Sep 2018
Sep 4, Tuesday
1:10 pm (Cosmology/ BCCP)
Joe DeRose, Stanford
Campbell 131
Modeling the Non-Linear Universe using Cosmological Simulations
Current and upcoming cosmological surveys are turning their eyes towards the late time universe in an attempt to answer fundamental questions about the structure of our world and the physical laws that govern it. Given the non-linear nature of the observables studied by these surveys, perturbative modeling approaches have limited faculty. Thus, non-perturbative simulation approaches are necessary in order to connect theory with observation and wring the most information out of the deluge of data that will be collected in the coming years. In this talk I will discuss a number of efforts I am involved with to interpret data using simulations within the Dark Energy Survey, focusing on their use in our Y1 analyses, and how we are extending this for upcoming analyses. I will also discuss more forward looking efforts to build emulators for galaxy survey observables that promise to unlock the information contained in highly non-linear observables, such as the halo mass function, redshift-space distortions and small scale projected clustering and galaxy-galaxy lensing.
Sep 11, Tuesday
1:10 pm (Cosmology/ BCCP)
Yi-Kuan Chiang, JHU
Campbell 131
Broadband Intensity Tomography: Spectral Tagging the Cosmic UV Background
Most of the photons ever recorded by astronomers are in the form of images—or broadband intensity mapping, while the photons’ redshift and frequency information has largely lost. I will introduce a data-driven technique to recover these otherwise collapsed dimensionalities by exhausting information in the spatial fluctuations. As the first application, we measure the spectrum of the ultraviolet background (UVB) at 0 < z < 2 in GALEX Imaging surveys. We spatially cross-correlate photons in the FUV and NUV bands with spectroscopic objects in SDSS as a function of redshift, and use the observed shapes of the K-corrections to constrain the rest-frame spectrum of the UVB. We fit simultaneously a parameterized UVB volume emissivity and a clustering bias factor both as function of frequency and cosmic time. With minimum assumptions, our measured non-ionizing continuum emissivity is broadly consistent with that in the Haardt & Madau model. Cosmic Lya emission is tentatively detected at the 2sigma level at z=1, with the luminosity density consistent with being powered by cosmic star-formation with an effective Lya escape fraction of 10%. Our approach probes all sources of radiation without surface brightness thresholding, which includes potential IGM emission. The technique brings some of the future line-intensity mapping science to within the reach of existing/upcoming broadband data at all wavelengths.
Sep 13, Thursday
4:10 pm (Astronomy Colloquium)
George Becker, Riverside
1 LeConte Hall
Connecting Galaxies and the Intergalactic Medium Near Reionization
The reionization of hydrogen was a landmark event in cosmic history. Within one billion years of the Big Bang the first galaxies emitted enough ultraviolet photons to ionize the gas in deep space, permanently transforming the Universe. Determining exactly when and how reionization occurred is therefore central to our efforts to understand these early sources, as well as the physics that governs the interaction between galaxies and their environments. I will describe what we know about reionization from the study of quasar absorption lines and other probes of the high-redshift Universe. I will especially focus on what we’re learning about the intergalactic medium (IGM) shorty after reionization is believed to end. By combining observations of high-redshift quasars with wide-field galaxy surveys we are beginning to better appreciate the complexity of the IGM at this epoch, and recognize how it may help us to construct a more complete model of reionization.
Sep 18, Tuesday
1:10 pm (Cosmology/ BCCP)
Johannes Lange , Yale
Campbell 131
New Insights into Cosmology and the Galaxy-Halo Connection from non-linear Scales
Current and future galaxy surveys have the potential to transform our understanding of galaxy formation. Information on small, non-linear scales holds the greatest statistical power and potential insight into the galaxy-halo connection. In this talk, I will concentrate on constraints derived from galaxy clustering, galaxy-galaxy lensing and satellite kinematics, showing that there is consistent tension between those different probes. I will present new measurements of the tension between clustering and lensing in the Baryonic Acoustic Oscillations Survey (BOSS). The most promising explanations for this tension, baryonic feedback, assembly bias and cosmological parameters different from the Planck CMB constraints, are discussed. Furthermore, I will show that previous tension between satellite kinematics and other probes can be attributed to systematics in the modelling. We developed an updated, more robust analysis to extract constraints on the galaxy-halo connection from satellite kinematics. The accuracy of this approach is tested using a large number of realistic mock catalogs and shown to yield unbiased, highly competitive constraints. We then apply this updated analysis to the Sloan Digital Sky Survey (SDSS) and compare our inferences from satellite kinematics to those from previous studies.
Sep 20, Thursday
4:00 pm (LBL-RPM)
Martin White, UCB/LBL
LBL 50-5132
The cosmological legacy of Planck
The anisotropies in the cosmic microwave background radiation have
become our most important cosmological fossil. The study of these
"echoes of gravity" has revolutionized cosmology, stringently tested our models and allowed precise measurement of a host of important
cosmological parameters. I will discuss how far we've come since the
early detections of CMB anisotropies and in particular the cosmological legacy of the Planck mission.
Sep 24, Monday
12:10 pm (TAC)
Vadim Semenov, Chicago
Campbell 131
"How Galaxies Form Stars"
Observed star-forming galaxies convert their gas into stars
inefficiently. The typical time on which available gas in galaxies is
depleted is ~5-10 Gyr which is orders of magnitude longer than any
physical timescale relevant for star formation. The origin of long
depletion times is a long-standing puzzle. Many galaxy simulations can
reproduce observed long depletion times but the physical mechanism
controlling their values is not understood. In addition, some of the
simulations show rather counter-intuitive behavior: global depletion
times appear to be almost insensitive to the assumptions about local
star formation in individual star-forming regions, a phenomenon
described as "self-regulation." Yet another part of the puzzle is the
observed tight and near-linear correlation between star formation rates
and the amount of molecular gas on kiloparsec and larger scales. A
linear correlation implies that depletion time of molecular gas is
almost independent of molecular gas density on >kiloparsec scales, while
popular models of star formation in galaxies predict a strong
dependence. I will present results from a suite of isolated disk galaxy
simulations in which we systematically explored the behavior of
depletion times. Using insights from these simulations we formulated a
physical model that explains both the origin of long gas depletion times
in observed galaxies and the results of galaxy formation simulations.
