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

You are the result of a quantum fluctuation!

Galaxies, Planets, You

11. Inflation This lecture: Problems with the standard big bang What is inflation? How does it work? How does it solve the problems? Different inflation models Slide 2

Problems with the standard Big Bang The horizon problem large scale structure flatness the monopole problem Slide 3

The horizon problem 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 2 o are causally disconnected! Slide 4

The horizon problem Slide 5

Large-scale structure 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? Slide 6

Flatness Observationally we have  0 = but look how  evolves with time in Friedmann models: –Matter dominated: |  (t)-1|  t 2/3 –Radiation dominated: |  (t)-1|  t So for  =1.02 now we need  = 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? Slide 7

The monopole problem 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 haven’t seen any! Slide 8

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

Inflation explains the horizon problem Many different inflation models, but all have rapid expansion in early Universe Slide 9

Inflation explains the structure problem 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 can’t thermalise - they are ‘frozen in’. Suddenly we have large scale anisotropies, and so structures can form. Slide 10

Quantum fluctuations and cosmic structure

Inflation solves the flatness problem Consider a Universe undergoing a rapid period of inflation. –Universe expands, becomes locally flat. –We don’t notice the Earth’s curvature as we walk around. –Solves the fine tuning problem - start with any curvature, and inflation will dilute it to 1. Slide 11

Inflation explains the monopole problem At the time of inflation, we predict ~ 1 monopole per particle horizon. With standard Friedmann cosmology, we’d now have in excess of 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. Slide 12

What drives inflation? 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. Slide 13

Inflation: how it works The Inflaton has a constant and negative pressure Exponential runaway effect Slide 14

Inflation: how it works 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 Slide 14

Inflation: how it works 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 -> matter forms And there was a universe… Normal expansion continues Slide 15

Inflation models ‘Old’ inflation ‘New’ inflation Chaotic inflation Slide 16

‘Old’ inflation Alan Guth, 1981 First order phase transition (bubble nucleation) Bubbles too small to be our universe - visible universe would not be uniform enough. Slide 17

‘New’ inflation Linde, Albrecht, Steinhardt 1982 Second order phase transition (domains, like a ferromagnet) Need fine tuning to get enough inflation Slide 18

Chaotic inflation Linde, 1981 Inflation happens everywhere Different parts of Universe have different  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 That’s where we are according to the anthropic cosmological principle. Slide 19

Key points: 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. Slide 20