May 2015:
May 4, Monday
12:10 pm (TAC)
Mark Vogelsberger, MIT
Campbell 131A
The era of large-scale cosmological simulations
Progress in our understanding of galaxy formation, improved numerical algorithms, and increased computing power have recently lead to a number of impressive large-scale hydrodynamical simulations, which are able to reproduce key observables of the local and higher redshift Universe. These simulations allow us, for the first time, to study the interplay between large-scale structure and galaxy formation in detail. I will give an overview of these efforts and focus on the successes and failures of some of them.
May 5, Tuesday
1:10 pm (Cosmology/BCCP)
Casey Stark, UCB
Campbell 131A
"Resolving the intergalactic medium with tomographic reconstructions"
Despite keen interest in how clusters form, the study of early cluster formation is observationally limited: clusters are rare and surveying large volumes is expensive. Today, there are only a handful of known z > 2 protoclusters. At the same time, cosmic voids have received renewed interest as tracers for an AP test and testing modified-gravity models, but current void samples are limited to z < 1. I will discuss how we can use densely packed Lyman-alpha forest sightlines to directly map large scale structure at z = 2 - 3, and how we can find protoclusters and voids using the maps. Using our new Wiener Filter code on sightlines with average separations of 2 - 3 Mpc/h, we can create high-quality maps of the IGM. With a resolution of a few Mpc, the maps clearly pick up protoclusters and voids, both with radii of up to 10 Mpc/h at z = 2.5. I will present the characteristic signals of these objects in the maps and forecasts for samples in upcoming surveys.
May 12, Tuesday
1:10 pm (Cosmology/BCCP)
Dan Kelson, Carnegie
Campbell 131A
"Quasistatic Equlibrium Models of Galaxy Formation and the Consequences of Stochasticity:
The Central Limit Theorem, Memory, and Precision Galaxy Evolution from First Principles"
Incomplete knowledge of fundamental aspects of astrophysics severely limits the realism and accuracy
of star-formation histories in simulations, resulting in poorly modeled correlations of galaxy
properties with mass and environment, hampering our understanding of how galaxy populations have
evolved for the past 13 Gyr. We have now derived new insights from the observed correlation of
star-formation rates with stellar mass, dubbed the "main sequence of star-formation." Modern
mathematics allows us to newly understand that this correlation is emergent, not deterministic. I
will demonstrate that SFMS is a simple consequence of the central limit theorem for systems in
quasi-static equilibria, of the kinds described by models in which gas inflow and feedback are
nearly in balance for long periods of time. For fair, unbiased samples of (disk) galaxies, we
derive an expectation that the median of SFR/Integral(SFR) is independent of mass, identically equal
to 2/t, and has a relative scatter that is also independent of mass and time. This radically new
mathematical framework reproduces, with no prior inputs, several important observables, including
the early evolution of galaxy mass functions and the star-formation rate density, the median/mean
SSFRs of galaxies on the "flat part" of the SFMS over 0 < z < 10, as well as published estimates of the
scatter in SSFR at fixed mass and redshift. Modern mathematics now provides tools by which we can
successfully model the ensembles of galaxies, constructing exact distributions of galaxy
star-formation and assembly histories over the full extent of cosmic time.
June 2015:
June 5, Friday
12 noon (INPA)
Yu Feng, UCB
LBL 50-5026 INPA room
Milky way-mass disk galaxies in a z=8 universe
The size of large scale cosmological simulations have steadily
increased with time, catching up with the computing power of
state-of-art super-computing facilities. In this talk I will present the
first results on high-redshift galaxy morphology from the BlueTides
simulation, a hydrodynamic simulation of the early universe up to z=8
with nearly 700 billion particles in a 400 Mpc/h per side box. BlueTides
resolves galaxy formation in a volume that is several hundred times
bigger than the footprint of the largest Hubble Ultra Deep Field survey
(BoRG) at this redshift. The simulation predicts the existence of a
population of disk galaxies with masses comparable to our Milky-way
galaxy. Upcoming observations (e.g. with WFIRST) will be capable of observing
many such galaxies, and open up a new frontier in the physics of high-redshift
galaxy formation.
September 2015:
Sep. 1, Tuesday
1:10 pm (Cosmology/BCCP)
Ae Lei Sun, Princeton
Campbell 131A
Sep. 8, Tuesday
1:10 pm (Cosmology/BCCP)
Lam Hui, Columbia
Campbell 131A
Past Months
(
Previous Years
)
April 2015:
Apr. 3, Friday
12 noon (INPA)
Michael Blomqvist, Irvine
LBL 50-5026 INPA room
"Baryon Acoustic Oscillations in the BOSS Lyman Alpha Forest"
The Baryon Oscillation Spectroscopic Survey (BOSS) has opened the possibility to measure the expansion rate and geometry of the Universe at redshift z>2 using baryon acoustic oscillations (BAO) in the Lyman-alpha forest. I will present the latest measurements of the BAO scale using the Lyman-alpha forest auto-correlation and the quasar-Lyman-alpha forest cross-correlation based on more than 130,000 quasar spectra taken from SDSS-III Data Release 11. By combining the BAO results for the auto- and cross-correlation, the Hubble parameter and the angular diameter distance at z=2.34 are measured to 2% and 3% accuracy, respectively. New advances in the modeling of broadband distortions of the auto-correlation due to quasar continuum fitting enable us to also make significantly more accurate measurements of the large-scale linear bias and redshift-space distortion of the Lyman-alpha forest.
Apr. 6, Monday
12:10 pm (TAC)
Renyue Cen, Princeton
Campbell 131A
"Some Global and Local Trends Learned from High Resolution Cosmological Hydrodynamic Simulations"
Utilizing state-of-the-art adaptive mesh-refinement high resolution (~100pc) cosmological hydrodynamic simulations, analyses are performed to gain insights on how galaxies evolve with respect to their star formation activities and color in different environments globally. Locally, we track the histories of a large sample of simulated galaxies (x1000) to understand how they migrate from the blue cloud to the red sequence, and what physical processes are responsible for quenching star formation and keeping red galaxies red.
Apr. 7, Tuesday
1:10 pm (Cosmology/BCCP)
Matt Becker,Stanford/SLAC
Campbell 131A
Measuring Cosmic Shear with the Dark Energy Survey
I will present cosmic shear measurements from the Dark Energy Survey, a five-year, five-band imaging survey with DECam on the Blanco telescope. These measurements require detailed work, from the reduction of pixel data all the way through to the generation of ray traced simulations for the computation of the measurement covariances. I will touch on this full range of required steps, highlighting the extensive suite of null tests used to verify the fidelity of the measurements. Furthermore, I hope to demonstrate how ray traced simulations can play a vital role in the data analysis process. Finally, I will briefly discuss future work and simulations required to complete the analysis of the final DES dataset.
