My Research

Resources

Here, I would like to share some resources that I find useful for conducting research in cosmology and astronomy.

Textbooks and review papers

Introduction to Cosmology by Barbara Ryden: This textbook is an excellent starting book for those new to cosmology. The book very clearly introduces basic concepts and ideas about modern cosmology, and is suitable for advanced undergraduate students in physics and astronomy.

Modern Cosmology by Scott Dodelson and Fabian Schmidt: This book covers more advance topics in cosmology, including perturbation theory of the large-scale structure, CMB anisotropies, CMB polarization etc. The mathematical content is both rigorous and clear. The book is closely tied to practical analyses, especially the last chapter about statistical methods.

Extragalactic Astronomy and Cosmology: An Introduction by Peter Schneider: This book does not delve deeply into mathematics but features numerous figures (many in color) from cutting-edge observations, offering a comprehensive review of a wide range of topics in extragalactic astronomy and cosmology.

Galaxy Formation and Evolution by Houjun Mo, Frank van den Bosch, Simon White : The covered topics of this book is similar to Extragalactic Astronomy and Cosmology, but it provides a more thorough introduction to the underlying theories. It is an excellent reference book and I particularly enjoyed the chapter on the halo model.

Cosmology by Steven Weinberg : Like other books by Weinberg, this book goes solid mathematics while clearly discusses the underlying physics picture. This unique book provides deep understanding of theoretical cosmology.

Review papers on weak lensing cosmology: Weak Lensing for Precision Cosmology by Rachel Mandelbaum, Weak gravitational lensing by Matthias Bartelmann and Matteo Maturi.

Useful tools

Taotie is a great collection of resources for astronomers, including tutorials, database, programming, career development, and so on.

I'll list some tools that I find useful for doing research in observational cosmology. I am using them for my own research.

RAIL: Redshift Assessment Infrastructure Layers: a flexible open-source software library providing tools to produce at-scale photometric redshift data products.

Core Cosmology Library: a public standardized library of routines to calculate basic observables used in cosmology.

NaMaster: a python package to compute the angular power spectrum of masked fields with arbitrary spin using the so-called pseudo-Cl formalism.

TreeCorr: a python package a package for efficiently computing 2-point and 3-point correlation functions.

OneCovariance: a python package to calculate covariance matrices for observables of photometric large-scale structure surveys.

Other books about astronomy and physics

I enjoy reading about the history of physics and astronomy. Learning those histories not only satisfies my curiosity about my own field, but also helps me understand how new ideas emerge and develop to discoveries. I believe this is helpful for my research. Here are some books I find interesting:

The Cosmic Century by Malcolm S. Longair: This book introduces the major discoveries across a wide range of astrophysics topics during the 20th century. With detailed explanations of the theoretical background and observational results, it effectively links to the most recent research.

Cosmology’s Century by P. J. E. Peebles: This book introduces the history of modern cosmology, especially the recent studies on the large-scale structures. Many of the contents are based on the author's own research experience. The book also demonstrated the author's insights on the current status and future of cosmology.

The Dreams That Stuff Is Made Of edited by Stephen Hawking: This book is a collection of early papers, lectures and speeches on quantum physics. It compiles the essential works of scientists who sparked the paradigm shift that transformed our understanding of the world.

Going Big: A Scientist's Guide to Large Projects and Collaborations by Christopher W. Stubbs: Astronomers today often work on large projects and collaborations. This book introduces how those large collaboration operates. For early-stage researchers, this book provides a useful guide on how to effectively contribute to a large collaboration.

Cosmology in a nutshell

Some history

In Chinese, 'the Universe' is called 'Yu Zhou (宇宙)'. As an ancient Chinese philosopher Shi Jiao (390 B.C-330 B.C) put it:

四方上下曰‘宇’，往古来今曰‘宙’。

——尸佼《尸子》

Heaven, earth and the four quadrants (together) are called ‘yu’(宇); from the past to the present is called ‘zhou’(宙)'.

——Shizi by Shi Jiao

This is a perfect definition of "the Universe": the whole space and time. Cosmology is the study on the origin, evolution, and eventual fate of the Universe.

Human beings started to think about the Universe since prehistory but for most of time, cosmology belonged to philosophy. Ancient philosophers from different civilizations developed different models about the Universe based on imagination, speculation and limited observational evidence. For example, the ancient Chinese pictured the Universe as an egg and the earth as the yolk; the ancient Greeks believed that our earth was fixed in the Universe surrounded by a concentric celestial sphere of planets and stars; and ancient Indian philosophers described the Universe as ‘Turtles all the way down (cosmic turtle)’.

In 1917, Einstein published “Cosmological Considerations in the General Theory of Relativity.” that describes the geometric structure of the Universe using his new theory of spacetime: general relativity . This work marks the birth of physical cosmology . In 1929, Edwin Hubble discovered that galaxies are moving away from us and the further away they are, the faster they are moving. This motivates the idea of the “Big Bang Theory” which describes how the Universe expands from a very dense and hot state since about 14 billion years ago.

