Jia Liu

Jia Liu

Welcome to my research website! I am an NSF postdoctoral fellow in the Department of Astrophysical Sciences at Princeton University.

My current work focuses on weak lensing non-Gaussian statistics, using N-body ray tracing simulations as a tool to study the large scale structure of our universe. The ultimate goal of this work is to understand the nature of dark energy, the total mass of neutrinos, and other cosmological parameters.

I also spend a small amount of my time observing AGNs at optical telescopes, hoping to find supermassive black hole binaries!


[ Garden State Youth Correctional Facility (as a volunteer Prison Teaching Initiative instructor)]
2017 Spring: Instructor MATH020 - Elementary Algebra
2016 Fall: Instructor MATH015 - Pre-Algebra

[ Columbia University Undergraduate Courses ]
2014 Spring: TA C3602 Physical Cosmology (DETF slides)
2013 Fall: TA C1610 Theories of the Universe: From Babylon to the Big Bang
2013 Spring & Fall: Observing labs
2012 Fall: C1903 Astronomy Lab I - Earth, Moon, and Planets
2012 Spring: TA BC1753 Life in the Universe

[ Barnard College Undergraduate Courses ]
2011 Fall: BC2001 Physics Lab I - Mechanics
2011 Sprint: BC2002 Physics Lab II - Electromagnetism
2010 Fall: BC2001 Physics Lab I - Mechanics

Past and Future Talks

2017/05/18 Invited: Princeton Thunch, Princeton, NJ
2017/03/09 Contributed: Seminar at University of Chicago Kavli Institute for Cosmological Physics, Chicago, IL
2017/03/03 Invited: Black Hole Network: Supermassive Black Hole Binary Workshop at the Center for Computational Astrophysics, New York, NY
2017/02/23 Invited: Astrophysics Seminar at Rutgers University, New Brunswick, NJ
2016/10/27 Invited: Informal Seminar at IAS, Princeton, NJ
2016/10/27 Invited: HSC Science Discussion at Princeton University, Princeton, NJ
2016/10/05 Invited (slides): Seminar at University of Pennsylvania, ‎Philadelphia, PA
2016/09/13 Public outreach (slides): Amateur Astronomer's Association of Princeton, Princeton, NJ
2016/09/06 Invited: Seminar at Tsinghua University, Beijing, China
2016/09/05 Invited: Lunch talk at Peking University, Beijing, China
2016/09/02 Invited: Seminar at National Astronomical Observatories of China, Beijing, China
2016/07 Contributed: Celebrating a century of gravitational lensing, Leiden, Netherland
2016/05 Contributed: Cross-correlation Spectacular with LSST, Long Island, NY
2016/05 Invited: Columbia University Astronomy Colloquium, New York, NY
2016/03 Contributed: Rencontres de Moriond Cosmology, La Thuile, Italy
2016/01 Contributed: Essential Cosmology for the Next Generation, Cancun, Mexico
2016/01 Contributed: 227th AAS Dissertation Talk, Kissimmee, FL
2015/11 Invited: Astronomy Seminar at University of Hawaii, Honolulu, HI
2015/11 Contributed (slides): JSPS 6th Multidisciplinary Science Forum at UC Davis, Sacramento, CA
2015/11 Invited: Joint Princeton/IAS Cosmology Lunch, Princeton, NJ
2015/07 Contributed: Theoretical and Observational Progress on the LSS, Garching, Germany
2015/05 Invited: Joint Stony Brook/Brookhaven National Lab Cosmology Seminar, Stony Brook, NY
2014/11 Invited: Seminar at Jiaotong University, Shanghai, China
2014/10 Invited: Seminar at National Astronomical Observatories of China, Beijing, China
2014/03 Contributed: Rencontres de Moriond Cosmology, La Thuile, Italy
2014/02 Public outreach: Columbia Astronomy Public Outreach, New York, NY
2014/01 Contributed: Essential Cosmology for the Next Generation, Cabo San Lucas, Mexico
2014/01 Public outreach: St. Marcellinius Secondary School, Mississauga, Ontario, Canada
2012/04 Public outreach (slides): Gesamtkunstwerk 1020, New York, NY

Observing Runs

MDM Observatory
2016/03/03 - 03/07 MDM 2.4m Modspec + Echelle (Co-I)
2013/05/01 - 05/05 MDM 2.4m ModSpec + Templeton 1k (PI)
2011/06/26 - 07/04 MDM 2.4m CCDS
2010/12/28 - 2011/01/02 MDM 2.4m OSMOS
2010/06/07 - 06/08 MDM 2.4m CCDS (PI)
2010/03/12 - 03/16 MDM 2.4m CCDS
2009/12/14 - 12/20 MDM 2.4m CCDS & 1.3m Templeton
2009/08/19 - 08/25 MDM 2.4m CCDS & 1.3m Templeton
2009/05/25 - 05/28 MDM 2.4m CCDS with Retrocam
2009/03/17 - 03/18 MDM 2.4m CCDS (Part of an observing class)


