# Presentation: 116043

## We don’t understand most of the Universe

- Stars and galaxies are only ~0.5%
- Neutrinos are ~0.3–10%
- Rest of ordinary matter (electrons and protons) are ~5%
- Dark Matter ~30%
- Dark Energy ~65%

## We have ideas about dark matter

- Could be a WIMP (Weakly interacting massive particle)
- Or it could be a MACHO (Massive Astronomical Object)
- If it’s a particle we can produce it in the LHC and/or see it coming from the cosmos to experiments deep underground

## NO Understanding of Dark Energy

- Dark energy
- Hypothetical form of energy
- NO GOOD THEORY
- Effects are similar to force acting in opposition to gravity
- Has strong negative pressure
- Universe appears to be expanding at accelerating rate

## How do we know dark energy is there?

- Supernova provide the most direct evidence
- Supernova result from exploding white dwarf stars
- All supernova have the same apparent brightness
- Can obtain distance with 1/r2 law, and then you know how long ago the supernova exploded
- Measure wavelength of light emitted from object and distance to object to get picture of the universe

## Supernova are Standard Candles

## Measure wavelength of light from supernova

-  When light from a distant galaxy travels across space, space itself, like this balloon is expanding.  The wavelength of the light increases and its spectrum shifts toward red

- http://snap.lbl.gov

-  Redshift of astronomical objects is measured by comparing spectral lines of elements in them with spectral lines of the same elements in the lab.  The higher the redshift, the more distant the object that emitted the light.

## The Universe is Expanding

- The further away a galaxy is, the more rapidly it is moving away from us

## The universe is expanding

- Expansion dilutes the number of particles
- Early universe was hot and dense. It began expanding 14 billion years ago.
- Radio telescopes can detect the leftover radiation:  Cosmic Microwave background

## WMAP Experiment

- Measures cosmic microwave background (CMB)

## How to understand expansion?

- If universe is flat and expanding ever faster, some energy must be offsetting the pull of gravity
- One explanation is “cosmological constant”
- Cosmological constant is just energy contained in every cubic centimeter of space
- Fudge factor
- Why is it so small? All estimates are 50 orders of magnitude large than observations
- “Einstein’s Greatest Blunder”

## Galaxy Clusters

- Clusters of galaxies are most massive objects in the universe
- They contain large quantities of hot gas which emit X-rays
- We can infer cluster masses from temperature of X-rays and infer the total amount of dark matter

- MATTER

- DARK ENERGY

- A complete change of pace

## Strings, etc

- General relativity fails at very short distances
- Could string theory be the theory @ short distances?
- String theory contains general relativity, quantum mechanics, particle physics
- Promising, but no predictions yet

## What’s the Problem?

- General relativity is important for massive extended objects like stars and galaxies
- Gravity has small effects on atoms, and can be neglected
- Quantum mechanics crucial for studying atoms
- Remember Newton’s gravitational law
- At very small scales << atomic scales, gravity is enormous
- At these scales, we need both quantum mechanics and general relativity
- Relevant scale is Planck Scale, rp10-33 cm

- At these very small scales can’t use quantum mechanics
- Anything energetic enough to probe 10-33 cm would be sucked into  a black hole
- Gravity should be communicated via a particle like the other forces
- This particle is the Graviton
- Theory with graviton is mathematically inconsistent at small distances
- Need to understand these small distances to understand the big bang

## Can String Theory Solve These Problems?

- The basic objects are strings
- These are vibrating, one-dimensional objects
- Everything (including electrons & quarks) are string oscillations
- Character of vibration determines particle properties

## Different Vibrational Modes of String are different particles

- Predicts infinite number of particles

## Strings

- Oscillations determine mass/spin/charge of particles
- Light particles from strings with fewest oscillations
- Heavy particles from higher vibrational modes

## Two Types of Strings

- Open (has end-points)
- Closed
- Energy excites modes of string
- When string has enough energy it corresponds to different particle types
- Resonant modes oscillate an integer number of times along a string length
- Can’t have waves which don’t complete an integer number of oscillations

## String Basics

- Elementary particles are not points.  They are one-dimensional loops.
- The characteristic length and mass can be constructed from the fundamental constants observed in nature: *h, G**N* and *c*.
- Planck length     rp = (h GN/c3)1/2   =  1.6 x 10-33 cm Planck mass     Mp = (h c/GN)1/2   =   1019 GeV/c2
- At energies << Mp c2, strings are well approximated by points, which is why quantum field theory is so successful.

## String Theory hopes to reconcile Quantum Mechanics & Gravity

- Superstring
- New kind of string
- Strings are very tiny (Planck scale)
- Contains supersymmetry
- Predicts graviton with right properties
- Might be able to work at all energies
- Only makes sense in 10 dimensions
- Only see the difference between string theory and particle physics @ Planck length
- New particles predicted by string theory will be too heavy to make at colliders

- A weird and wacky universe!