Cosmology and extragalactic astronomy

1
Cosmology and extragalactic astronomy
Mat Page
Mullard Space Science Lab, UCL
10. Inflation
2

• You are the result of a quantum fluctuation!

3
Galaxies, Planets, You
4
(No Transcript)
5
11. Inflation
Slide 2

• This lecture
• Problems with the standard big bang
• What is inflation?
• How does it work?
• How does it solve the problems?
• Different inflation models

6
Problems with the standard Big Bang
Slide 3

• The horizon problem
• large scale structure
• flatness
• the monopole problem

7
The horizon problem
Slide 4

• Light from the CMB is just reaching us.
• Regions of the CMB in opposite directions are not
  in causal contact
• How come they are at the same temperature?
• In fact, regions of the CMB separated by more
  than 2o are causally disconnected!

8
(No Transcript)
9
The horizon problem
Slide 5
10
Large-scale structure
Slide 6

• How come galaxies exist?
• Big Bang is isotropic and homogeneous, but
• galaxies, clusters, superclusters
• CMB anisotropies
• So anisotropy has existed since decoupling at
  least, but how did it get there?

11
Flatness
Slide 7

• Observationally we have W0 1.02 -0.02 but look
  how W evolves with time in Friedmann models
• Matter dominated W(t)-1 a t2/3
• Radiation dominated W(t)-1 a t
• So for W1.02 now we need W110-5 at
  decoupling, and very much closer to 1 at the
  Planck time.
• This is a fine-tuning problem why should the
  Universe just happen to be flat?

12
The monopole problem
Slide 8

• Fundamental forces were unified in the early
  Universe
• As time progresses, forces decouple
• phase transition in the Universe
• Many theories predict topological defects
• Domain walls
• Strings
• Magnetic monopoles
• Monopoles most common in theories - some
  calculate more monopoles than matter.
• But we havent seen any!

13

• Can solve ALL these problems in one go by having
  a very rapid (exponential) early expansion
  inflation

14
Inflation explains the horizon problem
Slide 9

• Many different inflation models, but all have
  rapid expansion in early Universe

15
Inflation explains the structure problem
Slide 10

• In the early Universe, we expect quantum
  fluctuations both in space-time itself and in the
  density of fields in space
• Rapid inflation expands these fluctuations vastly
  in size, and moves them out of causal contact
  with each other.
• No causal contact, so the fluctuations cant
  thermalise - they are frozen in.
• Suddenly we have large scale anisotropies, and so
  structures can form.

16
Quantum fluctuations and cosmic structure
17
Inflation solves the flatness problem
Slide 11

• Consider a Universe undergoing a rapid period of
  inflation.
• Universe expands, becomes locally flat.
• We dont notice the Earths curvature as we walk
  around.
• Solves the fine tuning problem - start with any
  curvature, and inflation will dilute it to 1.

18
Inflation explains the monopole problem
Slide 12

• At the time of inflation, we predict 1 monopole
  per particle horizon.
• With standard Friedmann cosmology, wed now have
  in excess of 10100 monopoles in the observable
  Universe.
• With inflation, all our observable Universe was
  in causal contact at early times, so we expect
  one monopole in the observable Universe
• Not only does inflation dilute curvature, it
  dilutes monopoles and other space-time defects
  too.

19
What drives inflation?
Slide 13

• We invent a new scalar field called the
  inflaton
• motivated by symmetry breaking in particle
  physics
• inflaton can be identified with one of the
  particle physics symmetry breakings but need not
  be.

20
Inflation how it works
Slide 14

• The Inflaton has a constant and negative pressure
  
• Exponential runaway effect

21
Inflation how it works
Slide 14

• Above Tc we have a normal universe
• At Tc a lower energy state becomes available
• Universe now is in state analagous to a
  supercooled liquid.
• False vacuum acts like pressure
• rapid expansion

22
Inflation how it works
Slide 15

• Some time later, expectation value of inflation
  finds minimum.
• Rapid phase transition, like supercool liquid
  freezing
• Inflation stops
• Lots of free energy
• Energy causes reheating of what is now basically
  empty space
• Lots of energy -gt matter forms
• And there was a universe
• Normal expansion continues

23
Inflation models
Slide 16

• Old inflation
• New inflation
• Chaotic inflation

24
Old inflation
Slide 17

• Alan Guth, 1981
• First order phase transition (bubble nucleation)
• Bubbles too small to be our universe - visible
  universe would not be uniform enough.

25
New inflation
Slide 18

• Linde, Albrecht, Steinhardt 1982
• Second order phase transition (domains, like a
  ferromagnet)
• Need fine tuning to get enough inflation

26
Chaotic inflation
Slide 19

• Linde, 1981
• Inflation happens everywhere
• Different parts of Universe have different f, so
  inflate differently at different times.
• Produce local regions of homogeneous isotropic
  universe, but on a larger (than observable)
  scale, universe is highly curved, inhomogeneous
• Many universes - some can have life
• Thats where we are according to the anthropic
  cosmological principle.

27
Key points
Slide 20

• Standard Big Bang theory has problems with fine
  tuning and causality
• Inflation solves these problems
• causality solved by observable universe having
  grown rapidly from a small region in causal
  contact
• fine tuning problems solved by the diluting
  effect of inflation
• many different models of inflation
• some kind of inflation appears to be required,
  but the exact model not decided on yet.