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?

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.

Strings, Branes and Cosmology • Why doesn’t 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 doesn’t 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 doesn’t 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 doesn’t 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 doesn’t 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 doesn’t 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 doesn’t 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 don’t 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

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 SLED: Observational Consequences Albrecht, CB, Ravndal & Skordis Potential domination when: Canonical Variables:

Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics 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;… log rvs log a

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 SLED: Observational Consequences Albrecht, CB, Ravndal & Skordis • L ~ 0.7 • m ~ 0.25 • andw vs log a Radiation Matter Total Scalar w Parameter: w ~ – 0.9

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 SLED: Observational Consequences Albrecht, CB, Ravndal & Skordis avs log a

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 SLED: Observational Consequences Albrecht, CB, Ravndal & Skordis log rvs log a

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

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 ABPQ

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 BQTZ, TBDH

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 BH

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 ABRS

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 TBDH

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? SLED: Present Status

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 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 P4[11169] ‘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 P4[11169] – giving rise to realistic inflationary fluctuations

String/Brane Cosmology