Abstract
We interpret the recent NANOGrav results in terms of a stochastic gravitational wave background from metastable cosmic strings. The observed amplitude of a stochastic signal can be translated into a range for the cosmic string tension and the mass of magnetic monopoles arising in theories of grand unification. In a sizable part of the parameter space, this interpretation predicts a large stochastic gravitational wave signal in the frequency band of ground-based interferometers, which can be probed in the very near future. We confront these results with predictions from successful inflation, leptogenesis and dark matter from the spontaneous breaking of a gauged B−L symmetry.
Highlights
The direct observation of gravitational waves (GWs) generated by merging black holes [1,2,3] has led to an increasing interest in further explorations of the GW spectrum
Astrophysical sources can lead to a stochastic gravitational background (SGWB) over a wide range of frequencies, and the ultimate hope is the detection of a SGWB of cosmological origin
Stringent upper bounds on a SGWB have been obtained by pulsar timing array (PTA) experiments which are sensitive to frequencies around fyr = 1/yr
Summary
The direct observation of gravitational waves (GWs) generated by merging black holes [1,2,3] has led to an increasing interest in further explorations of the GW spectrum. It has been demonstrated that GWs from a network of stable strings with an amplitude h2 gw(1/yr) ∼ 10−9 can account for the NANOGrav stochastic background [15,16] This signal is too small to be observed by Virgo [17], LIGO [18] and KAGRA [19] but will be probed by LISA [20] and other planned GW observatories. It has been shown that GWs emitted from a metastable cosmic string network can probe the seesaw mechanism of neutrino physics and high-scale leptogenesis [21] as well as the energy scale of grand unification [22,23] Such metastable cosmic strings arise when connecting hybrid inflation, high-scale leptogenesis and dark matter with gravitational waves through U(1)B−L breaking in a cosmological phase transition [24,25]. With the new NANOGrav data [7], κ and the scale of grand unification vGUT can be determined
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