Apr. 14, Tuesday
1:10 pm (Cosmology/BCCP)
John Peacock, ROE
Campbell 131A
"Gravitational lensing of the Cosmic Microwave Background:
flux conservation and growth of inhomogeneities"
Gravitational lensing is a rich source of cosmological information: it probes both the amplitude and evolution of metric fluctuations, plus the distance-redshift relation via the geometry of the distortion of background images. But in 2014, Clarkson et al. suggested that lensing had an average non-Newtonian "back-reaction" effect, which altered the effective distance to the CMB last-scattering surface, potentially by several percent - enough to cause substantial change in the cosmological parameters we infer. The first part of this talk discusses this issue from the point of view of flux conservation theorems due to Weinberg and to Kibble & Lieu, and a possible loophole in these theorems is identified. This loophole relates to the area of constant-redshift surfaces, and the solution to this puzzle is critical in order to decide whether the Clarkson et al. claims should be taken seriously. The second part of the talk presents some new results on cross-correlation between galaxy surveys (based on WISE & SuperCOSMOS) and CMB lensing data. These yield interesting measurements of the evolution of cosmological perturbations at redshift z<0.35.
Apr. 16, Thursday
4:10 pm (Astronomy Colloquium)
Dan Marrone, Arizona
Campbell 131A
Apr. 17, Friday
12 noon (INPA)
Miguel Zumalacarregui, Heidelberg
LBL 50-5026 INPA room
"Non-linear evolution of the BAO scale in alternative theories of gravity"
The scale of Baryon Acoustic Oscillations (BAO) imprinted in the matter distribution is known to suffer sub-percent deviations from being a perfect standard ruler due to non-linear corrections. I will give an overview of historical and recent developments in alternative theories of gravity and present a computation of the BAO shift based on perturbation theory (a la Bellini-Sawicki) and a peak-background split (a la Sherwin-Zaldarriaga), valid for a large class of models. The BAO shift is modified by the enhanced growth of linear perturbations, as well as by novel, non-linear gravitational effects. The prediction can be larger than the standard value by 45-82% at z=0 for simple alternatives to a cosmological constant, and in some cases presents a characteristic time evolution. Although small, this effect might be within the reach of future galaxy surveys, and might have implications for BAO reconstruction in theories other than Einstein's.
Apr. 20, Monday
12:10 pm (TAC)
Andrew Wetzel, Caltech
Campbell 131A
"Facets of cosmic accretion: dark matter, gas, and satellite galaxies"
I will present recent projects that address several aspects of cosmic accretion into halos. First, I examine the physical nature of cosmic accretion of dark matter versus gas. Recent work suggests that most halo growth at late cosmic time is not physical but rather is the byproduct of an evolving virial radius ("pseudo-evolution"), which raises the question: how much physical accretion of baryons do halos experience to fuel galaxy growth? Using a suite of cosmological hydrodynamic simulations, I explore systematically the differing physics that governs the accretion of dark matter versus gas, showing that while dark matter is subject to pseudo-evolution, gas is not. Second, I will discuss ongoing efforts using cosmological simulations in conjunction with observations to constrain the orbital histories of the satellite dwarf galaxies of the Local Group (Milky Way and M31), including the newly discovered dwarf galaxies near the LMC, with implications for star-formation quenching via the host-halo environment and cosmic reionization.
Apr. 21, Tuesday
1:10 pm (Cosmology/BCCP)
No Tuesday talk this week (go to Monday talk)
Apr. 28, Tuesday
1:10 pm (Cosmology/BCCP)
Mehmet Alpaslan, Ames
Campbell 131A
"Galaxy evolution along the Cosmic Web"
No galaxy is an island, and the evolutionary fate of these systems is often inextricably linked to their environment. This relationship is well understood at local scales; but what role do cosmic filaments play in the assembly of mass? Do galaxies in voids evolve differently to galaxies in clusters? These fundamental questions require not only detailed observations, but also thorough catalogues of large scale structure.
We have put together one such catalogue by combining data from the Galaxy And Mass Assembly (GAMA) survey and a modified minimal spanning tree (MST) algorithm. This catalogue classifies galaxies as belonging to filaments and voids, as well as a third class of interstitial structures called `tendrils,' which bridge the gap between filaments, or extend out into voids. We then use this catalogue to look for trends in the observational properties of galaxies in different environments; namely in their morphology, stellar content, and energy output. We further extend our analysis by looking at how the star formation rates of galaxies correlate with their placement along filaments; from their outskirts to their most central cores.
March 2015:
Mar. 3, Tuesday
1:10 pm (Cosmology/BCCP)
Simeon Bird, CMU
Campbell 131A
"What can DLAs and the gas around galaxies tell
us about galaxy formation?"
The distribution and abundance of absorber systems contain a wealth of
information on the way in which star formation within galaxies affects
its environment. I will show how to reproduce several previously
problematic properties of Damped Lyman-alpha systems, discuss their
connection to metal absorbers, especially SiII and CIV, and show how the
requirement to reproduce observations can act as a constraint on the
imagination of theorists.
Mar. 6, Friday
12 noon (INPA)
Simeon Bird, CMU
LBL 50-5026 INPA room
"Simulating Massive Neutrinos"
Massive neutrinos make up a fraction of the dark matter, but due to their large thermal velocities, cluster significantly less than cold dark matter (CDM) on small scales. An accurate theoretical modelling of their effect on structure formation is required to fully exploit large-scale structure data. I will discuss different methods to add neutrinos to cosmological simulations, finishing with one which is accurate for current cosmological mass bounds and requires minimal extra computational cost.
Mar. 10, Tuesday
1:10 pm (Cosmology/BCCP)
George Becker, STSCI
Campbell 131A
"A Consensus Picture of Reionization?"
When and how the intergalactic medium (IGM) became reionized carries fundamental implications for the formation of the first stars and galaxies. New results from the Planck satellite now suggest that the bulk of reionization occurred somewhat later than previously thought, potentially easing tensions with observed galaxy populations at high redshifts. A wide range of reionization histories are still allowed, however, and it is unclear whether the simplest models truly match observations. I will present new constraints on the ionizing output from galaxies and the timing of reionization based on quasar absorption line studies of the IGM over 2 < z < 7. The results help to clarify how and when reionization ended, but also pose significant challenges to current models.
Mar. 17, Tuesday
1:10 pm (Cosmology/BCCP)
Enrique Gaztanaga,CSIC
Campbell 131A
"Modeling bias on large scale galaxy clustering"
I will present new results on how galaxy bias, ie. the way
galaxies trace the underlaying mass distribution, can limit our ability to
measure cosmological parameters and how we could improve this
by using cross-correlations, multiple probes and overlapping surveys.
Another route is to constrain galaxy bias with measurements of
higher order correlations. We have recently explored this idea with
large cosmological simulations and will give a brief description of
the results we have found.
Mar. 20, Friday
12 noon (INPA)
Zvonimir Vlah, Stanford
LBL 50-5026 INPA room
Mar. 27, Friday
12 noon (INPA)
Irshad Mohammed, Zurich
LBL 50-5026 INPA room
Towards precision cosmology: the halo model and necessary modifications
There are several approaches to account for clustering of matter and its evolution in the Universe. One of the more successful frameworks is the halo model. In this approach, all the matter in the Universe is assumed to be in isolated haloes with mass defined by a threshold density. Despite its success over a decade and physical motivation, this approach is limited by its accuracy and precision. I will present necessary modifications in the halo model in order to achieve desired accuracy and precision in the underlying statistics of the distribution of matter, so called matter power spectrum, and to the extension weak lensing applications. Along with the non-linear clustering of dark matter, I will also target the corrections due to baryonic physics at small scales, full covariance matrix of the matter power spectrum and finally the cosmological implications due to these corrections.
