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Understanding the Dark Universe

Understanding the Dark Universe. 朱明中 Chu Ming-chung Department of Physics The Chinese University of Hong Kong. Science , Vol 302, Issue 5653, 2038-2039 , 19 December 2003. Science Magazine Breakthrough of the Year 2003. ‘Illuminating the Dark Universe’.

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Understanding the Dark Universe

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  1. Understanding the Dark Universe 朱明中 Chu Ming-chung Department of Physics The Chinese University of Hong Kong

  2. Science, Vol 302, Issue 5653, 2038-2039 , 19 December 2003 Science Magazine Breakthrough of the Year 2003 ‘Illuminating the Dark Universe’ ‘Portraits of the earliest universe and the lacy pattern of galaxies in today's sky confirm that the universe is made up largely of mysterious dark energy and dark matter. They also give the universe a firm age and a precise speed of expansion.’ Science, Vol 302, Issue 5653, 2038-2039, 19 December 2003. http://www.sciencemag.org/cgi/content/full/302/5653/2038

  3. new observations, theories Universe as we know it today • matter accounts for ~30% of total energy; ~70% are ‘dark energy’ (vacuum force) • ordinary matter takes up ~5% of total mass; ~95% of matter are dark matter • the universe is flat (k = 0) • many new discoveries new questions → < 2% total energy is known! New Era for Cosmology as a Science!

  4. Understanding the Dark Universe Summary • Dark Matter – evidences, some proposals • Dark Energy – evidences, some proposals • Related research at CUHK - neutrino stars - extra dimensions M. C. Chu, ‘Understanding the Dark Universe’; 「黑暗物質與黑暗能量」 http://www.phy.cuhk.edu.hk/gee/mctalks/mctalks.html

  5. I. Dark Matter

  6. Unless Newton’s Gravitation Law is wrong! Evidences for Dark Matter • If there were no dark matter: • Hot gas surrounding most galaxy clusters星系團would have escaped • Galaxy clusters would not have formed • Galaxy collisions would look different • Most stars would have escaped from galaxies There would be much less structure in the universe!

  7. http://antwrp.gsfc.nasa.gov/apod/ap020203.html 后髮座星系團 Coma Cluster >1000 bright galaxies distance ~ 2.8x108 l.y.s Photo credit: O. Lopez-Cruz (INAOEP) et al., AURA, NOAO, NSF

  8. 5o visual angle 室女座星系團 Virgo Cluster distance ~ 6x107 l.y.s, > 2000 galaxies. Milkway is being drawn there at several hundred km/s. M87 Photo credit:Digitized Sky Survey, Palomar Observatory, STScI

  9. 1. Hot Gas in Galaxy Clusters Visible light Radio X-ray http://chandra.nasa.gov/Photo Courtesy NASA Large amount of X-ray emitting hot gas (T~108 K) surrounding galaxy clusters, with total mass >> stars Hot gas mass ~9 times stellar mass Eg.: 3C295 Photo credit: NASA

  10. X-ray image T ~108 K http://antwrp.gsfc.nasa.gov/apod/ap020203.html 1.5 million l.y.s Coma Cluster Typical speed? Enclosed mass? must have large amount of dark matter to provide enough gravity Photo credit: Chandra X-ray Observatory http://chandra.harvard.edu/photo/2002/0150/

  11. Zwicky 2. galaxies’ speeds 2. Galaxy Motion in Clusters • Orbital velocities of galaxies inside a galaxy cluster →total mass of galaxy cluster • Zwicky, Smith (1930s) Virgo and Coma Clusters have much larger mass than visible mass How?

  12. 3. Gallaxy Collisions 銀河 Milkyway Tidal tail M31 仙女座星系 galaxy collisions: matter→gravity→matter distribution

  13. v (r) r 4. Galactic Rotation Curve Photo credit: NASA/STScI

  14. Milkyway r M = enclosed mass 105 l.y.s v (km/s) 1pc ~ 3.3 lys r (kpc) → Milkyway extended to 3-6x105 l.y.s, but dark!

