arXiv subjects

C. J. A. P. Martins

Publications and source records attributed to C. J. A. P. Martins.

At least 19 recordsLinked to original sources

arXiv2026

Peering Beyond the Veil of Last Scattering: A View of the Universe with CMB Spectral Distortions

The frequency spectrum of the cosmic microwave background is the most precise blackbody ever measured in nature, with deviations constrained at the level of almost one part per million from the COBE satellite. Nevertheless, departures away from a perfect blackbody are present in standard $Λ$CDM cosmology, lurking just beneath the surface of our current observational bounds. These spectral distortions provide invaluable information on our thermal history in both the post- and pre-recombination epochs, allowing us to peer beyond last scattering and into the primordial Universe. Here, we present an overview of the underlying physics responsible for generating CMB spectral distortions at all epochs. As an illustration of this rich physics, we review a comprehensive set of mechanisms capable of generating distortions both within and beyond the standard $Λ$CDM paradigm. We also discuss the information that can be gleaned by going beyond the monopole (sky-averaged) spectrum and exploiting the spatial information present in anisotropic spectral distortions. To supplement our discussion of the diverse science of spectral distortions, we provide an overview of the upcoming and proposed experimental landscape. We highlight that the combination of the TMS, COSMO, and BISOU experiments will provide the first discovery of a monopole $y$-type distortion within the coming decade. From space, the proposed FOSSIL experiment is forecasted to improve upon the original COBE/FIRAS measurement by roughly three orders of magnitude in sensitivity, bringing with it the detection of the $Λ$CDM $μ$-type distortion sourced by the dissipation of small scale acoustic modes in the pre-recombination plasma. With transformational measurements on the horizon, CMB spectral distortions offer a uniquely sensitive probe of the thermal history of the Universe at redshifts $z \lesssim 2 \times 10^6$.

astro-ph.CO↗

arXiv2026

FOSSIL: A future mission for CMB spectral distortion measurements

We present FOSSIL, a proposed space mission which is designed to deliver unprecedented absolute spectroscopic measurements of the sky intensity over a broad frequency range (50GHz to 2THz) to accurately measure chemical-potential mu-type, Compton y-type, and Compton relativistic y-type CMB spectral distortions. FOSSIL will reach monopole sensitivities representing a gain of roughly three orders of magnitude over COBE/FIRAS. These capabilities will allow us to constrain primordial density fluctuations on scales k~(10 to 10^4)Mpc^-1, test inflationary models, probe dark matter annihilation/decay/interactions, constrain primordial black hole seed scenarios, and pin down feedback models in galaxy evolution and structure formation by measuring the cosmic thermal energy content. Furthermore, its 130 spectral channels will enable legacy astrophysics, including precise characterisation of the Cosmic Infrared Background, line intensity mapping during cosmic noon, and detailed Galactic dust mapping.

astro-ph.CO↗

arXiv2026

Cosmic microwave background temperature cosmography

The temperature-redshift relation, $T(z)=T_0(1+z)$, is a cornerstone of the standard cosmological model, but it is violated in many physically motivated extensions thereof, e.g. when photon number is not conserved. We apply the methodology of cosmography, a model-independent approach to cosmology, to this observable, and use astrophysical Sunyaev-Zeldovich and high-resolution spectroscopic measurements spanning the redshift range $0\le z\le6.34$, to constrain violations of the standard relation. We explore the impact of different cosmographic approaches and expansion variable choices and also compare the cosmographic constraints with those obtained, with the same data, in the canonical adiabatic extension model of Lima. While these choices have some impact, overall we find that such violations are constrained to percent level, and that a single non-standard parameter suffices to describe them. We also show that the directly fitted slope of the temperature-redshift relation is compatible with (but slightly smaller than) the one inferred from FIRAS, and that standard cosmography lore, based on scale factor expansions, does not apply to this observable, e.g. Padé approximants have no meaningful advantage over plain Taylor series.

