Daniel Eisenstein - Observing dark energy

- dark matter: required by motion of matter on the scales of galaxies and galaxy clusters
	- such matter seems to be of exotic form
	- can be seen with gravitational lensing

- accelerating/decelerating universe
	- size of universe vs. time
	- very hard to measure distance in astronomy
		- standard candles: SNIa (luminosities vary by x2-3, but correlate with time to brightening)
		- high-redshift SNs too faint indicate accelerating universe
		- SN constraints force Omega_lambda/Omega_matter away from 0/1 critical densities (Einstein-De Sitter)
- concordance cosmology: 73% dark energy, 23% cold dark matter, 4% standard matter
- is this true? farther SN could be fainter for other reasons, but:
	- stronger SN
	- CMB acoustic scale + arguments for low matter density
	- other evidence
- supernova systematics
	- observational biases are 1-2%
	- no spectral evolution across redshift
	- deceleration argues against dust extinction, etc.
- acoustic oscillations in CMB
	- preferred scale at 1 degree
	- before recombination: ionized universe, photons provide restoring radiation pressure, perturbations oscillate as acoustic waves
	- recombination: z ~ 1000, t = 400,000 yrs (?)
	- after recombination: photons travel freely, phase of oscillation a t_rec affect late time amplitude
	- acoustic peaks result from preferred physical scale (distance sound waves could travel before recombination)
- evidence for low Omega_matter
	- relative sizes of acoustic peaks
	- cluster baryon fractions
		- galaxy clusters trap gas and dark matter at the cosmic fraction
		- gas observed in X rays
		- measure gas mass and dark matter from pressure balance
	- abundances of clusters
	- galaxy-galaxy weak lensing
	- cosmic shear weak lensing
	- integrated Sachs-Wolfe effects
		- CMB temperature correlations with galaxies at z < 1, due to gravitational redshifting
	- age of the universe
	- large-scale structure
		- clustering of matter at low redshift
		- ...
- evidence for acceleration
	- to avoid dark energy or something bizarre:
		- need Omega_matter = 1 with horrible errors in SNe and large-scale structure
		- or need Omega_matter = 0.25 with curvature and rewriting of CMB acoustic scale
	- cause: cosmological constant, low-mass fields (quintessence), modified gravity, extra dimensions... all are exotic and not compelling
	- equation of state w(z) = p/rho; w = -1 for cosmological constant
	 	- change of w is important, rather than rho_DE
		- but aside from w = -1, constant value is not a physical model
		- difficult to measure: need 1% distances
	- prospects for dark energy
		- make precision measurements of scale factor
			- dark energy task force: SN, baryon acoustic oscillations, weak lensing
				- SN: much data coming, but almost systematics limited
				- baryon acoustic oscillations: large survey volumes required, good for z > 1
				- weak lensing: several surveys starting, rich data set, but large systematics
		- make precision measurements of growth of clustering
			- weak lensing, galaxy cluster abundances
				- galaxy clusters: mass estimation problematic
		- portfolio of comparable techniques, but with different systematics

- conclusions
	- acceleration demonstrated
	- current data consistent with CDM cosmology
	- but constraints on DE EOS are not strong
	- strong portfolio of opportunities