  15. UGC9242 from Vogt et al. NGC3198 from Begeman 1989 M/Mlum ~ 50 http://astrosun2.astro.cornell.edu/academics/courses//astro201/rotcurve.htm http://www.astro.queensu.ca/~dursi/dm-tutorial/rot-vel.html

  16. Evidences for Dark Matter • Hot gas surrounding most galaxy clusters • Galaxy motion in clusters • Galaxy collisions • Galactic rotation curves But: What are they? How are they distributed? Why are they there? …..

  17. Dark matter could be … • Baryonic dark matter: ordinary matter formed from protons, neutrons, electrons, etc.eg., planets、brown dwarfs、dark nebulae、black holes • Non-baryonic dark matter: neutrinos、axions、supersymmetric partners (neutralinos, photinos, …) They exist, but not enough! We don’t know whether they exist, and we don’t know their properties! Except neutrinos! We even know now they are massive!

  18. 朱明中, ＜中微子與中微子天文物理＞, http://www.phy.cuhk.edu.hk/gee/mctalks/mctalks.html http://www.ps.uci.edu/~superk/neutrino.html http://wwwlapp.in2p3.fr/neutrinos Neutrinos 中微子 • Elementary particles – no structure • 3 kinds﹕ • neutral • Only weak and gravity forces, no strong or EM forces • Penetrating: only 1 in 106 interacts (trapped) after passing through the entire Earth • Produced in Big Bang: ~300/cc left over

  19. How are dark matter distributed? • Make use of gravity: • X-ray telescopes can be used to measure hot gas distribution →matter distribution • Gravitational lens (重力透鏡): General Relativity → light distorted by gravity → gravity ~ lens image of a far galaxy →distorted, multiple images → reconstruct mass distribution

  20. Gravitational Lens Illustration credit: NASA/STScI

  21. Gravitational Lens Photo credit: NASA/STScI http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/08/

  22. A galaxy 13 billion l.y.s away, red shift ~ 7, farthest seen so far; first generation galaxy Gravitational Lens http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/08/ Credit: ESA, NASA, J.-P. Kneib (Caltech/Observatoire Midi-Pyrénées) and R. Ellis (Caltech)

  23. Tune matter distribution to fit lensing effects From T. Tyson

  24. Distribution of dark matter galaxy dark matter Galaxy Cluster CL0024+1654 From T. Tyson

  25. II.Dark Energy

  26. 1929 1999 Hubble’s Law v =Hr

  27. A, B, C distinguished by observing expansion of early universe Fate of the universe: are there enough matter to stop its expansion？ matter →gravity →decelerate more matter →measure far away objects But dim！

  28. Type IA Supernovae • Explosions of white dwarfs with mass just >1.4 Mo • →same initial conditions, standard and bright • →can be observed over long distance • Monitor spectra and light curves to identify types • Compare visual and absolute magnitudes→distance • redshift → receding speed v • Extend Hubble’s diagram (v vs. d) to ~10 billion l.y.s M. Chu, ‘量子星’ http://www.phy.cuhk.edu.hk/gee/mctalks/mctalks.html

  29. S. Perlmutter Science Magazine: Breakthrough of the year 1998

  30. What is dark energy? CUHK PHY Accelerating expansion • Found that the expansion of the universe is accelerating! Independently confirmed by Cosmic Microwave Background measurements. • → repulsive force > gravity by 2.3 times！ →Dark energy！

  31. L Einstein’s Cosmological Constant 宇宙常數is just what’s needed! Introduced originally to counteract gravity. Dark energy = Cosmological Constant?

  32. L Einstein (after knowing Hubble’s result): L = 0 Quantum Mechanics (vacuum energy): L = 1012 Everybody but Peebles (pre-1998): L = 0 Almost everybody (2004): L = 0.7 How come??????

