Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - GravityLS (Large Scale Structure Constraints of General Relativity)

Teaser

Current observations of the large scale structure of the Universe - of the distribution of galaxies and of the relic radiation left over from the Big Bang - have been used to place constraints on key properties of the universe, such as its geometry, the density of different...

Summary

Current observations of the large scale structure of the Universe - of the distribution of galaxies and of the relic radiation left over from the Big Bang - have been used to place constraints on key properties of the universe, such as its geometry, the density of different types of matter and forms of energy and even on the initial conditions. A key assumption is the the dominant force that drives the evolution of the Universe is gravity and that it is perfectly described by Einstein\'s General Theory of Relativity. The purpose of this project is to develop the tool that will allow us to test this assumption and in doing so, either greatly strengthen constraints on existing fundamental physics or allow us to unambiguously detect evidence for new physics, i.e. modifications to general relativity, on cosmological scales.

Thus far, we have made steady progress on a number of projects. For a start we have developed a mathematical framework that allows us to describe all possible deviations from General Relativity on large scales. This also allows us to figure out if there are interesting observational effects to look for in the data. We have also developed a publicly available piece of software that implements a great part of this framework so that we will be able to constrain it with observations. A key aspect has been to cross-check our codes with over a dozen other codes - this has been useful to correct errors in all the codes so that they are completely trustworthy.

An important development has been our links with observations which are not cosmological. In particular, the recent detection of a binary neutron star merger allowed us to place constraints on a vast swathe of different theories, allowing us to focus on the modifications to general relativity that really matter. We have also understood how to use measurements of gravitational wave signals to place constraints in a completely different regime (where the gravitational field is much stronger) but which can then be combined with cosmological measurements to place even tighter constraints.

We expect the next decade to bring us a wealth of cosmological data which will allow us to place even more stringent constraints on gravity. As preparation for this new wave of information, we have been exploring the various techniques that may be applied to, in some sense a forecast of what we might expect. But we have also taken stock and analyzed on of the current most comprehensive data sets, KIDS-450, and combined it with other data sets to place state of the art constraints on cosmological gravity.

Understanding the fundamental laws that operate on largest scales and placing stringent constraints on them will allow us to be confident about our model of the origin and evolution of the Universe.

Work performed

1- A comprehensive formalism for characterizing theories of gravity on cosmological scales.
2- A new, action based, approach for characterizing perturbations of black holes in general theories of gravity . T
3- The tightest constraints on cosmological theories of modified gravity from the G170817 BNS measurements.
4- A first, complete, characterization of the quasi normal mode (QNM) spectrum for black holes for cosmological Horndeski theories.
5- A new, independent, Einstein-Boltzman solver for scalar tensor theories of gravity – HiCLASS – which is publicly available.
6- A global code comparison project to calibrate most of the existing Einstein-Boltzman solvers currently available.
7- An analysis of a combination of existing data sets (KIDS, 2dfLens, etc) to find the most robust constraints on modified gravity parameters.
8- A complete formalism for the impact of large scale structure on the gravitational wave background.
9- An Estimate the effect of large scale structure on the polarization of gravitational waves through gravitational scattering.
10- A comprehensive analysis of both galaxies constituents and their morphology to place constraints on modified gravity (or fifth forces).
11- We have shown that the normal approach for constructing likelihoods is robust to the enlargement of model space and that it is unnecessary to include parameter dependence in the covariance matrix.
12 – The first, state of the art constraints on Beyond Horndeski scalar-tensor theories of gravity.
13- Quantification of the biases on cosmological constraints arising from relativistic corrections to observables.
14- The most comprehensive forecasts on cosmological scalar-tensor theories from future surveys.
15- The PI has completed a major review of the topic of this grant, with the title “Cosmological Tests of Gravity” to be published in the Annual Reviews of Astronomy and Astrophysics. Reviews published in this journal tend to have a large impact in the community with a long shelf life.
16- An extensive analysis of how global scale invariance, as a fundamental symmetry of nature, can link many of the current open questions in modern cosmology and particle physics.

Final results

We have, by now, a firm understanding of the theoretical panorama of gravitational theories - we know how to model their effects on larges scales at the linear level, with great accuracy and, with the constraints from the binary neutron star event GW170817, we have been able to severely restrict the space of allowed theories. We have also, in place, highly accurate and calibrated codes with which we can calculate cosmological observables on linear scales. Finally, we have been able to establish important links with black holes and gravitational wave physics. Moving forward, we wish to build on this work and analyse existing data sets, to link these constraints with what can be obtained from gravitational waves emerging from the aLIGO events and, as importantly, begin to explore the tremendous power of constraints on gravity on galactic scales. With these three probes into gravitational physics, it should be possible to obtain a comprehensive set of constraints on gravitational physics. This will also allow us to prepare for the stage IV surveys which come online towards the end of the grant.

Website & more info

More info: http://www.pedroferreira.co.uk.