accelerating universes and the emerging landscape of string theory

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Accelerating Universes and The Emerging Landscape of String Theory Sandip Trivedi Tata Institute of Fundamental Research, Mumbai, India.

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An Inflationary Model In String TheoryTheory
apart is increasing with time.
New Discovery
"for the discovery of the
accelerating expansion of the
suggests that the rate of expansion
should be slowing down. Not
Accelerating.
be dominating the universe.
It is called Dark Energy.
The acceleration is thought to be
driven by dark energy, but what that
dark energy is remains an enigma -
perhaps the greatest in physics
today.
is Accelerating?
• Cosmic Microwave background
Most of the Universe is made up of
unknown forms of matter and energy:
•67% : Dark Energy – completely
unfamiliar. Must have negative
LHC experiment that is now underway)
•4% Familiar Baryonic Matter.
2011 Nobel Laureates in
is unknown to science. And
everything is possible again.
Cosmic Microwave Background Radiation
•Discovered by Penzias and
any terrestrial experiment!
the Cosmic Microwave Background
sky have been measured, with a
precision:
NASA/WMAP science team
WMAP 7-Year Data
model with about six parameters
including:
Kowalski et. Al. Taken From Particle
Data Book
Dark Energy.
•If this energy is positive it gives rise
to acceleration

pressure.
Pressure
CMB, Green BAO (Kowalski et. Al. (2008)).
•Negative pressure is strange!
cause an implosion.
leads to acceleration!
•But required value 2X gm/cc very
small.
Cosmological Constant problem.
)29(10
(except for gravity).
calculate the answer.
from first principles we need a theory of
gravity consistent with the rules of
quantum mechanics.
Physics at the longest of distance
scales is intimately tied to physics
at the shortest of distance scales!
Another Motivation:
theory could not give rise to
accelerating universes.
Moduli Stabilisation.

Compactifications. (~100)
compact space
Internal
directions
Non-compact
directions
physical parameters, like
degeneracy.
degeneracy is lifted.
Moduli Stabilisation
Mechanics.
E.g.
lifts the classical degenracy.
is due to Supersymmetry.
supersymmetry must be broken
constant.
•But resulting potential is hard to
control.
be calculated one gets typically runaway like
situations.
minimum with a positive value for the
potential.
flat directions.
behavior in the potential.
Accelerating Universe and Moduli
•Turn on Fluxes along the small extra
directions.
(Including Higher Forms, )
on size and shape of compactification.
Internal
directions
Non-compact
directions
Any Value of R1,R2 Allowed: Moduli
R1
R2
sphere
S. Kachru, R. Kallosh, A.Linde,
S.P.Trivedi,
Result:
Constant Can Indeed Arise in String
Theory.
understanding these different
confidence that the construction, and
others like it, do make sense.
The Construction
said.
rigorously ensure deSitter vacua do
exist in string theory.
need a deeper understanding akin to
what we have today for negative
cosmological constant vacua.
Many, Many, Universes
different vacua.
•Preliminary estimate,
(Douglas, Denef,..).
thermal fluctuation in desitter space.
Bousso Polchinkski
Constant
natural units.
Cosmological Constant:
•But values very much bigger than the
one observed would not lead to life.
•Galaxies would not form. (Weinberg
1987).
•In what sense is this an explanation?
•Is it predictive?
the other constants of Nature as well?
•In what sense then is string theory
predictive?
Implications of the Landscape Cont’d
Personal Views:
of last resort.
symmetries, are superior because they are
predictive.
is no other reasonable explanation for the
cosmological constant.
string theory more fully.
time dependent situations
constants, especially small values.
help find vacua of the kind we live in
and allow us to test string theory.
• Cosmologists have discovered that the
universe is accelerating.
in string theory. Their construction is
based on turning on fluxes .
Summary
possibilities described by a landscape.
•And provides an unconventional
explanation for the cosmological
theoretical model characterised by
some value for , k,
and a few other
Personal Views:
many predictions.
thinking in particle physics, based on
symmetries, are quite likely to continue to
be of great relevance.
Six Parameters:
(reionisation depth)
100cr
1cr if W_0 <<1. With 10^(100) vacua this leaves
many possibilities.
•In what sense is String Theory
predictive?
Anthropic Principle?
Personal Views:
goals of string theory.
correct? For this we need to make a
prediction which is borne out.