General Relativity & Quantum Cosmology

For the last ten years I have been documenting various scenarios for the early universe in a YouTube series called ‘Before the Big Bang”. Having interviewed many of the leader figures of the field including Stephen Hawking, Roger Penrose, Alan Guth (and hosted debates between them), this will be a broad survey of inflation, it’s suggested prequels and alternatives. I shall highlight the strengths and weaknesses of the various proposals and give an inside track of the claims and counter claims in attempts to move beyond the standard Big Bang model.

Singularities in general relativity and quantum field theory are often taken not only to motivate the search for a more-fundamental theory (quantum gravity), but also to characterise this new theory and shape expectations of what it is to achieve. In this talk, I will explore how different types of singularities play a role in the search for quantum gravity, and how different `attitudes' towards singularities can lead to different scenarios for the new theory. [Based on joint work with Sebastian DeHaro].

String theory has not come close to a complete formulation after half a century of intense research. On the other hand, a number of features of the theory suggest that the theory in its complete form may be a final theory. The combination of conceptual incompleteness and allusions to finality seems difficult to grasp. Two main points are made in this talk. First, it is pointed out that finality claims in the context of string theory are motivated in a fundamentally different way than traditional claims of finality one finds in earlier physics. Second, it is argued that finality and chronic conceptual incompleteness may be related to each other in a string theory context in an interesting way. The talk ends with discussing possible implications of this situation for the long-term prospects of theory building in fundamental physics.

Effective field theory is a computationally powerful and flexible theoretical framework, finding application in many areas of physics. In particle physics, Weinberg’s folk theorem also promises that any theory that reproduces the predictions of the Standard Model will, at low energies, look like an effective field theory. In one sense the power of this framework should be a cause for pride in the progress of physics and discovery of real structural features of the world. But given the inability to fully unite quantum theory and gravity into a consistent theoretical picture, there is also cause for pessimism: indirect tests of candidate theories of quantum gravity will ultimately reduce to something like an effective field theory, undermining efforts to find low-energy windows into new physics. One way around this pessimism is to look at where and why effective field theory breaks down in current physical theories. I will point to familiar breakdowns (the cosmological constant, inflation, the hierarchy problem), offering a take on what these breakdowns tell us about the shape of physics beyond the Standard Model. Cracks in the wall of effective field theory allow for a dim glimpse of what might lie beyond. Zoom Link: https://pitp.zoom.us/j/95721802052?pwd=TE1iTGxGLzNqeTFSZlNGRXRYMHBCZz09

Effective field theory analysis are inadequate to describe the evolution of the very early universe. I will describe an approach to obtain emergent time, emergent space, an emergent metric and an emergent early universe cosmology starting from the BFSS matrix model, a proposed non-perturbative definition of superstring theory. In this approach there is no cosmological constant problem.

"After reviewing some key hints and puzzles from the early universe, I will introduce recent joint work with Neil Turok suggesting a rigid and predictive new approach to addressing them. Our universe seems to be dominated by radiation at early times, and positive vacuum energy at late times. Taking the symmetry and analyticity properties of such a spacetime seriously leads to a new formula for the gravitational entropy of our universe, and a picture in which the Big Bang may be regarded as a kind of mirror. I will explain how this line of thought suggests new explanations for a number of observed properties of the universe, including: its homogeneity, isotropy and flatness; the arrow of time (i.e. the fact that entropy increases *away* from the bang); several properties of the primordial perturbations; the nature of dark matter (which, in this picture, is a right-handed neutrino, radiated from the early universe like Hawking radiation from a black hole); the origin of the primordial perturbations; and even the existence of three generations of standard model fermions. I will discuss some observational predictions that will be tested in the coming decade, and some key open questions."