Post on 28-Mar-2015
The Nuts and Bolts of Inflation
Richard Barrett
Dark Energy Seminar Glasgow 29/11/2003
Inflation
• What is inflation? What is dark energy?
• Why was inflation ‘invented’?
• Some basic gravity physics
• The physics of inflation
• Sources of dark energy
• Branes and inflation
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Inflation is
• Any time when expansion of universe accelerates
• Originally meant inflation in the very early universe but now applies also to current inflationary epoch
• Both caused by same thing (scalar fields)
• The scalar field causing inflation now is called dark energy (because no-one’s seen it)
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Why was inflation ‘invented’?
• The horizon problem
• The flatness problem
Inflation invented by Alan Guth in 1981 to solve some serious problems with ‘standard’ cosmology:
Now known to be important at the present epoch
Happens in the very early universe
Dark Energy Seminar Glasgow 29/11/2003
Inflation
How do we explain the isotropy of the CMBR, when opposite sides of the sky were ‘causally disconnected’ when the CMBR photons were emitted?
The horizon problem
From Guth (1997)
CMBR
Big Bang
time
space
Our world line
Now
A B
Our past light cone
Dark Energy Seminar Glasgow 29/11/2003
Inflation
The flatness problem
In ‘standard’ cosmology:
• =1 means universe is flat• If 1, moves quickly away from 1 after big bang• Today universe is close to flat
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Present day ‘closeness’ of matter density to the critical density appears to require an incredible degree of ‘fine tuning’ in the very early universe.
This is the FLATNESS PROBLEM
Solution (first proposed by Alan Guth in 1981) is…
INFLATIONINFLATION
…a period of accelerated expansion in the very early universe.
Small, causallyconnected region
Limit of observable Universe today
INFLATION
Inflationary solution to the Horizon Problem
From Guth (1997)
Inflationary solution to the Flatness Problem
From Guth (1997)
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Basic gravity physics: matter and gravity
• No ‘force field’ for gravity: matter falls freely through (curved) spacetime
• BUT: matter generates the curvature
• ‘Normal’ matter always makes curvature that causes two objects to move towards each other
gravity is always attractive
Dark Energy Seminar Glasgow 29/11/2003
Inflation
More mass (density) = more spacetime curvature:
Black holeThe sun Neutron starFlat space
Dark Energy Seminar Glasgow 29/11/2003
Inflation
More curvature means stronger attraction:
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Basic gravity physics: the EP
• All matter moves in the same way under gravity (because gravity is just the shape of spacetime)
• So, if some matter generates gravity, all other matter feels it
The Equivalence Principle (EP) says
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Basic gravity physics: pressure
• ‘Normal’ cosmic matter is a gas with pressure (just like air pressure on earth): the gas pushes on itself
• Only pressure gradients give a net force on gas
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Gas elements
No net force Net force =
Forces
Pressure
Position Position
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Basic gravity physics: pressure
• In a homogeneous universe pressure is the same everywhere
pressure doesn’t push matter at all
Can’t make matter expand using pressure
Dark Energy Seminar Glasgow 29/11/2003
Inflation
• Pressure = Energy
• Energy = Mass (E = mc2 )
• Mass generates gravity that’s attractive
• So: pressure makes matter attract itself
Basic gravity physics: pressure
Does pressure have any effect?
High pressure slows the expansion of the universe
Dark Energy Seminar Glasgow 29/11/2003
Inflation
• Gas particles move (they have momentum)• Momentum transferred to walls gives force• BUT: Moving particles have kinetic energy too (E = 1/2 mv2)• Also works for photons, by special relativity (E = cp)
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Inflating the universe
• What if we could make matter (‘gas’) with negative pressure?
