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Physics/Astronomy C290C Cosmology and Cosmology-BCCP Seminar
The Physics/Astronomy C290C series consists of the Cosmology-BCCP LBNL-Physics-Astronomy Cosmology seminars held Tuesdays 1:10-2:00 pm in room B1 (also videoconferenced to 50-5026) Hearst Field Annex. Feel free bring your lunch.
Please contact Joanne Cohn to add to this list or to suggest speakers.

Speaker/Visitor Info is here.



ACBAR


BOSS and Nyx
(Image by C. Stark)

Note that there are also other talks which generally might be of interest, including:


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.

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

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

Oct. 21, Tuesday
1:10 pm (Cosmology/BCCP)
Josh Dillon, MIT
Hearst Field Annex B-1

Oct. 28, Tuesday
1:10 pm (Cosmology/BCCP)
Joseph Clampitt, Penn
Hearst Field Annex B-1

Oct 31, Friday
12:00 pm (INPA/Cosmology/BCCP)
Liang Dai, JHU
LBL 50-5026 INPA room


November 2014:

Nov. 4, Tuesday
1:10 pm (Cosmology/BCCP)
Colin Slater, Michigan
Hearst Field Annex B-1

Nov. 11, Tuesday

Veteran's Day, no talk


Nov. 18, Tuesday
1:10 pm (Cosmology/BCCP)
Emanuele Castorina, SISSA
HFA B1

Nov. 25, Tuesday
1:10 pm (Cosmology/BCCP)
Camille Avestruz, Yale
Hearst Field Annex B-1


December 2014:

Dec. 2, Tuesday
1:10 pm (Cosmology/BCCP)
Jes Ford, UBC
Hearst Field Annex B-1





Past Months

Cosmology Seminars in Previous Years

   
            
 
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