MG15 - Talk detail

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In 2010, Banados and Ferreira (BF) constructed a variant of Born-Infeld (BI) gravity with a simple matter coupling and demonstrated how the standard background FRW cosmology could become free of curvature singularities (big-bang). Further investigations revealed many interesting consequences of this BF version of BI gravity. Beginning with a toy 3D version, we first show some simple analytical solutions exhibiting the removal of singularities. Thereafter, we look at BI gravity coupled to scalar BI matter where we are able to find non-singular (loitering and bounce types) background solutions with late as well as early time acceleration. Finally, we elevate the BI gravity parameter to a space-time dependent field (in a novel Brans-Dicke like way) and demonstrate how cosmologies without singularities and with late as well as early-time acceleration can indeed arise quite naturally.

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Ever since their revival, three decades ago, in the seminal work of Morris and Thorne, Lorentzian wormholes in General Relativity have led an uncomfortable existence because they require matter which violates the well-known energy conditions. However, in scalar-tensor and other theories of gravity realistic wormholes can indeed exist with `normal matter'. We illustrate this with some known examples and also by explicitly constructing a zero Ricci scalar wormhole in a four dimensional scalar-tensor, on-brane gravity theory arising from the two-brane Randall-Sundrum model with one extra dimension. If such a wormhole could arise as the end-state of some astrophysical process, its ringdown may be studied using gravitational waves. With this aim, we obtain the scalar quasinormal modes in this class of wormholes and choose to identify them as for the `breathing mode' associated with gravitational waves in scalar-tensor theories. Finally, with the frequencies, time constants of the breathing mode as well as results arising out of GW150914, we estimate the errors in the wormhole parameters using standard tools. Our analysis suggests that it may be possible to constrain wormholes and distinguish them from black holes in this way. If in future observations we are able to confirm the existence of such wormholes, we would, at one go, have some indirect evidence of a modified theory of gravity as well as extra spatial dimensions.

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