Search Talks

Photometric surveys are often larger and extend to fainter magnitudes than spectroscopic samples, and can therefore yield more precise cosmological measurements. However, photometric data are significantly contaminated by multiple sources of systematics, either intrinsic, observational, or instrumental. These systematics affect the properties of the raw images in complex ways, propagate into the final catalogues, and create spurious spatial correlations. Some of these correlations may also be imprinted in spectroscopic catalogues, since the latter rely on targets selected from imaging data. Therefore, not just precise — but also accurate — cosmological inferences from imaging surveys require careful mitigation of spatially-varying systematics. I will present a new framework of extended mode projection to robustly mitigate the impact of such systematics on power spectrum measurements. I will demonstrate the effectiveness of the technique, showing constraints on primordial non-Gaussianity using the clustering of 800,000 photometric quasars from the Sloan Digital Sky Survey in the redshift range 0.5 < z < 3.5. Finally, I will present a framework to map the observing conditions of the Science Verification data in the Dark Energy Survey (DES) and incorporate them into end-to-end simulations of the DES transfer function. The considerations presented here are relevant to all multi-epoch surveys, and will be essential for exploiting future high-cadence surveys such as the Large Synoptic Survey Telescope (LSST), which will require detailed null-tests and realistic end-to-end image simulations for correct cosmological inferences.

A modified gravity (MOG) theory has been developed over the past decade that can potentially fit all the available data in cosmology and the present universe. The basic ingredients of the theory are described by an action principle determined by the Einstein-Hilbert metric tensor and curvature tensor. An additional massive vector field φµ is sourced by a gravitational charge $Q=\sqrt{\alpha G_N}M$, where $\alpha$ is a parameter, $G_N$ is Newton's gravitational constant and $M$ is the mass of a body. In addition, two scalar fields $G(x)$ and $\mu(x)$ are added to the action principle, where $G(x)$ describes a variable $G_N$ and $\mu$ is the effective mass-range parameter of the the vector field $\phi_\mu$. For a slow moving test particle in a weak gravitational field, a modified Newtonian acceleration law is derived. This acceleration law reduces to the Newtonian acceleration law for distance scales $d << \mu^{-1}$. This acceleration law is applied to predict the rotation curves of galaxies and globular clusters with excellent fits to data for $\alpha=8.89\pm 0.34$ and $\mu=0.042\pm 0.004\,{\rm kpc}^{-1}$ without dark matter. An application to galaxy clusters with the same values of $\alpha$ and $\mu$ results in fits to cluster dynamics data without dark matter. The colliding clusters "bullet cluster" is also explained without dark matter. The vector field $\phi_\mu$ coupling to standard model particles is of gravitational strength and the mass of the vector field is $m_\phi=2.6\times 10^{-28}\,eV$. This makes the vector field undetectable in the present universe. 

For early universe cosmology before the formation of stars and galaxies, the mass $m_\phi$, determined by the scalar field $\mu(x)$, is bigger, $m_\phi >> 2.6\times 10^{-28}\,eV$, and can act as an ultralight cold dark matter photon with gravitational strength coupling to matter. The modified gravity can fit the cosmological data up to the epoch when stars and galaxies are first formed, and when the $\phi_\mu$ field density $\rho_\phi$ is significantly diluted compared to the baryon density $\rho_b$. After the commencement of the star and galaxy formation epoch, the modified gravity without dark matter takes over. A prediction of the matter power spectrum is made of the first formation of galaxies that do not have dark matter halos. The lack of detectability of the gravitationally sourced dark photon can explain why no convincing detection has so far been made of dark matter particles in laboratory and satellite experiments.

The modified gravity theory vacuum field equations with a smooth vector field source energy-momentum tensor are solved to produce a black hole that differs from the Schwarzschild, Kerr and Reissner-Norstr\"om black holes when the parameter $\alpha\neq 0$. A modified gravity solution is also obtained from a nonlinear regular vector field solution that is regular at $r=0$. This solution can describe a black hole with two horizons as well as a no black hole solution with no horizon. The black hole MOG solutions possess a photosphere and they cast a shadow against a bright background. The sizes and deformations of these shadows can be detected by the VLBI and Event Horizon (EHT) project. These observations will be able to test general relativity for strong gravitational fields.  A traversable wormhole can be constructed using the modified gravity theory with a wormhole throat stabilized by the gravitationally sourced repulsive vector field.  

The Most Wanted Particle

In what promises to be a smashing public lecture, Jon Butterworth will give a personal account of what it is like to work on the largest experiment ever built, the Large Hadron Collider at CERN. He will recount the excitement and tension surrounding the quest, under unprecedented media and public attention, to find the famous Higgs boson. Butterworth, a physics professor at University College London (UCL), will explain the motivation and importance of the Higgs search, discuss his specific research at CERN’s ATLAS detector, and speculate on the exciting breakthroughs that still await in the world of high-energy physics.

There is an analogy between the propagation of fields in the vicinity of astrophysical black holes and the that of small excitations in fluids and superfluids. This analogy allows one to test, challenge and verify, in tabletop experiments, the elusive processes of black hole mass and angular momentum loss.

I will first present a brief overview on analogue black hole experiments, and then discuss in more detail some of my earlier and more recent experimental and theoretical results on the subject.