We investigate limitations of causality arguments from flat-spacetime amplitudes, based on the eikonal limit of gravitational scattering, to place constraints on modified gravity. We show that causality constraints are only valid in the weak-gravity regime even for trans-Planckian scattering, and that such constraints are much less stringent than astrophysical ones, obtained for example from gravitational waves emitted in black hole coalescence. Special attention is given to the weakness of causality constraints on dynamical Chern-Simons gravity, but our results apply to other modified gravity theories as well. In the context of that theory, we also discuss how to obtain a time-delay formula from black holes, neutron stars, and shock wave solutions. For scattering with compact objects, we explicitly show that time delays are greatly suppressed by the ratio of the object’s mass to the impact parameter, so time advances only occur greatly outside the cut off of the theory. For the shock wave solution, we find that the time delay is always positive within the regime of validity of the solution. We also comment on the impact of graviton nonlinearities for time-delay calculations in the nonlinear, strong gravity regime. We conclude that amplitude-based causality constraints on modified gravity are typically not stringent relative to other experimental and observational bounds.

United States

Stephon Alexander

Heliudson Bernardo

Nicolás Yunes

Can weak-gravity, causality-violation arguments constrain modified gravity?

The search for dark matter is one of the crucial open problems in both particle physics and cosmology. If dark matter scatters with Standard Model particles, it could accumulate inside the Earth and begin to annihilate, producing heat within the Earth’s core. While past work has been done on the effect that this heat would have once it reached the surface, we model the flow of heat through the Earth’s core by numerically solving the heat equation to model dark matter’s effect on the interior of the planet. We compute how long it takes for the core to come into thermal equilibrium and show that for a wide range of dark matter parameters, a substantial fraction of the inner core would be melted by dark matter annihilation. Our analysis produces new limits on dark matter annihilating in the Earth, points out important new effects that must be considered when studying planetary heating by dark matter, and suggests new dark matter observables that could be searched for in exoplanet populations.

Christopher Cappiello

Tansu Daylan

Can a dark inferno melt the Earth’s core?

We present a minimal and dynamically consistent formulation of non-relativistic bosonic string theory in a Newton-Cartan background. Starting from a reparametrization-invariant Nambu-Goto action, we develop the Hamiltonian framework and perform a complete Dirac constraint analysis. The resulting structure exhibits first-class constraints that generate worldsheet diffeomorphisms, confirming the internal gauge consistency of the model. Using an interpolating Lagrangian, we derive a Polyakov-type action that enables a direct comparison with symmetry-based constructions known as gauging-the-algebra (GTA) approaches, which promote non-relativistic symmetry algebras to local symmetries. In contrast to GTA formulations which require additional background fields to achieve algebraic closure — our model derives all necessary geometric data dynamically from the string evolution itself. This establishes that standard geometry is sufficient to support consistent non-relativistic string dynamics. Our results provide a conceptually transparent and technically robust foundation for future studies of non-relativistic string theory in curved backgrounds.

South Africa

Partha Nandi

India

Sk. Moinuddin

Abdus Sattar

Can non-relativistic strings propagate without geometric baggage? A minimal Hamiltonian formulation in Newton-Cartan geometry

We revisit the lore establishing the allowed space of field redefinitions and show that there are essentially no restrictions. Our conclusions hold to all orders in perturbation theory and for any dispersion relation. Field redefinitions can be nonlocal, symmetry breaking, or in certain cases have explicit dependence on spacetime. We address field redefinitions that can be resummed into the propagator, which demonstrates how to perform perturbative calculations away from the minimum in field space. Field redefinitions are used to derive higher-order Schwinger-Dyson equations, which imply multiparticle soft theorems. Non-standard field redefinitions are showcased using both relativistic and nonrelativistic examples.

