Problems of Standard Big Bang

1
Problems of Standard Big Bang

• Horizon Problem
• Why is CMB so isotropic if different parts of
  universe were not causally connected at
  recombination?
• Flatness Problem
• Why is the universe so flat now was flatter
  since
• Oc a-2
• OM a-3
• OR a-4
• O? a0
• Remnant Problem
• Why are there no monopoles

2
Inflationary Cosmology

• Universe undergoes a phase of accelerating
  exponential expansion in early times t 10-35 s
• Observable universe is expanded from a small,
  causally connected region
• Curvature flattened by expansion
• Relics diluted away

3
Early Universe as a Lab

• Early universe was hot and dense and behavior of
  matter and radiation under these extreme
  conditions is governed by fundamental
  short-distance (high-energy) physics
• Theories of three forces can take us back to the
  Planck time
• t 10-43 s
• T 10-19 GeV 1032 K
• At earlier times, quantum gravity effects are
  important

4
Issues

• General Relativity is incomplete. Cannot handle
  singularities yet develops these from smooth
  initial conditions. The Big Bang the explosive
  end of black hole evaporating by Hawking
  radiation

• Quantum effects are expected to become important
  in these extreme circumstances, but traditional
  quantization fails when applied to gravity
• This cannot be the whole story

5
String Theory

• String theory is (sort of) what you get if you
  imagine that instead of particles (electrons,
  quarks, etc) the fundamental building blocks in
  the world are strings
• Strings can vibrate!

6

• One string can, by being excited to various
  vibration states account for many particles
• Strings interact by splitting and joining which
  looks like particle interactions
• Can be open or closed

• Of course, electrons look a lot like particles
  and not at all like strings
• That is because strings are small of order 10-33
  cm
• So if we cant see them, why should we care?

7
Whats the Point?

• In a string theory, various particles are not
  just posited with the properties we find in
  nature. All observed particles come from the
  same string! Properties of the particle follow
  from the vibrational state.
• A particularly important one is our friend
• Because quantum string theory is well-behaved,
  strings provide a consistent quantum theory of
  gravity

• The particle this describes has mass zero and
  spin 2 and has not been seen
• It is, however, thought to exist and is the
  carrier of gravitational force the graviton

8
Singularities Tamed

• Near singularities the difference between strings
  and particles becomes meaningful. Strings
  wrapping the singularity are not like any
  particle states.
• Some singular spaces lead to reasonable string
  dynamics

9
Extra Dimensions

• String theories suggest 10 spacetime dimensions.
  We see 4.
• This is a theory of gravity, which means it
  describes the shape of space. So can imagine the
  extra dimensions are too small to be seen or
  measured

10
T-Duality

• Strings can wind around extra dimensions as well
  as move there.
• Winding modes have energy (mass) of order R
• T-duality exchanges winding and momentum modes,
  showing that in string theory a radius of R and
  of 1/R are the same! Relates different types of
  strings
• Size of space indeterminate

• For more general Calabi-Yau shapes T-duality
  (mirror symmetry) relates spaces that are
  topologically different yet lead to identical
  string theories
• Topology of space indeterminate

11
D-branes

• Strings are not the end of the story. As
  relativity predicts black holes, string theory
  predicts black branes
• Excitations of brane can be described by open
  strings ending on the brane
• D-branes are dynamical objects and interact with
  the closed string states
• D-branes wrapping the singularity tame
  singularities that strings cannot.

12
Brane Worlds

• D-branes offer a new way to reconcile the extra
  dimensions If all the matter we measure is made
  of open string excitations of a D-3 brane then we
  live in 4 dimensions, interact with the bulk
  only through gravitation.
• Space as we know it may be only a slice of the
  universe

13
Conclusions

• String theory, while not yet a theory, has given
  us many new ideas about spacetime
• There is reason to believe that when it becomes a
  theory, spacetime will not be an input but an
  emergent feature, its properties at every level
  determined by the dynamics
• There is a lot yet to understand!