Comparing IR DBI Brane Inflation to Observations
1 / 46

Comparing IR DBI Brane Inflation to Observations - PowerPoint PPT Presentation

  • Uploaded on
  • Presentation posted in: General

Comparing IR DBI Brane Inflation to Observations. Xingang Chen. 陈新刚. CTP, MIT. hep-th/0408084; hep-th/0501184; astro-ph/0507053; 0710.1812, with Rachel Bean, Hiranya Peiris, Jiajun Xu. Motivation. Large number of ongoing and forthcoming experiments:

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Download Presentation

Comparing IR DBI Brane Inflation to Observations

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript

Comparing IR DBI Brane Inflation to Observations

Xingang Chen



hep-th/0408084; hep-th/0501184; astro-ph/0507053;

0710.1812, with Rachel Bean, Hiranya Peiris, Jiajun Xu.


  • Large number of ongoing and forthcoming experiments:

  • WMAP, SDSS, SNLS, ACBAR, Planck, ACT, Spider, ...

  • Specifying inflation model and probing underlying

  • fundamental theory such as string theory

  • Signatures beyond the vanilla LCDM model:

  • Running of spectral index, Large non-Gaussianities,

  • Tensor modes, Cosmic strings, …


  • Scan parameter space with minimum requirement:

  • Enough inflationary e-folds.

  • Look for observational signatures in all parameter space

  • and compare with data.

  • Probing string theory through dynamics of our own vacuum

Observational signatures Specific stringy dynamics


  • Properties of brane inflation: Phase diagrams

  • Analytical and numerical properties of IR DBI

  • Comparison with data

Brane Inflation in

Warped Compactification

  • Brane inflation(Dvali, Tye, 98; )

  • Brane position as inflaton;

  • Brane annihilation or collision as ending.



(Gidding, Kachru, Polchinski, 01;

Klebanov, Strassler, 00; Verlinde, 99;

Randall, Sundrum, 99)

  • Warped compactification

  • 6 dimensional bulk

  • Warped space generated by

  • point-like (6d) sources


Phase diagram: UV models



(KKLMMT, 03; Silverstein, Tong, Alishahiha,03,04; )

  • Potential

  • Warped space



Slow-roll inflation:

DBI inflation:

(Silverstein, Tong, 03)




: multiplicative factor

from orbifolding

: Length scale of A-throat;

: Length scale of bulk

Geometric Conditions

(Burgess,,01; X.C,05; X.C.,Sarangi,Tye,Xu,06; Baumann,McAllister,07)

  • Planck mass: integration over compact space

  • Throats glued to the bulk

  • Maximum separation between branes

  • Clean separation b.t. Slow-roll and DBI:

  • Brane inflation is small field:




  • Slow-roll region: KKLMMT model, 03

Shape of the potential may be adjusted to fit the spectral index;

In the absence of sharp feature,

Non-Gaussianity and running spectral index are unobservable;

Tensor mode is too small to be observed.

(Berg, Haack, Kors, 04;

Baumann et al, 06;


Krause, Pajer, 07; …)

(Bean, Shandera, Tye, Xu, 07)

  • DBI region: STA model

(Silverstein, Tong, Alishahiha, 03,04)

Large non-Gaussianity:

Tensor mode:

But inconsistent within GKP-type warped compactification

--- no UV DBI inflation due to probe brane backreactions

(Bean, X.C., Peiris, Xu, 07)

  • Antibrane tension cannot drive inflation

So need

  • Excessive probe brane backreaction



Note: No comparison with data has been made.


Phase diagram: IR models

(X.C., 04,05; Bean, X.C., Peiris, Xu, 07)

  • Potential


  • Warped space

  • Multi-throat brane inflation(X.C. 04)

  • Antibrane-flux annihilation (Kachru, Pearson, Verlinde, 01)

  • Generate branes as candidate inflatons

  • Exit B-throat, roll through bulk, settle down in another throat

  • Enough warping: DBI inflation; Flat potential: slow-roll inflation.


Slow-roll inflation:

IR DBI inflation:

(X.C. 04, 05)

  • For ,

  • For ,







Geometric conditions are automatically satisfied:

  • UV DBI

  • Antibrane tension cannot drive inflation,

    since it is warped down by the same A-throat warp factor.