Switzerland

Timothy Cohen

Matthew Forslund

Andreas Helset

Field redefinitions can be nonlocal

In the present work we analyze two different models of interaction between dark energy and dark matter, also known as vacuum decay models or $$\Lambda (t)$$ CDM models. In both models, when the $$H_0$$ parameter is constrained by high-redshift data as the Planck distance priors (CMB) and transversal BAO, its value is compatible with a higher value of $$H_0$$ , in agreement with low-redshift data, as Pantheon+ &SH0ES (PS) and Cosmic Chronometers (CC). For one of the models, only a mild $$\sim 2.1\sigma $$ tension is found for $$\Omega _m$$ , which at least is an alleviation to the $$\gtrsim 5\sigma $$ $$H_0$$ tension in the context of standard $$\Lambda $$ CDM model. We find $$H_0=71.63\pm 0.60$$ km/s/Mpc for one model and $$H_0=71.67\pm 0.60$$ km/s/Mpc for the other one, by combining CC+PS+BAO+CMB. We also find the decay parameter to be $$\epsilon =0.0162^{+0.0029}_{-0.0026}$$ for one model and $$\epsilon =0.0209\pm 0.0036$$ for the other one. All constraints are at 68% c.l. From these analyses, a noninteracting model is excluded at least at $$5.8\sigma $$ c.l. We shall emphasize, however, that this result comes from an analysis that involves an approximated treatment to CMB, the Planck distance priors, as mentioned above. We have also verified that if the SH0ES prior is not included, the evidence for an interacting model decreases from $$5.8\sigma $$ to $$4 \sigma $$ , a result closer to other recent works in the literature. This shows that these types of models are promising in solving or at least alleviating the $$H_0$$ tension problem.

Brazil

L. Brito

J. Jesus

China

A. Escobal

S. Pereira

Can decaying vacuum solve the $$H_0$$ tension?

We investigate the impact of curvature corrections to Starobinsky inflation in light of the latest observational results from the Atacama Cosmology Telescope (ACT). While the pure Starobinsky model remains a compelling candidate for cosmic inflation, we explore how the higher-order curvature terms <math altimg='si12.svg'><msup><mi>R</mi><mn>3</mn></msup></math>, <math altimg='si13.svg'><msup><mi>R</mi><mn>4</mn></msup></math> and <math altimg='si4.svg'><msup><mi>R</mi><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow></msup></math> modify the inflationary predictions. Using the scalar-tensor formulation of <math altimg='si14.svg'><mrow><mi>f</mi><mo>(</mo><mi>R</mi><mo>)</mo></mrow></math> gravity, we derive the effective scalar potentials and compute the resulting scalar tilt <math altimg='si5.svg'><msub><mi>n</mi><mi>s</mi></msub></math>, its running index <math altimg='si15.svg'><msub><mi>&#945;</mi><mi>s</mi></msub></math> and the tensor-to-scalar ratio <math altimg='si6.svg'><mi>r</mi></math>. We show that those curvature corrections can shift the predictions to align better with the ACT data, thus providing a possible resolution to a minor discrepancy between the standard Starobinsky model and ACT observations. Our findings suggest that the modified Starobinsky models with the higher-order curvature terms offer a viable pathway to reconciling inflationary predictions with precision cosmological measurements. At the same time, measuring or constraining primordial tensor modes can help to discriminate between these corrections.

Italy

Andrea Addazi

Yermek Aldabergenov

Japan

Sergei V. Ketov

Curvature corrections to Starobinsky inflation can explain the ACT results

We investigate the harvesting of quantum steering and its asymmetry between two static detectors locally interacting with a vacuum massless scalar field near an infinite, perfectly reflecting boundary. The detectors are arranged either parallel or orthogonal to the boundary, with detector $\textit{B}$ assumed to have an energy gap greater than or equal to that of detector $\textit{A}$. It is interesting to observe that, with increasing distance between the detectors and the boundary, the boundary tends to suppress quantum steering in one direction while enhancing it in the opposite direction. In the case of identical detectors, steering is symmetric when they are aligned parallel to the boundary. However, orthogonal alignment breaks this symmetry due to their unequal spatial proximity to the boundary. For non-identical detectors in the parallel configuration, the steering from $\textit{A}$ to $\textit{B}$ ($\textit{A}$ → $\textit{B}$) generally surpasses that from $\textit{B}$ to $\textit{A}$ ($\textit{B}$ → $\textit{A}$). In contrast, when the detectors are oriented orthogonally to the boundary, the relative strength of $\textit{A}$ → $\textit{B}$ and $\textit{B}$ → $\textit{A}$ steerability depends on the interplay between the boundary effects and the detectors’ energy gap difference. Across most of the parameter space, the orthogonal alignment tends to enhance $\textit{B}$ → $\textit{A}$ steering while suppressing $\textit{A}$ → $\textit{B}$ steering compared to the parallel setup. These findings suggest that boundary configurations should be flexibly adjusted according to the directional dependence of steering harvesting in order to optimize quantum information extraction.

