How did nature choose its gravity model?

Dr. Nima Khosravi

Shahid Beheshti University

Saturday 30 Bahman 95, 3:00-4:00 pm

Ibn Al-Haytham Hall, Physics Department



0) “What really interests me is whether God had any choice in the creation of the World.” A. Einstein.
1) It is generally believed that we are in the golden era of cosmology because of very precise observational data. However to address the data we need to assume our universe is built by 70% dark energy, 25% dark matter and only 5% known (visible) matter. Specifically dark energy means our universe is currently in a phase of accelerated expansion. I find the theoretical darkness in the bright golden era of observations a kind of dramatic situation.
2) From standard model of particle physics we know there is a non-vanishing vacuum energy. This energy has no contribution in particle physics since this theory merely is sensitive to energy differences. However gravity will be affected by the vacuum energy in a way that it predicts very rapid accelerating expansion. Although this prediction was not acceptable 20 years ago, nowadays we believe we are in an accelerating expansion universe. However the predicted vacuum energy is 10^(60-120) times bigger than the observed one. This is the cosmological constant problem which is “the worst theoretical prediction in the history of physics” (quote from Hobson et al.).
3) To address the cosmological constant problem as well as a few tensions between the theory and observations one can think of alternative models. In this framework I focus on modification of Einstein-Hilbert gravity model. I will review what the meaning of modified gravity is and how one can do modified gravity. We will see there are many many models of modified gravity.
4) There is another (philosophical) question: if there are more than one theoretically consistent model of gravity then how did nature choose its own model? In this talk I will try to discuss this question and will give a proposal to solve it: “Ensemble Average Theory of Gravity”.