Cosmology: “Understanding Dark Universe Using

Large Scale Structure of the Universe”

David ParkinsonKorea Astronomy and Space Science Institute

Outline

• Introduction

• Cosmological tests

• Surveys

• Projects

• Summary

Introduction

Cosmology group

Yong-Seon SongJinn-Ouk Gong Arman ShafielooDavid Parkinson

Jacobo Asorey Benjamin L’Huillier Srivatsan Sridhar

Inwoo Han Eric LinderMinji Oh

Faculty

Postdoc

Student Associate

Ryan Keeley

Fundamental Questions

• The expansion of the Universe is accelerating

• Vacuum energy calculations imply cosmological

constant is 10120 times larger than its measured

value - too small

• Coincidence problem - why is density of matter

(1/a3) so close to density of dark energy

(~constant) today?

• What is the correct theory of gravity?

• Does the acceleration of the Universe imply a

breakdown of Einsteinian gravity on large-scales?

• What gave rise to the initial seeds of

cosmological structure?

• Was there a period of accelerated expansion in the

early Universe, known as ‘Inflation’?

5%27%

68%

Baryons

Dark Matter

Dark Energy

Approaches• Extending the model

• If we find results inconsistent with previous model (LCDM) can we improve fitting with new parameter? (Parameter

estimation)

• Testing the model

• How does a particular theoretical model compare with another in terms of all possible predictions? (Bayesian model

selection)

• Disproving the framework

• If the new measurements are inconsistent with all possible predictions, maybe the framework as a whole is wrong (Data tensions)

List of symbols• How do we know if our model of the Universe is wrong?

• Bayesian approach: parameters!

• Dynamical dark energy: change of density with redshift

• cosmological constant (as w=-1 for all z)

• Modified gravity: change of Poisson equation and lensing potential

• Einstein gravity has mu and gamma = 0 for all z and scales

• non-Gaussianity: quadratic contributions to initial density field

Measuring distances• Cosmological distances difficult

to measure directly

• Astronomers often use redshift (recession velocity due to expansion of Universe) as proxy

• Measure redshift, then use fixed/assumed cosmology to infer distance

• Standard candles/rulers/sirens can be used to directly measure distances, and so infer details about cosmology

• But total redshift not just due to expansion, so local velocity component added also

• Peculiar velocity, useful for testing gravity

star

torch

Standard candles Cepheid

Type Ia Supernova

Standard rulers BAO

Strong lensing time delay

Standards sirens Gravitational wave event

Tracing structure• Universe filled with density fluctuations

• Structure only only visible through galaxies (distribution) and photons (weak lensing)

• Galaxies and photons here are functioning as test particles - tracing out the gravitational field

• Most low-redshift surveys have not measured spectrum of density fluctuations

• Much more sensitive to transfer functions

• Need very large volumes to measure primordial power spectrum and determine initial conditions (independently from CMB)

• Cross-correlations - looking at joint statistics of two populations, which should follow same potential

Surveys

DESI• Dark Energy Spectroscopic Instrument• Participants: Yong-Seon Song, Arman Shafieloo, David Parkinson

• 5000 fibre multi-object spectrograph attached to 4m Mayall telescope at Kitt Peak observatory (Ariz., USA)

• Science objective: spectra of 35 million galaxies and quasars over 14000 sq degs

• 4 million Luminous Red Galaxies (LRGs), z<1

• 18 million Emission Line Galaxies (ELGs) , 0.5 < z < 1.5

• 2.4 million quasars (QSO), including 0.7 million quasars at z>2.2 for Lyman-alpha-forest

• Science projects:

• Measuring distances using Baryon Acoustic Oscillations

• Testing gravity using Redshift-space distortions

LSST• Large Synoptic Survey Telescope• Participants: Arman Shafieloo

• 8.4m telescope with ~9.5 sq deg field of view, located at Cerro Pachón, Chile

• Science goals: Imaging survey of the entire southern sky, every few nights

• Science projects:

