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We analyse the impact of various boundary conditions on the (minisuperspace) Lorentzian gravitational path integral. In particular we assess the implications for the Hartle-Hawking no-boundary wavefunction. It was shown recently that when this proposal is defined as a sum over compact metrics, problems arise with the stability of fluctuations. These difficulties can be overcome by an especially simple implementation of the no-boundary idea: namely to take the Einstein-Hilbert action at face value while adding no boundary term. This prescription simultaneously imposes an initial Neumann boundary condition for the scale factor of the universe and, for a Bianchi IX spacetime, Dirichlet conditions for the anisotropies. Another way to implement the no-boundary wavefunction is to use Robin boundary conditions. A sub-class of Robin conditions allows one to specify the Hubble rate on the boundary hypersurface, and we highlight the surprising aspect that specifying the final Hubble rate (rather than the final size of the universe) significantly alters the off-shell structure of the path integral. The conclusion of our investigations is that all current working examples of the no-boundary wavefunction force one to abandon the notion of a sum over compact and regular geometries, and point to the importance of an initial Euclidean momentum.

We derive constraints on millicharged dark matter and axion-like particles using pulsar timing and fast radio burst observations. For dark matter particles of charge e, the constraint from time of arrival (TOA) of waves is /m milli 10 −8 eV −1 , for masses m milli 10 −6 eV. For axionlike particles, the polarization of the signals from pulsars yields a bound in the axial coupling g/ma 10 −13 GeV −1 /(10 −22 eV), for ma 10 −19 eV. Both bounds scale as (ρ/ρ dm ) 1/2 for fractions of the total dark matter energy density ρ dm . We make a precise study of these bounds using TOA from several pulsars, FRB 121102 and polarization measurements of PSR J0437−4715. Our results rule out a new region of the parameter space for these dark matter models.Unraveling the nature of dark matter (DMa) is among the most urgent issues in fundamental physics. Indirect searches aim at detecting the effects of DMa in astrophysical observations, beyond its pure gravitational interaction. Given the feeble interaction of DMa with standard model fields, precise measurements are particularly promising for these searches. When one requires precision, a particular measurement stands out in astrophysics: the time of arrival (TOA) of radio waves from pulsars and fast radio bursts (FRBs). The use of pulsar timing has already been suggested to study the effects of dark matter [1][2][3][4][5][6][7][8][9]. In this Letter we present new results for DMa models directly coupled to light from the propagation of radio pulses from pulsars and FRBs. A more comprehensive exploration will be presented elsewhere [10].If DMa is coupled to the electromagnetic field, one expects modifications in the emission, propagation, and detection of radio pulses. We focus here on the effects during the propagation, which are robust under astrophysical uncertainties. In particular, we derive stringent constraints on millicharged DMa and axion-like particles (ALPs) based on dispersion measurements (DM) of radio signals from pulsars and FRBs, and on the modulation of the light polarization angle due to axion-like DMa in the Milky Way.We give a unified treatment, where the millicharged DMa and ALPs are considered as independent species. In the former case we consider that (a fraction of) the DMa is made of particles with mass m milli and electric charge q = e ( 1) [11][12][13][14][15][16][17][18]. As an example, this coupling arises in models where the DMa is charged under a dark photon, which is kinematically coupled to the visible photon [16,17]. In our analysis we remain agnostic to the origin of this term and other possible model-dependent signatures behind the charge of the DMa, and focus on constraining . Regarding ALPs, we assume the existence of axion-like [19][20][21][22][23], pseudo-scalar DMa of mass m a (represented by the field φ below).The relevant field equations readwhere g is the ALP-photon axial coupling, j ν is the ordinary electron current, whereas j ν milli is the current from millicharged particles. The role of this term in the propagation of radio-waves will...

We study the impact of gas accretion on the orbital evolution of black-hole binaries initially at large separation in the band of the planned Laser Interferometer Space Antenna (LISA). We focus on two sources: (i) stellar-origin black-hole binaries (SOBHBs) that can migrate from the LISA band to the band of ground-based gravitational-wave (GW) observatories within weeks/months; and (ii) intermediate-mass black-hole binaries (IMBHBs) in the LISA band only. Because of the large number of observable GW cycles, the phase evolution of these systems needs to be modeled to great accuracy to avoid biasing the estimation of the source parameters. Accretion affects the GW phase at negative (−4) post-Newtonian order, being thus dominant for binaries at large separations. Accretion at the Eddington or at super-Eddington rate will leave a detectable imprint on the dynamics of SOBHBs. For super-Eddington rates and a 10-years mission, a multiwavelength strategy with LISA and a groundbased interferometer can detect about 10 (a few) SOBHB events for which the accretion rate can be measured at 50% (10%) level. In all cases the sky position can be identified within much less than 0.4 deg 2 uncertainty. Likewise, accretion at 100% of the Eddington rate can be measured in IMBHBs up to redshift z ≈ 0.1, and the position of these sources can be identified within less than 0.01 deg 2 uncertainty. Altogether, a detection of SOBHBs or IMBHBs would allow for targeted searches of electromagnetic counterparts to black-hole mergers in gas-rich environments with future X-ray detectors (such as Athena) and/or radio observatories (such as SKA).

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