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Extra Dimensions and the Cosmological Constant

Problem

- Cliff Burgess

Partners in Crime

- CC Problem
- Y. Aghababaie, J. Cline, C. de Rham, H.

Firouzjahi, D. Hoover, S. Parameswaran,

F. Quevedo, G. Tasinato, A. Tolley, I. Zavala - Phenomenology
- G. Azuelos, P.-H. Beauchemin, J. Matias, F.

Quevedo - Cosmology
- Albrecht, F. Ravndal, C. Skordis

The Plan

- The Cosmological Constant problem
- Why is it so hard?
- How extra dimensions might help
- Changing how the vacuum energy gravitates
- Making things concrete
- 6 dimensions and supersymmetry
- Prognosis
- Technical worries
- Observational tests

Naturalness as an Opportunity

- Cosmology alone cannot distinguish amongst the

various models of Dark Energy. - The features required by cosmology are difficult

to sensibly embed into a fundamental microscopic

theory. - Progress will come by combining both

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

The Cosmological Constant Problem

Concordance cosmology points to several types of

cosmic matter

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

The Cosmological Constant Problem

N. Wright, astro-ph/0701584

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

w pDE/rDE -1 for a cosmological constant

The Cosmological Constant Problem

- Hierarchy Problems
- Why Extra Dimensions?

A cosmological constant is not distinguishable

from a Lorentz invariant vacuum energy vs

The Cosmological Constant Problem

- Hierarchy Problems
- Why Extra Dimensions?

A cosmological constant is not distinguishable

from a Lorentz invariant vacuum energy vs

in 4 dimensions

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

The observed dark energy density corresponds to

a very small vacuum energy implies

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

BUT particles of mass m contribute m m

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

BUT particles of mass m contribute m m

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

BUT particles of mass m contribute m m

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

BUT particles of mass m contribute m m

Must cancel to 32 decimal places!!

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

BUT particles of mass m contribute m m

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

BUT particles of mass m contribute m m

Must cancel to 40 decimal places!!

Technical Naturalness

- Given a small quantity r r0 dr
- In the fundamental theory, why should r0 be

small? - Given that r0 is small, why does it stay small as

one integrates out physics up to the scales for

which r is measured?

Technical Naturalness

- This may have to wait until we know the

fundamental theory.

- Given a small quantity r r0 dr
- In the fundamental theory, why should r0 be

small? - Given that r0 is small, why does it stay small as

one integrates out physics up to the scales for

which r is measured?

Technical Naturalness

- This may have to wait until we know the

fundamental theory. - This is serious because it involves physics we

think we understand

- Given a small quantity r r0 dr
- In the fundamental theory, why should r0 be

small? - Given that r0 is small, why does it stay small as

one integrates out physics up to the scales for

which r is measured?

The Cosmological Constant Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

Seek to change properties of low-energy

particles (like the electron) so that their

zero-point energy does not gravitate, even though

quantum effects do gravitate in atoms!

Why this? But not this?

Another Naturalness Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

r is a constant if it is vacuum energy

Another Naturalness Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

More complicated time-dependence is possible

Another Naturalness Problem

- Dark energy vs vacuum energy
- Why must the vacuum energy be large?

When time dependence is due to scalar field

motion, the scalar field mass must be very small

BUT contributions to mf are of order

so are too large if M gt 10-2 eV

The Plan

- The Cosmological Constant problem
- Why is it so hard?
- How extra dimensions might help
- Changing how the vacuum energy gravitates
- Making things concrete
- 6 dimensions and supersymmetry
- Prognosis
- Technical worries
- Observational tests

How Extra Dimensions Help

- 4D CC vs 4D vacuum energy
- Branes and scales

How Extra Dimensions Help

New Tools motivated by string theory (or

condensed matter) Particles can be localized

on surfaces (branes, or defects) within the extra

dimensions Gravity is not similarly localized

- 4D CC vs 4D vacuum energy
- Branes and scales

How Extra Dimensions Help

In higher dimensions a 4D vacuum energy, if

localized in the extra dimensions, can curve the

extra dimensions instead of the observed four.