• Detecting transients (e.g. SN-Ia, GRB, )

• Weak lensing shear

• MW and extra-galactic science

ASKAP & SKA• The Square Kilometre Array is a radio telescope will a collecting area of 106 m2

• Participants: David Parkinson

• It will be constructed in Australia (SKA-Low) and South Africa (SKA-Mid)

• Science goals: Imaging (radio continuum) and redshifts (21cm galaxies and intensity mapping) over a very wide area

• Science projects:

• BAO

• Weak lensing shear

• Angular correlations and cross-correlations

• Cosmic Dipole

• ASKAP is the Australian SKA Pathfinder, a precursor experiment, being built in Australia.

• The EMU survey will be an all-sky radio continuum survey, detecting roughly ~100 million galaxies.

• Science projects (EMU): angular correlations, cross correlation with CMB and optical

Projects

MG vs DE using LSS in redshift space

• PI: Yong-Seon Song

• To understand the cosmos and its dynamic we have to look at the universe and probe it in different wavelengths and scales and depth. To do so we have come to probe distances and growth structure using the spectroscopy wide deep field survey data.

• Simulation: Complete LSS simulations of DESI. Suggesting new RSD model to fit the future data. Verifying it using SDSS.

• Modelling: Extending RSD perturbation model to the exotic theoretical models, Modified Gravity.

• Applied to observations: Test GR at cosmological scale using spec & photo survey data. Probing initial conditions. Cross-correlating multi data

Physical Cosmology

• PI: Arman Shafieloo

• Search for evidences beyond the standard model of cosmology

• Phenomenology:

• Probing early universe with large scale structure (N-Body simulations)

• Testing isotropy, homogeneity, flatness and FLRW metric

• Testing statistical isotropy in CMB temperature and polarization maps

• Reconstructing the primordial power spectrum

• Cosmic expansion versus growth of structure

• Theory:

• Dark Energy Modelling

• Inflationary Scenarios

Cosmology with large-area surveys

• PI: David Parkinson

•Cosmology with radio continuum

•We will cross-correlate radio with optical (Dark Energy Survey) data to extend the volume and improve measurements

• Testing gravity by tracking the motion of massive (galaxies) and massless particles (photons) - do they follow the same potential?

• Cross-correlation of 2dFLens (redshift-space distortion) and KIDS (weak lensing shear) data over same region of sky

• Peculiar velocity - making mock catalogues of future surveys, such as TAIPAN/WALLABY and SKA, using simulations

Inflation and non-linear structure

formation• PI: Jinn-Ouk Gong

• Initial conditions of the Universe currently consistent with a initial spectrum of adiabatic, near-scale invariant, Gaussiandensity perturbations• No primordial gravitational waves yet detected

• Inflation predicts a tilt in the initial power spectrum (meausred by the CMB experiments WMAP and Planck)• Should also generate non-linear perturbations,

primordial non-Gaussianities, and gravitational waves

•Projects

• Inflationary model building

•Primordial gravitational waves

•Realistic descriptions of structure formation (from theory)

Astrostatistics• Development of statistical techniques:

• Classification:

• Star-quasar separation/classification

• Supernovae classification (photometric)

• Spectroscopic

• Estimation

• Estimation of rotation curve of the galaxies

• Time delay estimation in from strong lens systems

• Unsupervised learning - ‘unknown unknowns’

• Simple vs complex objects

Summary• Cosmology projects at KASI use measurements of the extra-

galactic universe to make inferences as to the nature of fundamental physics

• The main areas of research are:

• Dark Energy

• Modified Gravity

• The primordial power spectrum from Inflation

• The main observables used is the large-scale structure of the Universe, measured by optical imaging and spectroscopic surveys, and radio continuum surveys

• The next generation of large-area surveys will provide a huge volume of data, which will be used to improve and test our understanding of the standard model of cosmology, and hopefully move beyond it