arXiv subjects

Fernando Quevedo

Publications and source records attributed to Fernando Quevedo.

At least 19 recordsLinked to original sources

arXiv2025

Cosmology, Decoherence and the Second Law

We consider quantum decoherence and entropy increase in early universe cosmology. We first study decoherence in a discrete bipartite quantum system for which a single qubit gets entangled with an environment and the entropy increase is correlated with the decay of the off-diagonal terms of the reduced density matrix. We compare this system with continuous systems relevant for cosmology for which there is a natural external intervention, corresponding to the time-dependent separation between the sub- and super-horizon inflationary fluctuations. We find, in this case, that the off-diagonal terms of the density matrix, in a field basis, do not decay as sometimes assumed in cosmological set-ups. Nevertheless, following a recent treatment in terms of open Effective Field Theories (EFTs), we compute the entanglement entropy for a Gaussian state and show that it actually increases monotonically ($\dot S>0$) during the accelerated phases ($\ddot a>0$ with $a(t)$ the scale factor). We generalise this result to include non-Gaussian states and briefly discuss the relevance of computing the von Neumann entropy as compared to the thermodynamic entropy.

hep-th↗

arXiv2025

Rediscovering the Standard Model with AI

We investigate whether artificial intelligence can autonomously recover known structures of the Standard Model of particle physics using only experimental data and without theoretical inputs. By applying unsupervised machine learning techniques -- including data dimensionality reduction and clustering algorithms -- to intrinsic particle properties and decay modes, we uncover key organizational features of particle physics, such as the relative strength of different interactions and the difference between baryons and mesons. We also identify conserved quantities such as baryon number, strangeness and charm as well as the structure of isospin and the Eightfold Way multiplets. Our analysis then reveals that clustering can separate particles by interaction, flavor symmetries as well as quantum numbers. Additionally, we observe patterns consistent with Regge trajectories in baryon excitations. Our results demonstrate that machine learning can reproduce key aspects of the Standard Model directly from data, suggesting a promising path toward data-driven discovery in fundamental physics.

hep-ph↗

arXiv2025

Coexisting Flux String Vacua from Numerical K\"ahler Moduli Stabilisation

We present a comprehensive study of K\"ahler moduli stabilisation in Type IIB flux compactifications, combining advanced numerical techniques with analytical methods. Our JAX-based computational framework enables efficient scanning of the UV parameter space, while incorporating $\alpha'$ corrections, loop and non-perturbative effects, as well as uplift contributions to the scalar potential. The implementation features rigorous vacuum validation protocols derived from analytic results. We apply our methods to explicit flux compactifications on more than 80,000 Calabi-Yau threefolds with $h^{1,1}\leq 6$ K\"ahler moduli. By systematically scanning over a wide range of values of the flux superpotential $W_0$ and the string coupling $g_s$, we find explicit realisations of every established K\"ahler moduli stabilisation scenario: for $10^{-15} \leq |W_0| \leq 10^{-2}$ we obtain both KKLT-like and K\"ahler uplifted vacua, while for the broader range $10^{-1} \leq |W_0| \leq 10^2$ we recover LVS as well as LVS-like hybrid solutions. Notably, we discover significant parameter regions where multiple vacua coexist within a single flux potential, including novel configurations pairing AdS, Minkowski, and dS minima with different volume hierarchies. These findings enable, for the first time, the analysis of vacuum decay processes within fixed flux configurations, complementing the established theory of transitions between distinct flux vacua and decays towards decompactification.

hep-th↗

arXiv2025

Statistical Analysis of Scientific Metrics in High Energy, Cosmology, and Astroparticle Physics in Latin America

We perform a comprehensive statistical analysis of key scientific metrics to evaluate the productivity and impact of research conducted in Latin American countries within the fields of High Energy Physics, Cosmology and Astroparticle Physics (HECAP). Using data from the widely used open-access digital library INSPIRE-HEP, we provide a detailed assessment of the scientific contributions from the continent over the past 70 years. We provide data for the evolution of the overall productivity in the region relative to the rest of the world, comparing the productivity of each country, number of active researchers, number of publications, citations, h-index in total and relative to the population and number of researchers, as well as the productivity and impact compared to the percentage of Gross Domestic Product (GDP) invested in research, and the Human Development Index (HDI) of each country. We also analyse collaborations among the different countries, as well as collaborations with the rest of the world. Additionally, we studied the gender gap evolution over the same period. This pioneering analysis, which relies solely on open data, can serve as an essential resource for researchers and policymakers alike. It aims to empower scientists with insights into the significance of their contributions to both regional and global research. Moreover, it provides both researchers and policymakers with critical quantitative data, strengthening their understanding of the progress in scientific productivity over the years to better support scientific endeavours in Latin America.

