Do pulsar timing arrays observe merging primordial black holes?

In this letter we evaluate whether the gravitational wave background recently observed by a number of different pulsar timing arrays could be due to merging primordial supermassive black hole binaries. We find that for homogeneously distributed primordial black holes this possibility is inconsistent with strong cosmological and astrophysical constraints on their total abundance. If the distribution exhibits some clustering, however, the merger rate will in general be enhanced, opening the window for a consistent interpretation of the PTA data in terms of merging primordial black holes.Comment: 8 pages, 3 figure

Hunting WIMPs with LISA: Correlating dark matter and gravitational wave signals

The thermal freeze-out mechanism in its classical form is tightly connected to physics beyond the Standard Model around the electroweak scale, which has been the target of enormous experimental efforts. In this work we study a dark matter model in which freeze-out is triggered by a strong first-order phase transition in a dark sector, and show that this phase transition must also happen close to the electroweak scale, i.e. in the temperature range relevant for gravitational wave searches with the LISA mission. Specifically, we consider the spontaneous breaking of a $U(1)^\prime$ gauge symmetry through the vacuum expectation value of a scalar field, which generates the mass of a fermionic dark matter candidate that subsequently annihilates into dark Higgs and gauge bosons. In this set-up the peak frequency of the gravitational wave background is tightly correlated with the dark matter relic abundance, and imposing the observed value for the latter implies that the former must lie in the milli-Hertz range. A peculiar feature of our set-up is that the dark sector is not necessarily in thermal equilibrium with the Standard Model during the phase transition, and hence the temperatures of the two sectors evolve independently. Nevertheless, the requirement that the universe does not enter an extended period of matter domination after the phase transition, which would strongly dilute any gravitational wave signal, places a lower bound on the portal coupling that governs the entropy transfer between the two sectors. As a result, the predictions for the peak frequency of gravitational waves in the LISA band are robust, while the amplitude can change depending on the initial dark sector temperature.Comment: 29 pages, 12 figures + appendice

Turn up the volume: Listening to phase transitions in hot dark sectors

Stochastic gravitational wave (GW) backgrounds from first-order phase transitions are an exciting target for future GW observatories and may enable us to study dark sectors with very weak couplings to the Standard Model. In this work we show that such signals may be significantly enhanced for hot dark sectors with a temperature larger than the one of the SM thermal bath. The need to transfer the entropy from the dark sector to the SM after the phase transition can however lead to a substantial dilution of the GW signal. We study this dilution in detail, including the effect of number-changing processes in the dark sector (so-called cannibalism), and show that in large regions of parameter space a net enhancement remains. We apply our findings to a specific example of a dark sector containing a dark Higgs boson and a dark photon and find excellent detection prospects for LISA and the Einstein telescope

Do pulsar timing arrays observe merging primordial black holes?

In this letter we evaluate whether the gravitational wave background recently observed by a number of different pulsar timing arrays could be due to merging primordial supermassive black hole binaries. We find that for homogeneously distributed primordial black holes this possibility is inconsistent with the strong cosmological and astrophysical constraints on their total abundance. If the distribution exhibits some clustering, however, the merger rate will in general be enhanced, opening the window for a consistent interpretation of the PTA data in terms of merging primordial black holes

Does NANOGrav observe a dark sector phase transition?

Gravitational waves from a first-order cosmological phase transition, at temper-atures at the MeV-scale, would arguably be the most exciting explanation of the commonred spectrum reported by the NANOGrav collaboration, not the least because this would bedirect evidence of physics beyond the standard model. Here we perform a detailed analysisof whether such an interpretation is consistent with constraints on the released energy de-riving from big bang nucleosynthesis and the cosmic microwave background. We find that aphase transition in a completely secluded dark sector is strongly disfavoured with respect tothe more conventional astrophysical explanation of the putative gravitational wave signal interms of supermassive black hole binaries. On the other hand, a phase transition in a darksector that subsequently decays, before the time of neutrino decoupling, remains an intriguingpossibility to explain the data. From the model-building perspective, such an option is easilysatisfied for couplings with the visible sector that are small enough to evade current colliderand astrophysical constraints. The first indication that could eventually corroborate such aninterpretation, once the observed common red spectrum is confirmed as a nHz gravitationalwave background, could be the spectral tilt of the signal. In fact, the current data alreadyshow a very slight preference for a spectrum that is softer than what is expected from theleading astrophysical explanation