1. Cosmological ideas from string theory
Brane World
Cosmological ideas from string theory
Cosmology
String (M) theory
The vacuum in particle physics
Branes and brane world
Problems in quantum gravity
Holography

2. Cosmological Questions
What are space and time?
Why are there 3 spatial directions and 1 time?
How did the Universe begin?
How will the Universe end?
Why does it contain the sorts of matter it does?
How does that matter interact and produce galaxies etc?
Very ambitious! – Physics has been very successful though so can begin to address these issues

3. The Big Bang
In General Relativity space and time are dynamic – we believe we live in an expanding Universe
Explosion! - galaxies are moving apart
Flash – universe is full of remnant radiation
Smoke and Ash – correctly predict the abundances of
H, He, Be…

4. We need to know some particle physics!
Singular
Beginning
At very early times the whole Universe and contents squeezed into a tiny volume…
Describe as a hot, dense soup of the fundamental building blocks of nature
Observing the expansion we find it is slowing due to the gravitational attraction of matter
Critical
Ending
It seems to be balanced between expanding forever and eventual collapse…. But then 90% of matter must be new DARK MATTER
We need to know some particle physics!

5. The Particle World Forces Particles Electromagnetism Strong nuclear
Weak nuclear
Gravity
Particles
6 quarks electrons neutrinos

6. Quantum Rules of Motion
Energy comes in lumps
E = h f
Fields can look like particles
The photon is the quantum of the electromagnetic field/ light

7. Quantum Dynamics x p > h t E > h
The quantum in some sense travels by both paths….
There is an uncertainty in the position and momentum of the quantum
x p > h
Heisenberg’s Uncertainty Principle
t E > h
Or equally

8. The Quantum Vacuum E t > h E = mc
The vacuum can borrow energy for short periods
E = mc 2
The borrowed energy can be used to create particles
(You can’t just create an electron because of charge conservation - but can create electron positron pair)
The quantum vacuum is a seething mass of particles appearing and disappearing constantly….

9. How Can You Tell?
The “virtual” particle pairs interfere in electron scattering processes.
The effective charge seen in two electron scattering depends on the separation of the electrons.

10. The Strong Nuclear Force
The strong nuclear force is described by a theory that is similar to electromagnetism… except that the fields carry (colour) charge…..
This difference changes the way in which the vacuum is polarized so that…

11. Confinement The QCD Vacuum
You can never pull hard enough to liberate a quark from a proton…
The QCD Vacuum
Every so often quantum effects create a quark anti-quark pair.
The attractive force is so strong that
binding energy >> mass energy
The vacuum has lower energy if it fills itself with quark anti-quark pairs!

12. The vacuum is really full of quark anti-quark pairs with a density
like that of an atomic nucleus ( grams/cm ) !! 15 3
The Proton Mass
The quark pairs are responsible for the proton’s mass
Interaction energy provides proton mass

13. The Origin of Mass The Higgs Boson
The strong nuclear force cannot explain the mass of the electron though…
Or very heavy quarks such as the top quark
top mass = 175 proton mass
The Higgs Boson
We suspect the vacuum is full of another sort of matter that is responsible – the higgs….
To explain the top mass the higgs vacuum must be 100 times denser than nuclear matter!!

14. Quantum Gravity
If the vacuum is full of all this stuff shouldn’t we be pulled gravitationally by it?
Since it is uniformily distributed there is no net pull (equal space to all sides)
But General Relativity says the energy should uniformily curve space-time… the Universe should be the size of a grapefruit!!
There’s something big we don’t understand about quantum gravity
Gravity is so weak that we’ve never measured it’s effects between individual particles or on distance scales smaller than 1mm!

15. Gravity is different to the other forces – it’s only attractive…
In General Relativity this shows up in that gravitational waves have different polarizations to electromagnetic waves
What fundamental theory can encompass both types of fields?

16. String Theory String theory unifies gravity and other forces
String theory is A quantum theory of gravity.. Is it THE quantum theory of gravity? - entering realm of speculation!

17. Extra Dimensions Compactification
Surprisingly the mathematics of string theory only makes sense in 9 spatial dimensions and 1 time dimension!
A prediction…. But wrong!!
Compactification
We can imagine a space where directions are curled up
We study A string in this space not ALL the strings that make the space…. Why are 6 dimensions compact… by what mechanism? UNKNOWN!

18. Shrinking Dimensions Away
Remember particles are really quanta of fields… only very special field configurations make sense on a compact dimension
Energy = E, 2E, 3E, 4E, 5E,….
As we shrink the circumference away E grows – eventually you can’t make the extra states… it’s gone! 19
Standard string lore – E is proton mass

19. Wrapping Strings Dual Spatial Directions
Closed strings can do something new in a compact space
Wrapping modes have energies
E’, 2E’, 3E’, 4E’, 5E’…
As we shrink the circumference away these states are very low energy
Dual Spatial Directions
If we “confuse” winding modes and kinetic modes we can see a new dimension appear as another disappears!

20. Membranes Only closed strings see the new direction… not open strings…
Electromagnetic particles live on a sub-space or “brane”
“Existence proof” for such a world
Gravitons live in a higher dimension “bulk”

21. Could Our Universe Be A Brane?
The strength of gravity is determined by the number of spatial dimensions (gravitons spread out around mass) S U
F = G M m 2
D=3+1 r
2 R p
But…. we don’t know anything about gravity on length scales below 1mm… R could = 1mm… and we wouldn’t know it!
If so we’ve miscalculated the strength of gravity (G) – it could become strong in our particle accelerators at any new energy!!

22. Matter Loss To The Bulk
Our Universe will slowly lose matter into the bulk…
In particle physics gravity is too weak to ever see these events
In fact the energy loss from the centre of stars is too low to see too!

23. Other Branes
Why should there be only one brane in the higher dimension spacetime?
There could be entire Universes only mm away!
The matter on the other brane will only interact with our world gravitationally – it’s dark matter…. W S E U N
Until we can produce high energy gravitons that are strongly interacting there’s little way to directly probe this idea though

24. Brane Collisions
There’s no reason branes should be static in the extra dimensions…. So they could collide!
This would be catastrophic!
Huge amounts of energy would be dumped into our Universe….
Could that have triggered the currently observed expansion of the Universe??!!

25. Holography
General Relativity predicts there should be objects whose gravitational attraction is enough to stop even light escaping – BLACK HOLES
‘tHooft argued that any information dropped into a black hole must be
lost to our Universe
spread over the surface
If the surface can contain all the information of the contents the real theory of the Universe must be dimensional!

26. Strong Nuclear Force is Holographic
String theory grew out of attempt to describe confined quarks
The resulting string theory has recently been discovered to describe gravity in 4+1 dimensions!
Is the real world holographic??
If there is only as much information in Universe as on surface problem of over density of vacuum material may be solved…??

27. Overview
The large scale structure of Universe was determined by particle interactions just after the Big Bang
Our theories of particles don’t fit with theories of gravity
String Theory is an attempt to construct a sensible theory of both
We are realizing it predicts wild possibilities
Extra dimensions and other Universes
Or are we a hologram in fewer dimensions?
Many exciting possibilities but very hard to experimentally test because gravity is very very weak….

28. M Theory
If you study strings at scales where they look like particles you get a theory of particles… can construct these independently
10 dimensional theories naturally map on to strings
There is an 11d theory that we now realize naturally maps onto membranes
Compactifying gives string theory – fundamentally 11d?