Upcoming and Previous Seminars (Past months or Previous years)

Note that if the talk's pdf or ppt is available after the talk, you can get it by clicking on the talk title.

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 131 Campbell Hall or on ZOOM. It will vary from week to week. Please don't bring your lunch (it is hard to keep this room clean; this is a change).
Please mail Joanne Cohn to add to this list or to suggest speakers.

This seminar is intended for BCCP members and Berkeley graduate students pursuing their dissertation research in cosmology. Other LBL, Berkeley Astronomy and and Berkeley Physics Department members are welcome. If the talk is on ZOOM, members of the Berkeley Astronomy/Physics and Cosmology communities can email Joanne Cohn for information.

Speaker/Visitor Info is here.





BOSS and Nyx
(Image by C. Stark)

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


August 2021
Aug 31, Tuesday
1:10 pm (BCCP/Cosmology)
One minute intros
131 Campbell Hall


September 2021
Sep 7, Tuesday
1:10 pm (BCCP/Cosmology)
Xiaohan Wu, Harvard
131 Campbell Hall
Potential new insights about reionization from the cosmic microwave background(?) and the Lyman alpha forest
Reionization of the intergalactic medium remains one of the most mysterious questions in the research of cosmology. While the first generation of stars (Pop-III) likely ionize the universe at 1-10% level at z>15, the second generation of stars are believed to drive the bulk of reionization at z<12 and complete the reionization process in a patchy pattern. In this talk I will discuss potential new insights into Pop-III reionization at z>15 using the large-scale polarization of the CMB and imprints of patchy reionization at z<12 on the post-reionization Lyman-alpha forest. I will show that although future CMB surveys will be able to measure the large-scale CMB EE power spectrum at higher signal-to-noise and constrain the optical depth at z>15 much better than Planck, the detailed shape of the EE power spectrum is unlikely to add in more constraining power on our understanding of Pop-III star formation. On the other hand, temperature fluctuations owing to patchy reionization likely induce excess power on the large scales of the Lyman-alpha forest flux power spectrum, indicating a potential future observational constraint on the patchiness of reionization. We also found negligible impact of temperature fluctuations on the small-scale shape of the forest flux power spectrum, implying that patchy reionization is unlikely to bias current IGM temperature measurements and WDM/FDM constraints using the forest flux power spectrum.
Sep 14, Tuesday
1:10 pm (BCCP/Cosmology)
Sihao Cheng, JHU
131 Campbell Hall, speaker via ZOOM
The scattering transform in cosmology, or, a CNN without training
Patterns and non-Gaussian textures are ubiquitous in astronomical data but challenging to quantify. I will present a new powerful statistical tool, called the “scattering transform”. It borrows ideas from convolutional neural nets (CNNs) while retaining the advantages of traditional statistics. As an example, I will demonstrate its application to weak lensing cosmology, where it outperforms classic statistics and is on a par with CNNs. I will also show interesting interpretations of the scattering statistics. I argue that the scattering transform provides a powerful new approach in cosmology and beyond.