Mar. 31, Tuesday
1:10 pm (Cosmology/BCCP)
Andrew Pontzen, UCL
Campbell 131A
"How the large-scale Lyman-alpha forest will tell us about the galaxy luminosity function"
I will introduce a surprising relationship between the Lyman-alpha forest large-scale power spectrum and the luminosity function of galaxies and quasars [arXiv 1402.0506]. The forest is generally assumed to be a tracer of total matter on large scales but I will show how the assumed link between the dark and observable universe is substantially modified by radiative transfer effects. Future dark energy surveys like DESI will therefore present novel opportunities to learn more about the source of ionising photons in our universe [arXiv 1407.6367].
February 2015:
Feb. 2, Monday
12:10 pm (TAC)
Yuexing Li, Penn State
Campbell 131A
Bridging the Gap between Theory and Observations of Galaxies
Over the past decade, we have witnessed an explosion of multi-wavelength surveys on galaxies and quasars across cosmic time. In order to understand the observations, a comprehensive framework that fully accounts for the formation, evolution and multi-band properties of galaxies is desperately needed. I will discuss recent developments in the modeling of structure formation that combine cosmological simulations with multi-wavelength radiative transfer. I will present new results from the Illustris Radiative Transfer Project on the cosmic reionization, the origins of extragalactic background lights, and detectability of the first galaxies with the next generation instruments such as JWST and ALMA.
Feb. 3, Tuesday
1:10 pm (Cosmology/BCCP)
Kam Arnold,UCSD
Campbell 131A
"Fundamental Physics with Cosmic Microwave Background Polarimetry"
The polarization of the cosmic microwave background (CMB) offers a unique window onto cosmology that can provide information about neutrinos, dark matter, dark energy, large-scale structure formation, and physics at 10^16 GeV energy scales. Several important measurements of CMB polarization were reported last year, including the first-season results from POLARBEAR. With those measurements, we showed the gravitational lensing of the CMB by large-scale structure using CMB polarization data alone, and a measurement of a non-zero B-mode polarization angular power spectrum.
Going forward, our expansion of POLARBEAR - the Simons Array - will produce more precise observations in multiple spectral bands over a large fraction of the sky. The Simons Array, powerful as a standalone experiment, is also a technological pathfinder for both the CMB-S4 experiment described in the Particle Physics Project Prioritization Panel report, and for the LiteBIRD satellite, which we recently proposed to NASA as a partner mission with JAXA. Together, the Simons Array and LiteBIRD will measure the sum of the neutrino masses with the precision necessary to determine their mass hierarchy, and make a deep search for the inflationary B-mode signal, producing a detection with significance > 10 sigma of the B-modes predicted by all large-field inflation models.
Feb. 6, Friday
12 noon (INPA)
Blake Sherwin, UCB/BCCP
LBL 50-5026 INPA room
"Towards a Clear View of the Early Universe: Methods for Delensing Near-Future CMB Polarization Surveys"
Large-scale B-mode polarization is one of our most promising windows to probe the physics of inflation, and study the universe at the highest energies and earliest times. But this window's glass is blurred: gravitational lensing produces a background of non-primordial B-modes, which present a fundamental limit to constraints on inflationary B-mode polarization. After a brief review of CMB lensing science in general, I will discuss methods for getting around this limit by delensing the B-mode sky -- removing the non-primordial component by subtracting off an estimate of the lensing B-mode. In particular, I will discuss some recent work on delensing the CMB using the cosmic infrared background or CIB, the large scale structure tracer which is the closest known match to the true CMB lensing map. I will explain why delensing with the CIB is a promising method to enhance near-future B-mode surveys and strengthen the constraints on inflation they can provide.
Feb. 10, Tuesday
1:10 pm (Cosmology/BCCP)
Michael Niemack, Cornell
Campbell 131A
"Probing fundamental physics and cosmic structure by measuring the CMB"
The cosmic microwave background (CMB) has proven to be a powerful probe of the physics and cosmology of our universe. CMB observations are helping to address fundamental questions, such as the nature of dark energy and dark matter, and are being used to probe the physics of inflation at energies a trillion times higher than the Large Hadron Collider. Recent measurements led to several exciting first detections, including CMB lensing, massive galaxy clusters, the large-scale velocity field, and the "B-mode" component of the polarization field. I will discuss these results, novel superconducting detector and optics instrumentation developed for the 6-meter Atacama Cosmology Telescope, and unique capabilities of the recently-funded Advanced ACTPol project. I will also describe the science potential of next generation observatories, including the ultimate "Stage-IV" CMB survey, and how superconducting detector arrays of the future could revolutionize photon-detection capabilities spanning eight orders of magnitude in wavelength.
Feb. 13, Friday
Eric Linder, UCB/BCCP
12 noon (INPA)
LBL 50-5026 INPA room
"New Probes of Cosmology"
We explore a tasting menu of new advances in probing cosmology:
- DESI and gravitional waves
- DESI and inflation
- Direct acceleration through cosmic redshift drift
- Strong gravitational lensing
We also discuss some generally applicable new tools, such as our competition winning machine learning for crosscorrelating noisy data (e.g. measuring time delays, exoplanets), optimizing leverage under a fixed resource constraint (wide field survey followup), and the new likelihood mapper MultiTracer to replace MCMC.
Feb. 17, Tuesday
1:10 pm (Cosmology/BCCP)
Albert Stebbins, FNAL
Campbell 131A
"Crab nanoshots, fast radio bursts and cosmology"
We explore the brightness frontier in time domain radio astronomy and its possible usefulness for cosmology. I argue that the brightest known source of emission, Crab nanoshots, could be caused by Schwinger pair production. The same mechanism may be the source of Fast Radio Bursts (FRBs) if this emission is from coalescing neutron stars. It is then shown how using FRBs as triggers can extend the reach of gravitational radiation and neutrino telescopes. Finally we discuss how combining FRB monitoring with these other observations might provide a more accurate Hubble diagram.
Feb. 23, Monday
5:30 pm (Oppenheimer lecture)
Andrei Linde, Stanford
Chevron Auditorium, International House
"Universe or Multiverse"
Cosmological observations show that the universe is very uniform on the maximally large scale accessible to our telescopes. The best theoretical explanation of this uniformity is provided by the inflationary theory. I will briefly describe the status of this theory in view of recent observational data obtained by the Planck satellite. Rather paradoxically, this theory predicts that on a very large scale, much greater than what we can see now, the world may look totally different. Instead of being a single spherically symmetric balloon, our universe may look like a "multiverse", a collection of many different exponentially large balloons ("universes") with different laws of low-energy physics operating in each of them. The new cosmological paradigm, supported by developments in string theory, changes the standard views on the origin and the global structure of the universe and on our own place in the world.
Feb. 24, Tuesday
1:10 pm (Cosmology/BCCP)
Ue-Li Pen, CITA
Campbell 131A
"Cosmic neutrino wakes"
We present a neutrino-dark matter relative motion effect, which leads to dipolar wakes around dark matter halos, an dipole correlation anisotropy, and other potentially observable outcomes. We present recent attempts to simulate this non-linear effect.