In addition to the Hubble expansion, another key evidence of the Big Bang Theory is the Cosmic Microwave Background (CMB) discovered in 1965. The CMB is the the afterglow of the Big Bang, and is therefore the earliest light that we can observe. CMB contains a wealth of information about the early Universe. CMB observations have provided the most precise estimates of the Universe's age, composition, and geometry.

(An artist’s depiction of the standard model of cosmology. Credit: BICEP2 COLLABORATION/CERN/NASA)

The large-scale structure of the Universe

The Big Bang cosmology assumes that the Universe is statistically homogeneous and isotropic on large scales. However, the Universe is less uniform on small scales. Galaxies cluster together due to gravity, forming galaxy clusters; galaxy clusters are connected by filaments creating the 'cosmic web'. Some regions of space remain empty, known as voids. Collectively, these patterns constitute the large-scale structure (LSS) of the Universe.

The evolution of LSS evolution is primarily governed by gravity from invisible dark matter, but we can map the LSS by observing different 'tracers', including galaxy distribution, weak gravitational lensing, Sunyaev–Zeldovich effect , X-ray astronomy , Cosmic infrared background , CMB lensing, etc. Each tracer is sensitive to different physical processes on different scales.

Observations on the LSS has been growing since the 20th century. Large-scale surveys are observing tracers from stellar, dust, and gas components of millions of galaxies, while CMB surveys have mapped the anisotropic microwave sky originated from the early Universe. With these data, we are gaining more and more knowledge on different aspects of the Universe.

(The large-scale structure of the Universe mapped by DESI. Credit: Claire Lamman/DESI collaboration Image)

The biggest puzzles of cosmology

Observations show that ordinary matter constitutes less than 20% of the mass content of our Universe, with the remainder being invisible dark matter. Moreover, it has been discovered that the expansion of the Universe is accelerating, indicating the presence of mysterious dark energy that drives this acceleration. The natures of dark matter and dark energy are still barely known.

(Content of the universe. Image from here)

In 1980s, cosmological inflation was proposed to address several puzzles in standard cosmology. Inflation refers to a dramatic period of accelerated expansion in the early Universe, occurring when it was 10^-36 seconds old. During inflation, the Universe expanded by a factor of 10²⁶ in 10^-32 seconds. Inflation is thought to contain the seeds of the large-scale structure and the CMB anisotropies that we observe today. However, there is no direct evidence to prove its existence yet, nor do we understand mechanisms behind it.

Dark matter, dark energy and inflation are three major puzzles of modern cosmology. Besides, there are many unresolved issues across various aspects of the Universe. Cosmologists are trying to address them by pointing telecsopes deep in the sky to collect data from a vast number of distant galaxies and the CMB. The curiosity to these puzzles, improvement of observational techniques and the accumulation of high-quality data are the driving force that makes cosmology a lively field of modern physics and astronomy.

Why do I love cosmology?

My interest in cosmology naturally stems from my curiosity on how our Universe comes into being, but what truely motivates me to be a cosmologits is the way we explore it. The theories we use to discribe the Universe are rooted in the laws of physics and mathematics that comes from labs, pens and scratchs. It is truely remarkable that the ideas from the tiny human brain on from a small blue planet can be used to understand the entire Universe. As Einstein put it: "The most incomprehensible thing about the Universe is that it is comprehensible."

Astronomy is based on observations, meaning that astronomers cannot design and control the physical process as experimental physicists do in their labs. Instead, they observe the sky as it is. Unlike other fields in astronomy, we cosmologists do not focus on certain objects, but the Universe as a whole. We survey the sky and do 'demography' on millions of galaxies. I feel it fancinating that we can extract information about the birth, age, content, and fate of the Universe from seemingly stochastic data.

More about me

About my name

For the prononciation, check my Namecoach profile for a record.

My family name Yan (颜) is listed 143rd on the Hundred Family Surnames. It is the 112th most common surname in the China. A total of around 1.7 million people bear the name, mostly in the regions of Hunan, Guangxi and Hubei.

My first name Ziang（子昂）is from a famous Chinese poet Chen Ziang. It has two characters, of which Zi （子）means kid and Ang（昂）means 'to raise ones head'. So I'm a 'kid raising my head', which coincidently agrees with my current job as an astronomer.

My Hometown

I was born and raised in Xinhuang county of Hunan province in the middle-south of China. It is a beautiful town surrounded by rivers and mountains. My mothertongue is Southwestern Mandarin. We enjoy very spicy food.

(A picture of my hometown. The bridge in the distance is the traditional "Wind and Rain bridge" of my hometown. Photo taken by my friend.)

My life outside science

I enjoy classical music from a wide range of composers, spanning from the Baroque era to the late Romantic period, and I also play the violin for myself. Classical music helps me concentrate and provides peace, inspiration, and emotional support. By the way, the piece that I find best match my research is Symphony No.3 in d minor by Gustav Mahler —— a masterpiece with the vastness and complexity that remind me of the Universe. Click here for a performance by the Lucerne Festival Orchestra conducted by Claudio Abbado.

In my leisure time, I like to read books on the history of science, entomology, and linguistics. I also play table tennis, write traditional Chinese poems, go hiking, and watch anime.

You can find me on Facebook and Twitter. Let's chat about Bach, Beethoven, Miyazaki Hayao, and of course, the Universe.