cosmology the road aheadgravity.psu.edu/events/conferences/inaugural/talks/...igc inaugural...

IGC Inaugural ConferencePenn. State University

8/9/2007

Mark TroddenSyracuse University?CosmologyThe Road Ahead

Mark Trodden, Syracuse UniversityCosmology - The Road Ahead

IGC Inaugural ConferencePenn. State University, 8/9/2007

Outline

• Introduction.• The Universe Observed, Circa 2007• The Mysteries of the Pie Chart• What might our observations be revealing about fundamental physics? - Connections to particle physics

- The importance of gravity

- Some examples and constraints• Summary



Establishing the New Cosmology

10 100 500 10000

1000

2000

3000

4000

5000

600090° 2° 0.5° 0.2°

?



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.



• Originally noticed through galaxy rotation curves • One modern way to look for it - weak gravitational lensing

Dark Matter



How to Use itCan reconstruct thedensity in the cluster

What is this stuff?

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



Seems to Really be Particles!

Hubble

Chandra

HST, Chandra + Lensing



BSM Physics & Dark Matter

There is a very broad connection between models of beyondthe 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!



Dark Matter• A prime dark matter candidate is the WIMP - a new stable particle. • Number density n determined by

Dilution fromexpansion

dndt

= −3Hn − σv n2 − neq2( )

Exponentialdrop

Freeze out

Annihilations

• Initially, term dominates, so n ≈ neq.

• Eventually, n becomes so small that dilution term dominates

• Co-moving number density is fixed (freeze out).



Abundance of WIMPs• Weakly-interacting particles

w/ weak-scale masses give ΩDM • Strong, fundamental, and

independent motivation for new physics at weak scale

Universe cools, leaves residue of dark matter

with ΩDM ~ 0.1 (σWeak/σ)• 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 (withspectacular success) to our understanding of BBN via

adetailed understanding of nuclear physics



Dark MatterIn this case, the DM candidate is the LSP - here’s what we’d love to do (I’ve cheated slightly)

WMAP(current)

Planck(~2010)

LHC (“best case scenario”)

[Thanks to J. Feng]

ILC



Another Fundamental Problem

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

•Can construct a ladder of evidence



Baryogenesis

BBN and CMB have determined

the cosmic baryon content:

ΩBh2 = 0.024 ± 0.001

To achieve this a particle theoryrequires (Sakharov, 1968) :



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!



Cosmic Acceleration

Not only is the universe expanding:But it is accelerating!

ȧ≡ dadt

> 0

ä≡ ddtȧ> 0

- new particles and new symmetries!



Logical Possibilities for Acceleration

Gµν = 8πGTµνNew Mass/Energy

SourcesModifications of

Gravity

QuintessenceK-essence

Oscillating DE...

Inverse Curvature Gravity

DGP BraneworldsCardassian Models

...

Cosmological ConstantExtra Dimensions

BackreactionEnvironmental Selection

...

Other

[Carroll, Duvvuri, Trodden, Turner; Dvali, Gabadadze, Porrati; Freese, Lewis; De Felice, Easson; Flanagan; ...] [Kolb, Matarrese, Notari,

Riotto; Brandenberger; Abramo, Woodard; Weinberg; Vilenkin; Linde; Bousso, Polchinski; ...]

[Ratra, Peebles; Wetterich; Caldwell, Dave, Steinhardt; Freiman, Hill, Stebbins, Waga; Armendariz-Picon, Mukhanov; Bean; ... ......... Basically every cosmologist you can think of ... and most particle theorists as well.]



Dark Energy - Theory

Evolution of the universe governed by Einstein eqns

The Friedmann equation

The “acceleration” equation

Parameterize different types of matter by equations of state: pi=wiρi

When evolution dominated by type i, obtain

(wi ≠ -1)

H2 ≡(ȧa

)2∝ ρ

a(t) ∝ t2/3(1+wi) ρ(a) ∝ a−3(1+wi)

äa∝−(ρ+3p)



Cosmic Acceleration

So, accelerating expansion means

p



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



Data on w

We’ll come back laterto discuss what is really

being measured here



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? -

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

Still 1060 too big!

BUT

∞

At this stage, fair to say we are stuck! Help needed!

ρΛ ∼M4SUSYe-

γe+

γ



Quintessence - Dark EnergyMaybe there’s some principle that sets vacuum energy to zero. Then dark energy might be like low-scale inflation today.

V(φ)

φ

Difference: no minimum or reheating

Use scalar fields to source Einstein’s equation.

Homogeneity gives

Small slope

ρφ =12φ̇2+

12(∇φ)2+V (φ)

φ̈+3Hφ̇+dVdφ

= 0

ρφ ≈V (φ)≈ constant w=−[2V (φ)− φ̇2

2V (φ)+ φ̇2

]

L=12

(∂µφ)∂µφ−V (φ)



Are we Being Fooled by Gravity?

We don’t really measure w - we infer it from the Hubble plot via

Maybe, if gravity is modified, can infer value not directlyrelated to energy sources (or perhaps without them!)

we f f =−1

1−Ωm

(1+

23ḢH2

)

One example - Brans-Dicke theories

ω>40000 (Signal timing measurements from Cassini) We showed that (with difficulty) can measure w



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 needof any special hypothesis.”, SPAW, Nov 18, 1915



How Might We Modify Gravity?Write an Lagrangian - a scalar involving the object and its derivatives. What might this look like?

gµν

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

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 yieldconstraints, which make everything except non-dynamical!hµν

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



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

Second: They can lead to instabilities because they are ghost-like (have the wrong sign kinetic terms.

Best tests are from timing delays of signals fromdistant spacecraft.Particularly the Cassini mission.

These would lead, among other things, to the decayof the vacuum on a microscopic timescale

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



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.

What about Earlier Epochs?



• 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 thevalues of the required input parameters to the

wildly successful standard cosmology?

Where’s the Action At?



The Road Ahead

… and many, many more…

CDMS

Thank You!

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