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String/Brane Cosmology

- for those who have not yet
- drunk the Kool-Aid
- C.P. Burgess

with J.Blanco-Pillado, J.Cline, C. de Rham,

C.Escoda, M.Gomez-Reino, D. Hoover, R.Kallosh,

A.Linde,F.Quevedo and A. Tolley

Outline

- Motivation
- String Cosmology Why Bother?
- Branes and late-Universe cosmology
- Some Dark (Energy) Thoughts
- String inflation
- A Sledgehammer for a Nutcracker?
- Outlook

Strings, Branes and Cosmology

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

Strings, Branes and Cosmology

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

Science progresses because short- distance

physics decouples from long distances.

Strings, Branes and Cosmology

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

Inflationary fluctuations could well arise

at very high energies MI 10-3 Mp

Science progresses because short distance

physics decouples from long distances.

Strings, Branes and Cosmology

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

Inflationary fluctuations could well arise

at very high energies MI 10-3 Mp

Cosmology (inflation, quintessence, etc) relies

on finely-tuned properties of scalar potentials,

which are extremely sensitive to short distances.

Science progresses because short distance

physics decouples from long distances.

Strings, Branes and Cosmology

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

Inflationary fluctuations could well arise

at very high energies MI 10-3 Mp

Cosmology (inflation, quintessence, etc) relies

on finely-tuned properties of scalar potentials,

which are extremely sensitive to short

distances. Modifications to gravity

(MOND, Bekenstein, DGP, etc) are very strongly

constrained by UV consistency issues.

Science progresses because short distance

physics decouples from long distances.

Strings, Branes and Cosmology

Polchinski

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

D branes in string theory are surfaces on

which some strings must end, ensuring their

low-energy modes are trapped on the brane.

Strings, Branes and Cosmology

Ibanez et al

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

In some cases this is where the Standard Model

particles live.

Strings, Branes and Cosmology

Rubakov Shaposhnikov

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

Leads to the brane-world scenario, wherein we

are all brane-bound.

Strings, Branes and Cosmology

- Why doesnt string theory decouple from

cosmology? - Why are branes important for cosmology and

particle physics?

Identifies hidden assumptions which particle

physicists and cosmologists have been making eg

all interactions dont see the same number of

dimensions.

Branes and Naturalness

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

Branes and Naturalness

ADD

Shows that extra dimensions can be as large

as microns

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

Branes and Naturalness

Horava Witten, Lykken, Antoniadis

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

Branes and Naturalness

Randall Sundrum

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV Ordinary physics in extra

dimensions (eg warping) can have extraordinary

implications for the low-energy 4D theory.

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

Branes and Naturalness

ADKS, KSS

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV Ordinary physics in extra

dimensions (eg warping) can have extraordinary

implications for the low-energy 4D theory.

Shows that the vacuum energy need not be directly

tied to the cosmological constant, as had been

thought.

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

Branes and Naturalness

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV Shows that the vacuum

energy is not as directly tied to the

cosmological constant

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

In 4D the cosmological constant problem

arises because a vacuum energy is equivalent to a

cosmological constant, and so also to a curved

universe.

Branes and Naturalness

CG, ABPQ

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV Shows that the vacuum

energy is not as directly tied to the

cosmological constant

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

In higher D solutions exist having large 4D

energy, but for which the 4D geometry is

absolutely flat!

Branes and Naturalness

BH

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV Shows that the vacuum

energy is not as directly tied to the

cosmological constant

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

Are the choices required for 4D flatness

stable against renormalization? With SUSY,

quantum corrections are usually order M2/r2 but

can be as small as 1/r4 .

Branes and Naturalness

ABPQ

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV Shows that the vacuum

energy is not as directly tied to the

cosmological constant

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

This can be small enough because 1/r can be

as small as 10-3 eV (since r m m is possible)!!!

Are the choices required for 4D flatness

stable against renormalization? With SUSY,

quantum corrections are usually order M2/r2 but

can be as small as 1/r4

Branes and Naturalness

BMQ, ,ABB, BC

Shows that extra dimensions can be as large

as microns Shows that the string scale could

be as small as TeV Shows that the vacuum

energy is not as directly tied to the

cosmological constant

- Removal of such assumptions has allowed new

insights into low-energy naturalness problems.

Very predictive time-dependent Dark Energy

tests of GR at both micron and astrophysical

distances implications for the LHC etc

Are the choices required for 4D flatness

stable against renormalization? So far so

good quantum corrections are usually order M2/r2

but can be as small as 1/r4

SLED 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

SLED 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

SLED 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

SLED 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

SLED 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

SLED Present Status

- Stability against loops?
- What choices ensure 4D flatness?
- Are these choices stable against renormalization?
- Tuned initial conditions?
- Do only special initial conditions lead to the

Universe we see around us?

SLED Present Status

ABPQ

- 4D space is not flat for arbitrary brane - bulk

couplings.

- Stability against loops?
- What choices ensure 4D flatness?
- Are these choices stable against renormalization?
- Tuned initial conditions?
- Do only special initial conditions lead to the

Universe we see around us?

SLED Present Status

BQTZ, TBDH

- 4D space is not flat for arbitrary brane - bulk

couplings. - Most brane pairs do not produce static solutions.

- Stability against loops?
- What choices ensure 4D flatness?
- Are these choices stable against renormalization?
- Tuned initial conditions?
- Do only special initial conditions lead to the

Universe we see around us?

SLED Present Status

BH

- 4D space is not flat for arbitrary brane - bulk

couplings. - Most brane pairs do not produce static solutions.
- In some cases these choices appear to be stable

against renormalization.

- Stability against loops?
- What choices ensure 4D flatness?
- Are these choices stable against renormalization?
- Tuned initial conditions?
- Do only special initial conditions lead to the

Universe we see around us?

