Topic: How to build a dimension (or, a new look at massive gravity and dimensional deconstruction)

Speaker: Dr. Kieran Wood

Coordinates: PCFT C1124, 16:00, Thursday, November 5  

Abstract: In this talk, I will aim to shed new light on an old question, namely: “what does gravitational physics in an additional compact dimension look like to a lower dimensional observer?” The standard Kaluza-Klein story tells us that, at least in the gravitational sector, we should expect to see (infinite) towers of massive spin-2 fields; more recently, theories involving exactly these kinds of fields - so-called massive and multi-gravity theories - have undergone something of a resurgence, thanks to many interesting theoretical and phenomenological developments. These ideas are related: multi-gravity can be thought of as arising from higher dimensional GR upon discretising the extra dimension, via a framework known as ‘dimensional deconstruction’. However, deconstruction in its current form is more of a guiding principle than a concrete means of doing calculations, as any strict correspondence between the two theories only holds when one discretises the dimension *after* gauge fixing the higher dimensional lapse function. The lapse provides crucial structure to higher dimensional GR: it ensures full diffeomorphism invariance and enforces the Hamiltonian constraint. Thus, upon deconstruction, important information related to the extra dimension is missing in the resulting multi-gravity theory, and one could never hope to recover GR in its entirety upon taking the appropriate continuum limit; this manifests in four dimensions as the now well-documented strong coupling problem in massive gravity theories. Here, I develop an improved deconstruction procedure that maintains the free lapse, and show that the (tentatively) correct deconstructed theory should actually be multi-gravity equipped with additional dynamical scalar fields, whose field equations encode the higher dimensional Hamiltonian constraint. I use the example of Randall-Sundrum brane cosmology to demonstrate this correspondence explicitly. This opens up a number of interesting avenues for studies of new, viable theories of modified gravity, and potentially a means to probe higher dimensional physics through a lower dimensional lens.

Speaker Profile: Dr Kieran Wood is a new postdoc at the Peng Huanwu Center for Fundamental Theory at USTC, Hefei, having recently obtained his PhD from the University of Nottingham in the UK, his home country. Kieran's research is broadly in the field of modified gravity, with a primary focus on the physics of interacting and massive spin-2 fields. His work tries to understand both the theoretical underpinnings and phenomenological consequences of such theories.