astro-ph.CO↗

arXiv2026

Beyond ΛCDM with the SKA Observatory -- I: Probing Gravity on Cosmological Scales

General relativity (GR) is currently the best description of the gravitational interaction at our disposal and is one of the foundations of the concordance cosmological model. For as much as we know that GR is not the final theory of gravitation - we still lack an understanding of its fundamental, quantum nature - it has demonstrated a remarkable success in describing observed phenomena and predicting effects that have later been confirmed by laboratory experiments or astronomical observations. Since gravity is extremely weak compared to the other three fundamental interactions, it has so far been tested with exquisite precision only in the strong-field regime. On the immense scales of the cosmos, on the other hand, the gravitational field is extremely weak and spacetime curvature is almost negligible. But crucially, it is on these scales that we see hints at the need for exotic components, such as dark matter and dark energy. The question of whether they really exist or their presence is but an artefact of the incompleteness of our understanding of gravity on cosmological scales then naturally arises. It is therefore paramount to test the validity of GR on these scales, either to further confirm its robustness or to detect deviations that could lead us to the formulation of a more general and conclusive theory of gravitation. To this purpose, the SKA Observatory is especially suited, thanks both to the enormous volumes it will probe, and to the variety and complementarity of cosmological observables that its surveys will make available to us.

gr-qc↗

arXiv2026

The small transiting planet population revealed by ESPRESSO with extreme precision radial velocities

Small planets are extremely common in the Galaxy, including planets with masses and radii between those of Earth and Neptune. Characterizing these planets' masses requires ultra-precise radial velocities. The ESPRESSO spectrograph was designed and built for this purpose. We present an overview of the ESPRESSO Guaranteed Time Observations transit follow-up sub-program, aimed at confirming and characterizing small transiting planet candidates from the K2 and TESS space missions. We analyse the global stellar and planetary properties of the sample of 65 planets in 30 systems characterized by this sub-program. This includes six systems presented in this paper, for which we either obtain only upper mass limits, or provide updates to previously published parameters. We also place this sample in the context of the overall population of precisely characterized small planets. Separating the population into insolation regimes, we find a tentative mass threshold at $\simeq6\,M_\oplus}$ for the rocky to volatile-rich composition transition in the medium-insolation regime, and a population of likely stripped massive rocky planets in the high-insolation regime. We likewise find a correlation between planet mass and stellar metallicity, with more massive planets being hosted by more metal-rich stars. We also explore the radius valley, finding that planets below the gap have a tighter mass distribution. We compare planet masses with typical protoplanetary disk masses and draw tentative conclusions about likely formation conditions. Finally, we discuss the impact of our observing strategy on our results. The ESPRESSO transit follow-up sub-program has been highly productive, characterizing a diverse population of small planets that allows us to identify population-level features. Likewise, the lessons learned from this sub-program will be valuable for PLATO follow-up planning.

astro-ph.EP↗

arXiv2026

Star-planet interaction in the Proxima system

(Abridged) We search for evidence of star-planet magnetic interactions in the nearby Proxima Centauri planetary system using high-quality, high-spectral-resolution optical observations. We measure a photospheric stellar rotation period of 84.9 +/- 0.6 d and a half-rotation period of 44.3 +/- 0.2 d, consistent with previous studies. Using FeI absorption and emission lines, we find that Proxima Centauri was flaring during 4.8 +/- 4.7 % of the observing time, with significant statistical evidence (>99.8 %) of flare events likely phase-locked to the inner Mars-mass planet Proxima d. Modeling the star-planet interaction via the helicity-driven reconnection mechanism with the Poynting flux formalism, we estimate a likely polar magnetic field of -16 G for Proxima d (assuming a Mars-sized radius), with a plausible range of 3-280 G accounting for radial and dipolar stellar magnetic field configurations, planetary radii comparable to Mars and Earth, and the observed range of stellar flare intensities. This represents the first such estimate for a terrestrial exoplanet. Evidence for a potential star-planet interaction with the outer, Earth-mass Proxima b arises not from phase-locked flare clustering, but from modulation of flare intensities. Applying a prewhitening analysis to the full time series of combined chromospheric Halpha, NaI D1 and D2, and CaII H &K lines reveals peaks, in order, at half the stellar rotation period, Proxima b's orbital period, the full stellar rotation, and Proxima d's orbital period. All evidence suggests that both planets show magnetic interaction with their host star. Focusing on flaring epochs only, the periodogram of these chromospheric lines shows a peak consistent with the synodic period between half the stellar rotation and the mutual synodic period of Proxima b and d, implying prograde stellar rotation and planetary orbits.

