Primordial black holes, allegedly formed in the very early Universe, have been proposed as a possible viable dark matter candidate. In this work we characterize the expected gravitational wave signal detectable by the planned space-borne interferometer LISA and the proposed next generation spaceborne interferometer µAres arising from a population of primordial black holes orbiting Sgr A , the super-massive black hole at the Galactic center. Assuming that such objects indeed form the entire diffuse mass allowed by the observed orbits of stars in the Galactic center ( < ∼ 4 × 10 3 M within a radius of 10 −3 pc from Sgr A ), under the simplified assumption of circular orbits and monochromatic mass function, we assess the expected signal in gravitational waves, either from resolved and non-resolved sources. We estimate a small but non negligible chance of 10% of detecting one single 1 M primordial black hole with LISA in a 10-year-long data stream, while the background signal due to unresolved sources would essentially elude any reasonable chance of detection. On the contrary, µAres, with a 3 orders-of-magnitude better sensitivity at 10 −5 Hz, would be able to resolve 140 solar mass primordial black holes in the same amount of time, while the unresolved background should be observable with an integrated signal-to-noise ratio 100. Allowing the typical PBH mass to be in the range 0.01-10 M would increase LISA chance of detection to 40% towards the lower limit of the mass spectrum. In the case of µAres, instead, we find a "sweet spot" just about 1 M , a mass for which the number of resolvable events is indeed maximized.

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Primordial black holes (PBH), supposedly formed in the very early Universe, have been proposed as a possible viable dark matter candidate. In this work we characterize the expected gravitational wave (GW) losses from a population of PBHs orbiting Sgr A , the super-massive black hole at the Galactic center (GC), and assess the signal detectability by the planned space-borne interferometer LISA and by the proposed next generation space-borne interferometer µAres. Assuming that PBHs indeed form the entire diffuse mass allowed to reside within the orbit of the S2 star, we compute an upper limit to the expected GW signal both from resolved and non-resolved sources, under the further assumptions of monochromatic mass function and thermally distributed eccentricities. By comparing with our previous work where PBHs on circular orbits were assumed, we show how the GW signal from high harmonics over a 10 year data stream increases by a factor of six the chances of LISA detectability, from the ≈ 10% of the circular case, to ≈ 60%, whereas multiple sources can be identified in 20% of our mock populations. The background signal, made by summing up all non resolved sources, should be certainly detectable thanks to the PBHs with higher eccentricity evolving under two body relaxation. In the case of µAres, because of its improved sensitivity in the µHz band, one third of the entire population of PBHs orbiting Sgr A would be resolved. The background noise from the remaining non resolved sources should be detectable as well.

We hereby propose an alternative and additional angle on the nature of gravitational waves (GWs), postulating the theoretical and experimental possibility that GWs carry a deformation of the time component of spacetime, other than the spatial one. By explicitly working outside of the transverse-traceless gauge, we propose how events with well-defined time duration, when hit by a GW, would consequently be expected to show a difference in their characteristic time, as measured from the rest frame of an outside observer, whose clock is to remain unaffected by the GW. This constitutes a theoretically viable way in the sense of detecting the passing of the wave itself and may prove relevant as a standalone method for GWs detection other than laser interferometers, or as well be implemented as a complementary but independent system of signal triggering, improving the statistical significance of existing methods. A simple but physically realistic scenario in which the appropriate conditions for the generation and detection of GWs with time dilation are met is presented, along with the conceptual design of an experimental detector.

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