- 4D CC vs 4D vacuum energy
- Branes and scales

Arkani-Hamad et al Kachru et al, Carroll

Guica Aghababaie, et al

How Extra Dimensions Help

Gibbons, Guven Pope

- 4D CC vs 4D vacuum energy
- Branes and scales

- Most general 4D flat solutions to chiral 6D

supergravity, without matter fields. - l3 nonzero gives curvature singularities at

branes

How Extra Dimensions Help

To be useful it must be that extra dimensions

can be as large as the observed Dark Energy

density c/r 10-2 eV or r 1

m-metre This is possible! provided all known

particles except gravity are trapped on a brane,

since tests of Newtons law allow r lt 50

m-metre

- 4D CC vs 4D vacuum energy
- Branes and scales

Arkani Hamed, Dvali, Dimopoulos

Adelberger et al

How Extra Dimensions Help

Arkani Hamed, Dvali, Dimopoulos

If there are extra dimensions as large as r 1

m-metre then there can only be two of them

(although others could exist if they are much

smaller), or else the observed strength of

gravity would require the scale of

extra-dimensional physics to be smaller than

mw with n extra dimensions

- 4D CC vs 4D vacuum energy
- Branes and scales

How Extra Dimensions Help

- 4D CC vs 4D vacuum energy
- Branes and scales

These scales are natural using standard 4D

arguments.

How Extra Dimensions Help

Must rethink how the vacuum gravitates in 6D

for these scales. SM interactions do not

change at all!

- 4D CC vs 4D vacuum energy
- Branes and scales

Only gravity gets modified over the most

dangerous distance scales!

The Plan

- The Cosmological Constant problem
- Why is it so hard?
- How extra dimensions might help
- Changing how the vacuum energy gravitates
- Making things concrete
- 6 dimensions and supersymmetry
- Prognosis
- Technical worries
- Observational tests

The SLED Proposal

Aghababaie, CB, Parameswaran Quevedo

- Suppose physics is extra-dimensional above the

10-2 eV scale. - Suppose the physics of the bulk is supersymmetric.

The SLED Proposal

- Suppose physics is extra-dimensional above the

10-2 eV scale. - Suppose the physics of the bulk is supersymmetric.

- Bulk supersymmetry
- SUSY breaks at scale Mg on the branes
- Trickle-down of SUSY breaking to the bulk is

The SLED Proposal

Particle Spectrum

SM on brane no partners Many KK modes

in bulk

4D scalar ef r2 const

4D graviton

The CC Problem in 6D

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

The CC Problem in 6D

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Several 6D SUGRAs are known, including chiral and

non-chiral variants. - None have a 6D CC.

The CC Problem in 6D

Nishino Sezgin

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Several 6D SUGRAs are known, including chiral and

non-chiral variants. - None have a 6D CC.

The CC Problem in 6D

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Generates large 4D vacuum energy
- This energy is localized in the extra dimensions

(plus higher-derivatives)

The CC Problem in 6D

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Solve classical equations in presence of branes
- Plug back into action

The CC Problem in 6D

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Solve classical equations in presence of branes
- Plug back into action

The CC Problem in 6D

Chen, Luty Ponton

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Solve classical equations in presence of branes
- Plug back into action

Tensions cancel between brane and bulk!!

The CC Problem in 6D

Aghababaie et al.

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Solve classical equations in presence of branes
- Plug back into action

Smooth parts also cancel for supersymmetric

theories!!

The CC Problem in 6D

- The 6D CC
- Integrate out brane physics
- Integrate out bulk physics
- Classical contribution
- Quantum corrections

- Bulk is a supersymmetric theory with msb 10-2

eV - Quantum corrections can be right size in absence

of msb2 Mg2 terms! - Lifts flat direction.

