Cosmology The Road Ahead

Mark Trodden Syracuse? University

Penn. State University IGC Inaugural Conference 8/9/2007 Outline

• Introduction. • The Observed, Circa 2007 • The Mysteries of the Pie Chart • What might our observations be revealing about fundamental ? - Connections to - The importance of gravity - Some examples and constraints • Summary

Mark Trodden, IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Establishing the New Cosmology

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0 10 100 500 1000 Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Three Problems

• Three problems posed by observational cosmology - a great achievement, but raises many issues. • Need fundamental physics to understand what the universe is made of and why these observations look the way they do. • It seems inevitable that this will require new particles and new symmetries.

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Dark Matter • Originally noticed through galaxy rotation curves • One modern way to look for it - weak gravitational lensing

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 How to Use it Can reconstruct the density in the cluster What is this stuff?

• WIMPs? • SUSY particles? • Axions? • Remnants of GUTs? • …

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Seems to Really be Particles!

Hubble

HST, Chandra + Lensing

Chandra Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 BSM Physics & Dark Matter

There is a very broad connection between models of beyond the standard model physics (particularly those addressing the hierarchy problem) and dark matter • Almost any model involves new particles at the TeV scale, related to the SM particles through new symmetries (SUSY partners, KK partners, extra gauge and scalar partners, ...) • Typically, to avoid things like proton decay and precision EW tests, an extra new symmetry is required (R-parity, KK-parity, T-parity, ...). • This new symmetry renders stable some new particle at the weak scale Often, this stable new particle is an ideal WIMP candidate! Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Dark Matter

• A prime dark matter dn 2 2 candidate is the WIMP = −3Hn − σv n − neq - a new stable particle. dt ( ) • Number density n Dilution from Annihilations determined by expansion • Initially, <σv> term Exponential dominates, so n ≈ drop neq. • Eventually, n becomes so small that dilution term dominates Freeze out • Co-moving number density is fixed (freeze out).

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Abundance of WIMPs Universe cools, leaves • Weakly-interacting particles residue of dark matter w/ weak-scale masses give ΩDM with • Strong, fundamental, and independent motivation for ΩDM ~ 0.1 (σWeak/σ) new physics at weak scale • Could use the colliders as a dark matter laboratory • Discover WIMPs and determine their properties • Consistency between properties (particle physics) and abundance (cosmology) may lead to understanding of Universe at T = 10 GeV, t = 10-8 s. Can compare this program with the one that led (with spectacular success) to our understanding of BBN via a detailed understanding of nuclear physics

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Dark Matter In this case, the DM candidate is the LSP - here’s what we’d love to do (I’ve cheated slightly)

ILC LHC (“best case scenario”)

WMAP Planck (current) (~2010)

[Thanks to J. Feng] Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Another Fundamental Problem •Can construct a ladder of evidence

•Essentially, observable universe, out to the Hubble size, is made of matter and not antimatter

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Baryogenesis

BBN and CMB have determined the cosmic baryon content:

2 ΩBh = 0.024 ± 0.001

To achieve this a particle theory requires (Sakharov, 1968) :

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 A Connection to TeV Physics • However, an attractive and testable possibility is that the asymmetry is generated at the weak scale. • The Standard Model of particle physics, satisfies all 3 Sakharov criteria in principle, (anomaly, CKM matrix, finite-temperature phase transition) • Exciting, but turns out not enough CPV and a continuous EWPT. Therefore, cannot be sufficient to explain the baryon asymmetry! • This is a clear indication, from observations of the universe, of physics beyond the standard model - new particles and new symmetries!

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Cosmic Acceleration

da Not only is the universe expanding: a˙ > 0 d ≡ dt But it is accelerating! a¨ a˙ > 0 ≡ dt - new particles and new symmetries!

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Logical Possibilities for Acceleration Modifications of New Mass/Energy Gravity Sources Gµν = 8πGTµν Quintessence Inverse Curvature K-essence Gravity Other Oscillating DE DGP Braneworlds Cosmological Constant ... Cardassian Models Extra Dimensions ... Backreaction [Ratra, Peebles; Wetterich; [Carroll, Duvvuri, Trodden, Environmental Selection Caldwell, Dave, Steinhardt; Turner; Dvali, Gabadadze, ... Freiman, Hill, Stebbins, Waga; Porrati; Freese, Lewis; De Armendariz-Picon, Felice, Easson; Flanagan; ...] [Kolb, Matarrese, Notari, Mukhanov; Bean; ... Riotto; Brandenberger; ...... Abramo, Woodard; Basically every cosmologist Weinberg; Vilenkin; Linde; you can think of ... and most Bousso, Polchinski; ...] particle theorists as well.]

