UBC Faculty Research and Publications

The bright future of dark matter and dark energy searches 2008

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Ludovic Van Waerbeke Department of Physics and Astronomy University of British Columbia The Bright Future of Dark Matter and dark Energy searches •Where are we in cosmology? What are the next big questions? •What is weak gravitational lensing? •How does it help addressing the big questions? •What next?  The physical frame of the Big-Bang is General Relativity, and then there are many “free” parameters: •Particle content (radiation, baryons, neutrino, dark matter,…) •Initial conditions (structures, energy density perturbations, inflation,…) Cosmological models include a total of ~26-30 parameters which accounts for “everything” that we know. Expansion of the Universe: This is a pure geometrical effect, governed by the Hubble constant H(z) Structure growth: governed by the local gravity, which is in competition with the Hubble expansion.  Galaxies location if there was no expansion SNIa see geometry at low redshift Galaxies trace matter at low redshift? CMB sees dark matter AND geometry at z=1100 From WMAP Successes of the Big-Bang scenario (latest results from WMAP and other probes): 26 parameters known fairly accurately Overall very good self consistency of the model when comparing various cosmology probes (no more “tension” between parameters) Depressing lack of surprises! Open (big) questions: •Modified gravity? E.g. MOND, TeVeS, Q,… •Nature of dark energy? (equation of state tells us what it is not) •Nature of dark matter? (non interacting?) -> we need to probe the universe over a large angular scale and up to large Redshift -> calls for precision cosmology surveys (dedicated combined surveys?) Discussion between R.Kolb and S.White about the future of cosmology versus particle experiment type of science…   Cosmological  weak lensing: A probe of space-time geometry 1deg. Wide field imaging survey Weak lensing is about “sees” the dark matter of the Universe using lensed background galaxies, as opposed to galaxies which are a biased tracer of the matter. Searching for the excess of variance against the random alignment of distant galaxies, which is attributed to gravitational lensing.  Simulated 3x3 degrees projected mass map (convergence) Weak lensing statistics Convergence histogram for two different cosmological models High density Universe Low density Universe The Canada-France-Hawaii Telescope legacy survey Latest results from The CFHTLS survey And the 100sq.deg. Survey Fu et al. 2008 Benjamin et al. 2007 F l u c t u a t i o n  d e n s i t y  r . m . s . Average mass density WMAP Lensing CFHTLS The power spectrum normalisation is a key parameter which helps to break many degeneracies, e.g. the Neutrino mass. WMAP5 Ray tracing simulation ~50sq.deg. CFHTLS W1 mass reconstruction ~36 sq.deg. First mass map of large scale structures How does weak lensing probe dark energy and modified gravity? Today z=2 Dolag et al. 2003 Dark Matter clustering history is an efficient cosmological tool to probe dark energy. Weak lensing is a redshift dependent effect: Higher source redshift means higher distorsion We need an estimate of all source galaxies! -> photometric redshifts  Huterer & Linder 2006 Combining a probe of the geometry of the universe with A probe of structure growth will reveal any deviation From general relativity. Pressure= [constant(w0)+linear term (wa)] x density Systematics and contamination effects Shape measurement (PSF correction) Small scale physics (baryons cooling & heating) 9Intrinsic alignment Intrinsic alignment in clusters of galaxies… SNAP, a wide field imager in space: “Sloan Digital Sky Survey volume with Hubble Space Telescope image quality” What next: Lensing from space Supernovae Acceleration Probe SNAP NASA “Beyond Einstein Program” (launch before 2020) • 0.7 square degrees FOV • wavelength region, 0.35- 1.7 microns. • Fixed filter mosaic on top of the imager sensors. – 3 NIR bandpasses. – 6 visible bandpasses. • Coalesce all sensors at one focal plane. – 36 2k x 2k HgCdTe NIR sensors covering 0.9-1.7 μm. – 36 3.5k x 3.5k CCDs covering 0.35-1.0 μm. The largest optical survey from space: COSMOS treasury survey (2 deg2 in two filters with ACS on HST) Comparison of SNAP vs COSMOS SNAP Cosmos single filter areas are to scale! ~3000000 HDFs The bullet cluster: Bradac et al. 2006, Clowe et al. 2006. Cosmic train wreck Mahdavi et al. 2007 Bonus science “for free” what you learn from comparing the dark matter distribution to galaxies, gas, in general any other wavelengths observations. Summary/conclusions Future optical wide field surveys (lensing capable) will be able to: •Address the modified gravity issue •Measure the dark energy equation of state •Map the dark matter distribution •Provide strong insight on galaxy/structure formation in relation with their environment


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