StringBrane Cosmology

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String/Brane Cosmology

• Can Microscopic Physics and Cosmology Inform One
  Another?
• C.P. Burgess

with J.Blanco-Pillado, J.Cline, C.Escoda,
M.Gomez-Reino, R.Kallosh, A.Linde and F.Quevedo
(hep-th/0603129)
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Outline

• Motivation
• What can string theory and cosmology hope to
  teach one another?
• Branes and naturalness
• Rephrasing the cosmological constant problem
• String inflation
• Inflating the better racetrack
• Outlook

3
Outline

• Motivation
• What can string theory and cosmology hope to
  teach one another?
• Branes and naturalness
• Rephrasing the cosmological constant problem
• String inflation
• Inflating the better racetrack
• Outlook

4
Outline

• Motivation
• What can string theory and cosmology hope to
  teach one another?
• Branes and naturalness
• Rephrasing the cosmological constant problem
• String inflation
• Inflating the better racetrack
• Outlook

5
Outline

• Motivation
• What can string theory and cosmology hope to
  teach one another?
• Branes and naturalness
• Rephrasing the cosmological constant problem
• String inflation
• Inflating the better racetrack
• Outlook

6
Strings, Branes and Cosmology

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

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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.
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Strings, Branes and Cosmology

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

Inflationary fluctuations arise at very
high energies MI 10-3 Mp
Science progresses because short distance
physics decouples from long distances.
9
Strings, Branes and Cosmology

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

Inflationary fluctuations 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.
10
Strings, Branes and Cosmology

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

Inflationary fluctuations 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. Infrared modifications to
gravity (MOND, Bekenstein, DGP, etc) strongly
constrained by UV consistency issues.
Science progresses because short distance
physics decouples from long distances.
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Strings, Branes and Cosmology

• 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

• 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.
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Branes and Naturalness

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

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Branes and Naturalness
Shows that extra dimensions can be as large
as microns

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

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Branes and Naturalness
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.

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 as was thought

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

18
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 as was thought Allows
room for a restricted form of nonlocality in the
effective theories arising in cosmology and
particle physics.

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

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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 a vacuum energy is equivalent to a
cosmological constant, and so also to a curved
universe.
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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 a vacuum energy is equivalent to a
cosmological constant, and so also to a curved
universe. In higher D solutions exist having
large 4D energy but for which the 4D geometry is
flat.
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String Inflation

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

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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?

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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?

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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?

25
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?

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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.
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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
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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.
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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.
30
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.
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String Inflation

• 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. Giddings, Kachru
Polchinski
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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
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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. Plausibly estimated
KKLT models
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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. Calculable for P411169
The Better Racetrack Douglas Denef
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String Inflation

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

This potential can inflate, for some choices
for the properties of P411169 giving rise to
realistic inflationary fluctuations!
Closest yet to inflation from an explicit string
vacuum! Blanco-Pillado et.al.
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The Bottom Line

• Branes continue to be useful in identifying
  hidden assumptions in naturalness problems.
• Dark Energy, Inflation,possibly more.
• We are getting very close to finding inflation in
  explicit string calculations
• Seem not to solve fine-tunings
• Provide a context for sharpening anthropism
• New insights on reheating (eg cosmic strings)
• Signals largely robust, except near horizon exit

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fin