Dark Energy Cosmology INPE Winter School September 12-16, 2005 Robert Caldwell Dartmouth College

Cosmic Evolution

Cosmic Acceleration observations of type 1a supernovae indicate our universe is accelerating RC, Physics World, May 2005 data: Riess et al, ApJ 607 (2004) 665

Cosmic Acceleration observations of type 1a supernovae indicate our universe is accelerating k=0 (CMB) universe contains m, k,  spatially-flat RW spacetime acceleration due to  : Riess et al, ApJ 607 (2004) 665 basic picture

Cosmological Constant “The Cosmological Constant and the Theory of Elementary Particles” Zel’dovich, Sov. Phys. Uspekhi 11 (1968) 381 “A new field of activity arises, namely the determination of  ” “The genie (  ) has been let out of the bottle….” The quantum vacuum energy of scalar particles resembles a cosmological constant Problem: IR phenomenon has UV divergence

sketch of basic calculation Casimir Effect oscillator energy energy density with boundaries change due to plates pressure difference uniform stress-energy between the plates Milton, 2001

Mohideen & Roy, PRL 81 (1998) 4549 Bressi et al, PRL 88 (2002) Lamoreaux, PRL 78 (1997) 5 Casimir Effect: Measurements

Casimir Effect Vacuum Energy is not Immutable Energy density carried in vacuum fields distorts in response to a weak gravitational field Caldwell, astro-ph/ Casimir Effect in a Weak Gravitational Field

Cosmological Constant Are unattached vacuum loops real? Do they gravitate? Casimir Effect: a real manifestation of fluctuations of the QED vacuum Jaffe, PRD 72 (2005) Casimir effect without the vacuum Quantum vacuum contributes to inertia Jaekel, Lambricht, Reynaud, New Astron. Rev. 46 (2002) 727

Constant Problems Numerous sources of an effective  in physics beyond SM Everywhere you look in QFT (even standard model) Symmetry-breaking vacuum energy (Higgs-like fields) Supergravity 3-form (may be quantized) … Ways to get rid of it Cancellation (unstable to dynamics) Supersymmetry (broken) Euclidean QG (probabilistic) Any physics content? There must be fluctuations! Ghost Condensation… Arkan-Hamed et al, JHEP 0405 (2004) 074 Weinberg, Rev Mod Phys 61 (1989) 1

Action Principle Unimodular gravity Einstein 1919 Anderson & Finkelstein, Am J. Phys. 39 (1971) 901 van der Bij and van Dam, Physica 116A (1982) 307 a theory of (linear quantum) gravitation … subject to symmetries satisfying -det(g)=1 nearly equivalent formulation of Einstein’s GR trace:

Action Principle: Multiplication Linde, PLB 200 (1988) 272 Two spacetimes, x & y Identical matter content Antipodal symmetry: Result: zero vacuum and trace-free GR quantum effects? observable consequences?

Action Principle: Subtraction Kaplan & Sundrum, hep-th/ One spacetime Identical matter content Energy parity symmetry Result: zero vacuum but there are ghosts “ghost sector” must be underpopulated classically stable, but QG instability Slow instability with QG cutoff:  ~ eV  observable consequence: ~30  breakdown of gravitational potential

Cosmological Constant: a bookmark Regard  as a placeholder until observation, experiment, and theory can determine the underlying physical laws behind the dark energy phenomenon. Astrophysical phenomena not sensitive to the dark energy: Cosmological phenomena sensitive to the dark energy: use  as a simple, economical model Question observations / experiments Test theoretical foundation Search for conflicts with baseline  What are the alternatives?

Dark Energy What is it? The cosmological constant? A new particle / field? A new aspect of gravitation? A misinterpretation of observations? Astrophysical confusion?

Lives of the Cosmological Constant 1917Einstein, deSitter 1968Zel’dovich 1980s  + dark matter or baryonic models History: Kragh, “Cosmology & Controversy” Numerous authors investigated the cosmological consequences of an additional smooth, time-varying component of the cosmic fluid. Charlton & Turner, ApJ 313 (1987) 495 Kinematic Tests of Exotic, Flat Cosmological Models Cosmology with a Time-Variable Cosmological “Constant” Peebles & Ratra, ApJ 325 (1988) L17

Dark Energy close the system of equations evaluate classical tests of cosmology to constrain “w”

Dark Energy w: equation-of-state controls the slope of the dark energy curve future: unknown without a reliable model of the dark energy past: in the simplest model, the dark energy does not play a significant role in cosmic evolution before z~2 present: the time-evolution of w determines the time-evolution of the dark energy

Dark Energy The dark energy phenomena could be due to a new, very-light scalar field. Why a scalar field? This is arguably the simplest method of introducing new physics within the well-tested framework of quantum physics. Due to the similarity of the dark energy phenomena with inflation, it seems plausible that a similar mechanism is responsible. Scalar fields, while not necessarily fundamental, are ubiquitous in theories beyond the standard model of particle physics. (-+++)

Dark Energy The scalar field is a pioneer, sent out to explore new worlds of physics! Gravity and the Tenacious Scalar Field Carl Brans, gr-qc/ Nordstrom’s Scalar Gravity Kaluza-Klein Unification Dirac and Jordan’s Cosmology Scalar-Tensor Gravity Inflaton Quintessence … Waves, Optics Electrodynamics Quantum Mechanics Scalar QED Field Theory Symmetry Breaking Dilatons, Moduli … 