SLED Present Status

ABRS

- Initial conditions exist which lead to dynamics

which can describe the observed Dark Energy.

- Stability against loops?
- What choices ensure 4D flatness?
- Are these choices stable against renormalization?
- Tuned initial conditions?
- Do only special initial conditions lead to the

Universe we see around us?

SLED Present Status

TBDH

- Initial conditions exist which lead to dynamics

which can describe the observed Dark Energy. - Successful initial condition are scarce.

- Stability against loops?
- What choices ensure 4D flatness?
- Are these choices stable against renormalization?
- Tuned initial conditions?
- Do only special initial conditions lead to the

Universe we see around us?

SLED Present Status

- Initial conditions exist which lead to dynamics

which can describe the observed Dark Energy. - Successful initial condition are scarce.
- Explained by earlier dynamics (eg inflation)?

- Stability against loops?
- What choices ensure 4D flatness?
- Are these choices stable against renormalization?
- Tuned initial conditions?
- Do only special initial conditions lead to the

Universe we see around us?

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String Inflation

Inflationary models must be embedded into a

fundamental theory in order to explain

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String Inflation

Inflationary models must be embedded into a

fundamental theory in order to explain Why

the inflaton potential has its

particular finely-tuned shape (and if

anthropically explained, what assigns the

probabilities?)

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String Inflation

Inflationary models must be embedded into a

fundamental theory in order to explain Why

the inflaton potential has its

particular finely-tuned shape (and if

anthropically explained, what assigns the

probabilities?) What explains any special

choices for initial conditions

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String Inflation

Inflationary models must be embedded into a

fundamental theory in order to explain Why

the inflaton potential has its

particular finely-tuned shape (and if

anthropically explained, what assigns the

probabilities?) What explains any special

choices for initial conditions Why the

observed particles get heated once inflation ends.

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String Inflation

Inflationary models must be embedded into a

fundamental theory in order to explain Why

the inflaton potential has its

particular finely-tuned shape (and if

anthropically explained, what assigns the

probabilities?) What explains any special

choices for initial conditions Why the

observed particles get heated once inflation ends.

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Can identify how robust inflationary

predictions are to high-energy details, and so

also what kinds of very high-energy physics might

be detectable using CMB measurements.

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String theory has many scalars having very

flat potentials. These scalars (called

moduli) describe the shape and size of the

various extra dimensions

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String theory has many scalars having very

flat potentials. BUT their potentials are

usually very difficult to calculate.

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String theory has many scalars having very

flat potentials. BUT their potentials are

usually very difficult to calculate. A

convincing case for inflation requires knowing

the potential for all of the scalars.

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

String theory has many scalars having very

flat potentials. BUT their potentials are

usually very difficult to calculate. A

convincing case for inflation requires knowing

the potential for all of the scalars.

String Inflation

GKP

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

For Type IIB strings it is now known how to

compute the potentials for some of the low-energy

string scalars.

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Branes want to squeeze extra dimensions while

the fluxes they source want the extra dimensions

to grow. The competition stabilizes many of the

moduli

String Inflation

KKLT, KKLMMT

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

The moduli which remain after this

stabilization can also acquire a potential due to

nonperturbative effects. Plausibly estimated

KKLT models

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

The moduli which remain after this

stabilization can also acquire a potential due to

nonperturbative effects. Improved for P411169

The Better Racetrack Douglas Denef

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

The inflaton in these models can describe the

relative positions of branes or the volume or

shape of the extra dimensions.

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

The motion of several complex fields must

generically be followed through a complicated

landscape many possible trajectories for each

vacuum

String Inflation

The Racetrack Eight

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

The potential can inflate, e.g. for some

choices for the properties of P411169 giving

rise to realistic inflationary fluctuations

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Barger et al hep-ph/0302150

CMB measurements begin to distinguish

different inflationary models

- model comparisons

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

WMAP preferred

CMB measurements begin to distinguish

different inflationary models

- model comparisons

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

brane-antibrane

racetrack

Trajectories through string landscape predict

same regions as do their low-energy effective

theories.

- model comparisons

String Inflation

KKLMMT, BCSQ, Racetrack 8

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

WMAP preferred

KKLMMT

P411169

The measurements can already distinguish

amongst some stringy inflationary models.

- model comparisons

String Inflation

KKLMMT, BCSQ, Racetrack 8

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Most inflationary trajectories require fine

tuning as do their field theory counterparts

- model comparisons - naturalness

String Inflation

BCSQ, Conlon Quevedo

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Kahler moduli inflation may be an important

exception slow roll relies largely on generic

approximations.

- model comparisons - naturalness

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Although robust against most stringy details,

predictions for CMB can be sensitive to specific

kinds of physics near horizon exit

- model comparisons - naturalness -

robustness

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Although robust against most stringy details,

predictions for CMB can be sensitive to specific

kinds of physics near horizon exit

- model comparisons - naturalness -

robustness

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Although robust against most stringy details,

predictions for CMB can be sensitive to specific

kinds of physics near horizon exit

- model comparisons - naturalness -

robustness

String Inflation

- Why try to embed inflation into string theory?
- Why is it hard?
- What have we learned?

Although robust against most stringy details,

predictions for CMB can be sensitive to specific

kinds of physics near horizon exit

- model comparisons - naturalness -

robustness

Outlook

- Branes continue to provide a useful approach for

naturalness problems. - Dark Energy, Inflation,possibly more.
- We are getting very close to finding inflation in

explicit controlled string calculations - Possible progress on fine-tunings
- New insights on reheating (eg cosmic strings)
- Signals largely robust, except near horizon exit
- Possibly even more novel physics can arise!

fin