Orateur
Description
The second data release of the European Pulsar Timing Array (EPTA) collaboration provides
evidence for the presence of a gravitational-wave (GW) background. In this work, we explore a
potential cosmological interpretation of this signal in terms of inflationary scenarios. We parametrize
the tensor power spectrum in terms of the tensor-to-scalar ratio r, the tensor spectral index nt, the
reheating temperature Trh and the cut-off frequency fend. We incorporate all relevant observational
constraints, including those from Cosmic Microwave Background, Big Bang Nucleosynthesis and
LIGO–Virgo–KAGRA observations. We demonstrate that imposing these constraints consistently
reduces the region of parameter space that provides a viable interpretation of the EPTA signal, to
log10 r > −12, 1.2 ≲ nt ≲ 2.5, 10 MeV ≲ Trh ≲ 50 GeV and 10−8 Hz ≲ fend ≲ 1012 Hz at 95%
confidence level. This favours
the scenario in which the GW spectrum in the EPTA frequency band originates from tensor modes
that re-entered the horizon during the radiation-dominated era, allowing for a higher r and a flatter
spectrum. However, Trh must take very low values, which are challenging to explain theoretically.