Cosmology and extragalactic astronomy

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

2. Slide 2 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

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

4. Slide 4 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 2o are causally disconnected!

5. Slide 5 The horizon problem

6. Slide 6 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?

7. Slide 7 Flatness • 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 W=1.02 now we need W=1+10-5 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?

8. Slide 8 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!

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

10. Slide 10 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.

11. Slide 11 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.

12. Slide 12 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 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.

13. Slide 13 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.

14. Slide 14 Inflation: how it works • Above Tc we have a ‘normal’ universe • At Tc a lower energy state becomes available • Universe is in state analagous to a supercooled liquid. • False vacuum acts like pressure • rapid expansion

15. Slide 15 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

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

17. Slide 17 ‘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.

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

19. Slide 19 Chaotic inflation • 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 • That’s where we are according to the anthropic cosmological principle.

20. Slide 20 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.