# Seeing Dark Energy (or the cosmological constant which is the simplest form of DE) Professor Bob Nichol (ICG, Portsmouth)

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Seeing Dark Energy (or the cosmological constant which is the simplest form of DE) Professor Bob Nichol (ICG, Portsmouth)

Overview 1.Cosmology Primer 2.Standard Candles (Supernovae) 3.Standard Rulers (CMB) 4.My role in all this (SDSS) ISW effect BAO

COSMOLOGY PRIMER

1916 - 1929 This decade saw the birth of cosmology –Einstein gave us a new theory of gravity (GR), that works for the whole Universe –Hubble discovered the Universe, and its expands!

FRW Equation Assuming homogeneous and isotropic universe (RW metric), then GR gives: Hubble Parameter Average density of matter a is the scale factor (radius) of the Universe relative to today k is the curvature of space-time of the Universe (a constant) Cosmological constant, but could be fn of time & space w=p/ =-1

3 Solutions to FRW equation ( =0) R time Bang Never stop! Stop at infinity Big crunch! Value of decides the fate of Universe! Like throwing a stone into space Larger universe Later in Universe

Search for two numbers (H 0 and 0 ) Subscript 0 means today (R=1), but formula holds at other cosmic times. Total energy density ( )

Standard Candles

Luminosity Distance We cant measure distances in the Universe directly, so hard to measure geometry and expansion rate directly d L is the luminosity distance and depends on the cosmological parameters, z is the redshift

Supernovae II

(lookback time) (distance) Supernova are 20% fainter than they should be

Standard Rulers

Baryon Acoustic Oscillations Initial fluctuation in DM. Sound wave driven out by intense pressure at 0.57c. Baryons Photons

CMB After 10 5 years, we reach recombination and photons stream away leaving the baryons behind Preferred scale imprinted on CMB

0.57t

My research

Sloan Digital Sky Survey

Integrated Sachs-Wolfe Effect

My Experiment: SDSS

CMB as seen by WMAP

Combine them

Lens Experiment

What we measure

SDSSSDSS WMAPWMAP

No Signal - No DE Positive Signal - DE! Most direct evidence yet that dark energy exists we see its repulsive force counteracting gravity directly

It is one of the ultimate discoveries in basic science, 2003

baryons photons Today

Sullivan et al. (2003) m =0.256 +0.019 -0.023 Percival et al. (2006)

Supernovae CMB SDSS/LSS Supernovae CMB SDSS/LSS

So is w=-1? 99.74% detection Percival et al. (2006) 143k + 465k 79k z~0.35 z~0.2 Percival et al. 2007 Measure ratio of angular- diameter distance between these redshifts (D 0.35 /D 0.2 ) D 0.35 /D 0.2 = 1.812 ± 0.060 (ratio should be 1.67 for cosmological constant)

Future Questions Is it a Cosmological Constant? Better measurements, specifically control of systematics (new experiments) Is it just a breakdown of GR on large scales? Probe universe using different measures (growth of structure). Again limited by systematics Better theory (any theory!) Parallels with HEP - large careful experiments worrying about large datasets and systematics DES, SDSS-III, WFMOS, DUNE, SPACE, SNAP, ADEPT

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