physics.soc-ph↗

arXiv2025

Challenges and Opportunities of Gravitational Wave Searches above 10 kHz

The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines the challenges and gains expected in gravitational-wave searches at frequencies above the LIGO/Virgo band. The scarcity of possible astrophysical sources in most of this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising of these sources. We review several detector concepts that have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of a series of workshops on the topic of high-frequency gravitational wave detection, held in 2019 (ICTP, Trieste, Italy), 2021 (online) and 2023 (CERN, Geneva, Switzerland).

gr-qc↗

arXiv2024

Back to the origins of brane-antibrane inflation

We study a new framework for brane-antibrane inflation where moduli stabilisation relies purely on perturbative corrections to the effective action. This guarantees that the model does not suffer from the eta-problem. The inflationary potential has two contributions: the tension of an antibrane at the tip of a warped throat, and its Coulomb interaction with a mobile brane. This represents the first realisation of the original idea of brane-antibrane inflation, as opposed to inflection point inflation which arises when the moduli are fixed with non-perturbative effects. Moreover, we formulate the brane-antibrane dynamics as an F-term potential of a nilpotent superfield in a manifestly supersymmetric effective theory. We impose compatibility with data and consistency conditions on control over the approximations and find that slow-roll inflation can occur in a large region of the underlying parameter space. The scalar spectral index is in agreement with data and the tensor-to-scalar ratio is beyond current observational reach. Interestingly, after the end of inflation the volume mode can, but does not need to, evolve towards a late-time minimum at larger values.

hep-th↗

arXiv2024

Cambridge Lectures on The Standard Model

These lecture notes cover the Standard Model (SM) course for Part III of the Cambridge Mathematical Tripos, taught during the years 2020-2023. The course comprised 25 lectures and 4 example classes. Following a brief historical introduction, the SM is constructed from first principles. We begin by demonstrating that essentially only particles with spin/helicity $0, \frac{1}{2}, 1, \frac{3}{2}, 2$ can describe matter and interactions, using spacetime symmetries, soft theorems, gauge redundancies, Ward identities, and perturbative unitarity. The remaining freedom lies in the choice of the Yang-Mills gauge group and matter representations. Effective field theories (EFTs) are a central theme throughout the course, with the 4-Fermi interactions and chiral perturbation theory serving as key examples. Both gravity and the SM itself are treated as EFTs, specifically as the SMEFT (Standard Model Effective Field Theory). Key phenomenological aspects of the SM are covered, including the Higgs mechanism, Yukawa couplings, the CKM matrix, the GIM mechanism, neutrino oscillations, running couplings, and asymptotic freedom. The discussion of anomalies and their non-trivial cancellations in the SM is detailed. Simple examples of calculations, such as scattering amplitudes and decay rates, are provided. The course concludes with a brief overview of the limitations of the SM and an introduction to the leading proposals for physics beyond the Standard Model.

hep-th↗

arXiv2024

Gravitational Waves from High Temperature Strings

We study finite temperature effects in string cosmology and their potential gravitational wave signature. Expanding on our recent work arXiv:2310.11494 , we consider a general configuration of highly excited open and closed strings at high enough temperature to be in the Hagedorn phase in 3+1 dimensions, in order to explore its cosmological implications. We find conditions, which can be satisfied in compactifications with moduli stabilization, that allow the long strings to remain in equilibrium in a controlled effective field theory, with equilibration driven by the joining and splitting of the dominant open string population. We calculate the emission rate of gravitons by long open strings, which we show is determined by ten dimensional flat space transition amplitudes available in the literature, and then find the total gravitational wave spectrum generated by the gas of long strings. The gravitational wave spectrum has robust characteristics. It peaks at frequencies of order 50-100 GHz, the same as for gravitational waves from the reheating epoch of the Standard Model. But the amplitude of the string signal is significantly larger than predicted by the Standard Model and its field theoretic extensions. The amplitude and other physical observables (such as the contribution to $\Delta N_{\text eff}$) are directly proportional to the string scale $M_s$; indicating that a potential signal may also determine the string scale. Our calculations provide one of the few examples of a signal of stringy origin that dominates over the field theory predictions. We give a physical explanation of our results and discuss further implications.

