String/Brane Cosmology

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 doesn’t string theory decouple from cosmology?

• Why are branes important for cosmology and particle physics?




Science progresses because short- distance physics decouples from long distances.




Science progresses because short distance physics decouples from long distances.

* 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.






* 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.




D branes in string theory are surfaces on which some strings must end, ensuring their low-energy modes are trapped on the brane.

Polchinski




In some cases this is where the Standard Model particles live.

Ibanez et al




Leads to the brane-world scenario, wherein we are all brane-bound.

Rubakov & Shaposhnikov




Identifies hidden assumptions which particle physicists and cosmologists have been making: eg: all interactions don’t see the same number of dimensions.

Branes and Naturalness

• Removal of such assumptions has allowed new insights into low-energy naturalness problems.



* Shows that extra dimensions can be as large as microns;

ADD




* Shows that the string scale could be as small as TeV

Horava & Witten, Lykken, Antoniadis





* Ordinary physics in extra dimensions (eg: warping) can have extraordinary implications for the low-energy 4D theory.

Randall & Sundrum





* 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.

ADKS, KSS





* Shows that the vacuum energy is not as directly tied to the cosmological constant

In 4D the cosmological constant problem arises because a vacuum energy is equivalent to a cosmological constant, and so also to a curved universe.

0 RgT






In higher D solutions exist having large 4D energy, but for which the 4D geometry is absolutely flat!

CG, ABPQ






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 .

BH







With SUSY, quantum corrections are usually order M2/r2 but can be as small as 1/r4

ABPQ

This can be small enough because 1/r can be as small as 10-3 eV (since r ~ m is possible)!!!







So far so good: quantum corrections are usually order M2/r2 but can be as small as 1/r4

BMQ, ,ABB, BC

Very predictive: time-dependent Dark Energy; tests of GR at both micron and astrophysical distances;

implications for the LHC; etc

• 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

Albrecht, CB, Ravndal & Skordis

]exp[)( 2 cbaV

42 1

)](log)log([r

rMcrMbaV

2

22

r

rML pkin

)/exp(0' baMrifV

Potential domination when:

Canonical Variables:

SLED: Observational Consequences









• Astrophysics


log vs log a

Radiation

Matter

Total Scalar










• Astrophysics


Radiation

Matter

Total Scalar

w Parameter:

andw vs log a

~ 0.7

w ~ – 0.9

m ~ 0.25










• Astrophysics


vs log a

03.0










• 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?


• 4D space is not flat for arbitrary brane - bulk couplings.






ABPQ



• Most brane pairs do not produce static solutions.






BQTZ, TBDH



• Most brane pairs do not produce static solutions.

• In some cases these choices appear to be stable against renormalization.






BH


• Initial conditions exist which lead to dynamics which can describe the observed Dark Energy.






ABRS



• Successful initial condition are scarce.






TBDH



• Successful initial condition are scarce.

• Explained by earlier dynamics (eg inflation)?






String Inflation

• Why try to embed inflation into string theory?

• Why is it hard?

• What have we learned?

String Inflation


• Why is it hard?


Inflationary models must be embedded into a fundamental theory in order to explain:

String Inflation


• Why is it hard?



* Why the inflaton potential has its particular finely-tuned shape

(and if anthropically explained, what assigns the probabilities?)

String Inflation


• Why is it hard?





* What explains any special choices for initial conditions

String Inflation


• Why is it hard?






* Why the observed particles get heated once inflation ends.

String Inflation


• Why is it hard?






* Why the observed particles get heated once inflation ends.

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 is it hard?


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 is it hard?



BUT their potentials are usually very difficult to calculate.

String Inflation


• Why is it hard?



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 is it hard?


For Type IIB strings it is now known how to compute the potentials for some of the low-energy string scalars.

GKP

String Inflation


• Why is it hard?


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


• 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

KKLT, KKLMMT

String Inflation


• 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 P4[11169]

‘The Better Racetrack’Douglas & Denef

String Inflation


• 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 is it hard?


The motion of several complex fields must generically be followed through a complicated landscape: many possible trajectories for each vacuum

String Inflation


• Why is it hard?

• What have we learned? The potential can inflate, e.g. for

some choices for the properties of P4[11169] – giving rise to realistic inflationary fluctuations

The ‘Racetrack Eight’

String Inflation

CMB measurements begin to distinguish different inflationary models


• Why is it hard?


Barger et al hep-ph/0302150

- model comparisons

String Inflation

CMB measurements begin to distinguish different inflationary models


• Why is it hard?


WMAP preferred

- model comparisons

String Inflation

Trajectories through string landscape predict same regions as do their low-energy effective theories.


• Why is it hard?


brane-antibrane

racetrack

- model comparisons

String Inflation

The measurements can already distinguish amongst some stringy inflationary models.


• Why is it hard?


KKLMMT*

P4[11169]

WMAP preferred

- model comparisons

KKLMMT, BCSQ, Racetrack 8

String Inflation

Most inflationary trajectories require fine tuning as do their field theory counterparts…


• Why is it hard?


- model comparisons

- naturalness

KKLMMT, BCSQ, Racetrack 8

String Inflation

‘Kahler moduli’ inflation may be an important exception: slow roll relies largely on generic approximations.


• Why is it hard?


- model comparisons

- naturalness

BCSQ, Conlon & Quevedo

nn MbMBAV )/(exp)/(

srM

M 1,

String Inflation

Although robust against most stringy details, predictions for CMB can be sensitive to specific kinds of physics near horizon exit


• Why is it hard?


H-1(t) (t)

Inflation Post-Inflation

Length

Time

p

oscillations 60 e-foldings

10-30 e-foldings

- model comparisons

- naturalness

- robustness

String Inflation

Although robust against most stringy details, predictions for CMB can be sensitive to specific kinds of physics near horizon exit


• Why is it hard?


- 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!

String/Brane Cosmology

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