Justin Pierel (STScI)

June 02, 2022, 2:00 PM (ET)

Cosmological Tensions in the Era of Next Generation Telescopes

Type Ia Supernovae (SNe Ia) have been the source of many astronomical discoveries over the past several decades, perhaps most prominently the accelerated expansion of the universe by so-called “dark energy” in 1998. Today, our most precise cosmological measurements from SNe Ia in the local universe have diverged significantly from the predictions of our most successful cosmological model (ΛCDM). Upcoming surveys from the Vera C. Rubin Observatory and Nancy G. Roman Space Telescope will attempt to unravel the mysteries of dark energy, increasing our SN Ia sample by orders of magnitude. In order to fully leverage this new wealth of information, we must find new innovations for understanding and mitigating systematic uncertainties in cosmological measurements. I will discuss a series of efforts designed to reduce various systematics associated with SNIa standardization, as well as the promise of near-infrared observations provided by Roman. In addition to these traditional luminosity distance measurements, I will highlight a relatively new means of leveraging SNe for cosmology: gravitational lensing. The new era of next generation telescopes will truly enable these fully independent probes for cosmology, which provide a critical check on SNe Ia and unique constraints on dark energy, as we attempt to resolve the age of cosmological tensions. Weak lensing measurements by the Dark Energy Survey (DES) and the new generation of wide field galaxy surveys offer an exciting window on the properties and laws of the Universe. I will describe our approach to constrain non-standard cosmological models with the latest DES weak lensing data, especially focusing on tests of gravity on cosmological scales. I will then give my perspective on the lessons learnt and challenges for future tests, especially with the Rubin Observatory. I will end by presenting a new approach I propose to overcome one of these challenges and maximise the potential for discovery of new physics in the coming decade.

Agnès Ferté (Jet Propulsion Laboratory)

April 14, 2022, 2:00 PM (EST)

Tests of gravity with weak lensing surveys

Weak lensing measurements by the Dark Energy Survey (DES) and the new generation of wide field galaxy surveys offer an exciting window on the properties and laws of the Universe. I will describe our approach to constrain non-standard cosmological models with the latest DES weak lensing data, especially focusing on tests of gravity on cosmological scales. I will then give my perspective on the lessons learnt and challenges for future tests, especially with the Rubin Observatory. I will end by presenting a new approach I propose to overcome one of these challenges and maximise the potential for discovery of new physics in the coming decade.

Kyle Boone (eScience Institue, University of Washington)

August 3, 2021, 2:00 PM (EST)

Modeling supernova light curves with physics-enabled deep learning

Upcoming cosmological surveys such as the LSST at the Rubin Observatory will discover millions of supernovae and other transients. To take advantage of these large datasets, we have developed a deep generative model of transient light curves. Our model combines deep learning to model the unknown physics of the sources with an explicit physics model of how light propagates through the universe and is observed on a detector. I’ll discuss several applications of such a model, including classification of transients, identifying new kinds of transients, and how to perform supernova cosmology without classification.

Ross Cawthon (University of Wisconsin-Madison)

February 25, 2021,  2:00 PM (EST)

Calibrating Photometric Redshifts for Cosmic Surveys

Cosmic surveys like the Dark Energy Survey (DES) and the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST) observe hundreds of millions to billions of galaxies. These large surveys help us study the composition and history of the Universe as well as the properties of dark matter, dark energy, gravity, and potentially new physics. A major limitation to these surveys is redshift accuracy. These large imaging surveys attempt to get redshift information from just a few color bands, rather than full spectra. I will discuss three major classes of techniques to get redshift information in these surveys: photometric redshifts, which use color information, clustering redshifts, which use spatial information, and 'self-calibration' techniques, which infer redshifts from typical cosmological measurements like galaxy clustering and galaxy-galaxy lensing. I will highlight recent work in DES.

Kimmy Wu (SLAC)

December 10, 2020,  2:00 PM (EST)

Improved constraint on primordial gravitational waves with delensing

Abstract: Inflation generically predicts a background of primordial gravitational waves, which generate a primordial B-mode component in the polarization of the cosmic microwave background (CMB). The measurement of such a B-mode signature would lend significant support to the paradigm of inflation. Observed B modes also contain a component from the gravitational lensing of primordial E modes, which can obscure the measurement of the primordial B modes. We reduce the sample variance in the BB spectrum contributed from this lensing component by a process called 'delensing.' In this talk, I will show results from the first demonstration in an improved constraint on primordial gravitational waves with delensing using data from BICEP/Keck, the South Pole Telescope (SPT), and Planck. In addition, I will provide an outlook of joint-analysis efforts of the BICEP/Keck and the SPT collaborations (the South Pole Observatory) to constrain primordial gravitational waves.

Eli Rykoff (SLAC)

September 10, 2020, 2:00PM - 3:00PM (EST)

The Magic of Red Galaxies

Abstract:

Red elliptical galaxies are not exciting.  All the action is long past, their star formation ended billions of years ago, they have retired from their busy mergers, and now they are made up of old stars, fading away. These properties make them uniquely valuable tools for tracing the history of structure formation in the Universe.  Both on their own and gathered into galaxy clusters, the largest peaks in the cosmic density field, they track the growth of structure over a large mass range.  
In this talk I will review what red galaxies are, how we measure their redshifts, and how we can make use of them in large photometric surveys such as the Dark Energy Survey (DES) and the Rubin Observatory Legacy Survey of Space and Time (LSST).  And yes, I will also answer the question “but what about dust?”

Jonathan Blazek (Ecole Polytechnique Federale de Lausanne)

March 26 2020, 2:00PM - 3:00PM

Galaxies Far, Far Away...
A general description of intrinsic alignments and biasing

Abstract: Observational cosmology with galaxy surveys provides a powerful probe of the cosmological model. Current and next-generation projects such as DES, LSST, and DESI will allow us to measure geometry and growth of structure in the Universe and to compare with both theoretical predictions and complementary observations. However, these measurements rely on our ability to understand where and how galaxies form in order to use them as robust tracers of the underlying cosmic structure. I will discuss the questions of galaxy biasing and intrinsic alignments, which impact the observed galaxy positions and shapes, from a cosmologist’s perspective. Rather than starting with particular astrophysical processes relevant to galaxy formation, I will show how we can describe galaxy observables in a general and flexible expansion using nonlinear perturbation theory. I will discuss the standard effects of gravitational collapse as well as the impact of baryonic physics from the early Universe. Finally, I will discuss how these perturbative expansions can provide opportunities to explore physics beyond the usual models.