Sep 16, Thursday
12:40 pm (Astronomy Colloquium)
Steven Furlanetto, UCLA
131 Campbell Hall
Hints About the Earliest Galaxies
The Cosmic Dawn - the era when the first stars and galaxies formed - is one of the frontiers of extragalactic astrophysics. In this talk, I will describe how surprisingly simple models of galaxy evolution provide insight into the basic processes driving these galaxies (and others) while simultaneously highlighting the many ways in which these early, tiny sources may differ from their descendants and ways in which we may observe their properties. In particular, I will show how measurements of the intergalactic medium already provide important insights into these galaxies. Finally, I will describe how a host of new telescopes, including the James Webb Space Telescope and the Hydrogen Epoch of Reionization Array, will provide insights into this era.
Sep 21, Tuesday
1:10 pm (BCCP/Cosmology)
Oliver Philcox, Princeton
131 Campbell Hall
Large Scale Structure Beyond the 2-Point Function
According to the standard paradigm, the distribution of matter in the early Universe obeys Gaussian statistics, and is thus fully described by its power spectrum. Thanks to gravitational evolution, the current Universe is far from Gaussian, yet large scale structure (LSS) analysis is almost always restricted to the two-point function. This begs the question: what information lies in higher-order statistics?
In this talk, I will discuss the benefits of non-Gaussian correlation functions, both in the context of tightening constraints on ΛCDM parameters, and probing new physics in the early and late Universe. A number of technical difficulties must be overcome before the full power of the statistics can be realized: these include the modeling of window functions for Fourier-space statistics, and efficient estimation of the real-space correlators themselves. I will present practical methods that can obviate these difficulties, such as quadratic algorithms for measuring N-point correlation functions, regardless of N. Such techniques will facilitate a number of exciting tests of the cosmological model.
Sep 28, Tuesday
1:10 pm (BCCP/Cosmology)
Huangyu Xiao, Washington
131 Campbell Hall
Axion Dark Matter in the Sky
The axion, a well-motivated particle from the theoretical point of view, is also a viable dark matter candidate. If the initial axion field is not homogenized by inflation, it can naturally imprint large isocurvature fluctuations at extremely small scales and form substructures with subplanetary masses. There have been proposals that appear capable of detecting substructures with such masses, which open the window to detect axions indirectly.
In this talk, I will discuss the evolution of axion minihalos based on N-body simulations. The present-day abundance and density profiles of axion minihalos in the Universe are obtained by extrapolating the simulation results analytically, suggesting that axion minihalos are detectable in future observations.

