Physics 133: Extragalactic Astronomy and Cosmology. Week 8

Transcription

1 Physics 133: Extragalactic Astronomy and Cosmology Week 8

2 Outline for Week 8 Primordial Nucleosynthesis Successes of the standard Big Bang model Olbers paradox/age of the Universe Hubble s law CMB Chemical/Physical composition Problems of the standard Big Bang model It s too flat It s too isotropic Low density of domain walls (relics from symmetry breaking) The currently favored solution is called inflation Qualitative description Physics of inflation Predictions of inflation

3 Statistical Equilibrium Neutrons decay on a timescale of 890 s. At very early times reaction is in equilibrium (<<1s) [enough e- e+] Relative abundances given by kinetic equilibrium: N(n)/N(p)=(m n /m p ) 3/2 g n/ g p exp(-q/kt) High T favors neutrons Expansion/Cooling removes the neutrons Mass difference è Q = 1.29 MeV What temperature is this? Was the universe ever this hot?

4 but then Cross section for weak interactions decays very rapidly with temperature Eventually interaction rate drops below expansion rate: FREEZE OUT! Freeze at t=1 sec when T=9e9 K and n/p ~ 2/10 What s the maximum He mass fraction possible? Why is this a maximum?

5 After neutron freeze out Most remaining neutrons get captured by p forming deuterium [Saha s equation] Maximum number of non-h nuclei is set by abundance of n at freeze-out and their decay. Most of the non-h nuclei end up as He because it s the most stable nucleus

6 Primordial He Abundance Time delay until start of nucleosynthesis when T=8e8K and n D =n n. Additional neutron decays reduce n/p ratio from 0.2 to Maximum number of non-h nuclei is set by abundance of n at freeze-out and their decay. Most of the non-h nuclei end up as He because it s the most stable nucleus

7 Basics of Nuclear fusion/fission

8 Final outcome Nuclear reactions last as long as the expanding universe supports them (rate > expansion rate) By ~5min everything is over!

9 Baryon-to-photon ratio, h, is the critical parameter The yield is dominated by h (remember recombination?) High h starts BBN early and is more efficient at producing He So there are fewer leftovers.. Li is more complicated since there are competing channels

10 Composition of the Universe If we measure the abundance of light elements, we infer h. We measure D/H for example and obtain h = e-10 We know T(CMB), so we obtain n(baryons)!

11 How do we measure D/H? Stars burn D. The gas in the interstellar medium has been cycled through stars. Where can we find nearly primordial gas? Tytler 2001

12 Baryon-antibaryon asymmetry There are many more photons than baryons There are many more baryons than antibaryons What happened? Perhaps most of the baryons and antibaryons annihilated. What s left is believed to reflect a small asymmetry between quarks and antiquarks in the early universe.

13 Olbers paradox

14 Hubble s Law

15 Age of the Universe

16 Cosmic Microwave Background

17 Flatness Problem At present time the universe is very close to flat, 1-W 0 < 0.2 You can consider this as a coincidence, but the Friedmann Eqn. suggests it was much flatter in the past [blackboard] Why so close to W(t) = 1?

18 The CMB is VERY ISOTROPIC! Perhaps too isotropic? Regions too far away from each other on the sky are not causally connected, because their distance is larger than the horizon at the last scattering surface [blackboard] And there are 20,000 such patches. Horizon Problem

19 Problem: Low Density of Domain Walls

20 Phase transitions and symmetry breaking

21 Monopole Problem Grand unified theories When strong and electroweak forces break apart, magnetic monopoles are predicted to be left over with E~1e12 TeV Monopoles would dominate the energy density of the Universe! Monopoles have never been seen!

22 Inflation Solution. Period of Rapid Expansion The period of ultra-rapid expansion means that our present day horizon was tiny before inflation. There could be a lot of bubbles!

23 Inflation Solves all 3 Problems All regions are in causal contact initially because the universe is so small before inflation As space inflates (accelerates) the universe becomes flatter. The density of magnetic monopoles is greatly reduced.

24 Inflation. The universe expands fast!

25 Solution to the Horizon Problem Before inflation the universe was small enough to have been in causal contact. Note - as used here universe means the observable universe today.

26 Rapid Expansion Blows Submicroscopic Scales into Macroscopic Scales Ultra-rapid expansion means that our present horizon was tiny before inflation. It was a small fraction of the causally connected region, so there could be a lot of bubbles! The entire universe that we see today was crammed into a region just a meter in radius after inflation. And it was just m before inflation. Quantum mechanics ensures that the universe is very inhomogeneous on this scale!

27 Quantum Fluctuations Caused the Perturbations in the Gravitational Potential on the LSS

28 Inflation Solves the Flatness Problem Exponential expansion requires w=-1. When w < -1/3, the expansion accelerates and 1-W(t) becomes closer to unity with increasing t. Note that this also solves the monopole problem. It dilutes them.

29 Inflation solves the monopole Inflation dilutes monopoles so that there is of order one or less left for horizon, today (depending on the number of e- foldings) So there are monopoles, they are just not observable! Problem

30 Symmetry Breaking Causes Phase At about ~10-36 s after the Big Bang symmetry broke and strong and electroweak forces separated. A quantity called the inflaton field (similar to the Earth s magnetic field in some sense) found itself in a position of false vacuum, i.e. in a state that looked like a minimum but was not a minimum of energy Transitions

31 Inflation. The inflaton rolls down The inflaton wants to roll down to its true vacuum, i.e. the energy minimum While you roll down you release energy (the guy in the ball is speeding up!) by transforming potential energy into kinetic energy

32 Inflation. The inflaton rolls down The same thing happens for the inflaton!

33 Slow Roll of the Inflaton Field Causes Rapid Expansion The size of the universe grows exponentially as a ~ e Ht where H is the Hubble constant at that time. In just s the universe expands by a factor of 10 43

34 Inflation. What happens to the Inflation expands space so much that the temperature of the universe cools down to about e -N T GUT at the end of inflation (too cold) But at the end of this phase transition there is a bunch of latent heat released by the inflaton field that heats it back to the right temperature, about K It s similar to boiling water that you need to heat it to do the phase transition and it releases heat when it condenses back temperature?

35 Inflation. Reheating Rapid expansion cools the universe Oscillation of the inflaton field about the true vacuum is damped by the Hubble friction Coupling of the inflaton field to other fields radiates away some of the energy, thereby reheating the photons and baryons in the universe.

36 Inflation solved three known problems. How about predictions? Inflationary models can predict the amount of polarization of the CMB Inflationary models predict fossil gravitational waves, like the CMB but for gravitons. Inflationary models predict the power spectrum index is close to 1, but not exactly one. The measured large-scale structure (defined by galaxies and the IGM) is consistent with n~1. Polarization measurements of the CMB are currently starting to become interesting. We may not have to wait much longer for an answer from the Planck team (ESA mission Planck launched in May 2009). For fossil gravitational waves we ll have to wait..

37 Things to Think About [R] Illustrates why inflation requires W very close to 1. [R] What will the decay of the vacuum energy associated with Dark Energy today do to the universe?

38 Summary of Week 8 Inflation solves several problems with the standard big band model. Early rapid expansion flattens space All regions of the universe were in causal contact prior to inflation. The large expansion factor dilutes domain walls.