Feb. 27, Friday
Nicolas Tejos, Santa Cruz
LBL 50-5026 INPA room
"The intergalactic medium in the cosmic web"
I will present observational results on the properties and distribution of the intergalactic medium (IGM) in the cosmic web. I use proprietary and public data from HST/COS UV spectroscopy of background QSOs to observe the IGM in absorption, together with galaxies mapped in emission from optical spectroscopy. By cross-matching the position of individual HI absorption line systems to those of different large scale structures (LSS) traced by galaxy distributions (e.g. voids, filaments, clusters), I characterize the IGM in different cosmic environments. I will present results on the properties of the IGM: (i) around normal star-forming and non-star-forming galaxies; (ii) within and around galaxy voids at z< 0.1, that trace low- and mean-density environments respectively; and (iii) high-density environments traced by galaxy cluster pairs (intercluster filaments). With these datasets, we can directly test the modern paradigm for the cosmic web developed from cosmological simulations of structure formation.
January 2015:
Jan. 20, Tuesday
1:10 pm (Cosmology/BCCP)
Alex Richings, Leiden
Campbell 131A
"Non-equilibrium interstellar chemistry in simulations of galaxy formation"
I will present a method to follow the chemical evolution of interstellar gas in hydrodynamic simulations of galaxy formation. This model follows the abundances of 157 chemical species in total, including all ionization states of the 11 elements that are most important for cooling, and 20 molecular species including H2 and CO. This allows us to evolve the galaxy using gas cooling rates that are calculated using non-equilibrium abundances, and it allows us to predict the observable emission from individual species, such as CII and CO, without needing to assume chemical equilibrium. To explore how non-equilibrium chemistry can affect the gas cooling rate, we have applied this chemical model to idealized cases of a uniform gas cloud that is cooling at either constant density or constant pressure. We find that, at temperatures below 10^4 K, the cooling rate calculated using non-equilibrium abundances is enhanced by up to two orders of magnitude. Finally, I will present simulations of isolated disk galaxies that we have run using this chemical model. We find that the abundance of molecular hydrogen at low metallicities tends to be further from chemical equilibrium than at high metallicities. We also compute the emission from CO and we find that there is more scatter in the relation between H2 column density and CO intensity when we use non-equilibrium abundances.
Jan. 22, Thursday
4:00 pm (RPM)
Darcy Barron, UCSD
LBL 50A-5132
"Measuring the Cosmic Microwave Background Polarization with POLARBEAR"
POLARBEAR is a cosmic microwave background (CMB) polarization experiment located in the Atacama desert in Chile. POLARBEAR-1 started observations in 2012, and in 2014, the POLARBEAR team published results from its first season of observations on a small fraction of the sky. These results include the first measurement of a non-zero B-mode polarization angular power spectrum, measured at sub-degree scales where the dominant signal is gravitational lensing of the CMB. We also published a measurement of the large-scale gravitational structure deflection power spectrum derived from CMB polarization alone, which demonstrates a powerful technique that can be used to measure nearly all of the gravitational structure in the universe. Improving these measurements requires precision characterization of the CMB polarization signal over large fractions of the sky, at multiple frequencies. To achieve these goals, POLARBEAR has begun expanding to include an additional two 3.5 meter telescopes with multi-chroic receivers, known as the Simons Array. Phased upgrades to receiver technology will improve sensitivity and capabilities, while continuing a deep survey of 80% of the sky. POLARBEAR-2 is the next receiver that will be installed in 2015 on a new telescope, with a larger area focal plane with dichroic pixels, with bands at 95 GHz and 150 GHz, and a total of 7,588 polarization sensitive antenna-coupled transition edge sensor bolometers. The focal plane is cooled to 250 milliKelvin, and the bolometers will be read-out by SQUID amplifiers with 40x frequency domain multiplexing. The array is designed to have a noise equivalent temperature of 5.7 muK(s)^1/2.
Jan. 27, Tuesday
1:10 pm (Cosmology/BCCP)
no talk
Campbell 131A
December 2014:
Dec. 2, Tuesday
1:10 pm (Cosmology/BCCP)
Jes Ford, UBC
Hearst Field Annex B-1
"Galaxy Cluster Studies with Weak Lensing Magnification and Shear "
The magnification component of weak lensing provides complementary
information to the more commonly measured shear distortion. While low
redshift halos are better constrained by shear, at increasingly high
redshifts the magnification signal becomes quite competitive. We present
recent measurements of halo masses from the stacked magnification signal
of >10,000 galaxy clusters in the 154 deg^2 Canada-France-Hawaii-Telescope
Lensing Survey (CFHTLenS). We perform the first direct mass comparison
between magnification and shear, finding global agreement between the
independent methods, but with systematic effects influencing particular
cluster redshift ranges. We measure the mass-richness scaling relation of
the CFHTLenS 3D-MF cluster sample, and search for evidence of its
evolution with redshift.
Dec. 5, Friday
Daan Meerburg, Princeton
LBL 50-5026 INPA room
"Putting inflation to the test"
In this talk I will address 2 questions. How can we test inflation as a scientific paradigm and if inflation happened, what we can we learn from observations about the physical mechanism behind it. I will present work on the latter based on looking for the existence of specific deviations from scale invariance through the presence of features in the CMB correlation functions. Looking for features used to be a computationally demanding effort, and I will show that by assuming that these features are perturbative w.r.t. to the overall scale dependence, large chunks of parameter space can be covered very efficiently. I will discuss the implications of our findings. For the former I will present work on testing the inflation consistency condition using the CMB and ground based gravitational wave detectors. I will show that even without BICEP data, one can already effectively use ground based detectors as well as constraints coming directly from the expansion history of the Universe, to put interesting limits on the r-n_t plane.
Dec. 9, Tuesday
1:10 pm (Cosmology/BCCP)
Doug Watson, Chicago
Hearst Field Annex B-1
" Modeling Galaxy Star Formation Across Cosmic Time"
How galaxies form and evolve remains one of the most profound problems in astrophysics today. The backbone of all theories of galaxy formation is the favored Cold Dark Matter (CDM) model for structure growth. Galaxies are believed to be tracers of the CDM cosmic web and are inextricably tied to the evolution and properties of bound regions of dark matter, so-called dark matter halos. We are in a particularly exciting era for understanding galaxy formation thanks to precision dark matter N-body simulations coupled with a rapid influx of exquisite high-redshift data, such as 3D-HST, CANDELS and UltraVista, as well as existing low-redshift surveys such as the Sloan Digital Sky Survey (SDSS). This allows, for the first time, the exploration of truly high-precision models of galaxy formation out to z~3. Specifically, I will focus on the following topics. (1) The "age matching" model that is predicated on the co-evolution of galaxies and halos. This simple formalism has been shown to be remarkably powerful, yielding accurate predictions of SDSS color- and SFR- dependent clustering and galaxy-galaxy lensing, as well as a variety of galaxy group-based statistics. (2) New observational statistics that have the power to discriminate between competing galaxy formation models. (3) Future prospects for self-consistent, precision modeling across most of cosmic time.