October 2021
Oct 5, Tuesday
1:10 pm (BCCP/Cosmology)
Boryana Hadzhiyska, Harvard
131 Campbell Hall, speaker via ZOOM
Forward modeling in the era of cosmological surveys
Upcoming cosmological surveys will measure the large-scale distribution of galaxies at the subpercent level. In order to extract unbiased cosmological data while retaining valuable small-scale information, we need highly accurate models of the connection between galaxies and (dark) matter. While cosmological hydrodynamical simulations are too small and computationally expensive to directly use in the analysis of galaxy observations, they provide a detailed probe of the galaxy-halo link (under the assumptions of a particular, plausible galaxy formation model). We show that the simplest galaxy-halo model, the mass-only halo occupation distribution (HOD), fails to capture the galaxy clustering at the 15% level, which is well beyond the 1% requirement set by current and future experiments. We develop augmented models which reproduce multiple galaxy distribution statistics by the hydro simulation. We develop a pipeline for applying these models to observational data and show that in their crudest form, they manage to alleviate existing tensions (e.g., Lensing is low).
Oct 12, Tuesday
1:10 pm (BCCP/Cosmology)
Deaglan Bartlett, Oxford
speaker only via ZOOM (not in 131 Campbell today)
Galactic-Scale Tests of Fundamental Physics
Conventional probes of fundamental physics tend to consider one of three regimes: small scales, cosmological scales or the strong-field regime. Since LCDM is known to have several galactic-scale issues and novel physics (modified gravity, non-cold dark matter etc.) can alter galactic dynamics and morphology, tests of fundamental physics on astrophysical scales can provide tight constraints which are complementary to traditional techniques. By forward-modelling observational signals on a source-by-source basis and marginalising over models describing other astrophysical and observational processes, it is possible to harness the constraining power of galaxies whilst accounting for their complexity. In this talk I will demonstrate how these Bayesian Monte Carlo-based forward models can be used to constrain a variety of gravitational theories and outline ways to assess their robustness to baryonic effects.
Oct 15, Friday
12 noon (INPA seminar, LBL)
Peter Taylor, JPL
via ZOOM
The RSD Sorting Hat
Redshift-space distortions (RSD) and weak lensing (WL) studies yield some of the tightest cosmological constraints. Large overlapping data sets from Euclid, the Rubin Observatory, the Roman Space Science Telescope and the Dark Energy Spectroscopic Instruments will enable a joint RSD/WL analysis. Such an analysis could significantly outperform WL and RSD analyses performed in isolation. In this talk I show how to optimally extract information from the RSD signal using angular statistics. This is a first step towards combing WL and RSD.
Oct 19, Tuesday
1:10 pm (BCCP/Cosmology)
Abhishek Maniyar, NYU
via ZOOM only
Upgrading the CMB foreground and lensing analysis: improved halo models and a global minimum variance quadratic estimator
On very small angular scales, the CIB, tSZ, and their cross-correlation act as foregrounds while measuring the kinematic SZ effect from the CMB power spectrum. As the upcoming CMB experiments are going to probe these small scales with ever increasing precision, it is crucial to properly account for foregrounds. Thus, it is important to go beyond the current foreground prescriptions and have physically motivated models for them. I will present a new halo model for the CIB which is simplistic in nature with four parameters and is able to fit varied observational data. Along with a halo model for the tSZ, we can use this CIB model to calculate the CIB-tSZ correlation consistently. In recent years, weak lensing of the CMB has emerged as a powerful tool to probe fundamental physics. The prime target of CMB lensing surveys is the lensing potential, which is reconstructed from observed CMB temperature T and polarization E and B fields. In this talk, I will show that the classic Hu-Okamoto (HO02) estimator used for the lensing potential reconstruction is not the absolute optimal lensing estimator that can be constructed out of quadratic combinations of T, E and B fields. Instead, I will derive the global-minimum-variance (GMV) lensing quadratic estimator and show explicitly that the HO02 estimator is suboptimal to the GMV estimator.
Oct 26, Tuesday
1:10 pm (BCCP/Cosmology)
Shivam Pandey, Penn
via ZOOM only
Cosmology and astrophysics from small scales
Complex and poorly understood astrophysics impacts our ability to constrain cosmological and astrophysical models from the large scale structure. Two major sources of systematic errors are galaxy biasing (non-linear mapping between dark matter and galaxies) and baryonic feedback (impact of supernovae or AGN on LSS). In the first part of my talk, I will describe a hybrid perturbation theory model of galaxy biasing and show its validation at sub-percent accuracy. I will then describe the cosmological constraints obtained using this model on the measurements from the first three years of observations of the Dark Energy Survey (DES). In the second part of my talk, I will describe tomographic measurements and analysis of the cross-correlations between thermal Sunyaev-Zel'dovich (tSZ) effect and gravitational lensing. Using data from ACT, Planck, and DES, we obtain the highest significance (20 sigma) measurements to-date and use them to constrain models for the pressure profiles of halos across a wide range of halo mass and redshift. We find evidence for reduced pressure in low mass halos, consistent with predictions for the effects of increased feedback from AGN. We also infer the hydrostatic mass bias (B) from our measurement, finding a large value when adopting the Planck-preferred cosmological parameters. We additionally find hints of the redshift evolution of B, which can help explain the large mass bias needed to reconcile cluster count measurements with the Planck-preferred cosmology.