hep-th↗

arXiv2024

Personal Reminiscences of Steven Weinberg

My personal recollections are presented regarding my interactions with Steven Weinberg and the impact he had in my career from when I was his graduate student until the present.

physics.hist-ph↗

arXiv2023

Moduli Stabilization in String Theory

We give an overview of moduli stabilization in compactifications of string theory. We summarize current methods for construction and analysis of vacua with stabilized moduli, and we describe applications to cosmology and particle physics. This is a contribution to the Handbook of Quantum Gravity.

hep-th↗

arXiv2023

String Thermodynamics In and Out of Equilibrium: Boltzmann Equations and Random Walks

We revisit the study of string theory close to the Hagedorn temperature with the aim towards cosmological applications. We consider interactions of open and closed strings in a gas of D$p-$branes, and/or one isolated D$p$-brane, in an arbitrary number $d$ of flat non-compact dimensions and general compact dimensions. Leading order string perturbation theory is used to obtain the basic interaction rates in a flat background, which are shown to be consistent with the random walk picture of highly excited strings that should apply in more general backgrounds. Using the random walk interpretation we infer the structure of more general semi-inclusive string scattering rates and then write down the corresponding Boltzmann equations describing ensembles of highly excited closed and open strings. We organise the interaction terms in Boltzmann equations so that detailed balance becomes manifest. We obtain the equilibrium solutions and show that they reduce to previously computed solutions for $d=0$. We further study the behaviour of non-equlibrium fluctuations and find explicit analytic expressions for the equilibration rates (and for the number of open strings in $d=0$). Potential implications for an early universe with strings at high temperatures are outlined.

hep-th↗

arXiv2023

Higher Derivative Corrections to String Inflation

We quantitatively estimate the leading higher derivative corrections to ${\mathcal{N}}=1$ supergravity derived from IIB string compactifications and study how they may affect moduli stabilisation and LVS inflation models. Using the Kreuzer-Skarke database of 4D reflexive polytopes and their triangulated Calabi-Yau database, we present scanning results for a set of divisor topologies corresponding to threefolds with $1 \leq h^{1,1} \leq 5$. In particular, we find several geometries suitable to realise blow-up inflation, fibre inflation and poly-instantons inflation, together with a classification of the divisors topologies for which the leading higher derivative corrections to the inflationary potential vanish. In all other cases, we instead estimate numerically how these corrections modify the inflationary dynamics, finding that that they do not destroy the predictions for the main cosmological observables.

hep-th↗

arXiv2023

Quantum Transitions, Detailed Balance, Black Holes and Nothingness

We consider vacuum transitions by bubble nucleation among 4D vacua with different values and signs of the cosmological constant $\Lambda $, including both up and down tunnelings. Following the Hamiltonian formalism, we explicitly compute the decay rates for all possible combinations of initial and final values of $\Lambda $ and find that up-tunneling is allowed starting not only from dS spacetime but also from AdS and Minkowski spacetimes. We trace the difference with the Euclidean approach, where these transitions are found to be forbidden, to the difference of treating the latter spacetimes as pure (vacuum) states rather than mixed states with correspondingly vanishing or infinite entropy. We point out that these transitions are best understood as limits of the corresponding transitions with black holes in the zero mass limit $M\rightarrow 0$. We find that detailed balance is satisfied provided we use the Hartle-Hawking sign of the wave function for nucleating space-times. In the formal limit $\Lambda \rightarrow -\infty $, the transition rates for AdS to dS agree with both the Hartle-Hawking and Vilenkin amplitudes for the creation of dS from nothing. This is consistent with a proposal of Brown and Dahlen to define `nothing' as AdS in this limit. For $M\neq 0$ detailed balance is satisfied only in a range of mass values. We compute the bubble trajectory after nucleation and find that, contrary to the $M=0$ case, the trajectory does not correspond to the open universe slicing of dS. We briefly discuss the relevance of our results to the string landscape.

hep-th↗

arXiv2023

String Cosmology: from the Early Universe to Today

We review applications of string theory to cosmology, from primordial times to the present-day accelerated expansion. Starting with a brief overview of cosmology and string compactifications, we discuss in detail moduli stabilisation, inflation in string theory, the impact of string theory on post-inflationary dynamics (reheating, moduli domination, kination), dark energy (the cosmological constant from a string landscape and models of quintessence) and various alternative scenarios (string/brane gases, the pre big-bang scenario, rolling tachyons, ekpyrotic/cyclic cosmologies, bubbles of nothing, S-brane and holographic cosmologies). The state of the art in string constructions is described in each topic and, where relevant, connections to swampland conjectures are made. The possibilities for novel particles and excitations (axions, moduli, cosmic strings, branes, solitons, oscillons and boson stars) are emphasised. Implications for the physics of the CMB, gravitational waves, dark matter and dark radiation are discussed along with potential observational signatures.

