Relic gravitons and pulsar timing arrays: a theoretical viewpoint

Abstract

During the last three years the pulsar timing arrays reported a series of repeated evidences of gravitational radiation (with stochastically distributed Fourier amplitudes) at a benchmark frequency of the order of $30$ nHz and characterized by spectral energy densities (in critical units) ranging between $10^{-8}$ and $10^{-9}$. While it is still unclear whether or not these effects are just a consequence of the pristine variation of the space-time curvature, the nature of the underlying physical processes would suggest that the spectral energy density of the relic gravitons in the nHz domain may only depend on the evolution of the comoving horizon at late, intermediate and early times. Along this systematic perspective we first consider the most conventional option, namely a post-inflationary modification of the expansion rate. Given the present constraints on the relic graviton backgrounds, we then show that such a late-time effect is unable to produce the desired hump in the nHz region. We then analyze a modified exit of the relevant wavelengths as it may happen when the gravitons inherit an effective refractive index from the interactions with the geometry. A relatively short inflationary phase leads, in this case, to an excess in the nHz region even if the observational data coming from competing experiments do not pin down exactly the same regions in the parameter space. We finally examine an early stage of increasing curvature and argue that it is not compatible with the observed spectral energy density unless the wavelengths crossing the comoving horizon at early times reenter in a decelerated stage not dominated by radiation.

arXiv comment40 pages, 15 included figures; comments added and corrected typos to match the version in press

References in corpus (41)

• The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang
• Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array
• Loop Quantum Cosmology: A Status Report
• The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background
• Cosmological Signature of New Parity-Violating Interactions
• Gravitational Wave Production by Collisions: More Bubbles
• Effective Field Theory for Inflation
• On the Cosmological Implications of the String Swampland
• Gravitational Radiation from Colliding Vacuum Bubbles: Envelope Approximation to Many-Bubble Collisions
• Singularity-free cosmological solutions of the superstring effective action
• New BBN limits on Physics Beyond the Standard Model from He4
• Ekpyrotic and Cyclic Cosmology
• Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves
• Upper bounds on the low-frequency stochastic gravitational wave background from pulsar timing observations: current limits and future prospects
• The International Pulsar Timing Array second data release: Search for an isotropic Gravitational Wave Background
• On the evidence for a common-spectrum process in the search for the nanohertz gravitational-wave background with the Parkes Pulsar Timing Array
• Damping of Tensor Modes in Cosmology
• Common-red-signal analysis with 24-yr high-precision timing of the European Pulsar Timing Array: Inferences in the stochastic gravitational-wave background search
• Relic Gravitational Waves from String Cosmology
• Gravitational waves from vacuum first-order phase transitions: from the envelope to the lattice
• The Null Energy Condition and its violation
• New limits on cosmic strings from gravitational wave observation
• The cosmic gravitational wave background in a cyclic universe
• Generalized Galileons: instabilities of bouncing and Genesis cosmologies and modified Genesis
• Gravitational radiation from a bulk flow model
• Spikes in the Relic Graviton Background from Quintessential Inflation
• Dilaton Contributions to the Cosmic Gravitational Wave Background
• Comment on "Damping of Tensor Modes in Cosmology"
• Thermal history of the plasma and high-frequency gravitons
• Primordial backgrounds of relic gravitons
• Stochastic backgrounds of relic gravitons, T$\Lambda$CDM paradigm and the stiff ages
• Constant-roll inflation in scalar-tensor gravity
• An analytic description of the damping of gravitational waves by free streaming neutrinos
• Brane Constant-roll Inflation
• Scalar and Tensor Inhomogeneities from Dimensional Decoupling
• The refractive index of relic gravitons
• Scalar Field Oscillations Contributing to Dark Energy
• Effective energy density of relic gravitons
• Post-inflationary thermal histories and the refractive index of relic gravitons
• The propagating speed of relic gravitational waves and their refractive index during inflation
• Flat spectra of cosmic gravitons in the nHz and audio bands