Cosmology IV: The Early Universe. Lecture 30

Transcription

1 Cosmology IV: The Early Universe Lecture 30

2 Announcement Prelim #3 on Wednesday, Nov. 14 In class: 11:15am - 12:05pm (Uris Auditorium) Will emphasize lectures (Galactic Center to Habitability of Worlds) Closed notes and closed book 2

3 Lecture Topics Problems with the Big Bang The Early Universe Theory of everything Inflation Pair-production Nucleosynthesis Pillars of the Big Bang The Multiverse The Anthropic Principle What grade does Cosmology get? 3

4 Problems with Big Bang The Big Bang described thus far is very successful in may aspects. However, there are two major problems that need to be addressed The Horizon problem Why is the CMB so uniform? The Flatness problem Why are we so close to W k = 0 (a flat universe)? 4

5 What is a Horizon? Our horizon (in a Cosmological sense) is the maximum distance we can see out to in the Universe. More generally, for any point in the Universe, the horizon is the maximum distance from which light could have reached that point, within the age of the Universe. Nothing outside your horizon can have any effect on you, because it has never been in causal contact. 5

6 The Horizon Problem Looking at one part of the sky and looking in the opposite direction radio telescopes see the same CMB temperature to 1 part in 100,000 Suppose the Universe is 14 billion years old, then the two directions are separated by 28 billion lightyears 28 billion lightyears Earth 6

7 The Horizon Problem Looking at one part of the sky and looking in the opposite direction radio telescopes see the same CMB temperature to 1 part in 100,000 Suppose the Universe is 14 billion years old, then the two directions are separated by 28 billion lightyears Thus they should not be causally connected That is, they don t know about each other The two regions should not have the same temperature In the past the situation is even worse. 100,000 years after the Big Bang the separation would be 10 million lightyears 7

8 The Flatness problem Measurements of the curvature of the Universe indicate it is almost exactly flat. However, both the average density and critical density change with time. In the past, right after the BIG BANG if the average density was slightly larger or smaller we would have a very obviously closed or open Universe. At the beginning of the Big Bang the density would have to be very close to the critical value (1 part in 10 60!). 8

9 Epochs of the Universe From the Big Bang until now, the universe can be viewed as proceeding through different epochs (time periods). Distinguishing characteristics of epochs Each succeeding epoch is cooler and less dense. Different forces and/or particles may dominate! 9

10 t (sec) T (K) Present Epochs GUT GUT = E-M, Weak, & Strong forces unified Planck All four forces unified (Quantum Gravity) Radius (cm) 11

11 Theory of Everything Unite gravity with the other fundamental forces. Merging of gravity with quantum mechanics and other forces. We don t have a theory yet but the most promising ones involves string theory and higher dimensions String Theory suggests there are 11 dimensions (10 spatial + 1 time). 12

12 t (sec) Present T (K) Epochs Hadron Heavy particles in equilibrium with the radiation field (Pair Production) Inflation GUT GUT = E-M, Weak, & Strong forces unified Planck All four forces unified (Quantum Gravity) Radius (cm) 13

13 Inflation (Part I) At sec after the Big Bang the Universe cooled to K! This caused a phase transition Like ice changing into water The strong force split from the other forces releasing tremendous amounts of energy The Universe expanded by a factor of in sec! 14

14 Inflation (Part II) This rapid expansion phase is called inflation. Universe causally connected before inflation CMB will be the same in all directions afterward Solves Horizon Problem! Universe becomes flat Because of stretching of space Space will now be flat due to inflation Solves Flatness Problem! 15

15 t (sec) T (K) Present Epochs Lepton Electron, muons, etc. in equilibrium (Pair Production for low mass particles) Hadron Heavy particles in equilibrium with the radiation field (Pair Production) Inflation GUT GUT = E-M, Weak, & Strong forces unified Planck All four forces unified (Quantum Gravity) Radius (cm) 16

16 What is Anti-Matter? A particle and its anti-particle have the same mass but opposite charge. Many antiparticles can be created in laboratories. Positrons (anti-electrons) are used routinely in medicine to imagine internal organs using Positron Emission Tomography (PET). 17

17 Pair Production Particle-antiparticle annihilation occurs when matter and anti-matter destroy each other in a burst of g-rays. The reverse is called pair production: 2 g particle + anti-particle Pair production happens spontaneously, and depends upon the temperature. Higher T more energetic photons more massive particles produced 18