astro-ph.EP↗

arXiv2026

The atmosphere of the warm Neptune GJ 436 b probed with ESPRESSO

Aims. We aim to identify the presence of atomic and molecular species in the upper atmosphere of the warm Neptune-sized transiting planet GJ 436 b, which has a radiative equilibrium temperature of 690 K and a mass of 25.4 Earth masses. Methods. Using transmission spectroscopy, we observed two full transits of GJ 436 b with the ESPRESSO spectrograph, covering the wavelength range from 3800 to 7880 Angstrom. We searched for traces of atomic (H I, Li I, Na I, Mg I, V I, Cr I, Fe I, and Fe II) and molecular (TiO, VO) species by directly detecting planetary absorption features and by cross-correlating the planetary spectrum with theoretical spectra computed for each investigated species. Results. Our analysis reveals no strong planetary detection for any of the species, consistent with a featureless optical spectrum. We derived upper limits by combining all ESPRESSO observations. Post-transit stellar flares were detected on both nights, primarily affecting chromospheric lines. A tentative Fe I signal appears in the first transit (S/N = 3.4 +/- 0.2) at a wind velocity of about -18.6 km/s, which is unexpectedly large for a cool planet. This weak signal is not present in the second transit and, combined with its low significance, suggests an origin in noise. In the less probable scenario where the feature is suppressed during the second transit by the higher stellar activity state, the T1 tentative signal peaks at 1300 K, which is above the equilibrium temperature of GJ 436 b. Ultimately, this result would imply a neutral iron abundance comparable to or exceeding that of the host star.

astro-ph.EP↗

arXiv2026

Probing Unification Scenarios with Big Bang Nucleosynthesis

We extend a recently developed Big Bang Nucleosynthesis (BBN) code, {\tt PRyMordial}, to constrain a broad class of Grand Unified Theories to which BBN is sensitive, since these lead to varying fundamental couplings. A previously developed self-consistent perturbative analysis of the effects of these variations has been implemented in {\tt PRyMordial}, leading to robust constraints of the value of the fine-structure constant, $α$, at the BBN epoch using current observations of Helium-4 and Deuterium abundances. We explored two different viable scenarios, relying on alternative assumptions on the gravitational sector: the variation of the gravitational coupling can be implemented by varying either particle masses, or Newton's gravitational constant. For the variation of masses, we obtained at $68\%$ confidence level a constraint on the relative variation of $α$, between the BBN epoch and the present-day laboratory value, of $Δα/α=2\pm51$ ppm (parts per million), while for the variation of Newton's constant the analogous constraint is $Δα/α=2\pm22$ ppm. We also show that, given these constraints, these models do not provide a solution to the cosmological Lithium problem.