Dec. 9, Tuesday
4:00 pm (RPM)
Florent Leclercq, IAP
LBL 50A-5132
"How did Structure Appear in the Universe? - A Bayesian Approach"
Ideally, cosmological surveys should be analyzed in terms of the joint constraints they place on the initial conditions from which structure originates and on their subsequent gravitational evolution. In this talk, I will describe an innovative statistical data analysis approach designed for the ab initio simultaneous analysis of the formation history and morphology of the large-scale structure of the inhomogeneous Universe. As three-dimensional large-scale structure surveys contain a wealth of information that cannot be trivially extracted due to the non-linear dynamical evolution of the density field, I will discuss methods designed to improve upon previous techniques by including non-Gaussian and non-linear data models for the description of late-time structure formation.Through the talk, I will demonstrate the application of our inference framework to the Sloan Digital Sky Survey data release 7 and describe the primordial and late-time large-scale structure in the Sloan volume. I will show how the approach has led to the first quantitative reconstructions of the cosmological initial conditions from galaxies, an exceptionally detailed characterization of the dynamic cosmic web underlying the observed galaxy distribution, and a new, enhanced catalog of cosmic voids probed at the level of the dark matter distribution, deeper than with the galaxies. Finally, I will present research plans aimed at further physical investigations using cosmological reconstructions.
Dec. 11, Thursday
4pm (RPM)
Kyle Story, Chicago
LBL 50-5026
"Gravitational Lensing of the CMB with the SPTpol Experiment"
I will present a measurement of the cosmic microwave background (CMB) gravitational lensing potential using data from the first two years of observations with SPTpol, the polarimeter installed on the South Pole Telescope. Measurements of gravitational lensing of the CMB directly probe the projected mass in the universe out to high redshifts. Gravitational lensing encodes a wealth of information in the CMB about the growth and geometry of large-scale structure, which is sensitive to cosmic acceleration (dark energy), the expansion history of the universe and the properties of neutrinos. I will also discuss my work measuring the primary temperature anisotropies of the CMB and the resulting constraints on cosmic neutrinos.
Dec. 18, Thursday
4pm (RPM)
Marius Millea, Davis
LBL 50A-5132
"Planck 2014 Constraints on the Cosmic Neutrino(-like) Background"
The Planck 2014 results include the tightest measurements to-date of cosmic microwave background (CMB) temperature and polarization anisotropies up to few-arcminute angular scales. I will give a broad overview of the cosmology results from these data, with particular focus on what we have learned about the cosmic neutrino background (CNB). The new data allow more precise answers to questions such as 1) how much energy, parameterized by N_eff, is contained in the CNB? 2) what is the sum of the masses of the particles making up the CNB? and 3) are these particles really neutrinos, i.e. do they free-stream like neutrinos? One possibility I will explore is if some component of the CNB actually comes from axions or axion-like particles. Recent improvements in CMB and BBN data are shedding new light on this scenario. Finally, I will discuss the status of agreement between Planck results and other cosmological probes such as BAO, H0, and low redshift structure measurements, and how the CNB may play a role in resolving tensions between some of them.
November 2014:
Nov. 4, Tuesday
1:10 pm (Cosmology/BCCP)
Colin Slater, Michigan
Hearst Field Annex B-1
Satellite quenching and the life cycle of dwarf galaxies
In the past ten years the known population of Local Group dwarf
galaxies has expanded substantially, both to greater distances from
the Milky Way and to lower dwarf masses. This growing sample allows us
to study the dwarf system as a population, and ask if we can see in
aggregate the signs of processes that would otherwise be difficult to
trace in dwarfs individually. Following this strategy I will discuss
how the quenching of dwarf galaxies can be modeled and understood at
the population-level, and how we use that to constrain how possible
quenching mechanisms must work if they are to reproduce the Local
Group system that we see. I will also discuss work done with
Pan-STARRS to study the role of infalling satellites in disrupting the
outer disk of the Milky Way and creating the so-called "Monoceros
Ring".
Nov. 11, Tuesday
Veteran's Day, no talk
Nov. 14, Friday
Cora Uhlemann, LMU Munich
LBL 50-5026 INPA room
"Large scale structure formation with the Schroedinger method"
When describing large-scale structure formation of collisionless dark matter one is interested in the dynamics of a large collection of identical point particles that interact only gravitationally. Via gravitational instability initially small density perturbations evolve into eventually bound structures, like dark matter halos that are distributed along the cosmic web. Even though this problem seems quite simple from a conceptual point of view, no sufficiently general solution of the underlying equation, the collisionless Boltzmann equation coupled to the Poisson equation, is known. Therefore one usually has to resort to N-body simulations which tackle the problem numerically. Analytical methods to describe structure formation are in general based on the dust model which describes cold dark matter as a pressureless fluid characterized by density and velocity. This model works quite well up to the quasi-linear regime but eventually fails when multiple streams form that are especially important for halo formation but lead to singularities in the model. We employ the so-called Schröger method to develop a model which is able to describe multi-streaming and therefore can serve as theoretical N-body double and replacement for the dust model. As a first application we study the coarse-grained dust model, which is a limiting case of the Schröger method, within perturbation theory. On the basis of the Gaussian streaming model for redshift space distortions we predict the halo correlation function by generalizing the idea of the truncated Zeldovich approximation.
Nov. 18, Tuesday
1:10 pm (Cosmology/BCCP)
Emanuele Castorina, SISSA
Hearst Field Annex B-1
Massive neutrinos and the Large Scale Structures of the Universe
Massive neutrinos have peculiar effects on several observables in current and future redshift surveys. A precise determination of them is crucial not only to constraint properly neutrino masses but also to avoid potential systematic errors in the determination of other cosmological parameters, e.g. the dark energy equation of state.
In this talk, after a brief review of linear theory results, I will discuss, with the help of a large suite of N-body simulations, the effect of massive neutrinos on different cosmological probes, like the abundance of massive clusters, the non linear matter power spectrum and its relation to the galaxy power spectrum, redshift space distorsions, and the bispectrum.
Nov. 21, Friday
12:00 pm (INPA/Cosmology/BCCP)
Ryan Cooke, Santa Cruz
LBL 50-5026 INPA room
The primordial deuterium abundance and the search for new physics
We are currently in an exciting era of precision cosmology. With the release of the cosmic microwave background data recorded by the Planck satellite, we are now in a position to accurately test the standard model of cosmology and particle physics. In this talk, I will present several precise measurements of the primordial abundance of deuterium - the most accurate measurements to date - derived from redshift ~3 metal-poor damped Lyman-alpha systems. These data have offered a new insight into the physical laws of the Universe just minutes after the Big Bang. Such precise measures, when analyzed in conjunction with the Planck data, now place strong bounds on both the total amount of visible matter in the Universe and the effective number of neutrino species. These data further provide new limits on physics beyond the standard model. I will discuss our ongoing survey to obtain new precision measures of the primordial nuclei in the era of the 30m class telescopes.