An extra, steep, potential is needed to raise the inflationary energy:

with a large m :

  • IR DBI

  • Speed-limit and antibrane tension are independent of each other:

    Speed-limit: B-throat; Inflationary energy: A-throat.

Flexible shape of brane moduli potential:

: over ten orders of magnitude.

Main Difference Between UV and IR DBI Model

B-throat warp factor is smaller than

  • Non-trivial condition:

    Various back-reactions that chop off the IR end of throat

  • Probe brane back-reaction;

(Silverstein,Tong,03; X.C.,04)

Easy to satisfy in IR DBI model.

  • Back-reaction from expanding background.

(X.C.,05; X.C.,Tye,06)

Condition for IR DBI inflation:

  • Flux induced warp factor is exponentially small:


Very easy to satisfy the condition.

  • From the point of view of closed string creation


Closed string density

Source of the bkgd (N branes)

  • From the point of view of open string fluctuations

(X.C., Tye, 06)

Transverse scalar fluctuations on the source branes:

Throat is cut off at

Maximum number of DBI e-folds:

Back-reaction from Expanding Background


  • Properties of brane inflation: Phase diagrams

  • Analytical and numerical properties of IR DBI

  • Comparison with data

Two attractor solutions:

  • IR DBI inflation:

  • Non-relativistic roll, typically fast roll:

Brane Dynamics

(X.C.04,05; Bean,X.C.,Peiris,Xu,07)

  • : Field theory applies;

  • 2) : Open string creation

  • (Stringy quantum fluctuations);

  • 3) : Closed string creation starts;

  • 4) : Closed strings smooth out background

  • (de Sitter back-reaction cuts off the throat).





Density perturbations:

1) Field theory regime

2) Hubble-expansion-induced stringy phase

  • Stringy phase transition:

  • Hubble scale < string scale:

  • Fluctuation speed < speed of light:

Phase transition at:


Density Perturbations

(X.C. 04, 05)

  • Field theory regime

  • Density perturbations:

  • Spectrum index:

Field theory regime

Stringy regime


Hubble energy

Fluctuation speed

Relativistic (superluminal if naïve)


World volume


Scalars + strings (branes)

Estimate the Transition Behavior

(Bean, X.C., Peiris, Xu, 07)

  • Model: Brane transverse fluctuations:

  • Random-walk within the horizon, speed given by H;

  • Frozen outside of the horizon.

We generalize the behavior of brane transverse fluctuations


  • Spectral index

  • Regional large running

For example,


Results (in IR DBI region):

  • Power spectrum

  • Non-Gaussianities in general single field inflation

  • are characterized by 5 parameters:

(X.C., Huang, Kachru, Shiu, 06)

c.f. slow-roll inflation, 2 parameters:

(Maldacena, 02; Seery, Lidsey, 05)

  • Leading Non-Gaussianities:

Large non-Gaussianity

In the absence of sharp features (X.C., Easther, Lim, 06),

running is weak, shape has two categories:

Equilateral shape (DBI inflation)

Local shape (Slow-roll inflation)

Shape: dependence on the shape of momenta triangle

(Babich, Creminelli, Zaldarriaga, 04)

Running: dependence on the size of momenta triangle

(X.C. 05)

  • DBI inflation:


  • UV DBI inflation (STA model)

  • IR DBI inflation

(X.C. 05)

  • Different requirements on microscopic parameters.

Geometric conditions have no effect on IR DBI.

  • In IR DBI, the large non-G can be small enough to satisfy current bound.

Negative running:

Non-G tends to be the smallest in the entire DBI inflation trajectory.

is tiny in IR DBI inflation

Small Tensor Mode

  • Tensor to scalar ratio:

Lyth Bound:

(Lyth,96; Baumann,Mcallister,06; Lidsey,Huston,07)

(Bean, X.C., Peiris, Xu, 07)


  • Properties of brane inflation: Phase diagrams

  • Analytical and numerical properties of IR DBI

  • Comparison with data

Microscopic Parameters

  • Shape of inflaton brane moduli potential:

  • Charge of the B-throat:

  • Number of inflaton branes:

  • Fundamental string scale:

  • A-throat warp factor and number of antibranes:

  • Scale dependence of power spectrum:

Spectrum index and its running

  • Non-Gaussianity bound:

  • Several consistency conditions, for example:

DBI e-folds and scale of the transient large running of

  • Scale – e-fold relation:

  • Geometric constraint:

  • Number of inflaton branes


  • Amplitude of power spectrum:

Goal: Compare to data directly from microscopic parameters,

using Bayes’ theorem:

: data.