hep-ph↗

arXiv2026

Current-carrying string network evolution in an external magnetic field

Cosmic strings are topological defects arising in a variety of cosmological scenarios, as the Universe undergoes symmetry-breaking phase transitions, whose discovery would offer valuable insight into the high-energy physics that shaped the early Universe. To interpret such a detection, robust theoretical models are essential. The Velocity-dependent One-Scale (VOS) model is particularly prominent: it self-consistently treats the network as a thermodynamic system, characterizing its key properties, and predicting its large-scale evolution. This has recently been extended to include superconducting cosmic strings, which carry additional degrees of freedom, giving rise to the Charge-Velocity dependent One-Scale (CVOS) model. One limitation of the latter model is that it only included loss mechanisms for the charge and current. Here, we extend this model by phenomenologically including a possible energy source mechanism, specifically by allowing current-carrying strings interactions with an external magnetic field. We discuss how this coupling impacts the network's evolution, present and classify the physically allowed scaling solutions for this extended CVOS model, and comment on the different impacts of the external magnetic field on the evolution of the network's charge and current. Under our modeling assumptions, one of the ten physically plausible scaling solution enables the network to gain energy by this mechanism.

hep-ph↗

arXiv2026

Scaling solutions for varying tension strings

We use the Velocity-dependent One Scale Model for topological defect evolution to explore and classify the possible scaling solutions for string networks with time-varying tension, in cosmological and non-cosmological settings and under two different phenomenological assumptions for the behavior of these variations, which rely on different stretching and damping contributions to the string dynamics. We discuss how these assumptions impact the standard scaling solutions, as well as the evolution of the string network density. In addition to simple power-law cosmological epochs solutions, we also discuss the behavior of the network during the radiation-to-matter and matter-to-acceleration transitions. Overall, our results show that for the same amount of tension variation, a change in the stretching length scale tends to have a more significant impact on the network than a change in the damping length.

hep-ph↗

arXiv2026

Precise measurement of WASP-31 b's Rossiter-McLaughlin effect and characterization of the planet transmission spectra

Context. Hot Jupiters are ideal natural laboratories to investigate atmospheric composition and dynamics. However, high-resolution transmission spectroscopy is currently limited by our capability of removing planet-occulted line-distortion (POLD) contamination from the signal. Aims. In this paper, we aim to characterize the transmission spectrum of WASP-31 b from two and a half transits observed with the ESPRESSO spectrograph at the VLT. Methods. The Rossiter-McLaughlin (RM) signature was analyzed using the RM "revolutions" method. Before extracting the transmission spectrum of the planet, we corrected the dataset for telluric lines using molecfit and further modeled the POLD deformations using EvE. Results. We confirm the planet low sky-projected spin-orbit angle from previous studies and further refine its value to $λ= -0.09^{+0.31}_{-0.32}$ deg. We do not detect any species (including previously detected species such as K or CrH) in the planetary atmosphere. In most cases the non-detections are due to the strong POLDs contamination or lack of observable lines in the ESPRESSO wavelength range, and so previous detections cannot be ruled out. Conclusions. Planet-occulted line-distortion contamination continues to be the main limitation of high-resolution transmission spectroscopy for species present in both the star and the planet, hindering atmospheric detections even with state-of-the-art models, in particular for planets with a low sky-projected spin-orbit angle. Developing advanced techniques to isolate planetary signatures is of utmost importance in the advent of ELT-like observations.

astro-ph.EP↗

arXiv2026

How to Improve Portuguese Secondary Education

I share some personal thoughts on the status of Portuguese secondary education in general, and of the physics part thereof in particular, drawn from several decades of experience of organizing training activities for students and school teachers, as well as several hundred visits to secondary schools and similar numbers of interviews of secondary school students (from Portugal and elsewhere). I offer various suggestions for improving and modernising our current system.

physics.ed-ph↗

arXiv2025

Euclid preparation. Review of forecast constraints on dark energy and modified gravity