18 Pair production (cont d) In the early universe temperatures were high enough for pair production to take place. There was a sea of photons, particles and anti-particles. The threshold temperatures are: Temperature T ~ K T ~ K T < 10 9 K Particles Pairs proton, anti-proton electron, positron no pair production 19

19 Pair Production (cont d) Above these threshold temperatures, particles and anti-particles will exist in equilibrium (as many created as destroyed). As the universe expands and the plasma cools, we expect particles and anti-particles to annihilate one another leaving just photons. This didn t happen! We are here. This is because there are asymmetries between matter and anti-matter. Still not fully understood 20

20 t (sec) T (K) Present Epochs Nuclear Formation of light elements Lepton Electron, muons, etc. in equilibrium (Pair Production for low mass particles) Hadron Heavy particles in equilibrium with the radiation field (Pair Production) Inflation GUT GUT = E-M, Weak, & Strong forces unified Planck All four forces unified (Quantum Gravity) Radius (cm) 21

21 Fraction of total mass in the universe Big Bang Nucleosynthesis Predictions 10-1 Observations 4 He He 7 Li Expected from CMB Deuterium Present density of baryons (g/cm 3 ) 22

22 t (sec) T (K) Present Dark Energy Dominated Matter Dominated Stellar Galactic Atomic Nuclear Lepton First Galaxies Formation of light elements Epochs Atoms form, matter photon decoupling Electron, muons, etc. in equilibrium (Pair Production for low mass particles) Radiation Dominated Hadron Heavy particles in equilibrium with the radiation field (Pair Production) Inflation GUT GUT = E-M, Weak, & Strong forces unified Planck All four forces unified (Quantum Gravity) Radius (cm) 23

23 In-Class Question What were two problems with the Big Bang theory? a) Horizon and Bigness b) Flatness and Expansion c) Expansion and Bigness d) Horizon and Flatness e) There were no problems 25

24 In-Class Question What were two problems with the Big Bang theory? a) Horizon and Bigness b) Flatness and Expansion c) Expansion and Bigness d) Horizon and Flatness e) There were no problems What is the answer to these problems? a) Cosmic string theory b) Inflation c) Accelerating Universe d) All of the above e) None of the above 26 Sky is more uniform than it should be (not causally connected) We are very near a flat universe (W ~ 1)

25 In-Class Question What were two problems with the Big Bang theory? a) Horizon and Bigness b) Flatness and Expansion c) Expansion and Bigness d) Horizon and Flatness e) There were no problems What is the answer to these problems? a) Cosmic string theory b) Inflation c) Accelerating Universe d) All of the above e) None of the above 27 Sky is more uniform than it should be (not causally connected) We are very near a flat universe (W ~ 1) Expansion of space due to phase transition in the early universe.

26 Pillars of the Big Bang Hubble Expansion Universe expanding in all directions (necessary but not sufficient) Cosmic Background Radiation probes T ~ 1 ev, t ~ 10 5 years Light Element Abundances probes T ~ 1 MeV, t ~ 10 mins These two agree!!! 29

27 But how did it all begin? Quantum gravity emergence: Universe derives from quantum fluctuations The seeds of galaxies cannot be infinitely close together Multiverse: Maybe we are one of many universes continuously being created As above or intersection of higher dimensional spaces Laws of physics may be different in each universe See Michal Turner article in Sep 2009 Scientific American

28 The Multiverse Our observable Universe extends out to a distance of about 42 billion light-years. Our cosmic horizon, which represents how far light has been able to travel since the big bang (as well as how much the universe has expanded in size since then). Assuming that space does not just stop there and may well be infinitely big Cosmologists make educated guesses as to what the rest of it looks like. Us 42 billion light-years Observable Universe See Scientific American - Aug article by George Ellis

29 Level 1 Multiverse: Plausible: Our volume of space is a representative sample of the whole. Distant alien beings see different volumes but all of these look basically alike but we can t see each other. Level 2 Multiverse: Questionable Sufficiently far away, things look quite different from what we see. The laws of physics would differ from bubble to bubble, leading to an almost inconceivable variety of outcomes.

30 The Anthropic Principle Philosophical position, rather than hard science. (Not universally agreed on.) Essential it states we are here, so the Universe must have formed in such a way as to allow life. Can have powerful reasoning implications. 34

31 The Anthropic Principle If any of a number of fundamental constants were altered just slightly, the Universe wouldn t be capable of sustaining life. It may seem that the Universe is very well suited to us, but if it wasn t then there wouldn t be anyone around to ask the question, why is the Universe the way it is? 35

32 Theory Report Card From James Peebles (noted cosmologist), Sci. Am, Jan