Sophie Henrot-Versillé and Florent Robinet are research associates at the Laboratoire de l’Accélérateur Linéaire d’Orsay

Many cosmological models predict the existence of a stochastic Gravitational-Wave (GW) background produced just after the universe was born. As gravitational waves do not interact with matter, their detection would give us a unique and pristine probe to study the very first instants of the Universe: when it was 50 orders of magnitude younger than its age at the epoch of the photon decoupling. Such a detection would be as important as the discovery of the Cosmological Microwave Background (CMB). CMB studies tell us what the universe looked like when it became optically thin (~300,000 years after the Big Bang). They help us to establish the standard ΛCDM model of cosmology and to understand the important role of inflation.

Direct searches for the presence of a cosmological stochastic GW background have been carried out by kilometric interferometers LIGO, and Virgo and pulsar timing arrays. The GW energy density has been bounded by these experiments. Primordial GWs produced during inflation are also searched for in the CMB data as they would generate specific signatures in the B polarization of the CMB anisotropies.

On the other hand, indirect constraints can also be derived from different cosmological probes. In our CQG article, we have revisited one of these indirect constraints using the 2013 Planck measurements (temperature and lensing), WMAP low-l polarization, high-l data from ground based telescope ACT and SPT and the BAO from the BOSS spectroscopic survey. By combining the different datasets into a profile likelihood analysis we have constrained the GW energy density to be lower than 3.8×10^-6. This is a factor of 2 lower than what previous studies achieved. We have also shown that this result is robust against the neutrino mass models.

If one then assumes that the cosmological stochastic GW background was produced by a network of cosmic (super-)strings, the constraint on the GW energy density can be used to derive exclusion limits in the cosmic string parameter space. We show that the CMB and BAO data allow one to derive the most competitive limits in a scenario of small loops and for low reconnection probabilities.

Read the full article in Classical and Quantum Gravity:
Improved constraint on the primordial gravitational-wave density using recent cosmological data and its impact on cosmic string models
Sophie Henrot-Versille, Florent Robinet, Nicolas Leroy, Stephane Plaszczynski, Nicolas Arnaud, Marie-Anne Bizouard, Fabien Cavalier, Nelson Christensen, Francois Couchot, Samuel Franco, Patrice Hello, Dominique Huet, Marie Kasprzack, Olivier Perdereau, Marta Spinelli and M Tristram
Sophie Henrot-Versillé et al 2015 Class. Quantum Grav. 32 045003

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