arXiv subjects

Rafael R. Lino dos Santos

Publications and source records attributed to Rafael R. Lino dos Santos.

7 recordsLinked to original sources

arXiv2026

Gravitational waves from the early universe

Even though different models could already be constrained using CMB data, these models remained unconstrained at higher frequency scales, and, therefore, the knowledge of new physics at these scales remained limited, relying on theoretical assumptions and extrapolations. Recently, however, we experienced the advent of gravitational-wave and multi-messenger astronomy, including the outstanding detections by the LIGO-Virgo collaboration and the searches for Hellings-Downs correlations in pulsar timing data. Over the next decades, ongoing and future collaborations will explore different frequency ranges of the gravitational wave spectrum, granting access to regimes inaccessible by the CMB and collider experiments. These include ground-based, space-borne, and pulsar timing array searches for gravitational waves. By comparing their data with model-dependent gravitational-wave predictions, we can unprecedentedly constrain or eliminate different models or even discover new physics. In these lecture notes, as an entrance door for graduate students, we focus on how we can search for a cosmological background of gravitational waves and how this can be used to probe cosmology and beyond-Standard-Model physics. For this purpose, we review the formalism of gravitational waves in General Relativity, introduce stochastic background gravitational waves, and derive the Hellings-Downs correlation for pulsar timing array searches. We comment on detection efforts and present some of the most important cosmological sources that could produce such a background. These lecture notes were inspired by the course "Gravitational Waves from the Early Universe" given at the 27th W.E. Heraeus "Saalburg" Summer School 2021 by Valerie Domcke.

astro-ph.CO↗

arXiv2025

Stabilizing dark matter with quantum scale symmetry

In the context of gauge-Yukawa theories with trans-Planckian asymptotic safety, quantum scale symmetry can prevent the appearance in the Lagrangian of couplings that would otherwise be allowed by the gauge symmetry. Such couplings correspond to irrelevant Gaussian fixed points of the renormalization group flow. Their absence in the theory implies that different sectors of the gauge-Yukawa theory are secluded from one another, in similar fashion to the effects of a global or a discrete symmetry. As an example, we impose the trans-Planckian scale symmetry on a model of Grand Unification based on the gauge group SU(6), showing that it leads to the emergence of several fermionic WIMP dark matter candidates whose coupling strengths are entirely predicted by the UV completion.

hep-ph↗

arXiv2025

The NANOGrav 15 yr Data Set: Running of the Spectral Index

The NANOGrav 15-year data provides compelling evidence for a stochastic gravitational-wave (GW) background at nanohertz frequencies. The simplest model-independent approach to characterizing the frequency spectrum of this signal consists in a simple power-law fit involving two parameters: an amplitude A and a spectral index γ. In this paper, we consider the next logical step beyond this minimal spectral model, allowing for a running (i.e., logarithmic frequency dependence) of the spectral index, γ_run(f) = γ+ β\ln(f/f_ref). We fit this running-power-law (RPL) model to the NANOGrav 15-year data and perform a Bayesian model comparison with the minimal constant-power-law (CPL) model, which results in a 95% credible interval for the parameter βconsistent with no running, β\in [-0.80,2.96], and an inconclusive Bayes factor, B(RPL vs. CPL) = 0.69 +- 0.01. We thus conclude that, at present, the minimal CPL model still suffices to adequately describe the NANOGrav signal; however, future data sets may well lead to a measurement of nonzero β. Finally, we interpret the RPL model as a description of primordial GWs generated during cosmic inflation, which allows us to combine our results with upper limits from big-bang nucleosynthesis, the cosmic microwave background, and LIGO-Virgo-KAGRA.