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Dark Energy - Theory

Evolution of the universe governed by Einstein eqns a˙ 2 H2 ∝ ρ The Friedmann equation ≡ a ! " a¨ ∝ (ρ + 3p) The “acceleration” equation a − Parameterize different types of matter by equations of state: pi=wiρi When evolution dominated by type i, obtain

2/3(1+wi) 3(1+wi) a(t) ∝ t ρ(a) ∝ a− (wi ≠ -1)

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Cosmic Acceleration

So, accelerating expansion means a¨ ∝ (ρ + 3p) p<-ρ/3 or w<-1/3 a −

2/3(1+wi) 3(1+w ) Three Broad Possibilities a(t) ∝ t ρ(a) ∝ a− i

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Evolution of Dilutes slower Stays absolutely Increases with the Energy Density than any matter constant (Λ) expansion!! Evolution of Power-law Exponential Infinite value in Scale Factor quintessence expansion a finite time!!

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 A Cosmological Constant or Not?

“If it bleeds, we can kill it” Arnold Schwarzenegger as Dutch Schaeffer, Predator (1987) What the Governator is trying to tell us is that our best chance of testing the origin of acceleration is if it is not a cosmological constant

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Data on w

We’ll come back later to discuss what is really being measured here

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 The Cosmological Constant • A long-standing problem for fundamental theory. • Has a clear connection to particle physics • Vacuum is full of virtual particles carrying energy. • Should lead to a constant vacuum energy. How big? - ∞

BUT

• While calculating branching ratios - easy to forget SUSY is a space-time symmetry.

e+ γ γ 4 60 ρΛ MSUSY Still 10 too big! e- ∼

At this stage, fair to say we are stuck! Help needed! Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Quintessence - Dark Energy Maybe there’s some principle that sets vacuum energy to zero. Then dark energy might be like low-scale inflation today. Use scalar fields to source Einstein’s equation. V(φ) Difference: no minimum or reheating 1 L = (∂ φ)∂µφ V(φ) 2 µ − 1 1 ρ = φ˙2 + (∇φ)2 +V(φ) φ 2 2 φ dV φ¨ + 3Hφ˙ + = 0 Homogeneity gives dφ 2V(φ) φ˙2 Small slope ρφ V(φ) constant w = − ≈ ≈ − 2V(φ) + φ˙2 ! "

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Are we Being Fooled by Gravity?

(Carroll, De Felice & M.T., Phys.Rev. D71: 023525 (2005) [astro-ph/0408081]) We don’t really measure w - we infer it from the Hubble plot via 1 2 H˙ we f f = 1 + −1 Ω 3 H2 − m ! " Maybe, if gravity is modified, can infer value not directly related to energy sources (or perhaps without them!) One example - Brans-Dicke theories

4 ω µ 4 SBD = d x√ g φR (∂µφ)∂ φ 2V(φ) + d x√ gLm(ψi,g) − − φ − − Z ! " Z ω>40000 (Signal timing measurements from Cassini) We showed that (with difficulty) can measure w<-1, even though no energy conditions are violated.

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 In Fact, Maybe it’s All Gravity!

“[General Relativity] explains ... quantitatively ... the secular rotation of the orbit of Mercury, discovered by Le Verrier, ... without the need of any special hypothesis.”, SPAW, Nov 18, 1915

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 How Might We Modify Gravity?

Write an Lagrangian - a scalar involving the object g µ ν and its derivatives. What might this look like?

What are the propagating degrees of freedom? A first step is to identify the degrees of freedom in

Answer: turns out there are scalar and vectors as well as hµν

How come we don’t see all these in GR? - Depends on the action!

The equations of motion arising from the Einstein-Hilbert action yield constraints, which make everything except non-dynamical!

Almost any other action will free up some of the other degrees of freedom. These can yield new problems.

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Issues with new d.o.f.

A couple of different problems can arise with these new degrees of freedom. First: Geodesics within the solar system can be appreciably altered Best tests are from timing delays of signals from distant spacecraft. Particularly the Cassini mission.

Second: They can lead to instabilities because they are ghost-like (have the wrong sign kinetic terms. These would lead, among other things, to the decay of the vacuum on a microscopic timescale

(Carroll, Hoffman & M.T., Phys.Rev. D68: 023509 (2003) [astro-ph/0301273])

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 What about Earlier Epochs? Cosmologists admitting they want to work on the cosmology of the earliest times in the universe elicits interesting responses from:

Cosmologists : “Why work on this when there’s so much data to analyze?” QG Theorists : “What do cosmologists know about quantum gravity?” Experimentalists : “Is this physics?”

But we do need to put real effort into this because:

Particle cosmologists need to “tool up” for the new issues being raised in cosmology. The data and its rate of acquisition is breathtaking. During next 5-10 years we’ll know unprecedentedly precise facts about the universe. But what will they mean?? It is not enough just to parameterize the universe. Only through combined efforts will it become clear that this is physics.

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 Where’s the Action At? • How did the observable universe become so flat and homogeneous on large scales?

• What made galaxies form (metric perturbations)? • Why do we observe 3 spatial dimensions (and for that matter, 1 temporal one)?

• Can cosmological observations teach us anything about fundamental physics?

• Is the universe eternal or finite in time; How, if at all, did it begin?

More generally, how did fundamental physics set the values of the required input parameters to the wildly successful standard cosmology?

Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007 The Road Ahead

CDMS

… and many, many more… Thank You! Mark Trodden, Syracuse University IGC Inaugural Conference Cosmology - The Road Ahead Penn. State University, 8/9/2007