Nov. 24, Monday
12:10 pm (TAC)
Dylan Nelson, Harvard/CfA
Hearst Field Annex B-1
"How galaxies acquire their gas: perspectives from modern cosmological simulations"
Despite significant progress with numerical simulations over the past decade, the process by which galaxies acquire their baryonic matter through cosmic time remains poorly understood. I examine this question with a series of cosmological hydrodynamical simulations, in the context of the moving-mesh code Arepo and the Illustris simulation project. First, I will describe the "Monte Carlo tracer particle" technique we developed to follow the Lagrangian history of gas in grid-based simulations. Then, I will re-examine how galaxies accrete gas from the intergalactic medium, contrasting our findings to earlier simulations which identified 'cold flows' or 'cold mode accretion' as a primary driver of galaxy growth. Next, I will discuss how the inclusion of a comprehensive model for baryonic feedback processes changes the nature and rate of cosmological gas accretion. In this context we identify several key differences with respect to simple theoretical assumptions commonly employed in semi-analytical models of galaxy formation. I will conclude with some ongoing work -- a suite of single object, high-resolution "zooms" which exquisitely resolve the gas-dynamics of the circumgalactic regime and the interaction between filamentary inflow from the IGM and the quasi-static hot halo atmosphere.
Nov. 25, Tuesday
1:10 pm (Cosmology/BCCP)
Camille Avestruz, Yale
Hearst Field Annex B-1
"Cosmological Simulations of Galaxy Cluster Outskirts"
The observational study of galaxy cluster outskirts is a new territory to probe the thermodynamic and chemical structure of the X-ray emitting intracluster medium (ICM). Cluster outskirts are particularly important for modeling the Sunyaev-Zel'dovich effect, which is sensitive to hot electrons at all radii and has been used to detect hundreds of galaxy clusters with recent microwave cluster surveys. In cluster-based cosmology, measurements of cluster outskirts are an important avenue for estimating the cluster mass, as the outskirts are less sensitive to baryonic processes that dominate the cluster core. However, recent observations of cluster outskirts deviate from theoretical expectations, indicating that cluster outskirts are more complicated than previously thought. Computational modeling of cluster outskirts is necessary to interpret these observations. I will present cosmological simulations of galaxy cluster formation that follow the thermodynamic and chemical structures in the virialization regions of the ICM and transition to the IGM. Specifically, I will discuss how observational signatures of galaxy clusters are affected by gas flows, inhomogeneities in the ICM, and non-equilibrium physics.
Nov. 25, Tuesday
4pm (RPM)
Boris Leistedt, UCL
LBL 50A-5132
"Measuring primordial non-Gaussianity with photometric quasars"
Quasars
are highly biased tracers of the large-scale structure and therefore
powerful probes of the initial conditions and the evolution of the
universe. However, current spectroscopic catalogues are relatively small
for studying the clustering of quasars on large-scales and
over extended redshift ranges. Hence one must resort to photometric
catalogues, which include large numbers of quasars
identified using imaging data but suffer from significant stellar
contamination and systematic uncertainties. I will present a detailed
analysis of the photometric quasars from the Sloan Digital Sky Survey, and the resulting constraints on the quasar
bias and primordial non-Gaussianity. The constraints on $f_{\rm NL}$,
its spectral index, and $g_{\rm NL}$, are the tightest ever obtained
from a single population of quasars or galaxies, and are competitive with the results obtained with WMAP, demonstrating the potential of quasars
to complement CMB experiments. These results take advantage of a novel
technique, 'extended mode projection', to mitigate the complex
spatially-varying systematics present in the survey in a blind and
robust fashion. This work is a new step towards the exploitation of data
from the Dark Energy Survey, Euclid and LSST, which will require a
careful mitigation of systematics in order to robustly constrain new
physics.
October 2014:
Oct 3, Friday
12:00 pm (INPA/Cosmology/BCCP)
Yin Li, Chicago
LBL 50-5026 INPA room
'The power spectrum super-sample effect'
The impact of density fluctuations with wavelengths larger than a survey must be considered when extracting cosmological information from power spectrum measurements. These modes change the power spectrum in the same way as a change in the cosmological background does. Using a handful of separate universe simulations, we accurately capture this effect in terms of response of the matter power spectrum to a single mode --- the mean density fluctuation in the survey volume. The unknown amplitude of this mean density mode contributes to a (typically dominant) error in the matter power spectrum estimators. Alternatively, it can also be simply included in parameter estimation and forecasts by treating the mean density fluctuation as an additional cosmological parameter. Parameter degeneracies arise since the response of the power spectrum to the mean density mode and cosmological parameters share similar properties in changing the growth of structure and dilating the scale of features.
Oct. 7, Tuesday
1:10 pm (Cosmology/BCCP)
Benedikt Diemer, Chicago
Hearst Field Annex B-1
"The (non-)universality of halo density profiles"
The density profiles of dark matter halos are an essential input for models of galaxy formation, as well as for the interpretation of numerous observations such as weak and strong lensing signals. The profiles are commonly thought to follow a simple, universal shape, and only depend on two parameters, mass and concentration. Using a large suite of cosmological simulations, I will show that the outer halo density profiles depend on an additional parameter, the mass accretion rate, and present an accurate new fitting formula that takes this dependence into account. I will further discuss the question of universality, and show that the definition of the halo boundary plays a crucial role. Similarly, halo concentrations are usually described as a universal function of mass and redshift. Instead, I will present a model in which concentration depends on an additional parameter: the local slope of the matter power spectrum. I will demonstrate that this model accurately (to better than 10-15%) describes simulated concentrations over a large range of redshifts, halo masses and cosmological parameters, and is in excellent agreement with the recent observations of the CLASH cluster survey.
Oct. 14, Tuesday
1:10 pm (Cosmology/BCCP)
Simone Ferraro, Princeton
Hearst Field Annex B-1
'Cosmic Flows: cosmology and astrophysics from galaxy velocities'
The study of velocity fields is a powerful probe of structure formation and the energy content of our Universe. Additionally, the motion of ionized gas on intermediate scales can shed light on galaxy formation and feedback mechanisms. I will first review the importance of velocity fields as a cosmological tool, with particular focus on redshift-space distortions and the kinetic Sunyaev-Zel'dovich effect. I will then discuss new techniques that can be used both to constrain cosmology and measure the clustering of baryons, which should help resolve the "missing baryon" problem. Finally, I will present some preliminary observational results.
Oct 17, Friday
2:00 pm (INPA/Cosmology/BCCP)--time change for runaround
Roland de Putter, Caltech
LBL 50-5026 INPA room
"Constraining fundamental physics with weak lensing and galaxy clustering"
I will discuss examples of the importance of complementarity between cosmological data sets. I will first discuss bounds from current data on neutrino mass and the primordial power spectrum, and will show that combining cosmological probes is crucial for obtaining robust constraints. I will then discuss the complementarity between near-future weak lensing and galaxy clustering surveys such as SuMIRe (Subaru Measurements of Images and Redshifts), showing that the combination of these probes promises much tighter dark energy and modified gravity constraints than those from either probe alone. Finally, I will discuss (optimal) survey strategies for constraining primordial non-Gaussianity with galaxy clustering data.