email: jia //a// astro.princeton.edu

Weak Lensing

Weak Gravitational Lensing


One of the most pressing problems in modern physics is the nature of dark energy, responsible for the accelerated expansion of the universe. Weak gravitational lensing is a promising method to study dark energy. During their cosmic journey toward us, photons emitted at cosmological distances are deflected by the intervening matter. As a result, we see a distorted image of the source light distribution. Lensing distortions produce non-Gaussianity in maps of cosmic microwave background (CMB) temperature and polarization anisotropies. Lensed galaxies are magnified in brightness and weakly distorted from their intrinsic shape. By statistically measuring the lensing signal, the matter density fluctuations in the universe can be mapped, yielding constraints on the parameters of the background cosmological model.

Figure: simulated CMB lensing (left) and galaxy lensing (right).

Peak Counts

Matter fluctuation evolves non-linearly at small scales. Peaks in the convergence maps can capture non-Gaussian information beyond the two-point function. Yang et al. 2011 used simulations to find that high significance peaks are typically caused by one single massive halo along the line of sight, while medium/low significance peaks are associated with 4-8 small halos, and this is confirmed by our observational work using the CFHTLenS data (Liu & Haiman 2016).

In Liu et al. 2015, we obtained the first cosmological constraints using galaxy lensing peaks. We run N-body ray tracing simulations, tailored to the 6 million galaxies in the 154 deg2 CFHTLenS survey. We find that when the power spectrum and peak counts are combined, the area of the error "banana" in the (Ωm, σ8) plane reduces by a factor of ~two, compared to using the power spectrum alone.

Extending our work from galaxy lensing to CMB lensing, in Liu et al. 2016, we forecasted the cosmological constraints from CMB lensing peaks and PDF to be slightly worse than that from the power spectrum alone, but can help tighten the constraints on (Ωm, σ8) by 30% when the three are combined. We also discovered that the reconstruction noise is non-Gaussian due to the quadratic nature of the CMB lensing estimator, which has been overlooked in previous work.

Cross correlating galaxy lensing and CMB lensing maps

Such cross-correlation probes the matter density in an intermediate redshift z = 0.9, providing a tomographic outlook on the matter density evolution. Furthermore, the galaxy lensing survey and CMB survey systematics are generally uncorrelated, serving as a calibration tool for auto-correlations. In Liu & Hill 2015, we cross-correlate the CFHTLenS galaxy lensing convergence maps with Planck CMB lensing maps. Our results show two sigma tension with the constraints obtained from the Planck temperature measurements (similar but smaller tension has also been found by Hand et al. 2015 using CS82 and ACT data). In the paper, we also investigate possible sources of the tension, including intrinsic alignments, Photo-z uncertainties, and masking of tSZ in the CMB maps.

In a follow up paper (Liu, Ortiz-Vazquez, & Hill 2016), we study the multiplicative bias in CFHTLenS shear data and find a non-vanishing bias in the deepest galaxy sample. The level of the multiplicative bias can potentially explain the tension between parameters inferred from the CFHTLenS shear two-point statistics with that inferred from Planck CMB temperature data.

Magnification and Size Bias

In Liu et al. 2014, we investigate systematics caused by magnitude and/or size cuts on galaxy samples. We find that ignoring such effects in modeling can cause the constraints on (Ωm, σ8, w) to be biased by many sigmas for the LSST survey. Fortunately, we also found that combination of the power spectrum and peak counts can help mitigate the impact, as they are biased differently in each parameters.

Black Holes

Supermassive Black Hole Binaries

The Final Parsec Problem

Supermassive black hole binaries are thought to be a common, if not inevitable, outcome of the merger-driven evolution of galaxies. In the scenario described by Begelman et al. 1980, after two galaxies merge, their central black holes sink into the merger core through dynamical friction on a time scale of ∼108 years. The loosely bound binaries (∼kpc scale) later tighten to ∼1 pc scale through the scattering of nuclear stars, until the "inventory" is depleted. Thereafter, gravitational wave driven angular momentum loss will not be significant until the separation is 0.01-0.001 pc. The difficulty in shrinking the orbit after it reaches ∼1 pc is the so-called "final parsec problem". However, there is no direct evidence for existence of such close supermassive black hole binaries.

Double Peaked Emission Lines in Active Galactic Nuclei (AGNs)

In Liu, Eracleous, & Halpern 2015, we test for periodic motion in the broad emission lines of 13 long-term monitored AGNs. We do not find significant periodic signals. Therefore, we conclude that for these AGNs, any periods are significantly longer than our monitoring span, and/or mechanisms other than orbiting black holes are responsible for their double-peaked broad H-alpha lines and their line profile changes.

Figure: illustration of the binary black holes and the circumbinary disk around them (Bogdanovic et al. 2010, left) and the spectrum of the quasar SDSS J1536+0441 (Chornock et al. 2010, right).



Photos taken with my phone.