025/08/21 · arxiv updated 2025/09/17

Abstract

We investigate whether an Early-Universe stochastic gravitational-wave background (SGWB) can account for the common spectrum process reported by NANOGrav, while also being consistent with current and projected CMB measurements of extra radiation. We compute the contribution of effective number of relativistic species, ΔN_eff, for a number of Early-Universe models proposed to explain the pulsar timing array (PTA) spectrum. We demonstrate that models predicting ΔN_eff above the CMB limit would be firmly excluded, implying that the NANOGrav signal in tension with these bounds must instead arise from astrophysical sources. We find that current NANOGrav 15-year dataset, sensitive up to 60 nHz, gives a negligible contribution to ΔN_eff and remains well below the present and future CMB detection threshold. However, when we project future PTA capabilities reaching upto 1 μHz, even with our conservative estimate we find that Inflation, Scalar Induced Gravitational Waves (SIGW), and metastable cosmic strings can induce a ΔN_eff large enough for >3.5σ detection by the Simons Observatory.

Citations

• Tensor induced gravitational waves
• Primordial Gravitational Wave Background as a Probe of the Primordial Black Holes
• Probing small-scale primordial power spectra with induced gravitational waves
• Cosmological constraints on small-scale primordial non-Gaussianity
• The Bearable Inhomogeneity of the Baryon Asymmetry
• Can tensor-scalar induced GWs dominate PTA observations ?
• Probe of spatial geometry from scalar induced gravitational waves
• Constraints on primordial black holes from N_eff : scalar induced gravitational waves as an extra radiation component
• CMB and energy conservation limits on nanohertz gravitational waves
• Cosmic superstrings, metastable strings and ultralight primordial black holes: from NANOGrav to LIGO and beyond
• The Simons Observatory: Design, integration, and testing of the small aperture telescopes
• Constraining inflationary magnetogenesis and reheating via GWs in light of PTA data
• What is the source of the PTA GW signal?
• Requiem to "Proof of Inflation" or Sourced Fluctuations in a Non-Singular Bounce
• Footprints of Axion-Like Particle in Pulsar Timing Array Data and James Webb Space Telescope Observations
• The NANOGrav 15 yr Data Set: Search for Signals from New Physics
• The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background
• Does NANOGrav observe a dark sector phase transition?
• Optical design concept of the CMB-S4 large-aperture telescopes and cameras
• Lensing dispersion of SNIa and small scales of the primordial power spectrum
• Light Sterile Neutrinos: A White Paper
• Cosmological gravitational wave background from primordial density perturbations

Discussions

• #arXiv Disentangling the Origins of the NANOGrav Signal: Early Universe Models and ΔNeff Bounds arxiv.org/abs/2508.15134 NANOGrav 15-year dataset, sensitive up to 60 nHz, gives a negligible contributi [bsky, 3 points, 0 comments]
• #arXiv Disentangling the Origins of the NANOGrav Signal: Early Universe Models and ΔNeff Bounds https://arxiv.org/abs/2508.15134 NANOGrav 15-year dataset, sensitive up to 60 nHz, gives a negligible co [bsky, 0 points, 0 comments]
• # arXiv Disentangling the Origins of the NANOGrav Signal: Early Universe Models and ΔNeff Bounds https:// arxiv.org/abs/2508.15134 NANOGrav 15-year dataset, sensitive up to 60 nHz, gives a negligible [mastodon, 0 points, 0 comments]

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