Leandros Perivolaropoulos

We present Ph-[Formula: see text]CDM, a phantom-scalar realization within general relativity of the sign-switching cosmological-constant idea, [Formula: see text]CDM, in which a phantom scalar evolving on a bounded hyperbolic-tangent potential induces a smooth mirror AdS-to-dS transition in the late-time dark-energy density. The wrong-sign kinetic term, usually viewed as pathological, becomes the mechanism lifting the field from a negative- to a positive-energy vacuum-like regime. The construction also shows that the field can become repulsive while its energy density is still negative. The cosmology nevertheless remains controlled: total energy stays positive, the late-time attractor is de Sitter rather than a Big Rip, and the dynamics remain safely infrared. Ph-[Formula: see text]CDM thus offers a concrete late-time mechanism with the potential to address multiple cosmological tensions.

In this paper we quantify the ability of the Laser Interferometer Space Antenna (LISA) to test the presence of non-tensorial polarizations as well as modifications to the tensor ones in gravitational waves emitted from massive black hole binaries. We employ the Parametrized Post-Einsteinian (PPE) formalism to model deviations from General Relativity (GR) for tensor, vector, and scalar polarizations. Our PPE parametrization is inspired by post-Newtonian waveforms from four modified gravity theories: Horndeski, Einstein-æther, Rosen's bimetric, and Lightman-Lee. We consistently implement these modifications during the inspiral, ensuring proper waveform alignment and tapering at merger. Subsequently, we perform Fisher forecasts to derive expected constraints on deviations from General Relativity and map these constraints to the parameter spaces of the four gravity theories. For tensor polarizations, LISA achieves constraints on amplitude modifications ranging between ∼ 10 ^-4 - 10 ^-2 precision level, depending on the frequency evolution of the modifications, for systems with 10 ⁵ - 10 ⁷ M _⊙ at z = 1. We find that LISA can distinguish breathing and longitudinal scalar polarizations, a capability that current ground-based detectors do not possess, only for relatively light binaries with M ≲ 10 ⁴ M _⊙ , beyond which these modes become degenerate in the detector response. Importantly, constraints on vector polarizations are approximately 2–3 times more precise than for scalar polarizations. For both vector and scalar modes, amplitude measurements reach precisions ranging between ∼ 10 ^-8 - 10 ^-2 , depending on the frequency evolution of the modifications, for systems with 10 ⁵ - 10 ⁷ M _⊙ at z = 1. These results demonstrate LISA's potential to probe gravity in the emission process of gravitational waves.

Baryon acoustic oscillation (BAO) measurements play a key role in ruling out postrecombination solutions to the Hubble tension. However, because the data compression leading to these measurements assumes a fiducial Λ CDM cosmology, their reliability in testing late-time modifications to Λ CDM has at times been called into question. We play devil’s advocate and posit that fiducial cosmology assumptions do indeed affect BAO measurements in such a way that low-redshift acoustic angular scales (proportional to the Hubble constant H 0 ) are biased low and test whether such a rescaling can rescue postrecombination solutions. The answer is no. First, strong constraints on the shape of the z ≲ 2 expansion history from unanchored Type Ia Supernovae (SNeIa) prevent large deviations from Λ CDM . In addition, unless Ω m is significantly lower than 0.3, the rescaled BAO measurements would be in strong tension with geometrical information from the cosmic microwave background. We demonstrate this explicitly on several dark energy (DE) models ( w CDM , CPL DE, phenomenologically emergent DE, holographic DE, Λ s CDM , and the negative cosmological constant model), finding that none can address the Hubble tension once unanchored SNeIa are included. We argue that the Λ s CDM sign-switching cosmological constant model possesses interesting features which make it the least unpromising one among those tested. Our results demonstrate that possible fiducial cosmology-induced BAO biases cannot be invoked as loopholes to the Hubble tension “no-go theorem”, and highlight the extremely important but so far underappreciated role of unanchored SNeIa in ruling out postrecombination solutions.