Oct. 21, Tuesday
1:10 pm (Cosmology/BCCP)
Josh Dillon, MIT
Hearst Field Annex B-1
"Chasing the Cosmic Dawn with 21 cm Tomography"
Realizing the promise of 21 cm cosmology to provide an exquisite probe of astrophysics and cosmology during the cosmic dark ages and the epoch of reionization has proven extremely challenging. We're looking for a small signal buried under foregrounds orders of magnitude stronger. We know that we're going to need very sensitive, and thus very large, low frequency interferometers, which present their own set of difficulties. And, as I will explain, we're going to need a rigorous statistical analysis of the maps we make to extract interesting cosmological information. I will discuss the steps we've taken to overcome these obstacles with prototype data from the Murchison Widefield Array by isolating foregrounds to a region of Fourier space outside a clean "epoch of reionization window." Additionally, I will present some of most recent and exciting predictions for what 21 cm cosmology can tell us as we move to larger telescopes like the Hydrogen Epoch of Reionization Array and higher redshifts.
Oct. 28, Tuesday
1:10 pm (Cosmology/BCCP)
Joseph Clampitt, Penn
Hearst Field Annex B-1
Lensing Measurements of SDSS Voids and Filaments
I will describe measurements of weak lensing mass profiles of voids from a volume-limited sample of SDSS Luminous Red Galaxies (LRGs). The stacked shear measurement has been performed on ~10,000 voids and subvoids with radii between 15-40 Mpc/h and redshifts between 0.16-0.37. The characteristic radial shear signal of voids is detected with a statistical significance that exceeds 13-sigma. The mass profile corresponds to a fractional underdensity of about -0.4 inside the void radius and a slow approach to the mean density indicating a partially compensated void structure. Time permitting, I will also describe a stacked weak lensing detection of filaments between close pairs of LRGs.
Oct 31, Friday
12:00 pm (INPA/Cosmology/BCCP)
Liang Dai, JHU
LBL 50-5026 INPA room
Conformal Fermi coordinates and the local universe formalism
In an inhomogeneous Universe, the physical effect of long-wavelength perturbation on short distances should be such that short-wavelength perturbations effectively evolve in a modified homogeneous universe. We explicitly construct the so-called conformal Fermi normal coordinates (CFNC) through an expansion around the observer's geodesic, which describe the local spacetime as a quasi-FRW metric and are valid at all times. The CFNC formalism demonstrates that the zeroth-order picture is that local expansion rate and spatial curvature are renormalized by long-wavelength perturbations, and the general condition for the spatial curvature to be a constant is derived. Beyond this "separate universe" picture, CFNC allows for systematic extraction of additional local effects from long-wavelength perturbations that cannot be attributed to a re-definition of the background FRW cosmology. The formalism can be useful in the studies of tracer bias, intrinsic alignment and gravitational-wave "fossil" effect.
September 2014:
Sep. 2, Tuesday
1:10 pm (Cosmology/BCCP)
Sirio Belli, Caltech
Hearst Field Annex B-1
"Deep Keck spectroscopy of 1 < z < 2.5 quiescent galaxies: constraining the size growth and the mass assembly of the red sequence "
The most effective probe of the physical nature of quiescent galaxies is absorption line spectroscopy, which is particularly challenging at high redshift. Using the improved sensitivity of optical and infrared detectors at the Keck observatory, and the multiplex advantage of its new MOSFIRE spectrograph, we have undertaken a new spectroscopic survey of over 100 galaxies selected according to stellar mass and rest-frame optical color in the redshift range 1 < z < 2.5. Velocity dispersions and stellar ages derived from our spectra, together with HST-based sizes, provide valuable insight into the mass assembly of quiescent galaxies. We find that the stellar to dynamical mass ratio evolves with redshift, which might imply a change in the dark matter fraction or in the stellar initial mass function. We also find that recently quenched galaxies are systematically larger in size, which enables us to quantify how "progenitor bias" contributes to the observed size evolution. We conclude that at least half of the size evolution of the red sequence observed at z~1.5 is due to physical growth of individual galaxies.
Sep. 4, Thursday
4:10 pm (Astronomy Colloquium)
Linda Tacconi, MPE, Garching
2 LeConte Hall
"The Evolution of Molecular Gas and Star Formation from the Peak Epoch of Galaxy Formation to the Present"
Comprehensive and systematic studies of the molecular content of galaxies during the epochs that are associated with the peak (z~1-2), and subsequent winding down (z<1) of star formation in the Universe are enabling us to illustrate the important role that cold gas, , the fuel for star formation, has played in the assembly of galaxies across cosmic time. Surveys, including COLDGASS and PHIBSS1&2, already provide robust molecular gas detections in hundreds of normal, star forming galaxies, from redshifts 0-2.5. In this talk, we focus on results from PHIBSS, comprising two IRAM Large Programs, where we are we have are mapping the CO J=3-2 or J-2-1 line emission in ~200 such galaxies from z=0.5-2.5; we find that galaxies at these epochs are very gas rich, relative to their star-forming counterparts in the local Universe. We discuss scaling relations for massive star forming galaxies that we derive from these data, and the impact of all of these new observations on our understanding of galaxy evolution in the early Universe.
Sep. 5, Friday
12 noon (INPA)
Adam Solomon, Cambridge
LBL 50-5026
"Are Two Metrics Better Than One?: The Cosmology of Massive (Bi)gravity"
In recent years, massive gravity - which modifies general relativity by giving the graviton a mass - has garnered increasing attention as an alternative to #CDM which may not need to be fine-tuned against large quantum corrections. The ingredients of massive gravity are two metric-like tensors; the theory is often studied in its bimetric form, where both metrics are dynamical. It is this version of the theory which leads to viable FRW cosmologies.
I will discuss recent work on the cosmology of both massive gravity and bigravity, focusing on three aspects: 1) their stability, 2) their predictions for structure formation in the subhorizon regime, and 3) the possibility of extending the theory to couple matter to both metrics. Our findings on the first two of these picks out a particular submodel, infinite-branch bigravity (IBB), which is stable at all times and agrees with present observational constraints, yet presents unique signatures for structure formation. These should render massive bigravity testable in the near future by Euclid and SKA. The doubly-coupled theory is nascent; I will talk about some of its salient cosmological properties, and discuss the potential for nontrivial matter couplings to avoid the no-go theorem on FRW cosmologies in single-metric massive gravity.
Sep. 9, Tuesday
1:10 pm (Cosmology/BCCP)
Frederick Davies, UCLA
Hearst Field Annex B-1
"Ionization in the Intergalactic Medium Across Cosmic Time"
Understanding the ionizing background of the universe is crucial to interpreting observations of intergalactic gas in the context of large-scale structure. The epochs of H and He reionization, currently under intense observational and theoretical investigation, set the boundary conditions for the propagation of ionizing photons in the universe. Using novel 1D and 3D calculations, we find that fluctuations in the ionizing background due to rare or clustered sources can be very important, in contrast to previous work. We show that fluctuations in the radiation field cause the mean free path to vary, leading to large-scale correlations that may match recent observations. We have also modeled Lyman-alpha emission from quasar ionization fronts as a potential probe of reionization topology at high redshift, expanding upon previous work by performing ionizing radiative transfer along sightlines from a cosmological simulation and accounting for important causal effects. Our new estimate of the Lyman-alpha surface brightness is a factor of >3 smaller than originally expected, likely placing it out of reach of current and near-future instruments.
