Lecture 1 General relativity and cosmology. Kerson Huang MIT & IAS, NTU

Transcription

1 A Superfluid Universe Lecture 1 General relativity and cosmology Kerson Huang MIT & IAS, NTU

2 Lecture 1. General relativity and cosmology Mathematics and physics Big bang Dark energy Dark matter Robertson-Walker Schwarzchild Black hole 2

3 x (, t x, y, z) (c=1) 2 d d d ds g dx dx Flat space: ds dt dx dy dz g diagonal (-1, 1, 1, 1) Space is curved, when vector's direction changes upon "parallel displacement" (covariant derivative=0) in closed loop. 3

4 Mathematics Dv v v x (Covariant derivative) 1 g g g g 2 x x x (Connection, or Christoffel symbol) R x x R (Curvature tensor) (Scalar curvature) R Bernhard Riemann ( ) Elwin Bruno Christoffel ( ) Gregorio Ricci Curbastro ( ) 4

5 Physics Matter is source of curvature (gravitational field) T G 1 R g R 8 GT (Einstein's equation) 2 Energy-momentum tensor of matter cm g sec (Gravitational constant) Particle moves on geodesic (shortest distance in the geometry). Mathematica notebooks: JamesB B. Hartle, GRAVITY, An Introduction to Einstein's General Relativity (Addison Wesley) 5

6 The expanding universe a(t) Edwin Hubble Hubble s law: Distances between galaxies increase with time. Rate of increase proportional to distance. Extrapolation to distant past: The big bang. Hubble parameter: H 1 da 1 a dt yrs 6

7 Dark energy DeviationfromHubble s law: expansion is accelerating, as if driven by unseen energy. Evidence of accelerated expansion (velocity) 7

8 Dark matter: unidentified components of galaxies Andromeda Mass m moving in mass distr. M(r) m r mv 2 r Vera Rubin (1975) GmM r r 2 M(r) v GM r r 8

9 The bullet cluster : colliding galaxies Blue spot: galaxy cluster 1E X ray gas Overall view Dark matter 9

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11 Bright tness (1 0 ergs/sec/cm /steradian/cm ) CMB (cosmic microwave background) Early universe was a plasma of ionized atoms. Photons were being scattered back and forth among ions and electrons, and cannot propagate. At about 10⁵ yrs, temperature drops below 10³ K, and neutral hydrogen was formed. Photons decoupled from matter to become CMB. Uniform 3 degree black body radiation. Small angular fluctuations contain info on early universe. 1.2 T=2.73 K Frequency (cycles/cm) 11

12 Cosmic inflation What is the reason for the uniformity on large scale, when different parts of the present universe lie outside of each other s light cone? Inflation scenario: matter was uniformly created in a small universe after the big bang. The universe inflates rapidly (by factor in s), and matter remains uniform. Models of inflations makes use of scalar field, inspired by the Higgs field in particle theory. In our theory, such a complex scalar field gives rise to superfluity.

13 Galactic voids Though uniform on scale of 10 9 light years, galactic distribution is full of voids on smaller scale. The tik stick man 10 8 light years The Great Wall 13

14 Robertson Walker metric Uniform universe, co moving coordinates. ds 2 dt 2 a 2 t dr 2 Einstein s equation reduces to 1 kr 2 r 2 d 2 r 2 sin 2 d 2 Curvature parameter: k = 0, 1, 1 ä a ȧ a ȧ a 2 k a 2 2 k T 00 a g ij 2T ij Uniform fluid: T 00 T ij g ij p T j0 0 i, j 1,2,3 Conservation law ( T ; 0 ) : 3ȧ a p 0 14

15 FLRW model (Friedmann Lemaitre Robertson Walker) H a / a Ḣ k p 2 a H 2 k a Constraint equation 1 k a 2 H 2 2 3H 2 3H p 1 k A. Friedmann G. Lemaitre H.P. Robertson ( ) ( ) ( ) A.G. Walker ( ) 15

16 Temperature of CMB has angular dependence across the sky Theory: T f l P l cos l peak 200 l peak 200 Thus 1 K = 0 (flat universe) 16

17 Spherically symmetric metric ds 2 f r dt 2 dr2 2 f r r2 d 2 sin 2 d 2 Substitute into Einstein s equation to find f(r). De Sitter metric: (vacuum solution with cosmological constant) f r 1 br 2 This leads to radius = exp (Ht) Accelerated expansion dark energy Willem de Sitter ( ) 17

18 Fine tuning problem Radius of universe = exp (Ht) Hubble parameter: H = O(1) on Planck scale, naturally Planck length c 3 4 G m Planck time G 43 c 4 G s Planck energy c 5 4 G GeV Theory: H = s 1 Observed: H = (Age of universe) 11 = (15 billion yrs) 11 = s 11 We would have to fine tune the theory by 60 orders of magnitude! 18

19 Schwarzschild metric f r 1 2M r Vacuum solution (c = G =1) Reduces to Newtonian gravity at large r, with mass M at center. Schwarschild horizon: r = 2M. Star lying inside horizon will collapse into black hole. Correctionsto to Newtonian gravity: Bending of light by star Precession of perihelion of planetary orbit Karl Schwarzschild ( ) 19

20 Schwarzchild metric and black hole ds 2 1 2M r dt 2 dr2 1 2M r r 2 d 2 sin 2 d 2 20

21 Black hole: gravitational collapse Oppenheimer Snyder model Robert Oppenheimer ( ) Initial radius = Schwarzschild horizon ( R = 2M ). Solve Einstein s equation for time evolution. Join metrics at horizon. Hartland Snyder ( ) 21

22 Inside solution 2 dt 2 2 dr ds a t 2 r 2 d 2 r 2 sin 2 d kr Put k=1. Put pressure p=0. Einstein s equation reduces to 2ä a ȧ a ȧ a 2 1 a a ȧ a c 0 a 3 Solution is the cycloid ( a(0)=1 ). ȧ 2 2c a 1 a 0 2 3a 3 a 1 1 cos ȧ 2 a 0 2 a 1 t sin 2 k 22

23 Radius of star Collapse to black Hole Inside time Starradiuscollapses radius to zero infinite inside time. Joining of metrics gives relation between inside and outside time. To an observer outside, tid the collapse takes tk infinite it time. Light emitted from the surface of the star will never reach outside. 23