astro-ph.HE↗

arXiv2024

From quantum gravity to gravitational waves through cosmic strings

New physics beyond the Standard Model can give rise to stochastic gravitational-wave backgrounds, for example through cosmic strings. In this way, gravitational-wave searches with pulsar-timing arrays as well as existing and future laser interferometers may provide information on particle physics beyond the Standard Model. Here, we take one additional step and link particle physics beyond the Standard Model to quantum gravity. We investigate whether particle physics models that may give rise to cosmic strings can be embedded into an asymptotically safe theory of quantum gravity and matter. We focus on models where cosmic strings arise from U(1)-symmetry-breaking in an extended Yukawa-Abelian-Higgs sector that may be part of a dark sector. We find a negative answer for the simplest model that can give rise to cosmic strings and also find constraints on an extended model. We tentatively conclude that cosmic strings are difficult to accommodate in asymptotically safe models. This fits well with the latest 15-year dataset and search for new physics from the NANOGrav collaboration, which disfavors a stable-cosmic-string interpretation. In that sense, the recent data provide an indirect, albeit at present rather tentative, hint about the quantum theory of gravity.

gr-qc↗

arXiv2023

The NANOGrav 15-year Data Set: Search for Signals from New Physics

The 15-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) shows positive evidence for the presence of a low-frequency gravitational-wave (GW) background. In this paper, we investigate potential cosmological interpretations of this signal, specifically cosmic inflation, scalar-induced GWs, first-order phase transitions, cosmic strings, and domain walls. We find that, with the exception of stable cosmic strings of field theory origin, all these models can reproduce the observed signal. When compared to the standard interpretation in terms of inspiraling supermassive black hole binaries (SMBHBs), many cosmological models seem to provide a better fit resulting in Bayes factors in the range from 10 to 100. However, these results strongly depend on modeling assumptions about the cosmic SMBHB population and, at this stage, should not be regarded as evidence for new physics. Furthermore, we identify excluded parameter regions where the predicted GW signal from cosmological sources significantly exceeds the NANOGrav signal. These parameter constraints are independent of the origin of the NANOGrav signal and illustrate how pulsar timing data provide a new way to constrain the parameter space of these models. Finally, we search for deterministic signals produced by models of ultralight dark matter (ULDM) and dark matter substructures in the Milky Way. We find no evidence for either of these signals and thus report updated constraints on these models. In the case of ULDM, these constraints outperform torsion balance and atomic clock constraints for ULDM coupled to electrons, muons, or gluons.

astro-ph.HE↗

arXiv2023

Shift-symmetric Horndeski gravity in the asymptotic-safety paradigm

Horndeski gravity is a popular contender for a phenomenological model of dynamical dark energy, and as such subject to observational constraints. In this work, we ask whether Horndeski gravity can be more than a phenomenological model and instead become a fundamental theory, which extends towards high energy scales and includes quantum effects. We find that within the asymptotic-safety paradigm, an ultraviolet completion of a simple class of models of Horndeski gravity is achievable, but places strong constraints on the couplings of the theory. These constraints are not compatible with dynamical dark energy. Further, we find a similar result in an effective-field theory approach to this class of models of Horndeski gravity: under the assumption that there is no new strongly-coupled physics below the Planck scale, quantum gravity fluctuations force the Horndeski couplings to be too small to achieve an explanation of dynamical dark energy.

gr-qc↗

arXiv2022

Are there ALPs in the asymptotically safe landscape?

We investigate axion-like particles (ALPs) in the context of asymptotically safe gravity-matter systems. The ALP-photon interaction, which facilitates experimental searches for ALPs, is a dimension-5-operator. Quantum fluctuations of gravity lower its scaling dimension, and the ALP-photon coupling can become asymptotically free or even asymptotically safe. However, quantum fluctuations of gravity need to be strong to overcome the canonical scaling and this strong-gravity regime is in tension with the weak-gravity bound in asymptotic safety. Thus, we tentatively conclude that fundamental ALPs can likely not be accommodated in asymptotically safe gravity-matter systems. In turn, an experimental discovery of an ALP would thus shed valuable light on the quantum nature of gravity.

gr-qc↗