November 2021
Nov 2, Tuesday
1:10 pm (BCCP/Cosmology)
Chris Pedersen, NYU
ZOOM only
Cosmological interpretation of the Lyman-alpha forest
The Lyman-alpha forest (Lyaf) is a unique probe of structure formation on small scales. This information is particularly powerful when combined with observations of the CMB in constraints of parameters that affect the relative amplitude of small and large scales. Two prominent examples are the shape of the primordial power spectrum, and the mass scale of neutrinos. Over the next few years, DESI will produce the largest Lyaf dataset yet with the potential to provide unprecedented constraints on these parameters, and it is therefore important to now determine the best approach to interpret this data. I will describe recent and ongoing work in this area concerning the construction of cosmological emulators for the Lyaf. Finally, I will present an approach to performing the analysis which will make the small-scale clustering information from the Lyaf readily accessible to the wider cosmological community.
Nov 4, Thursday
12:30 pm (Astronomy Colloquium)
Jorge Moreno, Pomona
131 Campbell Hall
Galaxies lacking dark matter
The standard Lambda-Cold-Dark-Matter model predicts that galaxies form within dark matter halos and that low-mass galaxies are more dark-matter dominated than massive ones. The unexpected discovery of two low-mass galaxies lacking dark matter immediately provoked concerns about the standard cosmology and ignited explorations of alternatives, including self-interacting dark matter and modified gravity. Apprehension grew after several cosmological simulations using the conventional model failed to adequately form numerical analogs (with comparable internal characteristics: stellar-masses, sizes, velocity dispersions and morphologies). In this talk I will show that the standard paradigm naturally produces galaxies lacking dark matter with internal characteristics in agreement with observations. Using a state-of-the-art cosmological simulation with the FIRE-2 physics model and a meticulous galaxy-identification technique, we find that extreme close-encounters with massive neighbors can be responsible for this. We predict that ∼ 30% of massive central galaxies (with at least 1e11 solar masses in stars) harbor at least one dark-matter deficient satellite (with 1e8 – 1e9 solar masses in stars). This distinctive class of galaxies opens an additional layer to our understanding of the role of interactions in shaping galactic properties. Future observations surveying galaxies in the aforementioned regime will provide a crucial test of this scenario.
Nov 9, Tuesday
1:10 pm (BCCP/Cosmology)
Huanqing Chen, Chicago
131 Campbell Hall
Probing Cosmic Reionization with Quasar Proximity Zones
In the first billion years of the Universe, the first galaxies and quasars formed and their ionizing photons brought about a major phase transition of the IGM from a mostly neutral to a mostly ionized state. The exploration of this crucial epoch, called cosmic reionization, is entering a golden era with the exponential progress in computational ability and with JWST and 30m-class telescopes on the horizon. The regions around the first quasars, called quasar proximity zones, are among the first targets that JWST will observe. They are unique since they are thought to trace the densest environments where many galaxies reside. Besides, the quasar spectra carry an immense amount of information about the IGM during reionization. In this talk, I will show two aspects of what we can learn from the quasar proximity zones. First, I will show how to use the absorption spectra to recover the density field at z~6 and further constrain cosmology and quasar properties. Second, I will talk about my suite of quasar proximity zone simulations and show how galaxies form in this biased field with strong radiation. This study will help us interpret future JWST data and answer key questions about the quasar environment and radiative feedback. I will conclude with an outlook of synergizing JWST and ground-based observations of quasar proximity zones to learn more about reionization.
Nov 16, Tuesday
1:00 pm (LBL RPM/BCCP Cosmology seminar) **note special start time!**
Taylor Hoyt, Chicago
ZOOM
"Using Astrophysical Distance Indicators to Test Standard Cosmology"
Measurements of the universe's present-day expansion rate, or the Hubble constant (H0), that use a Cepheid variable star calibration of Type Ia supernovae (SNe Ia) are in >4σ disagreement with values predicted by the standard, Lambda cold dark matter (LCDM) model of the universe. In this talk, I will review the evidence for this “Hubble Tension” and discuss in particular my work on an alternative calibration of the SNe Ia using the Tip of the Red Giant Branch (TRGB), a standard candle that can return distances precise to 2% when observed in ancient populations of stars. Anchored by the TRGB, we derived in the Carnegie Chicago Hubble Program a SN value of H0 that is significantly less in tension with base LCDM (< 2σ) than the Cepheid-calibrated SN H0, which raises the question of underestimated uncertainties and softens evidence for new physics. I will identify likely causes of this Cepheid-TRGB divergence, present paths to a potential resolution, and highlight how the astrophysical distance scale can converge on a self-consistent, 1% determination of the Hubble constant with near-future technologies.
