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Seeing Dark Energy (or the cosmological constant which is the simplest form of DE). Professor Bob Nichol (ICG, Portsmouth). Overview. Cosmology Primer Standard Candles (Supernovae) Standard Rulers (CMB) My role in all this (SDSS) ISW effect BAO. COSMOLOGY PRIMER. FRW Equation.
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Seeing Dark Energy(or the cosmological constant which is the simplest form of DE) Professor Bob Nichol (ICG, Portsmouth)
Overview • Cosmology Primer • Standard Candles (Supernovae) • Standard Rulers (CMB) • My role in all this (SDSS) • ISW effect • BAO
FRW Equation • Assuming homogeneous and isotropic universe (RW metric), then GR gives: Cosmological constant, but could be fn of time & space w=p/=-1 Average density of matter ‘a’ is the scale factor (“radius”) of the Universe relative to today Hubble Parameter k is the curvature of space-time of the Universe (a constant)
3 Solutions to FRW equation (=0) Never stop! Stop at infinity R Larger universe Big crunch! Bang time Later in Universe Value of decides the fate of Universe! Like throwing a stone into space
Search for two numbers (H0 and 0) Subscript “0” means today (R=1), but formula holds at other cosmic times. Total energy density ()
Luminosity Distance • We can’t measure distances in the Universe directly, so hard to measure geometry and expansion rate directly • dL is the luminosity distance and depends on the cosmological parameters, z is the redshift
(distance) Supernova are 20% fainter than they should be (lookback time)
Baryon Acoustic Oscillations Photons Baryons Initial fluctuation in DM. Sound wave driven out by intense pressure at 0.57c.
Preferred scale imprinted on CMB CMB After 105 years, we reach recombination and photons stream away leaving the baryons behind
Most direct evidence yet that dark energy exists we see it’s repulsive force counteracting gravity directly Positive Signal - DE! No Signal - No DE
baryons photons Today
Percival et al. (2006) Sullivan et al. (2003) m=0.256+0.019-0.023
Supernovae m < • CMBm + • SDSS/LSS m =0.25
So is w=-1? Measure ratio of angular-diameter distance between these redshifts (D0.35 /D0.2) D0.35 /D0.2 = 1.812 ± 0.060 (ratio should be 1.67 for cosmological constant) 99.74% detection 143k + 465k z~0.2 79k z~0.35 Percival et al. (2006) Percival et al. 2007
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