  33. new observations, theories Universe as we know it today • Ordinary matter takes up ~5% of total mass; 95% of matter are dark matter • matter accounts for 0.3 of total energy; 0.7 are ‘dark energy’ (vacuum force) • the universe is flat (k = 0) • many new discoveries new questions New Era for Cosmology as a Science!

  34. III. Our Crazy Ideas • Neutrino stars • Extra-dimensional cosmology

  35. Related work at CUHK • Chan Man Ho: neutrino stars could exist, be stable, and provide the necessary gravity to explain various structures in the universe – galaxies, galaxy clusters, hot gas • Cheung Kai Chung, Li Baojiu, Alfred Tang: extra spatial dimensions (1+3+n) can cause the accelerating expansion of the universe, without the cosmological constant • Ngai Wah Kai, Alfred Tang: Daya Bay Neutrino Oscillation Experiment

  36. radius of visible Milkyway M.H.Chan neutrino density ~1/r2 m-3 r (kpc) distance from center 1kpc~3.3kly Neutrino Star (中微子星) • Can massive neutrinos form a stable ‘star’？ • Yes. Hydrostatic equilibrium: gravity balanced by degenerate pressure (Pauli Exclusion Principle)

  37. Neutrino Star data v Neutrino Star theory km/s r ( kpc) M.H. Chan Calculate trajectories of stars inside a neutrino star →rotation curve →most galactic dark matter = neutrinos？ →We live inside a star？

  38. Neutrino Star? Density distribution of dark matter by gravitational lensing observation

  39. Formation of a Neutrino Star: Hydrodynamics Always form (t ~ 6 Gyrs) a stable star at hydrostatic equilibrium with some oscillations t = 5.5by t = 4.9by t = 0 neutrino star model Provides just the right gravity to hold the hot gas in galactic clusters with the correct density distribution

  40. Physics with Extra Dimensions Generalize standard physics to (1+3+n) dimensions Kaluza + Klein (1920’s) – General Relativity in (1+3+1) dimensions →gravity + Maxwell Eq. String theory (1990’s) – consistent only for D =11, 26 Brane model (1990’s) – our universe is in only one 4-d brane of the multi-dimension universe But we haven’t observed the extra dimensions! Could it be that we need to look at either very large or very small scales to see the extra dimensions? Our proposal: dark energy is a signature of extra dimensions!

  41. spacetime curvature energy-momentum 1+3+n Cosmology

  42. Effects of extra dimensions (geometric) Generalized Friedmann Equations

  43. a deceleration acceleration t (Gyr) cosmic age ~ 13 GYr today Evolution of the universe Li et al., CUHK

  44. Curves reconstructed from SN data in Alam etal. q Cosmological constant CUHK Theory calculated by Li et al., z Deceleration Parameter z = 0: today z > 0: past Alam et al., MNRAS 354, 275 (2004).

  45. Curves reconstructed from SN data in Alam etal. w CUHK Theory calculated by Li et al. Cosmological constant z Dark Energy EOS Alam et al., MNRAS 354, 275 (2004).

  46. The CUHK Cosmological Model • Pure GR, + extra closed spatial dimensions • No cosmological constant added in by hand; essentially no free parameter (n = 7 preferred, but not a must) • Explains: deceleration, acceleration; Fits: deceleration parameter, dark energy EOS, cosmic age ~13 Gyr (n = 7) • Features robust w.r.t. initial conditions, n, EOS • Spontaneous compactification of extra dimensions • Extra dimensions were large in early universe: signatures of extra dimensions in cosmology!

  47. Understanding the Dark Universe Summary • Dark Matter – evidences, some proposals • Dark Energy – evidences, some proposals • Related research at CUHK - neutrino stars - extra dimensions M. C. Chu, ‘Understanding the Dark Universe’; 「黑暗物質與黑暗能量」 http://www.phy.cuhk.edu.hk/public_lectures/

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