Xiao-Li Huang

Xiao-Ying Jiang

Yu-Xuan Wang

Si-Yu Liu

Zejun Wang

Shu-Min Wu

Can boundary configuration be tuned to optimize directional quantum steering harvesting?

Like general relativity, metric-affine gravity should be a viable effective quantum theory; otherwise, it is a mathematical curiosity without physical application. Assuming a perturbative quantum field theory, the universal, flat limit of metric-affine gravity offers a good foundation for model-building only when symmetry constraints are themselves sufficient to get rid of ghosts and tachyons in the spectrum of propagating particle states, without requiring any further tuning of the couplings. Using this symmetry-first criterion, we find that for parity-preserving models with a totally symmetric distortion, only massless spin-one and spin-three modes are possible besides the graviton. Moreover, no viable models result from gauge symmetries generated by a scalar field.

Czechia

Will Barker

Estonia

Carlo Marzo

Chile

Austria

Alessandro Santoni

Can metric-affine gravity be saved?

Three possible strategies have been advocated to solve the strong CP problem. The first is the axion, a dynamical mechanism that relaxes any initial value of the CP violating angle $$ \overline{\theta} $$ to zero. The second is the imposition of new symmetries that are believed to set $$ \overline{\theta} $$ to zero in the UV. The third is the acceptance of the fine tuning of parameters. We argue that the latter two solutions do not solve the strong CP problem. The $\textit{θ}$ term of QCD is not a parameter — it does not exist in the Hamiltonian. Rather, it is a property of the quantum state that our universe finds itself in, arising from the fact that there are CP violating states of a CP preserving Hamiltonian. It is not eliminated by imposing parity as a symmetry since the underlying theory is already parity symmetric and that does not preclude the existence of CP violating states. Moreover, since the value of $\textit{θ}$ realized in our universe is a consequence of measurement, it is inherently random and cannot be fine tuned by choice of parameters. Rather any fine tuning would require a tuning between parameters in the theory and the random outcome of measurement. Our results considerably strengthen the case for the existence of the axion and axion dark matter. The confusion around $\textit{θ}$ arises from the fact that unlike classical mechanics, the Hamiltonian and Lagrangian are not equivalent in quantum mechanics. The Hamiltonian defines the differential time evolution, whereas the Lagrangian is a solution to this evolution. Consequently, initial conditions could in principle appear in the Lagrangian but not in the Hamiltonian. This results in aspects of the initial condition such as $\textit{θ}$ misleadingly appearing in the Lagrangian as parameters. We comment on the similarity between the $\textit{θ}$ vacua and the violations of the constraint equations of classical gauge theories in quantum mechanics.

David Kaplan

Tom Melia

Surjeet Rajendran

What can solve the strong CP problem?

We present a minimal setup within the framework of Horndeski gravity that can describe a nonpathological Genesis scenario, namely, the strong-coupling energy scale is always well above the classical energy scale. Our setup allows for a fully stable transition to the kination epoch, during which General Relativity (GR) is restored. This Genesis scenario circumvents the no-go theorem at the cost of encountering the risk of strong coupling in the past. Interestingly, our scenario admits two different regimes for the background solution for Hubble parameter at the Genesis stage: power-law behavior and manifestly non-power-law behavior. We explicitly show that, in both regimes, our model remains within unitarity bounds. In most cases, the tensor spectrum is blue-tilted. Then, we adopt a mechanism with a spectator field that allows for a red-tilted scalar power spectrum. We also suggest a deformation of the model that enables us to achieve sufficiently small values for the tensor-to-scalar ratio. Finally, we discuss the geodesic (in)completeness of the current model.