This tutorial provides tips on how to adapt presentations for different goals. The tutorial is mainly meant for scientists, but several tips can be useful for other types of talks.
The notion of self acceleration has been introduced as a convenient way to theoretically distinguish cosmological models in which acceleration is due to modified gravity from those in which it is due to the properties of matter or fields. In this paper we review the concept of self acceleration as given, for example, by , and highlight two problems. First, that it applies only to universal couplings, and second, that it is too narrow, i.e. it excludes models in which the acceleration can be shown to be induced by a genuine modification of gravity, for instance coupled dark energy with a universal coupling, the Hu-Sawicki f(R) model or, in the context of inflation, the Starobinski model. We then propose two new, more general, concepts in its place: force-acceleration and field-acceleration, which are also applicable in presence of non universal cosmologies. We illustrate their concrete application with two examples, among the modified gravity classes which are still in agreement with current data, i.e. f(R) models and coupled dark energy.
As noted already for example in [35, 36], we further remark that at present non-universal couplings are among the (few) classes of models which survive gravitational wave detection and local constraints (see  for a review on models surviving with a universal coupling). This is because, by construction, baryonic interactions are standard and satisfy solar system constraints; furthermore the speed of gravitational waves in these models is cT = 1 and therefore in agreement with gravitational wave detection. It has also been noted (see for example [37–39] and the update in ) that models in which a non-universal coupling between dark matter particles is considered would also solve the tension in the measurement of the Hubble parameter  due to the degeneracy beta - H0 first noted in Ref. .
Reference: L.Amendola, V.Pettorino "Beyond self-acceleration: force- and fluid-acceleration", Physics Letters B, in press, 2020.
Date: July 8-14 2019
Venue: Basel, CH
Conference App will be announced on the blog.
EuroPython is an annual conference hosting ~1200 participants from academia and companies, interested in development and applications of python programming language. It’s also a good opportunity for students and postdocs who wish to find a job outside academia.
For more info, contact: Valeria Pettorino
Dark energy may be triggered by neutrinos with varying mass (https://arxiv.org/abs/1608.02358). The internship is meant to use a MonteCarlo COSMOMC Boltzmann code to test a simplified framework for this scenario using a prescription we have developed already. The student will be able to use data to test theories, using MCMC simulations. It will involve collaboration with IAP (F.Fuhrer) and Heidelberg (C.Wetterich), potentially leading to a scientific paper. Availability for 6 months is preferred.
Required skills: python and one language between C or fortran. Previous use of CAMB/CLASS/COSMOMC would be an asset.
Date: July 23-29 2018
Venue: Edinburgh, UK
Conference App: https://ep2018.europython.eu/en/events/conference-app/
Tutorial on “Comprendre l’infiniment grand: cosmology and large scales in the Universe” by Valeria Pettorino
Cosmic shear, the weak gravitational lensing caused by the large-scale structure, is one of the primary probes to test gravity with current and future surveys. There are two main techniques to analyse a cosmic shear survey; a tomographic method, where correlations between the lensing signal in different redshift bins are used to recover redshift information, and a 3D approach, where the full redshift information is carried through the entire analysis. Here we compare the two methods, by forecasting cosmological constraints for future surveys like Euclid. We extend the 3D formalism for the first time to theories beyond the standard model, belonging to the Horndeski class. This includes the majority of universally coupled extensions to LCDM with one scalar degree of freedom in addition to the metric, which are still in agreement with current observations. Given a fixed background, the evolution of linear perturbations in Horndeski gravity is described by a set of four functions of time only. We model their time evolution assuming proportionality to the dark energy density fraction and place Fisher matrix constraints on the proportionality coefficients. We find that a 3D analysis can constrain Horndeski theories better than a tomographic one, in particular with a decrease in the errors on the Horndeski parameters of the order of 20 - 30%. This paper shows for the first time a quantitative comparison on an equal footing between Fisher matrix forecasts for both a fully 3D and a tomographic analysis of cosmic shear surveys. The increased sensitivity of the 3D formalism comes from its ability to retain information on the source redshifts along the entire analysis.
A new paper has been put on the arXiv, led by Alessio Spurio Mancini, PhD student of CosmoStat member Valeria Pettorino in collaboration with R. Reischke, B.M. Scháefer (Heidelberg) and M. Zumalacárregui (Berkeley LBNL and Paris Saclay IPhT).
The authors investigate the performance of a 3D analysis of cosmic shear measurements vs a tomographic analysis as a probe of Horndeski theories of modified gravity, setting constraints by means of a Fisher matrix analysis on the parameters that describe the evolution of linear perturbations, using the specifications of a future Euclid-like experiment. Constraints are shown on both the modified gravity parameters and on a set of standard cosmological parameters, including the sum of neutrino masses. The analysis is restricted to angular modes ell < 1000 and k < 1 h/Mpc to avoid the deeply non-linear regime of structure growth. Below the main results of the paper.
The signal-to-noise ratio of both a 3D analysis as well as a tomographic one is very similar.
- 3D cosmic shear provides tighter constraints than tomography for most cosmological parameters, with both methods showing very similar degeneracies.
- The gain of 3D vs tomography is particularly significant for the sum of the neutrino masses (factor 3). For the Horndeski parameters the
gain is of the order of 20 - 30 % in the errors.
- In Horndeski theories, braiding and the effective Newton coupling parameters (\alpha_B and \alpha_M) are constrained better if the kineticity is higher.
- We investigated the impact on non-linear scales, and introduced an artificial screening scale, which pushes the deviations from General Relativity to zero below its value. The gain when including the non-linear signal calls for the development of analytic or semi-analytic prescriptions for the treatment of non-linear scales in ΛCDM and modified gravity.