Dark Conclusions John Peacock Dark Energy X 10 STScI, May          

Outline  Confessions of a dark skeptic  What should we try to measure?  How should we go about it?  What do we really expect to find?  And what would it mean?

History: the CDM argument for  Why wasn’t this correct argument immediately accepted?

1996 Leiden School

1996: The CMBFAST revolution Flat  m = 0.3 (vacuum dominated) Open  m = 0.3 (no vacuum)

Dark Energy: observables  Want to know if DE density is independent of time –is dw/da a sensible thing to focus on? –why not  (a) in bins?  Want to know if DE is homogeneous –is there a speed of sound in DE?  Generalization to testing GR via growth important and healthy development  Other methods (variable  etc.) –More scope for improving limits?

Studying dark energy – why?

Dark Energy: the enemy of astronomy?  A fair point if your experiment uses a lot of telescope time and only measures w  But well known to workers in the field  In fact, DE is good for astronomy: N 1/2 arguments lead to surveying the entire universe in a general way

10 8 All-sky galaxies: SuperCOSMOS UKST + POSS2

2MASS XSC: BRJHK photoz map

Foreground cosmology Predicted Compton signal in CMB (Francis et al. 2008)

Observing dark energy – how?

Figure of merit for Nobel Prize: minimal transverse size? Lensing SNe BAO Avoiding the JDEM hole

But even getting near the hole is hard

Measuring the vacuum Vacuum affects H(z): H 2 (z) = H 2 0 [  M (1+z) 3 +  R (1+z) 4 +  V (1+z) 3 (1+w) ] matter radiation vacuum Alters D(z) via r = s c dz/H And growth via 2H d  /dt in growth equation Both effects are (1)Small (need D to 1% for w to § 0.05) (2)Degenerate with changes in  m Lensing more like a rule of 10 Rule of 5 distance growth

The vacuum: present knowledge Combined: Future probes need to achieve <1% accuracy in D(z)

Issues  Need to understand photo-z systematics at << 1%  Need to calibrate photo-z’s: >10 5 spectroscopic z’s over different sky regions, with extremely high success rate and confidence.

Set 1: CDFS, UBVRIzJHK+IRAC

Set 2: CDFS, minus z, other templates

Comparison of 2 sets; objects with spec z

Comparison of 2 sets; all objects

What will we find? w =

Proving  with help from Bayes Trotta: decide if you need a new parameter, p, based on Evidence ratio: E = LR X (  /  p) where  is your accuracy and  p is the prior range So an accurate but inconclusive experiment can say ‘enough is enough’ Maybe  (w)=0.01 is just enough

Cosmic puzzles if it isn’t   Dark aether: w not -1 defines a preferred frame  Dynamical DE still has to solve the classical  problem

Two cosmic puzzles if it is  The Scale Problem: Surely E max is > 100 GeV, not 2.4 meV? The why now problem time density matter vacuum now future is vacuum dominated Zeldovich 1967

The answer to ‘why now’ must be anthropic  One-universe anthropic –Life (structure) only after matter-radiation equality –Not controversial –k-essence would do –But need to solve classical  =0 problem  Many-universe anthropic –Predates landscape, but requires new physics for variable  –Can we ‘detect’ the ensemble? –Sound logic (exoplanets)

Weinberg’s prediction

Conclusions Huge progress in efficiency of surveying universe: QDOT: 10 scientists for 2163 z’s 2dFGRS: 33 scientists for 220k z’s Pan-STARRS: 160 scientists for 1 billion (photo)z’s ) 500 scientists for all universe in 2020 Which will either rule out  or demonstrate w = -1 to 1%, and will test GR up to 100 Mpc Either way, need a solution to the classical  problem, or will have to accept an ensemble picture