Astrophysics & Cosmology

Berkeley Lab is actively advancing state-of-the-art astrophysics and cosmology simulations. Our code developments enable the modeling of different phenomena, from single supernovae explosions to structure formation on the largest observable scales in the universe. We implement state-of-the-art physics models using grid and particle representation in our simulations; we use adaptive mesh refinement (AMR) techniques to enhance resolution where it’s needed. We particularly focus on code performance and scalability on world-largest supercomputers.

Our astrophysics research focuses on explosive astronomical phenomena such as supernovae and neutron star mergers that create the elements that comprise all of life. We simulate X-ray burst flames on neutron star surfaces, creating the most accurate computational estimates for X-ray burst flame speeds on rotating neutron stars. To better understand explosion mechanisms, we also simulate low-Mach number hydrodynamics of convecting stars preceding supernova explosions. We are developing new techniques to use machine learning surrogate models to speed up nuclear burning calculations. We also study neutron star mergers using a particle-in-cell code for simulating three-flavor neutrino quantum kinetics in full three-dimensional detail. Our neutrino flavor simulations have shed light on how lepton flavor evolves in neutron star merger ejecta and influences the elemental composition of the ejected material.

In cosmology, we study the formation and evolution of the Universe and of the structures in it. We focus mainly on simulations of the Cosmic Microwave Background (CMB), the Lyman alpha forest, and the intergalactic medium (IGM). These simulations have been used for a number of recent research discoveries, including constraining the parameters of the standard cosmological model, using CMB lensing and tomographic reconstruction to map cosmic structure, detecting cosmic voids at high redshifts, putting constraints on reionization history, measuring density-temperature relation in the IGM, measuring small-scale fluctuations in the baryonic gas, inferring the evolution of ionizing background, constraining quasar lifetimes, determining thermodynamical evolution of IGM gas, and others. Our simulations currently play an important role in the design of the CMB-S4 and Simons Observatory experiments and the Lyman alpha working group of the ongoing Dark Energy Spectroscopic Instrument (DESI) sky survey.

Projects

Astrophysical Plasmas

Kinetic simulations offer a way to investigate fundamental microscale processes that dominate many fascinating and important astrophysical processes including accretion disks around black holes, gamma-ray bursts, pulsar-wind nebulae, core-collapse supernovae and neutron star mergers.  Despite the large differences between these environments, the kinetic processes that determine particle acceleration, transport properties and non-thermal distribution of ionized plasma are similar.  Lab researchers are extending the wave propagation methodology developed for accelerator modeling in WarpX to model pulsar magnetosphere and relativistic magnetic reconnection relevant to various astrophysical phenomena. Contact: Revathi Jambunathan

Computational Cosmology

Modern cosmological observations carried out with large-scale sky surveys are unique probes of fundamental physics. They have led to a remarkably successful model for the dynamics of the universe and several breakthrough discoveries. Three key ingredients – dark energy, dark matter, and inflation – are signposts to further breakthroughs because all reach beyond the known boundaries of the Standard Model of particle physics. Sophisticated large-scale simulations of cosmic structure formation are essential to this scientific enterprise. The DOE's Exascale Computing Project's ExaSky project is extending existing cosmological simulation codes to work on exascale platforms to address this challenge. Nyx is an adaptive mesh, massively parallel, cosmological simulation code based on the AMReX software framework that solves the equations of compressible hydrodynamics describing the evolution of baryonic gas coupled with an N-body treatment of the dark matter in an expanding universe. Nyx's hydrodynamics are based on formulation in Eulerian coordinates. Contact: Zarija Lukic

Compressible Astrophysics

CASTRO is an adaptive mesh, astrophysical compressible (radiation-, magneto-) hydrodynamics simulation code for massively parallel CPU and GPU architectures based on the AMReX software framework.. This code specializes in near-sonic and supersonic flows, where reactions can be an important driver of the dynamics. Radiation and magnetic contributions are supported. CASTRO emphasizes accurately coupling reactions and hydro, with a variety of time-stepping techniques available.  Castro is part of the Exascale Computing Project's ExaStar project which focuses on developing a new component-based multiphysics code suite with adaptive mesh refinement (AMR) that can accurately simulate coupled hydrodynamics, radiation transport, thermonuclear kinetics, and nuclear microphysics for stellar explosion simulations. In addition, StarKiller Microphysics is a set of publicly available microphysics modules designed to enable simulations of stellar explosions Contact: Donald Willcox

Low Mach Number Astrophysics

Many astrophysical phenomena of interest occur in the low Mach number regime, where the characteristic fluid velocity is small compared to the speed of sound. Some well-known examples are the convective phase of Type Ia supernovae, classical novae, convection in stars, and Type I X-ray bursts. Such problems require a numerical approach capable of resolving phenomena over time scales much longer than the characteristic time required for an acoustic wave to propagate across the computational domain.  MAESTROeX is an AMReX-based low Mach number hydrodynamics code using adaptive mesh refinement (AMR)  that includes stellar equations of state and nuclear reaction networks. MAESTROeX solves a reformulation of the equations of hydrodynamics that filters out sound waves, while retaining the compressibility effects important to the problem at hand.  This formulation allows to efficient long-time integration that is not possible with fully compressible codes.. Contact: Andy Nonaka

TOAST

TOAST is a suite of tools to generate mock datasets for Cosmic Microwave Background experiments and to reduce mock and real data to maps of the sky. Simulations are critical to validate and verify both experiments and their analysis pipelines and to quantify the uncertainties and biases in their results. TOAST was developed for the Planck satellite mission but has since been extended to support ground-based experiments, including Simons Observatory and CMB-S4, as well as next-generation satellites such as LiteBIRD. Contact: Julian Borrill (Borrill on the Web) 

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