Person: Monjo Agut, Robert
Loading...
First Name
Robert
Last Name
Monjo Agut
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Matemáticas
Department
Álgebra, Geometría y Topología
Area
Geometría y Topología
Identifiers
7 results
Search Results
Now showing 1 - 7 of 7
Item Geometric perspective for explaining Hubble tension: theoretical and observational aspects(Classical and Quantum Gravity, 2023) Monjo Agut, Robert; Campoamor Stursberg, Otto-RudwigThe Universe expansion rate has two different but very precise values (67.4 ± 0.5 and 73.30 ± 1.04 km s−1Mpc−1) that are not compatible. This problem, known as a Hubble tension, adds to other cosmological questions such as the origin of dark energy and the flatness problem. In turn, alternative models have attempted to explain similar phenomena but without dark energy. The aim of this work was to explain the Hubble tension by using a geometrical interpretation of observational viewpoints in embedded manifolds. Our technique consists of a set of parametric projections of radially inhomogeneous metrics, linking indistinguishable behaviours of accelerated flat and non-accelerated closed universes. A dark-energy-like phenomenon emerges from the distortion of matter-independent hyperconical metrics. To contrast our model, numerical solutions of dark energy/matter densities and Hubble parameter were obtained and compared to the standard model fitted to the Pantheon Supernovae Ia sample and in contrast to the SH0ES LMC Cepheid findings. Finally, Hubble tension is modelled by the different extrinsic/intrinsic viewpoints of the manifold. Compared to the Planck Legacy’s 2018 release of H0 = 67.4 ± 0.5 km s−1Mpc−1, we found ΛCDM-dependent (intrinsic) ranges between 66.38 and 68.87 km s−1Mpc−1, which were theoretically derived by setting local compatibility of metrics. The ΛCDM-independent extrinsic viewpoint resulted in a Hubble parameter between 73 and 74 km s−1Mpc−1 (compared to H0 = 73.30 ± 1.04 km s−1Mpc−1 of SH0ES). Datasets of 1048 Pantheon Type Ia supernovae (0.0101 < z < 2.26) and 34 cosmic chronometers combined with 7 radial baryon acoustic oscillation sizebased samples (0.0708 < z < 1.965) were used to constraint the model. According to this geometrical perspective, dark parameters (energy and matter) could partially or totally be considered ‘apparent physical quantities’, a consequence of the stereographic projection of the extrinsic curvature.Item Galaxy rotation curve in hyperconical universes: a natural relativistic MOND(Classical and Quantum Gravity, 2023) Monjo Agut, RobertModified Newtonian dynamics (MOND) and similar proposals can (at least partially) explain the excess rotation of galaxies or the equivalent mass-discrepancy acceleration, without (or by reducing) the requirement of dark matter halos. This paper develops a modified gravity model to obtain local limit to the general relativity (GR) compatible with a cosmological metric different to the standard Friedmann–Lemaître–Robertson–Walker metric. Specifically, the paper uses a distorted stereographic projection of hyperconical universes, which are 4D hypersurfaces embedded into 5D Minkowski spacetime. This embedding is a key in the MOND effects found in galactic scales. To adequately describe the mass-discrepancy acceleration relation, centrifugal force would present a small time-like contribution at large-scale dynamics due to curvature of the Universe. Therefore, the Lagrangian density is very similar to the GR but with subtracting the background curvature (or vacuum energy density) of the perturbed hyperconical metric. Results showed that the proposed model adjusts well to 123 galaxy rotation curves obtained from the Spitzer Photometry and Accurate Rotation Curves database, using only a free parameter.Item Anomalous radial acceleration of galaxies and clusters supports hyperconical modified gravity(2014) Monjo Agut, Robert; Banik, IndranilGeneral relativity (GR) is the most successful theory of gravity, with great observational support at local scales. However, to keep GR valid at over cosmic scales, some phenomena (such as the flat galaxy rotation curves and the cosmic acceleration) require the assumption of exotic dark matter. The radial acceleration relation (RAR) indicates a tight correlation between dynamical mass and baryonicmass in galaxies. This suggests that the observations could be better explained by modified gravity theories without exotic matter. Modified Newtonian Dynamics (MOND) is an alternative theory that was originally designed to explain flat galaxy rotation curves by using a new fundamental constant acceleration a0, the so-called Milgromian parameter. However, this non-relativistic model is too rigid (with insufficient parameters) to fit the large diversity of observational phenomena. In contrast, a relativistic MOND-like gravity naturally emerges from the hyperconical model, which derives a fictitious acceleration compatible with observations. This study analyses the compatibility of the hyperconi cal model with respect to RAR observations of 10 galaxy clusters obtained from HIFLUGCS and 60 high-quality SPARC galaxy rotation curves. The results show that a general relation can be fitted to most cases with only one or two parameters, with an acceptable chi-square and p-value. These findings suggest a possible way to complete the proposed modification of GR on a cosmic scale.Item Fractal geometry in precipitation(2024) Monjo Agut, Robert; Meseguer Ruiz, OliverRainfall, or more generally the precipitation process (flux), is a clear example of chaotic variables resulting from a highly nonlinear dynamical system, the atmosphere, represented by a set of physical equations such as the Navier-Stokes equations, energy balances and hydrological cycle among others. As a generalization of the Euclidean (ordinary) measurements, chaotic solutions of these equations are characterized by fractal dimensions, which are non-integer values that represent the complexity of variables like the precipitation. However, observed precipitation is measured as an aggregate variable over time, thus physical analysis of the observed fluxes is very limited. Therefore, this review aims to go through the different approaches used in the identification and analysis of the complexity of the observed precipitation, taking advantage of its geometry footprint. To