: parameters;

Possible obstacles: Nonlinear and non-transparent relation

between microscopic parameters and observables

Non-Gaussian posterior distributions, curved likelihood surface, etc.

Difficult to search the likelihood surface efficiently

Solution: Reparameterization:

Implementing Markov Chain Monte Carlo


Full expressions:

have to be solved numerically;

However, approximate expression for observational window:

can be obtained.

Effective parameters:

General Procedures


1) Extract isolated expression for a small window

in terms of smaller number of parameters

2) Run a trial MCMC with the effective parameters ,

to ensure that these parameters have simple likelihood surface.

3) Express (approximately) in terms of microscopic parameters ,

which provides guidance to the reparameterization .


Using the efold – scale relation:

We approximate:

The reparameterization:

4) Run the full MCMC with .

Analytical approximation dropped, observables calculated numerically.

5) Transform the likelihood surface of to the space of the original

parameters .

Re-weighted to impose any desired priors on .

These parameters will have simple likelihood surface.

The results

Data cannot distinguish


but is able to give interesting


  • Shape of moduli potential:

Data picks out O(1) value from 10 orders of magnitude that allows IR DBI.

  • Fundamental string scale:

Intermediate string scale, intermediate large volume compactification

  • B-throat charge:

  • Number of inflaton branes:

Flux number , small number of inflatons is ruled out.

  • A-throat minimum warp factor:

A-throat tends to be short; tunneling reheating is possible.

Summary of MCMC Results

Microscopic parameters:

  • The stringy phase transition:

The stringy phase transition happens at the largest scales in the sky;

but its impact extends to shorter scales, generating transient large

running of .

  • Inflation scale:

This gives a tiny tensor to scalar ratio:

  • Cosmic string tension:

is tension of D-string left over in A-throat after brane annihilation;

F-string tension:

Secondary derived parameters:

  • Inflationary phases: the last e-folds come from

  • non-relativistic fast-roll inflation.

  • Large, but regional, running of spectral index:

In future experiments, Planck is expected to reach .

(Planck bluebook)

Observational predictions:

Better theoretical understanding and experimental measurement

may lead to finer structures.

Reconstructed Power Spectrum

Dashed lines: 1) Single-field slow-roll; 2) Empirical power law ansatz.

(Peiris, Easther, 06)

In future experiments: on CMB scales, Planck can achieve ;

on LSS scales, high-z galaxy surveys can reach similar or better resolutions.

(Smith, Zaldarriaga, 06; Sefusatti, Komatsu, 07)

  • Large non-Gaussianities:

However, large running of can be achieved by engineering the potential:

adding mild features, such as periodic ripples.

(Bean, X.C., Peiris, Xu, 07)

  • Helps to sustain the inflation

  • Generating large running of spectral index

varies between

To distinguish, use the non-Gaussianity:

Distinguishing IR DBI and other models

  • Slow-roll potential with mild features

Usual slow-roll gives negligible running of spectral index:

  • Non-Bunch-Davies vaccum

(Martin, Brandenberger, 00; ……)

Generalize slow-roll results

to case with arbitrary speed of sound

(Danielsson, 02; Polarski, Starobinsky, 95)

(Bean, X.C., Peiris, Xu, 07)

Running spectral index:

  • Slow-roll with non-BD: have much smaller , or have frequent oscillations

  • IR DBI with non-BD: frequent oscillations

  • Main difference:

  • Non-BD case: new physics energy scale M >> Hubble parameter H,

    so field theory apply

  • Phase transition in IR DBI: new physics (stringy) scale is

    comparable or larger than Hubble parameter H


  • Multi-throat brane inflation and IR DBI:

  • Phase diagram of brane inflation;

  • Comparision with UV models.

  • Warp compactification:

  • Speed-limit: DBI inflation;

  • Warped string scale: stringy phase transition.

  • Comparing to data:

  • Current data gives interesting constraints to microscopic parameters.

  • Observational predictions:

  • Regional large running of spectral index; Large non-Gaussianities.

String theory making testable predictions with distinctive signatures;

Probing string theory using cosmological observations.

  • Login