StringBrane Cosmology

1
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
2
Outline

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

3
Strings, Branes and Cosmology

• Why doesnt string theory decouple from
  cosmology?
• Why are branes important for cosmology and
  particle physics?

4
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.
5
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.
6
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.
7
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.
8
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.
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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.
10
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.
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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.
12
Branes and Naturalness

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

13
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.

14
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.

15
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.

16
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.

17
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.
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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!
19
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 .
20
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
21
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
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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
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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
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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
25
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
26
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
27
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?

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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?

29
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?

30
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?

31
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?

32
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?

33
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?

34
String Inflation

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

35
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?

36
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?

37
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?

38
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?

39
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.
40
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
41
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.
42
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.
43
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.
44
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.
45
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
46
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
47
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
48
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.
49
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
50
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
51
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
52
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
53
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
54
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
55
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
56
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
57
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
58
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
59
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
60
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
61
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!

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fin