The Euclid mission has been designed to provide, as one of its main deliverables, information on the nature of the gravitational interaction, which determines the expansion of the Universe and the formation of structures. Thus, Euclid has the potential to test deviations from general relativity that will allow us to shed light on long-lasting problems in the standard cosmological model, $Λ$CDM. Euclid will mainly do this by using two complementary probes: weak gravitational lensing and galaxy clustering. In this paper we review pre-launch Euclid analyses for dark energy and modified gravity. These include forecast constraints with future Euclid data on cosmological parameters for different cosmological models, such as a time-varying dark energy component, phenomenological modifications of the perturbation sector and specific modified gravity models, with further extensions that include neutrino physics and the coupling to the electromagnetic sector through the fine-structure constant. We review the study of the impact of nonlinear clustering methods on beyond-$Λ$CDM constraints with Euclid. This is of fundamental importance to efficiently predict the large-scale clustering of matter and dark matter halos, given that we will have access to a wealth of information on scales beyond the linear regime. We inspect the extension of theoretical predictions for observable quantities in alternative cosmologies to $Λ$CDM at fully nonlinear scales by means of $N$-body simulations. We discuss the impact of relativistic corrections in extended cosmological models. Overall, this review highlights the significant potential of the Euclid mission to tightly constrain parameters of dark energy and modified gravity models, or perhaps to detect possible signatures of a $Λ$CDM failure.

astro-ph.CO↗

arXiv2025

Cosmologically Viable Solutions in Geometric Modified Gravity

The discovery of the accelerated expansion of the universe highlighted General Relativity's inability to naturally account for dark energy without invoking a finely tuned cosmological constant. In response, a wide range of alternative paradigms have been proposed. Among these, Teleparallel Gravity and Symmetric Teleparallel Gravity, which depart from the Riemannian framework of General Relativity and instead rely on torsion or non-metricity to describe gravitational interactions, have gained increasing attention in recent years. We explore extensions of these non-Riemannian approaches, aiming to replicate the observed late-time acceleration of the universe by emulating the cosmological constant's role. We also evaluate the consistency of these theories with local gravity constraints by studying their static, spherically symmetric solutions. We show that although some models can reproduce the desired cosmological behavior, they often fail to meet Solar System observational bounds, particularly through deviations in the predicted Eddington parameter. Our findings underscore the need for a unified approach that tests modified gravity theories across both cosmological and local scales.

gr-qc↗

arXiv2025

Unveiling the nature of HE 0107-5240

The vast majority of the most iron-poor stars in the Galaxy exhibit a strong carbon enhancement, with C/H ratios only about two orders of magnitude below solar. This unusual chemical composition likely reflects the properties of the gas cloud from which these stars formed, having been enriched by one, or at most a few, supernovae. A remarkable member of this stellar class, HE 0107-5240 with [Fe/H]=-5.56, has been identified as part of a binary system. To constrain its orbital parameters, radial velocity monitoring has been carried out using the ESPRESSO spectrograph. Radial velocities were derived using cross-correlation with a template, taking advantage of the strong G-band feature. Combining all observations yielded a high signal-to-noise spectrum, which has been used to refine our understanding of the stellar chemical composition. Additionally, a co-added UVES spectrum in the blue was used to complement the wavelength coverage of ESPRESSO. Observations of HE 0107-5240 over a span of more than four years have yielded a revised orbital period of about 29 years. Updated elemental abundances have been determined for Sc, Cr, Co, and, tentatively, Al, along with a new upper limit for Be. The iron abundance has been derived from ionised Fe lines. Significant upper limits have been established for Li, Si, and Sr. The star is confirmed to be a long-period binary. Iron abundances derived from neutral and ionised lines are consistent with local thermodynamical equilibrium (LTE) assumption, casting doubt on published deviation from LTE corrections for Fe for this star. The heavy elements Sr and Ba remain undetected, confirming the classification of HE 0107-5240 as a carbon enhanced metal-poor and non enhanced in heavy elements (CEMP-no) star and supporting the absence of an n-capture element plateau at the lowest metallicities.

astro-ph.SR↗

arXiv2025

TOI-283 b: A transiting mini-Neptune in a 17.6-day orbit discovered with TESS and ESPRESSO

