Nanohertz gravitational waves from NEC violation in the early universe

Abstract

We study nanohertz gravitational waves relevant to pulsar timing array experiments from quantum fluctuations in the early universe with null energy condition (NEC) violation. The NEC violation admits accelerated expansion with the scale factor $a\propto (-t{)}^{-p}$ ($p>0$), which gives the tensor spectral index $n_t=2/(p+1)>0$. To evade the constraint from Big Bang nucleosynthesis (BBN), we connect the NEC-violating phase to a subsequent short slow-roll inflationary phase which ends with standard reheating, and thereby reduce the high frequency part of the spectrum. An explicit model is constructed within the cubic Horndeski theory which allows for stable violation of the NEC. We present numerical examples of the background evolution having the different maximal Hubble parameters (which determine the peak amplitude of gravitational waves), the different inflationary Hubble parameters (which determine the amplitudes of high frequency gravitational waves), and different durations of the inflationary phase (which essentially determine the peak frequency of the spectrum). We display the spectra with $n_t=0.8$, $0.9$, and $0.95$ for $f\lesssim 1/\mathrm{yr}$, which are consistent with the recent NANOGrav result. We also check that they do not contradict the BBN constraint. We discuss how the nearly scale-invariant spectrum of curvature perturbations is produced in the NEC-violating phase.

arXiv comment11 pages, 8 figures

References in corpus (54)

• k-Inflation
• Generalized G-inflation: Inflation with the most general second-order field equations
• From k-essence to generalised Galileons
• Gravitational Wave Experiments and Early Universe Cosmology
• Imperfect Dark Energy from Kinetic Gravity Braiding
• Horndeski theory and beyond: a review
• One percent determination of the primordial deuterium abundance
• Starting the Universe: Stable Violation of the Null Energy Condition and Non-standard Cosmologies
• G-inflation: inflation driven by the Galileon field
• BICEP2 / Keck Array x: Constraints on Primordial Gravitational Waves using Planck, WMAP, and New BICEP2/Keck Observations through the 2015 Season
• Galilean Genesis: an alternative to inflation
• Solid Inflation
• A new cosmic microwave background constraint to primordial gravitational waves
• Relating gravitational wave constraints from primordial nucleosynthesis, pulsar timing, laser interferometers, and the CMB: implications for the early universe
• B-Inflation
• From NANOGrav to LIGO with metastable cosmic strings
• Generic instabilities of non-singular cosmologies in Horndeski theory: a no-go theorem
• The Effective Field Theory of nonsingular cosmology
• Resilience of the standard predictions for primordial tensor modes
• Phantom Inflation and Primordial Perturbation Spectrum
• Generalized Galileons: instabilities of bouncing and Genesis cosmologies and modified Genesis
• Nearly Scale-Invariant Spectrum of Adiabatic Fluctuations May be from a Very Slowly Expanding Phase of the Universe
• Blue-tilted Tensor Spectrum and Thermal History of the Universe
• Stability of Geodesically Complete Cosmologies
• The PHLEK Survey: A New Determination of the Primordial Helium Abundance
• Subluminal Galilean Genesis
• Non-Bunch-Davies Initial State Reconciles Chaotic Models with BICEP and Planck
• The Effective Field Theory of nonsingular cosmology: II
• A Galileon Design of Slow Expansion
• DBI Realizations of the Pseudo-Conformal Universe and Galilean Genesis Scenarios
• Inflation with violation of the null energy condition
• DBI Genesis: An Improved Violation of the Null Energy Condition
• Primordial gravitational waves in supersolid inflation
• Super-inflation in Loop Quantum Cosmology
• Distinctive signatures of space-time diffeomorphism breaking in EFT of inflation
• Generalised tensor fluctuations and inflation
• Elastic Inflation
• The gravitational wave background from super-inflation in Loop Quantum Cosmology
• Galilean Creation of the Inflationary Universe
• Inflation from Minkowski Space
• From Satisfying to Violating the Null Energy Condition
• Blue-tilted Primordial Gravitational Waves from Massive Gravity
• Propagating speed of primordial gravitational waves and inflation
• A Galileon Design of Slow Expansion: II
• Is there an effect of a nontrivial $c_T$ during inflation?
• The Primordial Perturbation Spectrum from Various Expanding and Contracting Phases
• Generalized Galilean Genesis
• The slow expansion with nonminimal derivative coupling and its conformal dual
• Scale-invariant perturbations from NEC violation: A new variant of Galilean Genesis
• Primordial Perturbations During a Slow Expansion
• Revisiting slow-roll dynamics and the tensor tilt in general single-field inflation
• Gravitational Wave Background from Phantom Superinflation
• Let Effective Field Theory of Inflation flow: stochastic generation of models with red/blue tensor tilt
• Galilean Islands in Eternally Inflating Background

Cited by in corpus (20)

• Cosmic String Interpretation of NANOGrav Pulsar Timing Data
• Has NANOGrav found first evidence for cosmic strings?
• Inflationary interpretation of the stochastic gravitational wave background signal detected by pulsar timing array experiments
• Primordial gravitational waves from NANOGrav: a broken power-law approach
• Decoding the phases of early and late time reheating through imprints on primordial gravitational waves
• Blue-tilted inflationary tensor spectrum and reheating in the light of NANOGrav results
• Can we observe the QCD phase transition-generated gravitational waves through pulsar timing arrays?
• Scalar and Tensor Perturbations in DHOST Bounce Cosmology
• Intermittent null energy condition violations during inflation and primordial gravitational waves
• Broken blue-tilted inflationary gravitational waves: a joint analysis of NANOGrav 15-year and BICEP/Keck 2018 data
• Probing The Early Universe Cosmology With NANOGrav: Possibilities and Limitations
• Nonsingular cosmological models with strong gravity in the past
• Escape from supercooling with or without bubbles: gravitational wave signatures
• Hunting the stochastic gravitational wave background in pulsar timing array cross correlations through theoretical uncertainty
• Generating enhanced parity-violating gravitational waves during inflation with violation of the null energy condition
• Generating enhanced primordial GWs during inflation with intermittent violation of NEC and diminishment of GW propagating speed
• Charting the Nanohertz Gravitational Wave Sky with Pulsar Timing Arrays
• Tensor Perturbations from Bounce Inflation Scenario in f(Q) Gravity
• Can Horndeski Genesis be Nonpathological?
• Testing No slip model with pulsar timing arrays: NANOGrav and IPTA