Cosmological simulations with disformally coupled symmetron fields

We investigate statistical properties of the distribution of matter at redshift zero in disformal gravity by using N-body simulations. The disformal model studied here consists of a conformally coupled symmetron field with an additional exponential disformal term. We conduct cosmological simulations to discover the impact of the new disformal terms in the matter power spectrum, halo mass function, and radial profile of the scalar field. We calculated the disformal geodesic equation and the equation of motion for the scalar field. We then implemented these equations into the N-body code ISIS, which is a modified gravity version of the code RAMSES. The presence of a conformal symmetron field increases both the power spectrum and mass function compared to standard gravity on small scales. Our main finding is that the newly added disformal terms tend to counteract these effects and can make the evolution slightly closer to standard gravity. We finally show that the disformal terms give rise to oscillations of the scalar field in the centre of the dark matter haloes.Comment: Updated version to reflect the journal accepted paper. Added one figure. 7 pages, 7 figure

Conocimiento de matemáticas y la competencia de reconocer el desarrollo del razonamiento "up and down" en los estudiantes

El objetivo de este estudio es aportar información sobre el papel que desempeña el conocimiento de matemáticas de los estudiantes para maestro (EPM) cuando piensan en el aprendizaje de las matemáticas de los estudiantes de primaria. Nuestro estudio se centra en el razonamiento "up and down" que es una de las componentes que facilitan el desarrollo del razonamiento proporcional. 92 EPM resolvieron una tarea en la que tenían que interpretar las respuestas de estudiantes de educación primaria a un problema que implicaba el razonamiento "up and down". Identificamos tres perfiles de EPM caracterizados por la relación entre el conocimiento de matemáticas y la competencia de reconocer el desarrollo del razonamiento up and down en los estudiantes

Consensus on community guidelines: an experimental study on the legitimacy of content removal in social media

The popularization of social media has led to a considerable increase in the importance of discursive expressions of violence, especially when directed at vulnerable communities. While social media platforms have created rules to regulate such expressions, little information is available on the perception of the legitimacy of these rules in the general population, regardless of the importance of the former for the latter. It is therefore the objective of this study to analyze the perception of the seriousness of such content and the degree to which the population has established a consensus on the withdrawal of restricted discursive behaviour on three major social media platforms (Facebook, Instagram and Twitter). For this purpose, 918 participants were immersed in an experimental paradigm in three different groups (n1 = 302; n2 = 301; n3 = 315). Each was presented with stimuli containing discursive behaviour that is banned by community guidelines. The stimuli were presented differently to each group (i.e., description of the banned behaviour, description and accompanying example, example only). Our experimental data reveals that the degree of consensus on the need to remove content is quite high, regardless of the style of presentation. It furthermore suggests that the behaviour in question is perceived as very serious, due to the harm that our participants presume it to cause. These results have important implications for the debate on freedom of expression on the Internet and its regulation by private actors

Lopsidedness of cluster galaxies in modified gravity

We point out an interesting theoretical prediction for elliptical galaxies residing inside galaxy clusters in the framework of modified Newtonian dynamics (MOND), that could be used to test this paradigm. Apart from the central brightest cluster galaxy, other galaxies close enough to the centre experience a strong gravitational influence from the other galaxies of the cluster. This influence manifests itself only as tides in standard Newtonian gravity, meaning that the systematic acceleration of the centre of mass of the galaxy has no consequence. However, in the context of MOND, a consequence of the breaking of the strong equivalence principle is that the systematic acceleration changes the own self-gravity of the galaxy. We show here that, in this framework, initially axisymmetric elliptical galaxies become lopsided along the external field's direction, and that the centroid of the galaxy, defined by the outer density contours, is shifted by a few hundreds parsecs with respect to the densest point.Comment: accepted for publication in JCA

Atypical Phimosis Secondary to a Preputial Metastasis from Rectal Carcinoma

Background: Cutaneous metastases from colorectal cancer are uncommon, accounting for 6.5% of all secondary skin lesions. They occur in advanced disease. The most common site is the abdomen. The penis is a rare site. Case Report: We report the case of a 79-year-old patient who presented, two years after rectocolectomy for a rectal adenocarcinoma, obstructive renal failure secondary to a presacral recurrence, as well as symptomatic phimosis associated with papulonodules invading the penis, scrotum and pubis. After hemodialysis, percutaneous pyelostomy, and double-J catheter placement, the patient underwent circumcision and a pubic skin biopsy. On histology and immunohistochemistry, a cutaneous metastasis originating from the rectal adenocarcinoma was diagnosed. Conclusions: Penile invasion by colorectal cancer revealed by phimosis is a rare occurrence but needs to be recognized and promptly diagnosed by biopsy in order to propose appropriate adjuvant therapy. These rare lesions can occur without any liver or lung involvement

Rayleigh-wave attenuation by a semi-infinite two-dimensional elastic-band-gap crystal

In this paper, we report experiments on the scattering of surface-elastic waves by a periodic array of cylindrical holes. The experiments were performed in a marble quarry by drilling cylindrical holes in two different configurations: honeycomb and triangular lattices. The attenuation spectra of the surface waves show the existence of absolute band gaps for elastic waves in these semi-infinite two-dimensional crystals. Results are compared with theoretical calculations based on a scalar-wave approach. The scaling property of the underlying theory has led us to explore the possible application of the results obtained to the attenuation of surface waves in seismic movements.This work was partially supported by the Comisión Interministerial de Ciencia y Tecnología of Spain, Contract No. MAT97-0698-C04. We also thank the Consejo Superior de Investigaciones Científicas for the special funding to perform this experiment.Peer reviewe

Long-Term Optical Monitoring of Eta Carinae. Multiband light curves for a complete orbital period

The periodicity of 5.5 years for some observational events occurring in Eta Carinae manifests itself across a large wavelength range and has been associated with its binary nature. These events are supposed to occur when the binary components are close to periastron. To detect the previous periastron passage of Eta Car in 2003, we started an intensive, ground-based, optical, photometric observing campaign. We continued observing the object to monitor its photometric behavior and variability across the entire orbital cycle. Our observation program consisted of daily differential photometry from CCD images, which were acquired using a 0.8 m telescope and a standard BVRI filter set at La Plata Observatory. The photometry includes the central object and the surrounding Homunculus nebula. We present up-to-date results of our observing program, including homogeneous photometric data collected between 2003 and 2008. Our observations demonstrated that Eta Car has continued increasing in brightness at a constant rate since 1998. In 2006, it reached its brightest magnitude (V ~ 4.7) since about 1860s. The object then suddenly reverted its brightening trend, fading to V = 5.0 at the beginning of 2007, and has maintained a quite steady state since then. We continue the photometric monitoring of Eta Car in anticipation of the next "periastron passage", predicted to occur at the beginning of 2009.Comment: Accepted by A&A. The paper contains 3 figures and 2 table