The Plan

- The Cosmological Constant problem
- Why is it so hard?
- How extra dimensions might help
- Changing how the vacuum energy gravitates
- Making things concrete
- 6 dimensions and supersymmetry
- Prognosis
- Technical worries
- Observational tests

Prognosis

- Theoretical worries
- Observational tests

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Classical part of the argument
- What choices must be made to ensure 4D flatness?
- Quantum part of the argument
- Are these choices stable against renormalization?

The Worries

Tolley, CB, Hoover Aghababaie Tolley, CB, de

Rham Hoover CB, Hoover Tasinato

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Classical part of the argument
- What choices must be made to ensure 4D flatness?

- Now understand how 2 extra dimensions respond to

presence of 2 branes having arbitrary couplings. - Not all are flat in 4D, but all of those having

only conical singularities are flat. - (Conical singularities correspond to absence

of dilaton couplings to branes)

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Quantum part of the argument
- Are these choices stable against renormalization?

- So far so good, but not yet complete
- Brane loops cannot generate dilaton couplings if

these are not initially present - Bulk loops can generate such couplings, but are

suppressed by 6D supersymmetry

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

The Worries

Albrecht, CB, Ravndal, Skordis Tolley, CB,

Hoover Aghababaie Tolley, CB, de Rham Hoover

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Most brane properties and initial conditions do

not lead to anything like the universe we see

around us. - For many choices the extra dimensions implode or

expand to infinite size.

The Worries

Albrecht, CB, Ravndal, Skordis Tolley, CB,

Hoover Aghababaie Tolley, CB, de Rham Hoover

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Most brane properties and initial conditions do

not lead to anything like the universe we see

around us. - For many choices the extra dimensions implode or

expand to infinite size. - Initial condition problem much like the Hot Big

Bang, possibly understood by reference to earlier

epochs of cosmology (eg inflation)

Prognosis

- Theoretical worries
- Observational tests

The Observational Tests

- Quintessence cosmology

The Observational Tests

- Quintessence cosmology
- Modifications to gravity

The Observational Tests

- Quintessence cosmology
- Modifications to gravity
- Collider physics

The Observational Tests

- Quintessence cosmology
- Modifications to gravity
- Collider physics

SUSY broken at the TeV scale,

but not the MSSM!

The Observational Tests

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics?

The Observational Tests

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics?
- And more!

Summary

- It is the interplay between cosmological

phenomenology and microscopic constraints which

will make it possible to solve the Dark Energy

problem. - Technical naturalness provides a crucial clue.

Summary

- It is the interplay between cosmological

phenomenology and microscopic constraints which

will make it possible to solve the Dark Energy

problem. - Technical naturalness provides a crucial clue.
- 6D brane-worlds allow progress on technical

naturalness - Vacuum energy not equivalent to curved 4D
- Are Flat choices stable against renormalization?

Summary

- It is the interplay between cosmological

phenomenology and microscopic constraints which

will make it possible to solve the Dark Energy

problem. - Technical naturalness provides a crucial clue.
- 6D brane-worlds allow progress on technical

naturalness - Vacuum energy not equivalent to curved 4D
- Are Flat choices stable against

renormalization? - Tuned initial conditions
- Much like for the Hot Big Bang Model.

Summary

- It is the interplay between cosmological

phenomenology and microscopic constraints which

will make it possible to solve the Dark Energy

problem. - Technical naturalness provides a crucial clue.
- 6D brane-worlds allow progress on technical

naturalness - Vacuum energy not equivalent to curved 4D
- Are Flat choices stable against

renormalization? - Tuned initial conditions
- Much like for the Hot Big Bang Model.
- Enormously predictive, with many observational

consequences. - Cosmology at Colliders! Tests of gravity

Detailed Worries and Observations

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics?

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

Backup slides

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

The Worries

Salam Sezgin

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Classical flat direction corresponding to

combination of radius and dilaton

ef r2 constant. - Loops lift this flat direction, and in so doing

give dynamics to f and r.

