Astrophysics

Primordial Magnetic Fields (PMFs), long studied as potential relics of the early Universe, can accelerate recombination and have been proposed as a possible solution to the Hubble tension. In this seminar, I will present the latest observational evidence supporting this idea, based on comprehensive magnetohydrodynamic (MHD) simulations of recombination in the presence of PMFs. Current Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillation (BAO) data exhibit a mild preference for PMFs while predicting a higher Hubble constant. Notably, the inferred field strengths align with those required to explain galaxy cluster magnetic fields purely from primordial origins, eliminating the need for additional dynamo amplification or stellar contributions. Future high-resolution CMB temperature and polarization measurements will be crucial in confirming or further constraining the role of PMFs during recombination.

Vacuum polarisation can destabilise dark matter models made from ultralight, weakly coupled scalar fields. The dark matter condenses into quantum droplets, like those seen in Bose Einstein condensates, and could contribute to explaining some cosmological conundrums.

Part I: OGRePy: Object-Oriented General Relativity in Python   I will present a detailed introduction to my new Python package, OGRePy: (O)bject-Oriented (G)eneral (Re)lativity for (Py)thon, a port of my popular Mathematica package OGRe, which is used by many researchers in general relativity and related areas. I will demonstrate the package's usage and features, including its ability to manipulate tensors of arbitrary rank using an intuitive interface, and calculate arbitrary tensor formulas involving any combination of addition, multiplication, trace, contraction, and partial and covariant derivatives - while automatically figuring out the proper index configuration and coordinate system to use for each tensor, eliminating user error.   Part II: Time Travel Paradoxes and Entangled Timelines   If time travel is possible, it seems that it would inevitably lead to paradoxes, indicating an internal inconsistency in our current theories of nature. Can these paradoxes be resolved by new laws of physics, or perhaps even existing ones? I will first review the different types of time travel paradoxes and their proposed resolutions. Then I will present the results of my 3 recent papers (1911.11590, 2110.02448, 2303.07635) discussing different aspects of time travel paradoxes from the perspectives of both general relativity and quantum mechanics. I will argue that generic time travel paradoxes can only be resolved using the concept of parallel timelines, and suggest possibilities for how such timelines may manifest themselves.

The standard model of cosmology is built upon the assumptions of homogeneity and isotropy. Invoking backreaction of inhomogeneities leads to an alternative model, the timescape cosmology. It is homogeneous and isotropic on a statistical level but departs from average Friedmann-Lemaître-Robertson-Walker evolution and replaces dark energy by kinetic gravitational energy and its gradients.   In this talk, I will give an overview of the timescape cosmology and present a statistical analysis of the Pantheon+ Type Ia Supernovae spectroscopic comparing the timescape and spatially flat ΛCDM cosmological models. This analysis is based on the Tripp equation for supernova standardisation alone, thereby avoiding any potential correlation in the stretch and colour distributions and finds very strong evidence (ln B > 5) in favour of timescape over ΛCDM when considering the entire Pantheon+ sample.