Lectures on String Cosmology: Lecture 1 The Beginning of the Universe

Transcription

1 Lectures on String Cosmology: Lecture 1 The Beginning of the Universe Henry Tye Hong Kong University of Science and Technology Cornell University 19 Jan 2012, IAS-CERN School, NTU, Singapore 1

2 IAS-HKUST 2

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4 Background : Always Light (X-ray, sun light, microwave,...) travels at the speed of light: 299,792,458 meters per second. Nothing travels faster than the speed of light. Einstein s Theory of Special Relativity (1905) (E=Mc 2 ) 4

5 Gravity Newton s apple tree Gravitational force. 5

6 Planets move according to Newton s gravitational force law Einstein asked : Since there is nothing in between the earth and the sun except empty space, how does the earth know about the gravitational pull of the sun? There is space! 6

7 Einstein s relativity theory (1915). Space-time is an active player: warps, curves, expands, shrinks, 7

8 There are about 400 billion stars in our galaxy. Our sun is a star close to the edge. A galaxy like ours Hubble Space Telescope 8

9 9 There are many billions of galaxies in our universe.

10 The Big Bang Theory Georges Lamaitre first proposed his theory in Einstein : Your math is correct, but your physics is abominable. Einstein has his own solution by introducing a cosmological constant into his theory. Later, he supposedly said that this was his biggest blunder. 10

11 Hubble(1929): The Universe is Expanding The further away a galaxy is, the faster it moves away from us. The expansion of the Universe is similar to the expansion of a balloon: while objects are fixed at their comoving locations, their relative distances are increasing in all directions. 11

12 Historical tidbits: To explain the expansion, there was an alternative theory called the steady state universe. The big bang theory was largely ignored till Fred Hoyle coined the name big bang as a joke. The Vatican embraced the big bang theory. 12

13 Big Bang Theory Our universe is expanding and cooling today Going back in time, the universe is small and very hot. Prediction : Today, it should have a temperature of about 3K= C. Note : 0K= C is absolute zero. 13

14 Big Bang Theory predicts (shown by Gamow etc in late 1940s...) : Today, it should have a temperature of about 3K= C. This means we are living in a thermal bath of cosmic microwave radiation. In 1964, Penzias and Wilson measured the intensity of microwave at 7.35cm. 14 They received the Nobel prize in 1978.

15 The age of our universe is 13.7 billion years. 7.35cm Mather etc Nobel prize in T=2.725K As our universe expands, space is created. 15

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17 What is in Our Universe today? Are all we see in the sky all there are? Atoms, molecules, electrons, photons,... 17

18 Matter in Our Universe Ordinary matter = atoms and molecules Other matter? 15% 85% 18

19 Contents in Our Universe 4% Dark energy 23% Dark matter Ordinary matter 73% Mysterious 19

20 What is the difference between dark matter and dark energy? * * * * * * While the objects move away from each other, matter * density goes down, but not the dark energy (blueness) density. 20

21 As universe expands : Past Today Future Blue : dark energy Red : dark matter Yellow : ordinary matter Our future universe will be mostly space filled with nothing but dark energy. 21

22 When the universe is very hot, it is simply a gas (or soup). What is this hot soup like? 1. Look for remnants of the hot soup in the sky. T=2.725K 2. Create in the laboratory the condition for the hot soup and see what happens. 22

23 Recreating the early moments of the universe To find out what is dark matter CERN LHC 23 (Large Hadron Collider)

24 Going back in time, the size of our universe becomes smaller and smaller, eventually becomes a point. We learned that the universe would have been a point about 13.7 billions years ago. Squeezing everything into a point is not possible, but a very small region is possible. When squeezed to a very small region, the universe was very hot, and the description becomes very simple. The history of our universe can be traced all the way back to second old. 24

25 Asymptotic freedom D. Gross, F. Wilczek, D. Politzer, 1973 Strong couplings/forces become weaker at high temperature (short distances) atoms become electrons and nuclei nuclei become quarks and gluons.. a gas of elementary particles in thermal equilibrium 25

26 H 2 = Λ + k a 2 + ρ m a 3 + ρ r a 4 H 2 =(ȧ a )2 Λ a(t) e Ht After a while, Λ ρ 26

27 What started the hot big bang? The Inflationary Universe 涨 论 Alan Guth The universe started with a large dark energy density, so it expands very rapidly, or inflates. This resulting dark energy is then converted to radiation and matter, leaving behind the very small dark energy of today. 27

28 Dark energy comes from the inflaton potential Quantum Fluctuations V(!) Slow!Roll Region Damped Oscillations, Reheating! The quantum fluctuation resulted in density and temperature fluctuations. 28

29 The Inflationary Universe Scenario says : All matter comes from inflation (converted from dark energy). Space is created by inflation. Ultimate free lunch It is even possible that a single quantum fluctuation started the inflationary universe. Everything comes from nothing 無中生有 29

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31 Inflationary Universe also predicts : Quantum effect creates a density fluctuation which seeds the structure formation. This leads to a temperature fluctuation that can be measured and tested. WMAP

32 Now that inflation seems to be strongly supported by data, what causes inflation, or how does it happen? This leads us to Superstring Theory 32

33 String Theory 33

34 Superstring theory contains p-branes (1995) J. Polchinski,... A p-brane has p spatial dimensions: a point particle is like a 0-brane; a string is like a 1-brane; a membrane is a 2-brane We are living inside a 3-brane. Outside our branes is the bulk, which is compactified. 34

35 Brane world Extra dimensions We live in a 3-brane, with 3 spatial dimensions Multiverse ~ multiple universes 35

36 Brane world r " m W + G µ! How to test this picture? 36

37 Calabi-Yau A 2-dimensional look of a typical CY 3-manifold (6-dim.) 37

38 Z boson emitting a graviton open string closed string field theory point particle TIME string theory open string oneloop = closed string tree diagram 38

39 How is inflation realized in brane world? Brane inflation Inflaton is an open string mode Inflaton potential comes from the closed string exchange 39 Dvali and HT 1998

40 Brane inflation INFLATION t predicts some details of inflation that will be tested. 40

41 Brane Inflation in String Theory Search for cosmic strings in the sky 41

42 More on brane inflation and on Search for Cosmic Superstrings in Lecture 4 42

43 Many ways to study the universe 43 and many ground based telescopes and other detectors as well as satellites

44 Our big bang is like a single fire cracker in a firework show Eternal firework show? 44

45 Eternal Inflation? Inflation is still going on outside our observable universe Multiverse or Multiple Universes 45

46 Cyclic Universe? Branes colliding and separate repeatedly 46

47 Does the universe have a beginning? Our observable universe surely has. But our observable universe may be only a tiny speck of the whole multiverse. Does the multiverse have a beginning? If we can never see the other parts of the multiverse, is this question scientific? Philosophy? Religion? 47

48 Copernicus Principle Eternal Inflation There is no god Anthropic anthropic anthropic anthropic Multiverse God is Laws of Nature There must be a god?????? 48 Cyclic Universe Genesis-Bible There is a beginning

49 As we learn more about nature, we find that nature can be more amazing than we have imagined more questions and puzzles are raised Science can be very exciting and its results as well as spin-offs will continue to transform our society. 49

50 Next 3 lectures will examine some of the issues raised in this introductory lecture: Lecture 2 & 3 : On the string/cosmic landscape Lecture 4 : Brane inflation and cosmic strings 50