Search Talks

LIGO's first observing run which ended in January 2016 yielded two unambiguous gravitational wave signals (GW150914 and GW151226) from the merger of binary black holes as well as a possible third signal (LVT151012). I will review our current estimates of the parameters of the source systems as well as possible formation scenarios. I will discuss how joint analyses of GW150914 and GW151226 allow us to place bounds on departures from general relativity, infer astrophysical binary black hole formation rates, and constrain the mass distribution of coalescing black hole systems. Additionally, I will review the status of searches for other types of gravitational wave signals in data from LIGO's first observing run as well as the implications of the nondetection of gravitational waves from the merger of binary neutron star and neutron-star-black-hole systems. Finally, with the start of LIGO's second observing run within the next few weeks, I will outline our expectations for new detections and scientific prospects.

Entanglement is fundamental to quantum mechanics. It is central to the EPR paradox and Bell’s inequality. Tensor network states constructed with explicit entanglement structures have provided powerful new insights into many body quantum mechanics. In contrast, the saddle points of conventional Feynman path integrals are not entangled, since they comprise a sequence of classical field configurations. The path integral gives a clear picture of the emergence of classical physics through the constructive interference between such sequences, and a compelling scheme for adding quantum corrections using diagrammatic expansions.  We combine these two powerful and complementary perspectives by constructing Feynman path integrals over sequences of matrix product states, such that the dominant paths support a degree of entanglement. We develop a general formalism for such path integrals and give a couple of simple examples to illustrate their utility [arXiv:1607.01778]. 

Dark energy, the driver of the accelerated expansion of the universe, remains a conundrum. New physics may well be needed to explain it; from Lorentz invariance, this new physics should contain scalar fields, which should be straightforward to detect — so where are they? Physicists have now realized that scalar fields can hide from detection by three distinct screening mechanisms, which respectively rely on nonlinear features in the scalar’s kinetic energy, potential energy, or coupling to normal matter. These fields go by names such as galileons, chameleons, f(R) gravity, or symmetrons and range from theoretically well-motivated and natural to speculative. We will present new techniques to detect these particles, relying on interferometry with cold atoms in vacuum. The low density of such atoms compared to bulk matter avoids triggering some screening mechanisms, while the high sensitivity of the interferometer overcomes other screening mechanisms by brute force. We will show limits ruling out substantial regions of parameter space for chameleons and symmetrons [Science 349, 849 (2015)] and argue that an interferometer with 10,000 fold improved sensitivity will be able to search the gamut of screened scalar fields. A positive identification would revolutionize particle physics and cosmology, but even a null result would have important implications for our understanding of the dark sector. 

In the coming decade, ground based CMB telescopes could face a substantial upgrade, to so-called CMB-S4. There are two main science drivers behind this initiative: B-modes, and neutrino mass, and I will focus on the latter. Thought of more generally, constraints on neutrinos can be thought of as generic tests of dark matter. I will discuss the prospects for CMB-S4 in the dark sector, with emphasis on searches for axions and neutrinos. It will be possible to detect percent level departures from standard cold dark matter at many sigma over a wide range of scales: a vast improvement over Planck (+ existing ground based). A large part of the improvement over Planck comes from precision lensing measurements at high multipole. I will briefly discuss some technical challenges in this measurement, based on modelling of dark matter clustering in the non-linear regime.