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

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

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COSMOLOGY PRIMER

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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!

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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

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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

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

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Standard Candles

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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

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Supernovae II

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

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Standard Rulers

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

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

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0.57t

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My research

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Sloan Digital Sky Survey

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Integrated Sachs-Wolfe Effect

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My Experiment: SDSS

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CMB as seen by WMAP

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Combine them

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Lens Experiment

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What we measure

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SDSSSDSS WMAPWMAP

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

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

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baryons photons Today

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

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Supernovae CMB SDSS/LSS Supernovae CMB SDSS/LSS

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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)

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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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