Sep. 11, Thursday
4:10 pm (Astronomy Colloquium)
Chao-Lin Kuo, Stanford
2 LeConte Hall
BICEP and Keck
South Pole-based BICEP/Keck program is a series of experiments focusing on making deep measurements of degree-scale B-mode polarization in the cosmic microwave background. This feature is a unique probe of inflationary gravitational waves (tensor modes) and fundamental physics at energy scales inaccessible to terrestrial particle colliders. This spring, BICEP2 reported a significant detection of degree-scale B-modes. I will discuss the measurements, implications, and the upcoming BICEP3 experiment, the latest major expansion of this series that will be deployed in this austral summer.
Sep. 15, Monday
4:15 pm (Physics Colloquium)
Adrian Lee, Berkeley
1 Le Conte Hall
"Gravitational Lensing and the Search for Inflation in the Cosmic Microwave Background Polarization"
Observations of temperature fluctuations in the Cosmic Microwave Background (CMB) have played a central role in establishing our current model of cosmology. The next frontier in CMB observations is the measurement of polarized fluctuations, which could determine (and may have already have) if an epoch of cosmic inflation put the Bang in the Big Bang. Also, measurements of polarized CMB gravitational lensing have the potential to weigh the sum of neutrino masses by their effect on structure formation.
The POLARBEAR experiment has made some of the first measurements of the faint and hard-to-detect polarized gravitational lensing effect of the CMB. I will describe our new measurements and the detector technology we developed to achieve the required sensitivity, as well as the design aspects of the experiment to control systematic errors.
POLARBEAR is also searching for the signature of the Inflation in the early Universe. I will describe the current status of the field, including the recent BICEP2 results and the importance of separating a cosmic signal from galactic foreground emission. In the next few years, the single-telescope POLARBEAR experiment will be expanded to become the Simons Array, consisting of three telescopes. The Simons Array will have higher sensitivity than any current generation experiment, broad frequency coverage to separate galactic foreground emission, and stringent control of systematic errors. Finally, I will discuss long-term plans for CMB polarization measurements focusing on the ground-based CMB-S4 experiment and the LiteBIRD space mission.
Sep. 16, Tuesday
1:10 pm (Cosmology/BCCP)
Shea Garrison-Kimmel, Irvine
Hearst Field Annex B-1
"Lessons in Near-Field Cosmology from Simulating the Local Group"
Studies of the Milky Way (MW) and Andromeda (M31) galaxies, along with their associated satellites and nearby dwarf galaxies, have proven immensely useful for constraining the cosmology of the Universe, particularly on small scales. I will present a number of simulations, many of which are a part of the ELVIS Suite, cosmological zoom-in simulations of Local Group-like volumes of MW/M31 pairs. Using these, and other simulations, I will highlight existing tensions within the LCDM paradigm, as well as illustrate how simulations can provide links between near-field and deep-field observations.
Sep. 17, Wednesday
2 pm
Leonardo Senatore, Stanford
LBL 50A-5132
"The Effective Field Theory of Cosmological Large Scale Structures"
The Effective Filed Theory of Large Scale Structures provides a novel framework to analytically compute the clustering of the Large Scale Structures in the weakly non-linear regime in a consistent and reliable way. The theory that describes the long wavelength fluctuations is obtained after integrating out the short distance modes and adding suitable operators that allow to correctly reconstruct the effect of short distance fluctuations at long distances. A few observables have been computed so far, and the results are extremely promising. I will discuss the formalism and the main results so far.
Sep. 18, Thursday
4:10 pm (Astronomy Colloquium)
Evan Kirby, Caltech
2 LeConte Hall
"Dwarf Galaxies: The Nexus of Dark Matter and Chemical Evolution"
The Local Group's dwarf galaxies are near enough for exquisitely detailed, resolved stellar spectroscopy and diverse enough to conduct experiments on dark matter and chemical evolution. I have collected medium-resolution spectra for thousands of stars in many dwarf galaxies in the Local Group. Innovative techniques applied to these spectra recover velocities precise to a few km/s and detailed abundances precise to 0.1 dex. Although Milky Way satellites and field dwarf galaxies are different in many ways, their velocity dispersions show that both types of galaxy pose a serious challenge to cold dark matter. Both types also obey the same mass-metallicity relation despite the large diversity of star formation histories and detailed abundance ratios. I will show how those detailed abundances reveal the star formation histories. I will closely examine one galaxy, Segue 2, because it is the least massive galaxy known.
Sep. 23, Tuesday
1:10 pm (Cosmology/BCCP)
Joe Hennawi, MPIA
Hearst Field Annex B-1
"Three Unresolved Problems in Studies of the Circumgalactic Medium"
The physical conditions of diffuse gas in the outskirts of
galaxies, known as the circumgalactic medium, regulates the supply of
fuel for star-formation in galaxies. The conventional wisdom is that
hydrodynamics of this moderate overdensity gas is resolved by current
simulations of galaxy formation. I will present three different
observations suggesting that the observed properties of the
circumgalactic medium are in conflict with the predictions of these
simulations, casting serious doubts on the predictive power of the
current generation of models. While the galaxy formation community has
focused a tremendous amount of effort on sub-grid prescriptions to
describe unresolved processes like star-formation and feedback, I will
argue that because the circumgalactic medium sets the initial
conditions for galaxy formation, resolving these discrepancies likely
poses a more pressing problem.
Sep. 25, Thursday
4:10 pm (Astronomy Colloquium)
Tom Abel, Stanford
2 LeConte Hall
"Dark Matter Dynamics"
Computational Physics allows us to study extremely non-linear systems with fidelity. In astrophysical hydrodynamics and studies of galaxy formation much
of the last two decades we have explored various discretization techniques and found subtle differences in some applications. Interestingly numerical studies of collisionless fluids such as e.g. the collapse of cold dark matter to form the large scale structure of the Universe has only been studied meaningfully with one approach; N-body Monte Carlo techniques. I will introduce a novel simulation approach, and demonstrate its feasibility, that for the first time can study a collisionless system in the continuum limit in multi-dimensions. I will also show this new technique opens a new window in making sense of structure formation as well as plasma physics. In this context we have developed a novel rasterization/voxelization algorithm applicable in computational geometry, computational physics, CAD design and other fields. I show how these approaches allow also for much improved predictions for gravitational lensing, dark matter annihilation, properties of cosmic velocity fields , and many other applications.
Sep. 30, Tuesday
no Tuesday talk,
Astronomy Dept move
August 2014:
Aug. 22, Friday
12 noon (INPA)
David J.E. Marsh, Perimeter
LBL 50-5026
Quintessence in a quandary: on prior dependence in dark energy models
The archetypal theory of dark energy is quintessence: a minimally coupled scalar field with a canonical kinetic energy and potential. By studying random potentials we show that quintessence imposes a restricted set of priors on the equation of state of dark energy. Focusing on the commonly-used parametrisation, $w(a)\approx w_0+w_a(1-a)$, we show that there is a natural scale and direction on the $(w_0, w_a)$ plane that distinguishes quintessence as a general framework. We calculate the expected information gain for a given survey and show that, because of the non-trivial prior information, it is a function of more than just the figure of merit. This allows us to make a quantitative case for novel survey strategies.
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