This model also sheds light on the origin of the observed linear
correlation between star formation rates and molecular gas.
Sep 25, Tuesday
1:10 pm (Cosmology/ BCCP)
Christiane Lorenz, Oxford
Campbell 131
'Neutrino cosmology and large scale structure'
In this talk, I will present studies of the model-dependence of cosmological neutrino mass constraints. In particular, I will focus on two phenomenological parameterizations of time-varying dark energy (early dark energy and barotropic dark energy) that can exhibit degeneracies with the cosmic neutrino background over extended periods of cosmic time, and show how the combination of multiple probes across cosmic time can help to distinguish between the two components. In addition, I will discuss how neutrino mass constraints can change in extended neutrino mass models, and how current tensions between low- and high-redshift cosmological data might be affected in these models. Finally, I will discuss whether lensing magnification and other relativistic effects that affect the galaxy distribution contain additional information about dark energy and neutrino parameters, and how much parameter constraints can be biased from neglecting them.
October 2018
Oct 2, Tuesday
1:10 pm (Cosmology/ BCCP)
Kris Pardo, Princeton
Campbell 131
Oct 5, Friday
12:00 pm (INPA)
Arka Banerjee, Stanford
LBL 50-5132
Oct 9, Tuesday
1:10 pm (Cosmology/ BCCP)
Hung-Jin Huang, CMU
Campbell 131
Oct 16, Tuesday
1:10 pm (Cosmology/ BCCP)
Gigi Guzzo, Milan
Campbell 131
Oct 23, Tuesday
1:10 pm (Cosmology/ BCCP)
Fred Davies, UCSB
Campbell 131
Oct 30, Tuesday
1:10 pm (Cosmology/ BCCP)
Alex Malz, NYU
Campbell 131
November 2018
Nov 5, Monday
12:10 pm (TAC)
Ue-Li Pen, CITA
Campbell 131
Nov 6, Tuesday
1:10 pm (Cosmology/ BCCP)
Denise Schmitz, Caltech
Campbell 131
Nov 9, Friday
12:00 pm (INPA)
Doug Finkbeiner, Harvard
LBL 50-5132
Nov 13, Tuesday
1:10 pm (Cosmology/ BCCP)
held
Campbell 131
Nov 26, Monday
4:10 pm (physics colloquium)
Shirley Ho, CCA
Campbell 131
Nov 27, Tuesday
1:10 pm (Cosmology/ BCCP)
Campbell 131
Nov 30, Friday
12:00 pm (INPA)
Simon Foreman, CITA
LBL 50-5132
Past Months
July 2018
July 19, Thursday
4 pm (RPM)
Laura Newburgh, Yale
LBL 50-5132
“New Probes of Old Structure: Cosmology with 21cm Intensity Mapping and the Cosmic Microwave Background”
Current cosmological measurements have left us with deep questions about our Universe: What caused the expansion of the Universe at the earliest times? How did structure form? What is Dark Energy and does it evolve with time? New experiments like CHIME, HIRAX, and ACTPol are poised to address these questions through 3-dimensional maps of structure and measurements of the polarized Cosmic Microwave Background. In this talk, I will describe how we will use 21cm intensity measurements from CHIME and HIRAX to place sensitive constraints on Dark Energy between redshifts 0.8 — 2.5, a poorly probed era corresponding to when Dark Energy began to impact the expansion history of the Universe. I will also discuss how we will use data from new instruments on the ACT telescope to constrain cosmological parameters like the total neutrino mass and probe structure at late times.
Aug 2018
Aug 9, Thursday
2:00 pm (LBL special seminar)
John Walsh, Unlearn.AI, Inc.
LBL 50-5026
Machine Learning
I will give a broad overview of machine learning, defining the main areas of supervised, unsupervised, and reinforcement learning, including a brief history and a view on the current state of the field. I will discuss what aspects of machine learning are interesting to me as a physicist and data scientist, highlighting some open problems in supervised learning before delving into unsupervised learning. Physics-inspired machine learning models have the potential to make a tremendous impact on unsupervised learning, and I will present some of my recent work on a class of models known as Boltzmann machines.
Aug 28, Tuesday
1:10 pm (Cosmology/ BCCP)
Matt Lewandowski, IPhT (Saclay)
Campbell 131
Analytic IR-resummation for the BAO peak
We develop an analytic method for implementing the IR-resummation of arXiv:1404.5954, which allows one to correctly and consistently describe the imprint of baryon acoustic oscillations (BAO) on statistical observables in large-scale structure. We show that the final IR-resummed correlation function can be computed analytically without relying on numerical integration, thus allowing for an efficient and accurate use of these predictions on real data in cosmological parameter fitting. In this work we focus on the one-loop correlation function, where the challenge is to reproduce the BAO peak. We show that, compared with the standard numerical integration method of IR-resummation, the new method is accurate to better than 0.2% of the numerical result, and is quite easily improvable. We also give an approximate resummation scheme which is based on using the linear displacements of a fixed fiducial cosmology, which when combined with the method described above, is about six times faster than the standard numerical integration. Finally, we show that this analytic method is generalizable to higher loop computations.
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