JinnOuk Gong
It has been only one century since cosmology, the study
of the origin and evolution of our own universe, has become
a subject of physical sciences rather than that of
philosophy and theology. After the advent of general relativity,
people could deduce the physical laws that govern
the evolution of the universe. The standard cosmological
model, the hot big bang picture, was established following
the discovery of the cosmic microwave background
(CMB), which is the afterglow of the hot plasma in the
very early universe, and has been tested in the last decades.
The most recent cosmological observations on CMB and
the large scale distribution of the galaxies (large scale
structure; LSS) tell us that the current observable universe
is very well described by the hot big bang cosmology
with a certain combination of matter contents, and
that it is consistent with a special period in the very early
moment, primordial inflation; to explain the universe as
we observe it now according to the hot big bang picture,
we need extremely finetuned initial conditions, as much
as 1 part of 10^{60}. Primordial inflation, a period
of accelerated expansion caused by one or more scalar
fields, the inflations , is an addon to provide the necessary
initial conditions. Furthermore, during inflation the
expansion is so fast that the quantum fluctuations on
subatomic scales become classical perturbations on cosmological
distances. Thus, observing CMB and LSS, we
can infer the physics relevant for the very early universe.
The primordial perturbations later have become, after
inflation, the seeds for the all observable structure, like
the temperature fluctuations in the CMB and galaxies.
Hence, to make full use of cosmological observations for
the very early universe, we have to understand how this
primordial perturbation is generated and their properties.
The Junior Research Group "Generation and Evolution
of Cosmic Structure" aims to develop cosmological
perturbation theory to understand the nature of these
perturbations and their novel observational signatures.
This is in particular timely since a number of sensitive
observational programs are being made and under progress
in the following decade.
We will be studying the following topics in the next few
years:
1. Nonlinear nature of cosmological perturbation:
In
the simplest scenario of inflation, the primordial perturbation
is essentially a free field so that it is linear and
Gaussian. But the nonlinear nature of gravity tells us
that it possesses intrinsic nonlinearity, despite the fact
that it may be too small to be observed. Indeed, the most
recent observation of the CMB gives the bound of the
nonlinear parameter, a convenient parametrization of
nonlinearity, f_{NL}<100, which means the universe
is Gaussian up to 99.9%. However, the future observations
are sensitive enough to discriminate f_{NL} as
small as 5. This appears as a nonvanishing 3point correlation
function, or the bispectrum, which may have a
very complex shape and size. We will employ different
approaches which complement each other to study the
nonlinear nature of cosmological perturbation.
2. Effective theory of inflation:
A homogeneous and
isotropic background postulates that the primordial perturbation
is associated with time translational symmetry.
This strongly restricts the form of the Lagrangian of the
perturbation in such a way that a constant solution is always
allowed, and we can write the effective operators of
the primordial perturbation consistent with the requirement
of its constancy. Further, this observation is based
on the symmetry principle and thus should be valid
nonperturbatively. Thus, the amplitude of the nth
order correlation function is given by the coefficients
of the nth order action. All this is independent of the
specific model detail and is completely general. We are
interested in extending this effective theory approach to
broader contexts, like multifield inflation, loops effects
and so on.
3. signatures of early universe in lss:
Essentially, the
relation between LSS and the primordial perturbation
is the same as that between that of CMB: the primordial
perturbation is related to matter density perturbation
produced by the annihilation and/or decay of the inflation
through the gravitational potential. Matter perturbation
evolves by the gravitational interaction, and baryon
is further concentrated to eventually form galaxies and
galaxy clusters which we can observe. Thus, to study the
early universe with LSS we should identify the relations
between the primordial, gravitational, matter perturbations
and the distribution of galaxies. The discrepancy
between the distributions of dark matter and galaxies
is parametrized by the socalled bias. We will study the
contributions of nonlinearity to the bias, which induces
a novel scale dependence. The full general relativistic
description of the bias is also of great interest.

Jinnouk Gong received his Ph.D. in
physics from KAIST in 2005. After
postdoctoral research experiences most
recently at CERN, Switzerland, he joined
APCTP in November 2012 as a Junior
Research Group Leader. His main research
fi eld is theoretical cosmology. 