Nov 19, Friday
12 pm (LBL INPA)
Charlotte Ward, Maryland
ZOOM
Using the Zwicky Transient Facility and Vera C. Rubin Observatory To Study Black Hole Seeds, Discover High-Z Quasars, and Measure Time Delays From Lensed Supernovae
Over the last 3 years, the Zwicky Transient Facility (ZTF) has demonstrated the potential of wide-field transient surveys for the discovery of black hole populations which trace the formation of the first BH seeds and their merger-driven growth. For instance, our search for off-nuclear AGN in ZTF revealed 9 supermassive black holes (SMBHs) which may have been ejected from their host galaxy by gravitational wave recoil from SMBH mergers with misaligned spins. Likewise, our search for faintly variable intermediate mass black holes (IMBHs) in dwarf galaxies with ZTF and WISE found 200 new IMBH candidates, most of which could not have been found via their spectroscopic or multi-wavelength signatures. We found these rare BH populations by using the latest techniques in forward modeling to improve photometric sensitivity, resolve blended sources and measure the separations between variable objects and their host galaxies. Our work is an exciting precursor to the Legacy Survey of Space and Time (LSST), which we expect to detect large populations of recoiling SMBHs, IMBH candidates, and high-z quasars—especially when combined with large spectroscopic surveys. LSST is also predicted to find thousands of strongly gravitationally lensed, multiply-imaged supernovae, and application of the forward modeling techniques we previously demonstrated for ZTF will enable improved measurements of the Hubble constant.
Nov 23, Tuesday
12:00 pm (LBL RPM)
Oliver Philcox, Princeton
ZOOM
" Large Scale Structure Beyond the 2-Point Function"
Quantum fluctuations in inflation provide the seeds for the large scale distribution of matter today. According to the standard paradigm, these fluctuations induce density perturbations that are Gaussian distributed. In this limit, all the information is contained within the pairwise distribution of galaxies, usually represented by a power spectrum. Today, the distribution of matter is far from Gaussian, with structures forming across a vast range of scales. To date, almost all spectroscopic analyses have used only the two-point function. This begs the question: can we extract more information using higher-point statistics?
In this seminar, I will present a pedagogical overview of the leading-order non-Gaussian statistics, and demonstrate how they can be used both to sharpen constraints on known physical parameters, and to provide stringent tests of new physics occurring in the early Universe. One of the major barriers to constraining cosmology from the higher-point functions is computational: measuring the statistics with conventional techniques is infeasible for current and future datasets. I will discuss new methods capable of reducing the computational cost by orders of magnitude, and show how this facilitates a number of exciting new tests of the cosmological model. Such techniques are already being applied to data from BOSS; the corresponding pipelines can be simply reapplied to DESI data, and will lead to sharper parameter constraints without additional observing time.
Nov 30, Tuesday
1:10 pm (BCCP/Cosmology)
Farnik Nikakhtar, Penn
ZOOM
Precision Tests of CO and [CII] Power Spectra Models against Simulated Intensity Maps
Line intensity mapping (LIM) is an emerging technique with a unique potential to probe a wide range of scales and redshifts. Realizing the full potential of LIM, however, relies on accurate modeling of the signal. In this talk, I introduce an extended halo model for the power spectrum of intensity fluctuations of CO rotational lines and [CII] fine transition line in real space, modeling nonlinearities in matter fluctuations and biasing relation between the line intensity fluctuations and the underlying dark matter distribution. To establish the accuracy of the model, we generate the first cosmological-scale simulations of CO and [CII] intensity maps, at redshifts 0.5 <= z <= 6, using halo catalogs from Hidden-Valley simulations, and painting halos according to mass-redshift-luminosity relations for each line. We show that at z=1 on scales k_max <~ 0.8 Mpc^-1 h, the model predictions of clustering power are in agreement with the measured power spectrum at better than 5%. At a higher redshift of z=4.5, this remarkable agreement extends to a smaller scale of k_max <~ 2 Mpc^-1 h. Furthermore, we show that on large scales, the stochastic contributions to CO and CII power spectra are non-Poissonian, with amplitudes reproduced reasonably well by the halo model prescription. Lastly, we assess the performance of the theoretical model of the baryon acoustic oscillations (BAO) and show that hypothetical LIM surveys probing CO lines at z = 1, which can be deployed within this decade, will be able to make a high significance measurement of the BAO. On a longer time scale, a space-based mission probing [CII] line can uniquely measure the BAO on a wide range of redshifts at an unprecedented precision.