The Worries

Kantowski Milton Albrecht, CB, Ravndal, Skordis

CB Hoover Ghilencea, Hoover, CB Quevedo

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

Potential domination when

Canonical Variables

The Worries

Albrecht, CB, Ravndal, Skordis

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

Potential domination when

Hubble damping can allow potential domination

for exponentially large r, even though r is not

stabilized.

Canonical Variables

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Weinbergs No-Go Theorem
- Steven Weinberg has a general objection to

self-tuning mechanisms for solving the

cosmological constant problem that are based on

scale invariance

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Weinbergs No-Go Theorem
- Steven Weinberg has a general objection to

self-tuning mechanisms for solving the

cosmological constant problem that are based on

scale invariance

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Weinbergs No-Go Theorem
- Steven Weinberg has a general objection to

self-tuning mechanisms for solving the

cosmological constant problem that are based on

scale invariance

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Nimas No-Go Argument
- One can have a vacuum energy m4 with m

greater than the cutoff, provided it is turned on

adiabatically. - So having extra dimensions with r 1/m does

not release one from having to find an

intrinsically 4D mechanism.

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Nimas No-Go Argument
- One can have a vacuum energy m4 with m

greater than the cutoff, provided it is turned on

adiabatically. - So having extra dimensions with r 1/m does

not release one from having to find an

intrinsically 4D mechanism.

- Scale invariance precludes obtaining \mu greater

than the cutoff in an adiabatic way

implies

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Post BBN
- Since r controls Newtons constant, its

motion between BBN and now will cause

unacceptably large changes to G.

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Post BBN
- Since r controls Newtons constant, its

motion between BBN and now will cause

unacceptably large changes to G. - Even if the kinetic energy associated with r

were to be as large as possible at BBN, Hubble

damping keeps it from rolling dangerously far

between then and now.

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Post BBN
- Since r controls Newtons constant, its

motion between BBN and now will cause

unacceptably large changes to G. - Even if the kinetic energy associated with r

were to be as large as possible at BBN, Hubble

damping keeps it from rolling dangerously far

between then and now.

log r vs log a

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Pre BBN
- There are strong bounds on KK modes in models

with large extra dimensions from - their later decays into photons
- their over-closing the Universe
- their light decay products being too

abundant at BBN

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- Pre BBN
- There are strong bounds on KK modes in models

with large extra dimensions from - their later decays into photons
- their over-closing the Universe
- their light decay products being too

abundant at BBN - Photon bounds can be evaded by having

invisible channels others are model dependent,

but eventually must be addressed

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- A light scalar with mass m H has several

generic difficulties - What protects such a small mass from large

quantum corrections?

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- A light scalar with mass m H has several

generic difficulties - What protects such a small mass from large

quantum corrections? - Given a potential of the form
- V(r) c0 M4 c1 M2/r2 c2 /r4
- then c0 c1 0 ensures both small mass and

small dark energy.

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- A light scalar with mass m H has several

generic difficulties - Isnt such a light scalar already ruled out

by precision tests of GR in the solar system?

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- A light scalar with mass m H has several

generic difficulties - Isnt such a light scalar already ruled out

by precision tests of GR in the solar system?

The same logarithmic corrections which enter the

potential can also appear in its matter

couplings, making them field dependent and so

also time-dependent as f rolls. Can arrange these

to be small here now.

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- A light scalar with mass m H has several

generic difficulties - Isnt such a light scalar already ruled out

by precision tests of GR in the solar system?

The same logarithmic corrections which enter the

potential can also appear in its matter

couplings, making them field dependent and so

also time-dependent as f rolls. Can arrange these

to be small here now.

a vs log a

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- A light scalar with mass m H has several

generic difficulties - Shouldnt there be strong bounds due to

energy losses from red giant stars and

supernovae? (Really a bound on LEDs and not on

scalars.)

The Worries

- Technical Naturalness
- Runaway Behaviour
- Stabilizing the Extra Dimensions
- Famous No-Go Arguments
- Problems with Cosmology
- Constraints on Light Scalars

- A light scalar with mass m H has several

generic difficulties - Shouldnt there be strong bounds due to

energy losses from red giant stars and

supernovae? (Really a bound on LEDs and not on

scalars.) - Yes, and this is how the scale M 10 TeV for

gravity in the extra dimensions is obtained.