Super-Earths and mini-Neptunes are missing from our Solar System, yet they appear to be the most abundant planetary types in our Galaxy. A detailed characterization of key planets within this population is important for understanding the formation mechanisms of rocky and gas giant planets and the diversity of planetary interior structures. In 2019, NASA's TESS satellite found a transiting planet candidate in a 17.6-day orbit around the star TOI-283. We started radial velocity (RV) follow-up observations with ESPRESSO to obtain a mass measurement. Mass and radius are measurements critical for planetary classification and internal composition modeling. We used ESPRESSO spectra to derive the stellar parameters of the planet candidate host star TOI-283. We then performed a joint analysis of the photometric and RV data of this star, using Gaussian processes to model the systematic noise present in both datasets. We find that the host is a bright K-type star ($d = 82.4$ pc, $\mathrm{T}_\mathrm{eff} = 5213 \pm 70$ K, $V = 10.4$ mag) with a mass and radius of $\mathrm{M}_\star = 0.80 \pm 0.01\; \mathrm{M}_\odot$ and $\mathrm{R}_\star = 0.85 \pm 0.03\; \mathrm{R}_\odot$. The planet has an orbital period of $P = 17.617$ days, a size of $\mathrm{R}_\mathrm{p} = 2.34 \pm 0.09\; \mathrm{R}_\oplus$, and a mass of $\mathrm{M}_\mathrm{p} = 6.54 \pm 2.04\; \mathrm{M}_\oplus$. With an equilibrium temperature of $\sim$600 K and a bulk density of $ρ_\mathrm{p} = 2.81 \pm 0.93$ g cm$^{-3}$, this planet is positioned in the mass-radius diagram where planetary models predict H$_2$O- and H/He-rich envelopes. The ESPRESSO RV data also reveal a long-term trend that is probably related to the star's activity cycle. Further RV observations are required to confirm whether this signal originates from stellar activity or another planetary body in the system.

astro-ph.EP↗

arXiv2025

Stellar-activity analysis of the nearby M dwarf GJ 526. Multi-dimensional Gaussian-process modelling of RV, FWHM and S-index

M dwarfs are the most abundant stars in the Galaxy, and their low masses make them natural favourites for exoplanet radial-velocity (RV) searches. Nevertheless, these stars often demonstrate strong stellar activity that is yet to be fully understood. We use high-precision ESPRESSO, HIRES, and HARPS spectroscopy to perform a stellar-activity analysis on the nearby early M dwarf GJ 526 ($d=5.4$ pc). We carry out joint modelling of: (i) stellar rotation in RV, FWHM, and Mount Wilson S-index through Gaussian processes, (ii) long-term trends in these three physical quantities, (iii) RV planetary signals. We constrain the stellar-rotation period to $P_\text{rot}=48.7\pm 0.3$ d, and discover a weak sinusoidal signature in RV, FWHM and S-index of period $P_\text{cyc}=1680^{+50}_{-40}$ d. We propose phase-space trajectories between RV and activity indicators as a novel means to visualise and interpret stellar activity. Current evidence does not support planetary companions of GJ 526.

astro-ph.SR↗

arXiv2025

Varying fundamental constants cosmography

Cosmography is a model-independent phenomenological approach to observational cosmology, relying on Taylor series expansions of physical quantities as a function of the cosmological redshift or other analogous variables. A recent work developed the formalism for a cosmographic analysis of astrophysical and local measurements of the fine-structure constant, $α$, and provided first constraints on the corresponding parameters. Here we update the earlier work, both by including more recent measurements of $α$, and by extending it to other fundamental dimensionless couplings. We find no statistically significant evidence for such variations, and place stringent constraints on the first two terms of all these cosmographic series: the linear coefficient in the $α$ series is constrained to parts per billion level, thanks to recently improved atomic clock constraints, while the other coefficients are constrained to parts per million level. Additionally, we use the same data to place cosmographic constraints on models from a broad class of Grand Unified Theories in which varying fundamental constants occur, and highlight how future data can provide a discriminating cosmographic test between freezing and thawing dark energy models.

astro-ph.CO↗