December 2021
Dec 1, Wednesday
12 pm (LBL RPM)
Boryana Hadzhiyshka, Harvard

"Uncovering physics from Stage-IV cosmological experiments with accurate galaxy models"
Over the next few years, cutting-edge cosmological experiments such as DESI, Rubin and CMB-S4 will provide an exquisite probe of the accelerated expansion of the Universe, structure formation, and general relativity, and thus bring us closer to revealing the nature of dark energy, dark matter, inflation and neutrinos. One of the most critical issues with these experiments will be the connection between observed galaxies and the underlying matter field. My research program offers a viable path for constructing accurate models of the galaxy-matter connection and applying them to observational analysis, with the goal of recovering the missing pieces of our cosmological model. In particular, I will share my contributions to the development of state-of-the-art cosmological simulations and analysis tools and propose readily reachable goals for extracting cosmological information from the ongoing DESI and CMB experiments. Future breakthroughs will likely be the product of collaborative efforts across all of cosmology, galaxy formation and particle physics. My broad-scaled research proposal will bring together diverse ideas and aid LBNL science goals at the crossroads of cosmological discovery.
Dec 10, Friday
12 noon (LBL INPA)
Postponed,
LBL 50-5132 and ZOOM

Dec 17, Friday
12 noon (LBL INPA)
postponed,
LBL 50-5132 and ZOOM






Past Months



June 2021
June 3, Thursday
4 pm (LBL RPM)
Julien Guy, LBL
The Dark Energy Spectroscopic Instrument (DESI): start of the five year survey.
The Dark Energy Spectroscopic Instrument (DESI) started its main survey on May 14. Over 5 years, it will measure the spectra and redshifts of about 30 millions galaxies and quasars over 14,000 square degrees. This 3D map will be used to reconstruct the expansion history of the universe up to z=3.5, and measure the growth rate of structure in the redshift range 0.7-1.6 with unequaled precision (< 1.5% per redshift bin of 0.1). The start of the survey marks the end of a successful survey validation period during which more than one million cosmological redshifts were measured. This data set, along with many commissioning studies, has demonstrated the project meets its science requirements written many years ago. I will present how we have validated the target selection, the instrument performance, the spectroscopic pipeline, operations, and the survey strategy.
June 8, Tuesday
1:10 pm (BCCP/Cosmology)--postponed April talk
Alexandra Amon, SLAC/Stanford
The Dark Energy Survey Year 3 Results from Weak Lensing and Clustering
I will present the cosmological weak lensing and clustering results from the Dark Energy Survey (DES) using its first three years of data taken using the Dark Energy Camera on the 4m Blanco telescope at CTIO. This analysis spans the full DES footprint, more than 4000 sq. deg. of sky, with the final shear catalogue containing more than 100 million galaxies, constituting the most powerful weak lensing dataset to date. The comparison of DES cosmological constraints on dark matter and dark energy from WL and LSS in the low-redshift Universe to CMB constraints provides an unprecedented test of the standard cosmological model, across high and low redshift. These DES results from the low-redshift Universe are consistent with those from the cosmic microwave background (CMB) and support the standard cosmological model, LCDM. I will mention the main challenges that our analysis is susceptible to, and the summarise the DES-Year 3 approach to account for these and deliver robust cosmological constraints.

July 2021
July 22, Thursday
4 pm (RPM LBL)
Anna Porredon, OSU By ZOOM
Dark Energy Survey Year 3 Results from Galaxy Clustering and Weak Lensing
The cosmological information extracted from photometric surveys is more robust when multiple probes of the large-scale structure of the universe are used. Two of the most sensitive probes are galaxy clustering and weak lensing. I will present cosmological results from the Dark Energy Survey first three years of observations combining those two probes, using an optimized lens sample of 11 million galaxies for the clustering measurements. The shear catalog used for weak lensing contains more than 100 million galaxies, constituting the largest dataset to date of this kind. I will show how we optimized the selection of this lens sample in terms of the forecasted cosmological constraints. Last, I will explain how we validated the analysis to deliver robust cosmological constraints.


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