Observational Consequences

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

Observational Consequences

Albrecht, CB, Ravndal Skordis Kainulainen

Sunhede

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Quantum vacuum energy lifts flat direction.
- Specific types of scalar interactions are

predicted. - Includes the Albrecht-Skordis type of potential
- Preliminary studies indicate it is possible to

have viable cosmology - Changing G BBN

Observational Consequences

Albrecht, CB, Ravndal Skordis

- Quantum vacuum energy lifts flat direction.
- Specific types of scalar interactions are

predicted. - Includes the Albrecht-Skordis type of potential
- Preliminary studies indicate it is possible to

have viable cosmology - Changing G BBN

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

Potential domination when

Canonical Variables

Observational Consequences

Albrecht, CB, Ravndal Skordis

Radiation Matter Total Scalar

- Quantum vacuum energy lifts flat direction.
- Specific types of scalar interactions are

predicted. - Includes the Albrecht-Skordis type of potential
- Preliminary studies indicate it is possible to

have viable cosmology - Changing G BBN

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

log r vs log a

Observational Consequences

Albrecht, CB, Ravndal Skordis

- L 0.7

- Quantum vacuum energy lifts flat direction.
- Specific types of scalar interactions are

predicted. - Includes the Albrecht-Skordis type of potential
- Preliminary studies indicate it is possible to

have viable cosmology - Changing G BBN

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- m 0.25

- and w
- vs log a

Radiation Matter Total Scalar w Parameter

w 0.9

Observational Consequences

Albrecht, CB, Ravndal Skordis

- Quantum vacuum energy lifts flat direction.
- Specific types of scalar interactions are

predicted. - Includes the Albrecht-Skordis type of potential
- Preliminary studies indicate it is possible to

have viable cosmology - Changing G BBN

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

a vs log a

Observational Consequences

Albrecht, CB, Ravndal Skordis

- Quantum vacuum energy lifts flat direction.
- Specific types of scalar interactions are

predicted. - Includes the Albrecht-Skordis type of potential
- Preliminary studies indicate it is possible to

have viable cosmology - Changing G BBN

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

log r vs log a

Observational Consequences

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- At small distances
- Changes Newtons Law at range r/2p 1 mm.
- At large distances
- Scalar-tensor theory out to distances of order

H0.

Observational Consequences

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- At small distances
- Changes Newtons Law at range r/2p 1 mm.
- At large distances
- Scalar-tensor theory out to distances of order

H0.

Observational Consequences

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Not the MSSM!
- No superpartners
- Bulk scale bounded by astrophysics
- Mg 10 TeV
- Many channels for losing energy to KK modes
- Scalars, fermions, vectors live in the bulk

Observational Consequences

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Can there be observable signals if Mg 10 TeV?
- Must hit new states before E Mg . Eg string

and KK states have MKK lt Ms lt Mg - Dimensionless couplings to bulk scalars are

unsuppressed by Mg

Observational Consequences

Azuelos, Beauchemin CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Not the MSSM!
- No superpartners
- Bulk scale bounded by astrophysics
- Mg 10 TeV
- Many channels for losing energy to KK modes
- Scalars, fermions, vectors live in the bulk

Dimensionless coupling! O(0.1-0.001) from

loops

Observational Consequences

Azuelos, Beauchemin CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Not the MSSM!
- No superpartners
- Bulk scale bounded by astrophysics
- Mg 10 TeV
- Many channels for losing energy to KK modes
- Scalars, fermions, vectors live in the bulk

Dimensionless coupling! O(0.1-0.001) from

loops

- Use H decay into gg, so search for two hard

photons plus missing ET.

