Opening

The 500 m Precision Floor in Neutron Star Radius Measurements

• CESAR AUGUSTO Zen Vasconcellos (UFRGS/ICRANet)

abstract. Multi-messenger observations of neutron stars aim to constrain the dense-matter equation of state via mass-radius measurements. We demonstrate that, for a canonical 1.4 M⊙ star, current crust-modeling uncertainties impose a persistent∼500 m precision floor in radius predictions. This threshold is comparable to—and in some cases exceeds—the radius differences predicted for quark-hadron transitions, hyperonic cores, dark matter admixture, and modified gravity. Consequently, in the one-dimensional mass-radius plane, these scenarios remain effectively degenerate. Breaking this degeneracy requires either reducing crustal uncertainties to σR ≪500 m—a challenge for current missions—or shifting toward multi-observable tomography. The 500 m precision floor thus reframes the inverse problem, motivating a move beyond radius measurements alone.

M Alcubierre

F S Guzmán

VPOS from multistate Scalar Field Dark Matter

• Tula Bernal (Universidad Autónoma Chapingo)

Observations have confirmed that satellite galaxies of the Milky Way, as well as those of Andromeda and Centaurus A, exhibit a non-homogeneous distribution known as the Vast Polar Structure (VPOS), where satellites show orbits aligned perpendicular to the host galaxy's plane. Conventional Cold Dark Matter (CDM) models face significant challenges in explaining this anisotropic distribution across multiple galaxies. In this work, we propose that the quantum nature of the Scalar Field Dark Matter (SFDM) model offers a natural explanation for the VPOS. By incorporating finite temperature corrections for a complex, self-interacting scalar field at early cosmological epochs, we demonstrate that the SFDM halo behaves as a macroscopic gravitational atom. We show that the system's quantum character allows for the formation of ground and excited states, specifically, p-states that resemble lobes along the north-south direction, which can explain the observed VPOS. By fitting these multistate SFDM solutions to rotation curves for the Milky Way, Andromeda, Centaurus A, and additional galaxies, we find that this model effectively accounts for the anisotropic distribution of satellite galaxies, suggesting that this structure may be a general characteristic of galaxies in the Universe.

M Hernández

L Sánchez

Lam Hui

A González

Constructing de Sitter space and Dark Matter with Dynamical Tension Strings

• Eduardo Leon Guendelman (Ben Gurion University)

The string tensions can be dynamical in the modified measure formalism and appear as an additional dynamical degrees of freedom . These tensions may not be universal, instead, each string generates its own tension. We then consider a new bulk field that can couple to the strings, the tension scalar which changes locally the tension along the world sheet. In the case with two string tensions there is a braneworld solution which gives rise to an induced de Sitter space in the brane, avoiding swampland constraints of the standard string theory. Strings with different tension to ours can appear also as Dark Matter and since they share the same space and compactifications as visible matter, they should lead to Dark copies of the standard model,

L Bixano

Gravitational Waves from Mergers of Asymmetric Dark Stars

• Boris Betancourt Kamenetskaia (Institute for Basic Science CTPU-CGA)

A strongly self-interacting component of asymmetric dark matter (DM) particles can form compact dark stars (DSs). These objects have a broad spectrum of masses and radii, with distinct evolution histories from both neutron stars and black holes (BHs). We argue that these differences allow a population of DSs to contribute significantly to the astrophysical merger rate in unique and discernible ways. Specifically, their merger rate could dominate at low redshifts over other sources, while their mass function may populate windows outside known astrophysical processes. We investigate the structure and formation of DSs within a dissipative model, and calculate the enhancement of their merger cross-section due to tidal deformation effects. From this, we derive the cosmological DS merger rate and discuss the DM parameter space available for observation. These findings open a new window to probe DM substructure and particle interactions through present and future gravitational wave (GW) observatories.

S Mendoza

A León

S Mendoza

L Ureña

J Cervantes

M G Aspeitia

Deformed phase-space for a non-singular cosmology

• ERI ATAHUALPA MENA BARBOZA (Centro Universitario de la Ciénega, Universidad de Guadalajara)

We present the implications of incorporating noncommutativity into f(R) gravity. In particular, we explore the proposed framework in a flat FRLW background. We introduced a deformation in a 2n-dimensional phase space.

Reparametrization Invariance and the Phenomena of Dark Energy & Dark Matter

• Vesselin Gueorguiev (NCIS, RIIS 2.0, and IAPS)

The nature of dark energy (DE) challenges our comprehension of the cosmos, appearing as the source responsible for the Universe’s accelerating expansion. This work considers the Einstein Cosmological Constant (ΛE) as a manifestation of DE, interpreted through the lens of Reparametrization Invariant Scaling Symmetry (RISS). Within this paradigm, ΛE emerges as a “kinetic energy” term, derived from relative temporal motion, setting it apart from the conventional kinetic energy based on spatial relative motion. Central to this exploration is the scale factor λ(t) representing a reparametrization capable of rendering ΛE dynamically within the extended equations of Einstein’s General Relativity(EGR) as Λ=ΛE*λ^2. Through meticulous derivations, the governing equations of λ(t) and its interplay with ΛE are articulated. Imposing reparametrization symmetry on the equations of motion reveals a new avenue for addressing the missing mass problem evident at galactic and extragalactic scales. Here, improper/non-affine (non co-moving) temporal parameterizations introduce fictitious forces, whose presence is reconciled through the symmetry framework. This symmetry-based approach naturally yields the MOND-like relationship, g^2 ∼ (a0gN), where g denotes gravitational acceleration, a0 represents the fundamental MOND acceleration, and gN is the Newtonian acceleration. The theoretical predictions for ΛE and a0 demonstrate remarkable alignment with their observed magnitudes, lending credence to this interpretation. This synthesis underscores a potential unifying principle in our understanding of dark energy and dark matter phenomena.

Cesar Zen