address the review, it ranges from classical perspectives of fractal-based techniques to new perspectives at temporal and spatial scales as well as for classification of climatic features, including monofractal dimension, multifractal approaches, Hurst exponent, Shannon entropy and time scaling in intensity-duration-frequency curves.Item What if the Universe Expands Linearly? A Local General Relativity to Solve the “Zero Active Mass” Problem(The Astrophysical Journal, 2024) Monjo Agut, RobertModern cosmology presents important challenges such as the Hubble Tension, El Gordo’s collision, or the impossible galaxies (z > 10). Slight modifications to the standard model propose new parameters (e.g., the early and dynamical dark energy). On the other hand, alternatives such as the coasting universes (e.g., the hyperconical model and the spatially flat Rh = ct universe) are statistically compatible with most of the observational tests, but still present theoretical problems in matching the observed matter contents since they predict a “zero active gravitational mass.” To solve these open issues, we suggest that general relativity might be not valid at cosmic scales, but it would be valid at local scales. This proposal is addressed from two main features of the embedding hyperconical model: (1) the background metric would be independent of the matter content, and (2) the observed cosmic acceleration would be fictitious and because of a distorted stereographic projection of coordinates that produce an apparent radial inhomogeneity from homogeneous manifolds. Finally, to support the discussion, standard observational tests were updated here, showing that the hyperconical model is adequately fitted to Type Ia supernovae, quasars, galaxy clusters, baryon acoustic oscillations, and cosmic chronometer data sets.Item Estimation of future extreme rainfall in Barcelona (Spain) under monofractal hypothesis(International Journal of Climatology, 2023) Monjo Agut, Robert; Locatelli, Luca; Milligan, John; Torres, Luis; Velasco, Marc; Gaitán, Emma; Pórtoles, Javier; Redolat, Darío; Russo, Beniamino; Ribalaygua, JaimeClimate change effects on subdaily rainfall (from 5 min to a few hours) can hardly be measured in mid-latitude climates due to the high natural variability of the precipitation patterns and their effects on local topography. The goal of this study was to obtain change projections of intensity–duration–frequency (IDF) curves, for up to 2-h precipitation events, comparing two approaches that use the daily outputs of the downscaled Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model projections: (a) direct scaling of the expected probable precipitation, from 2-year to 500-year return periods of daily rainfall and (b) a new semi-stochastic approach, built by combining the physically forced outputs of climate models (on a daily scale) and stochastic simulation given by the probability distribution of a concentration index (n-index) for individual rainfall events (on a subdaily scale). The approaches were applied to a set of 27 stations located around Barcelona, Spain, including a long reference series (with 5-min rainfall records since 1927), representing the highly variable Mediterranean climate. The validation process showed a systematic error (bias) generally smaller than 10%, especially for rainfall extreme events with durations of less than 2 h. The concentration n-index and IDF curves were projected by 10 downscaled CMIP5 climate models under 2 emission scenarios (RCP4.5 and RCP8.5), obtaining a consensual increase in both relative concentration and absolute intensities in Barcelona. Ensemble projection of rainfall concentration (n-index) showed an increase up to 10% by 2071–2100 and about 20% (15%–30% range) for maximum intensities of 2-year to 500-year return periods. Results provide robustness in decision-making regarding the design of stormwater management infrastructure at a local scale.Item Effects of Climate Change on Water Quality in the Jucar River Basin (Spain)(Water, 2021) Gómez-Martínez, Gabriel; Galiano, Lorena; Rubio, Toni; Prado-López, Carlos; Redolat, darío; Paradinas Blázquez, César; Gaitán, Emma; Pedro-Monzonís, María; Ferriz-Sánchez, Sergio; Añó Soto, Miguel; Monjo Agut, Robert; Pérez-Martín, Miguel Ángel; Almenar Llorens, Pura; Macián Cervera, JavierThe Mediterranean region is a climate change hotspot, especially concerning issues of hydrological planning and urban water supply systems. In this context, the Jucar River Basin (Spain) presents an increase of frequency, intensity and duration of extreme meteorological phenomena, such as torrential rains, droughts or heat waves, which directly affect the quantity and quality of raw water available for drinking. This paper aims to analyze the effects of climate change on the raw water quality of the Jucar River Basin District, which mainly supplies the city of Valencia and its metropolitan area, in order to adapt drinking water treatments to new conditions and opportunities. For this purpose, we used observed data of water quality parameters from four stations and climate drivers from seven Earth system models of the latest Coupled Model Intercomparison Project—Phase 6. To model water quality (turbidity and conductivity) in the past and future scenarios, this study employs a backward stepwise regression taking into account daily values of mean temperature, maximum temperature, total rainfall and minimum and maximum relative humidity. Results showed that the model performance of the water quality simulation is more adequate for short moving-average windows (about 2–7 days) for turbidity and longer windows (about 30–60 days) for conductivity. Concerning the future scenarios, the most significant change was found in the projected increase of conductivity for the station of the Júcar river, between 4 and 11% by 2100, respectively, under the medium (SSP2–4.5) and pessimistic (SSP5–8.5) emission scenarios. The joint use of these types of management and monitoring tools may help the managers in charge of carrying out the different water treatments needed to apply a better plan to raw water and may help them identify future threats and investment needs to adapt the urban water supply systems to the changing conditions of raw water, such as turbidity or conductivity, as a consequence of climate change.