Periodic Reporting for period 3 - ModGravTrial (Modified Gravity on Trial)
Période du rapport: 2021-11-01 au 2023-04-30
My research is driven by the following fundamental questions: What sets gravity apart from the other fundamental interactions? What are the essential and fundamental properties of gravity? How can gravity and the quantum theories be consistently combined? How can we extract fundamental knowledge from cosmological and particle physics observations? My ERC grant plays a key role in answering these questions.
My research activities following from these questions fall into four main areas: (1) Different representations of gravity allow alternative and simpler approaches to essential problems of general relativity (GR), e.g. to the calculation of energy and entropy of gravitational fields or their canonical quantization. For this reason, I am studying GR from a variety of geometrical and field-theoretical concepts. (2) Generalizations of gravity theory beyond GR underly severe restrictions. I have completed the most extensive classes of such generalizations, and I am studying these with respect to their theoretical consistency, e.g. in view of background stability, absence of ghosts, causality, and well-posedness. (3) Gravity as we observe it is a low-energy effective field theory of an underlying quantum or string theory. I study the implications of quantum corrections for the low-energy sector. This includes also the renormalizability properties of classical Lagrangians, the structure of the arising counterterms, and stringy implications for cosmology. (4) The consequences of the theoretical foundations of gravity for astrophysics, cosmology, and particle physics need to be confronted with empirical evidence. I am working out the observational implications of theoretical ideas, e.g. on gravitational waves, cosmic structures, astrophysical compact objects, and particle physics beyond the standard model. These activities are encapsulated in 6 work packages (WP) of my ERC grant.
My research activities following from these questions fall into four main areas: (1) Different representations of gravity allow alternative and simpler approaches to essential problems of general relativity (GR), e.g. to the calculation of energy and entropy of gravitational fields or their canonical quantization. For this reason, I am studying GR from a variety of geometrical and field-theoretical concepts. (2) Generalizations of gravity theory beyond GR underly severe restrictions. I have completed the most extensive classes of such generalizations, and I am studying these with respect to their theoretical consistency, e.g. in view of background stability, absence of ghosts, causality, and well-posedness. (3) Gravity as we observe it is a low-energy effective field theory of an underlying quantum or string theory. I study the implications of quantum corrections for the low-energy sector. This includes also the renormalizability properties of classical Lagrangians, the structure of the arising counterterms, and stringy implications for cosmology. (4) The consequences of the theoretical foundations of gravity for astrophysics, cosmology, and particle physics need to be confronted with empirical evidence. I am working out the observational implications of theoretical ideas, e.g. on gravitational waves, cosmic structures, astrophysical compact objects, and particle physics beyond the standard model. These activities are encapsulated in 6 work packages (WP) of my ERC grant.
In modern theoretical cosmology, finding a consistent effective field theory of gravity is a fundamental question. The construction of these theories undergoes a higly non-trivial fine-tuning of the involved parameters. We have studied whether some of the prominent gravity theories (such as Galileons, Horndeski and Generalized Proca) are subject to strong renormalization by quantum loops. A possible detuning of the interactions within the scale of the effective field theory could reintroduce a ghost instability, that would render these theories sick. Proving the technical naturalness of their classical parameters will be a crucial precondition for their physical viability. This has allowed us to classify the allowed theories based on purely theoretical grounds. For this, we have performed an exhaustive one-loop calculation. We have showed that effective field theories, such as Horndeski and Generalized Proca are stable under quantum corrections. After having accomplished this theoretical part, we have explored the cosmological application of these theories. We have systematically classified the type of cosmological solutions and study exhaustively the conditions to have accelerated solutions. This assessment allowed us to filter out the most promising models for dark energy and inflation.
The outstanding improvement in our understanding of gravity and, thus, of the composition, evolution and structure of the universe has only been possible thanks to the continuous efforts in designing more advanced ground-based and satellite experiments that permitted to test our more accurate theoretical developments. Each new generation of such experiments required pushing forward not only the technology, but also the necessary techniques to be able to analyse enormous amounts of data at an ever increasing level of adequate precision. The ESA satellite Planck has provided the most precise measurements of the CMB anisotropies and, complemented by other observations, has led to a very accurate understanding of the linear evolution of structures in the universe. My group uses these data to constrain the corresponding
regime of gravity theories. The next ESA mission to advance in our comprehension of the universe will be the Euclid satellite. The analysis of Euclid data will require very advanced techniques and, in particular, the availability of N-body simulations to model the non-linear formation of structures in the universe for different cosmological scenarios will be crucial. Our simulations will complement those already existing and those under development by including novel effects derived from more general theories. Similarly, the recently inaugurated field of GWs astronomy by LIGO makes use of extremely advanced interferometers able to detect strains with an accuracy corresponding to one hydrogen-atom radius compared to the astronomical unit. This technology will be pushed even further with the ESA project LISA (whose pathfinder already gave extraordinary results). These measurements also require an exquisite control of the theoretical predictions to properly analyse the data. In particular, it is necessary to have full understanding of the whole process of the merging of compact objects. In the present project, we crucially contribute to an intense exploitation of the products of these two foremost European experiments, which represent two major items in the long-term strategic plans of ESA.
regime of gravity theories. The next ESA mission to advance in our comprehension of the universe will be the Euclid satellite. The analysis of Euclid data will require very advanced techniques and, in particular, the availability of N-body simulations to model the non-linear formation of structures in the universe for different cosmological scenarios will be crucial. Our simulations will complement those already existing and those under development by including novel effects derived from more general theories. Similarly, the recently inaugurated field of GWs astronomy by LIGO makes use of extremely advanced interferometers able to detect strains with an accuracy corresponding to one hydrogen-atom radius compared to the astronomical unit. This technology will be pushed even further with the ESA project LISA (whose pathfinder already gave extraordinary results). These measurements also require an exquisite control of the theoretical predictions to properly analyse the data. In particular, it is necessary to have full understanding of the whole process of the merging of compact objects. In the present project, we crucially contribute to an intense exploitation of the products of these two foremost European experiments, which represent two major items in the long-term strategic plans of ESA.