• Would generate gravity that’s repulsive
• Universe would expand even faster
INFLATION
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Inflating the universe
• Not possible with ‘normal’ matter
• Need something a bit exotic
A SCALAR FIELD
(Physically scalar field isn’t really like a gas, it only makes curvature in the same way as a gas, but like a gas with negative pressure would)
Dark Energy Seminar Glasgow 29/11/2003
Inflation
The physics of inflation
• ‘Field’ means something that is distributed smoothly through space (like electric field), rather than made of particles
• ‘Scalar’ just means the value of the field at any point is just a number (in contrast, the electric field is like an arrow at each point)
• Call this value . can vary from place to place and from time to time: = (x,t)
Dark Energy Seminar Glasgow 29/11/2003
Inflation
The physics of inflation
How does a scalar field generate curvature (gravity)?
• All matter generates gravity through its stress-energy tensor Tab
• Einstein’s equations are Gab = 8Tab (CURVATURE = MATTER DENSITY)
• Tab has the same form for gases and scalar fields
• Scalar fields are only like gases in terms of their gravity
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Scalar field: Tab = a b - [a a/2 + V()]gab
Normal gas: Tab = (+p)uaub + pgab
Energy part Pressure part
• The important bit is V(), the scalar potential. Its value at any point just depends on
• It does exactly what pressure p does for gases
• For gases p must be positive, BUT: V() can have any value at all
• We get negative pressures with the right V()
Dark Energy Seminar Glasgow 29/11/2003
Inflation
The physics of inflation
To get inflation we need V() to be large.
How can this happen?
• Spontaneous Symmetry Breaking (in the very early universe)
• Scalar fields (dilatons) predicted by string theory can have inflationary potentials
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Spontaneous Symmetry Breaking• At high energies (early universe) laws of physics have
high symmetry
• At lower energies the preferred state has less symmetry
• Some scalar field that previously was at the minimum of V() is now not at the minimum
• The scalar field has vacuum energy, V() > 0
moves to the new minimum and symmetry is broken
• This is a phase transition
• Inflation occurs
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Spontaneous Symmetry Breaking
Original inflation Slow-roll inflation
• V() has a minimum at = 0 before SSB
• V() has a minimum at > 0 after SSB
quantum tunnels from 0 to the new minimum
Dark Energy Seminar Glasgow 29/11/2003
Inflation
The cosmological constant
• Original explanation of inflation was in term of the cosmological constant
• This is just like a scalar field = that is constant everywhere at all times
• Gives pressure p = -
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Sources of dark energy
• Observational evidence (SNIa, CMBR, SDSS,) provides a consistent picture: the expansion is accelerating
• We must have a scalar field in action
• This is the mysterious dark energy
• Where does it come from?
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Sources of dark energy
• Spontaneous symmetry breaking phase transition (like in the early universe)?
• String theory: all predict a scalar field (the dilaton) in the ‘low-energy limit’ that could act as an inflaton
• Branes: string theory also suggests that our universe is a brane in a higher dimensional space, and that a scalar field that could cause inflation
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Branes and inflation
• String theories predict universe actually has 4 space dimensions
• Our universe is a 3D brane moving in this 4D ‘bulk’ (a 3D sheet floating in 4D space)
• Gravity propagates in the bulk (4D), other forces only on the brane
• The inflaton scalar field comes from the part of gravity propagating through the bulk
Dark Energy Seminar Glasgow 29/11/2003
Inflation
A brane is ‘soliton’ (a stable solution of the string theory equationswhere matter and the nongravitational fields are nonzero only in a very thin layer)
All the physics that we are familiar with happens on this brane
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Branes and inflation
In one string theory the universe is a 4D bulk with a 3D brane at each end:
• One brane is the real universe, the other a ‘mirror world’
• We are aware of the mirror world only through its gravity
• This gravity looks like a scalar field to us• The two branes can get closer together (and
even cross over)• This gives rise to the cyclic model of Steinhardt
and Turok
Dark Energy Seminar Glasgow 29/11/2003
Inflation
Branes and inflation
Brane cosmology simulation