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.

11 pages, 8 figures

References in corpus (23)

• Imperfect Dark Energy from Kinetic Gravity Braiding
• Horndeski theory and beyond: a review
• Starting the Universe: Stable Violation of the Null Energy Condition and Non-standard Cosmologies
• G-inflation: inflation driven by the Galileon field
• Galilean Genesis: an alternative to 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
• The Effective Field Theory of nonsingular cosmology
• Resilience of the standard predictions for primordial tensor modes
• 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
• DBI Realizations of the Pseudo-Conformal Universe and Galilean Genesis Scenarios
• Super-inflation in Loop Quantum Cosmology
• Generalised tensor fluctuations and inflation
• The gravitational wave background from super-inflation in Loop Quantum Cosmology
• Inflation from Minkowski Space
• Generalized Galilean Genesis
• 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

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