UBC Faculty Research and Publications

The bright future of dark matter and dark energy searches Van Waerbeke, Ludovic Apr 11, 2008

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Ludovic Van WaerbekeDepartment of Physics and AstronomyUniversity of British ColumbiaThe Bright Future ofDark Matter and dark Energysearches•Where are we in cosmology? What are thenext 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 expansionSNIa see geometry at low redshiftGalaxies trace matter at low redshift?CMB sees dark matter ANDgeometry at z=1100From WMAPSuccesses of the Big-Bang scenario (latestresults from WMAP and other probes):26 parameters known fairly accuratelyOverall 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 largeRedshift-> calls for precision cosmology surveys (dedicated combined surveys?)Discussion between R.Kolb and S.White about the future of cosmologyversus particle experiment type of science…Cosmological  weaklensing:A probe of space-timegeometry1deg.Wide field imaging surveyWeak lensing is about “sees” the dark matter of theUniverse using lensed background galaxies, as opposedto galaxies which are a biased tracer of the matter.Searching for the excess of varianceagainst the random alignment of distant galaxies, which is attributed togravitational lensing.Simulated 3x3 degrees projectedmass map (convergence)Weak lensing statisticsConvergence histogramfor two differentcosmological modelsHigh densityUniverseLow densityUniverseThe Canada-France-Hawaii Telescope legacy surveyLatest results fromThe CFHTLS surveyAnd the 100sq.deg. SurveyFu et al. 2008Benjamin et al. 2007Fluctuation density r.m.s.Average mass densityWMAPLensingCFHTLSThe power spectrum normalisation is a key parameterwhich helps to break many degeneracies, e.g. theNeutrino mass.WMAP5Ray tracing simulation ~50sq.deg.CFHTLS W1 mass reconstruction~36 sq.deg.First mass map of large scale structuresHow does weak lensingprobe dark energy andmodified gravity?Todayz=2Dolag et al. 2003Dark Matter clustering history is an efficient cosmological tool to probedark energy.Weak lensing is a redshift dependent effect:Higher source redshift means higher distorsionWe need an estimate of all sourcegalaxies!-> photometric redshiftsHuterer & Linder 2006Combining a probe of the geometry of the universe withA probe of structure growth will reveal any deviationFrom general relativity.Pressure= [constant(w0)+linear term (wa)] x densitySystematics and contamination effectsShape measurement (PSF correction)Small scale physics (baryons cooling & heating)9Intrinsic alignmentIntrinsic 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 spaceSupernovae Acceleration ProbeSNAP 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 COSMOSSNAP Cosmossingle filterareas are to scale!~3000000 HDFsThe bullet cluster: Bradac et al. 2006, Clowe et al. 2006.Cosmic train wreckMahdavi et al. 2007Bonus science “for free” what you learn from comparing the dark matterdistribution to galaxies, gas, in general any other wavelengths observations.Summary/conclusionsFuture 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/structureformation in relation with their environment


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