hep-th↗

arXiv2023

Testing BSM Physics with Gravitational Waves

The Cosmic Gravitational Wave Background (CGWB) is an irreducible background of gravitational waves generated by particle exchange in the early Universe plasma. Standard Model particles contribute to such a stochastic background with a peak at $f\sim 80$ GHz. Any physics beyond the Standard Model (BSM) may modify the CGWB spectrum, making it a potential testing ground for BSM physics. We consider the impact of general BSM scenarios on the CGWB, including an arbitrary number of hidden sectors. We find that the largest amplitude of the CGWB comes from the sector that dominates the energy density after reheating and confirm the dominance of the SM for standard cosmological histories. For non-standard cosmological histories, such as those with a stiff equation of state $\omega >1/3$, like in kination, BSM physics may dominate and modify the spectrum substantially. We conclude that, if the CGWB is detected at lower frequencies and amplitudes compared to that of the SM, it will hint at extra massive degrees of freedom or hidden sectors. If it is instead measured at higher values, it will imply a period with $\omega >1/3$. We argue that for scenarios with periods of kination in the early Universe, a significant fraction of the parameter space can be ruled out from dark radiation bounds at BBN.

hep-ph↗

arXiv2022

Light Axiodilatons: Matter Couplings, Weak-Scale Completions and Long-Distance Tests of Gravity

We consider the physical implications of very light axiodilatons motivated by a novel mechanism to substantially reduce the vacuum energy proposed in arXiv:2110.10352. We address the two main problems concerning the light axiodilaton that appears in the low-energy limit, namely that the axion has a very low decay constant $f_a \sim $ eV (as read from its kinetic term) and that the dilaton is subject to bounds that are relevant to tests of GR once $\rho_{\rm vac} \leq 10^{-80} M_p^4$. We show that eV scale axion decay constants need not be a problem by showing how supersymmetric extra dimensions provide a sample unitarization for axion physics above eV scales for which non-anomalous matter/axiodilaton couplings can really have gravitational strength, showing how naive EFT reasoning can mistakenly overestimates axion interactions at eV. When axions really do couple strongly at eV scales we identify the dimensionless interaction in the UV completion that is also O(1), and how axion energy-loss bounds map onto known extra-dimensional constraints. We find a broad new class of exact exterior solutions to the vacuum axiodilaton equations and knowledge of axiodilaton-matter couplings also allows us to numerically search for interior solutions that match to known exterior solutions that can evade solar-system tests. We find no examples that do so, but also identify potential new candidate mechanisms for reducing the effective dilaton-matter coupling to gravitating objects without also undermining the underlying suppression of $\rho_{\rm vac}$.

hep-ph↗

arXiv2022

Formal Theory: Status and Outlook

A very brief overview is presented on some of the current most active areas of research in formal aspects of High Energy Physics and Cosmology. Including the recent breakthrough on the black hole information paradox, developments on amplitudes, the bootstrap and swampland programmes as well as progress towards realistic UV complete models of particle physics and cosmology. Perspectives on future contact with observations are discussed, emphasising the long term prospect for ultra high frequency gravitational waves to test early universe physics beyond the Standard Model.

hep-ph↗

arXiv2022

Vacuum Transitions in Two-Dimensions and their Holographic Interpretation

We calculate amplitudes for 2D vacuum transitions by means of the Euclidean methods of Coleman-De Luccia (CDL) and Brown-Teitelboim (BT), as well as the Hamiltonian formalism of Fischler, Morgan and Polchinski (FMP). The resulting similarities and differences in between the three approaches are compared with their respective 4D realisations. For CDL, the total bounce can be expressed as the product of relative entropies, whereas, for the case of BT and FMP, the transition rate can be written as the difference of two generalised entropies, ultimately enabling to circumvent the need to resort to detailed balance. By means of holographic arguments, we show that the Euclidean methods, as well as the Lorentzian cases without non-extremal black holes, provide examples of an AdS$_2$/CFT$_1 \subset $ AdS$_3$/CFT$_2$ correspondence. Such embedding is not possible in the presence of islands for which the setup corresponds to AdS$_2$/CFT$_1 \not\subset $ AdS$_3$/CFT$_2$. We find that whenever an island is present, up-tunnelling is possible.

hep-th↗