Observational Consequences

Azuelos, Beauchemin CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Not the MSSM!
- No superpartners
- Bulk scale bounded by astrophysics
- Mg 10 TeV
- Many channels for losing energy to KK modes
- Scalars, fermions, vectors live in the bulk

- Standard Model backgrounds

Observational Consequences

Azuelos, Beauchemin CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Not the MSSM!
- No superpartners
- Bulk scale bounded by astrophysics
- Mg 10 TeV
- Many channels for losing energy to KK modes
- Scalars, fermions, vectors live in the bulk

Observational Consequences

Azuelos, Beauchemin CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Not the MSSM!
- No superpartners
- Bulk scale bounded by astrophysics
- Mg 10 TeV
- Many channels for losing energy to KK modes
- Scalars, fermions, vectors live in the bulk

- Significance of signal vs cut on missing ET

Observational Consequences

Azuelos, Beauchemin CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Not the MSSM!
- No superpartners
- Bulk scale bounded by astrophysics
- Mg 10 TeV
- Many channels for losing energy to KK modes
- Scalars, fermions, vectors live in the bulk

- Possibility of missing-ET cut improves the reach

of the search for Higgs through its gg channel

Observational Consequences

Matias, CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be chosen to agree with

oscillation data. - Most difficult bounds on resonant SN

oscillilations.

Observational Consequences

Matias, CB

- 6D supergravities have many bulk fermions
- Gravity (gmn, ym, Bmn, c, j)
- Gauge (Am, l)
- Hyper (F, x)
- Bulk couplings dictated by supersymmetry
- In particular 6D fermion masses must vanish
- Back-reaction removes KK zero modes
- eg boundary condition due to conical defect at

brane position

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

Observational Consequences

Matias, CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

Dimensionful coupling l 1/Mg

Observational Consequences

Matias, CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

- SUSY keeps N massless in bulk
- Natural mixing with Goldstino on branes
- Chirality in extra dimensions provides natural L

Dimensionful coupling l 1/Mg

Observational Consequences

Matias, CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

Dimensionful coupling! l 1/Mg

Observational Consequences

Matias, CB

t

Constrained by bounds on sterile neutrino emission

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

Dimensionful coupling! l 1/Mg

Require observed masses and large mixing.

Observational Consequences

Matias, CB

t

Constrained by bounds on sterile neutrino emission

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

- Bounds on sterile neutrinos easiest to satisfy if

g l v lt 10-4. - Degenerate perturbation theory implies massless

states strongly mix even if g is small. - This is a problem if there are massless KK modes.
- This is good for 3 observed flavours.
- Brane back-reaction can remove the KK zero mode

for fermions.

Dimensionful coupling! l 1/Mg

Require observed masses and large mixing.

Observational Consequences

Matias, CB

- Imagine lepton-breaking terms are suppressed.
- Possibly generated by loops in running to low

energies from Mg. - Acquire desired masses and mixings with a mild

hierarchy for g/g and e/e. - Build in approximate Le Lm Lt, and Z2

symmetries.

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

S Mg r

Observational Consequences

Matias, CB

- 1 massless state
- 2 next- lightest states have strong overlap with

brane. - Inverted hierarchy.
- Massive KK states mix weakly.

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

Observational Consequences

Matias, CB

Worrisome once we choose g 10-4, good masses

for the light states require e S k

1/g Must get this from a real compactification.

- 1 massless state
- 2 next- lightest states have strong overlap with

brane. - Inverted hierarchy.
- Massive KK states mix weakly.

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

Observational Consequences

Matias, CB

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- SLED predicts there are 6D massless fermions in

the bulk, as well as their properties - Massless, chiral, etc.
- Masses and mixings can be naturally achieved

which agree with data! - Sterile bounds oscillation experiments

2

- Lightest 3 states can have acceptable 3-flavour

mixings. - Active sterile mixings can satisfy incoherent

bounds provided g 10-4 or less (qi g/ci).

Observational Consequences

- Quintessence cosmology
- Modifications to gravity
- Collider physics
- Neutrino physics
- Astrophysics

- Energy loss into extra dimensions is close to

existing bounds - Supernova, red-giant stars,
- Scalar-tensor form for gravity may have

astrophysical implications. - Binary pulsars