Galaxies and Cosmology

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Galactic dynamics

The next decade will be a golden age for galactic dynamics. The GAIA satellite will provide kinematical information for one billion of stars while SDSS-IV will provide resolved kinematics for 10,000 nearby galaxies. How to go from these datasets to dynamic models of galaxies remains a huge challenge theorists are yet to meet. We are particularly interested in exploring these huge data sets to understand the mass (including dark matter) distribution in galaxies, and how we can use the dynamical information to understand the formation and evolution of galaxies.

Contact: Shude Mao
Gravitational lensing

Gravitational lensing, loosely speaking, refers to the fact that light rays from a distance source are deflected and distorted by intervening mass distributions. Gravitational lensing is often divided into strong lensing, weak lensing and microlensing. We are primarily interested in using strong lensing to probe the dark matter distribution around galaxies, in particular understanding substructure lensing and its implication on whether the dark matter particles are cold or warm. In the future, we are interested in identifying more gravitational lenses in large surveys, and how we can use them for a variety of purposes.

Contact: Shude Mao
Cosmic reionization and 21-cm cosmology

The epoch of reionization is a period in the history of the Universe during which the intergalactic hydrogen gas went from being neutral to ionized. This watershed event was powered by the emergence of the first luminous objects (“cosmic dawn”) when the Universe was a few hundred million years to a billion years old. We quest the answer to the big question “how and when was cosmic dawn?” Specifically, we investigate the observational signatures of cosmic reionization on the 21-cm line of atomic hydrogen, by running cosmological reionization simulations using our radiative transfer code, as well as running semi-numerical simulations

Contact: Yi Mao
Cosmic microwave background

The cosmic microwave background (CMB) is one of the 3 pillars of modern cosmology. It is a relic of photons released early in the recombination era, which carry information from both the early universe and the entire expansion history. CMB observations have provided constraints of incomparable precision on cosmological parameters. We are specially interested in the B-mode polarization of CMB, which is an imprint of primordial gravitational wave and can shed light on inflation physics. We've been seeking a good Northern Hemisphere site, e.g., Ali in Tibet, for B-mode polarization observation, complementary to the ongoing observations at the South Pole.

Contact: Charling Tao
BAO and cosmology

Baryon Acoustic Oscillations (BAO) refers to the baryonic matter density perturbations in the pre-recombination epoch, leaving distinctive features manifested by a series of peaks in the CMB angular power spectrum, and a peak in the matter correlation function of large scale structures. The acoustic length scale is a “standard ruler” which provides an angular diameter distance measurement independent of the supernova Ia “standardizable” candle method. BAO provides additional information about the expansion history of the universe. While the BAO feature is primarily detected in the galaxy-galaxy correlation function obtained by galaxy surveys, Lyα forest autocorrelation function and quasar- Lyα forest cross correlation function can be used as promising tools to study BAO in higher redshifts, since the absorption in the Lyα forest traces underlying mass fluctuations. We expect improved precision in the measurements of BAO to put better constraints on cosmological models and look deeper into the nature of the accelerated expansion of the universe.

Contact: Charling Tao
Circumgalactic and intergalactic media

One of the triumphs of the Big Bang Nucleosynthesis (BBN) theory is that its predicted abundances of primordial isotopes agree with the measured values. Moreover, the predicted baryonic mass seems to be accounted for at high redshifts (z > 2-3) observationally. In the present-day universe, however, only about two thirds of the BBN baryons are detected; this is the "missing baryon problem". The common wisdom is that those baryons are not actually missing, but are hidden in some warm-hot gas of very low density, which makes it difficult to detect; cosmological hydrodynamic simulations support this view. Such gas may be "seen" through the emission or absorption lines of its highly ionized constituents. We are carrying out spectroscopic studies of CGM/IGM with existing data, which are highly limited. To make progress, an X-ray spectrometer of high throughput and high resolution would likely be required. We are developing microcalorimeters for a spectroscopic satellite mission that is being conceptualized to significantly advance the study of CGM/IGM (and thus the formation and evolution of galaxies).

Contact: Wei Cui
Dark matter

Dark matter (DM) refers to the matter we cannot observe via electromagnetic signals. Its existence and properties are inferred from its gravitational effects such as the motions of visible matter, gravitational lensing, its influence on the universe's large-scale structure, and its effects on the cosmic microwave background. The DM hypothesis plays a central role in current modeling of cosmic structure formation, galaxy formation and evolution, etc. The standard model of cosmology indicates that DM constitutes more than 4/5 of total mass of the Universe, and about 27% of the mass-energy. Yet, there is no direct observation of DM so far. The most widely accepted hypothesis for the DM candidate particles are the weakly interacting massive particles (WIMPs). Among the existing detection methods on detecting the DM (or WIMPs) we are most interested in direct detection. Directional detection takes advantage of the Solar system rotation around the Galactic center to show correlation of the Solar motion with respect to the DM halo. We are now working on 3D reconstruction of low energy nuclear recoil tracks using the MIcro-tpc MAtrix of Chambers (MIMAC) detector. We are also starting a spherical TPC project (STPC) based on radial geometry with spherical proportional amplification read-out.

Contact: Charling Tao