The Gravity of Darkness and the Dark Side of Gravity 朱明中 Chu Ming-chung Department of Physics The Chinese University of Hong Kong

Hello Darkness my old friend, I ’ ve come to talk about you again …

Science, Vol 302, Issue 5653, , 19 December 2003 Summary of this Article Summary of this Article PDF Version of this Article PDF Version of this Article Related commentary and articles in Science products Related commentary and articles in Science products Download to Citation Manager Download to Citation Manager Alert me when:new articles cite this articleAlert me when:new articles cite this article Search for similar articles in: Science Online PubMed Search for similar articles in: Science Online PubMed Search Medline for articles by:Seife,C.Search Medline for articles by:Seife,C. This article appears in the following Subject Collections:Astronomy This article appears in the following Subject Collections:Astronomy Science Magazine Breakthrough of the Year ‘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, , 19 December 2003.

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 energy) the universe is flat (k = 0) many new discoveries new questions new observations, theories New Era for Cosmology as a Science!

The Gravity of Darkness and the Dark Side of Gravity Dark Matter – evidences Dark Energy – evidences The CUHK Models - Neutrino Stars - Dark Energy as a Signature of Extra Dimensions Preprints, powerpoint downloadable from

I. Dark Matter

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

Coma Cluster 后髮座星系團 Photo credit: O. Lopez-Cruz (INAOEP) et al., AURA, NOAO, NSFO. Lopez-CruzINAOEPAURANOAONSF >1000 bright galaxies; each galaxy ~ stars distance ~ 2.8x10 8 l.y.s

Virgo Cluster 室女座星系團 Photo credit: Digitized Sky Survey, Palomar Observatory, STScI Digitized Sky SurveyPalomar ObservatorySTScI distance ~ 6x10 7 l.y.s, > 2000 galaxies. 5 o visual angle M87 Milkway is being drawn there at several hundred km/s.

1. Hot Gas in Galaxy Clusters Large amount of X-ray emitting hot gas (T ~ 10 8 K) surrounding many galaxy clusters Hot gas mass ~ 10 times stellar mass, typically must have large amount of dark matter to provide enough gravity to hold the hot gas mass in stars << mass in hot gas << total mass mostly dark matter! 1 : 10: 200typically

Photo credit: Chandra X-ray Observatory du/photo/2002/0150/ must have large amount of dark matter to provide enough gravity Coma Cluster 1.5 million l.y.s X-ray image T ~10 8 K

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 Zwicky 2. galaxies’ speeds

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

UGC9242 from Vogt et al. es//astro201/rotcurve.htm NGC3198 from Begeman 1989 M/M lum ~ 50

Milkyway 銀河 r r (kpc) v (km/s) → Milkyway extended to 3-6x10 5 l.y.s, but dark! 10 5 l.y.s 1pc ~ 3.3 l.y.s

4. Gallaxy Collisions galaxy collisions: matter → gravity → matter distribution Milkyway M31

Evidences for Dark Matter Hot gas surrounding most galaxy clusters Galaxy motion in clusters Galactic rotation curves Galaxy collisions But: What are they? How are they distributed? Why are they there? ….. Dark matter are responsible for the emergence of structures in an initially uniform universe!

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 there are too little of them! We don’t know whether they exist, and we don’t know their properties! Except neutrinos! We even know now they are massive!

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

Density distribution of dark matter Galaxy Cluster CL Galaxies are embedded into a glob of dark matter, which also has some structure. galaxy dark matter From T. Tyson, using gravitational lensing data

II. Dark Energy

Hubble’s Law v = HrH = Hubble’s ‘constant’ 1929 Velocity (km/s) Distance (Mpc) 0 2 v r r (Mpc) (10 4 km/s) v 1 pc ~ 3.3 light years

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

Type IA Supernovae 超新星 Explosions of white dwarfs with mass just >1.4 M o →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. r) to ~10 billion l.y.s Maximum luminosity ~ 10 9 – L o

Accelerating expansion Found that the expansion of the universe is accelerating! Independently confirmed by Cosmic Microwave Background measurements and large scale structure surveys. → Dark energy ！ → repulsive force > gravity by 2.3 times ！ What is dark energy?

 Einstein’s Cosmological Constant ~ what’s needed! Dark energy = Cosmological Constant? Introduced originally to counteract gravity.

Wilkinson Microwave Anisotropy Probe Microwave Background = leftover radiations from Big Bang; almost uniform, T ~ K Microwave Anisotropies = fluctuations around mean temperature ~ K; pattern →cosmological parameters

C.L. Bennett et al., 2003, ApJS, 148, 1 WMAP results of Cosmological parameters

Large Scale Structure Survey Clustering of matter gives information on cosmological parameters, especially matter content Obtained redshifts of 220,000 galaxies, 23,000 quasars

Sloan Digital Sky Survey Aims to measure the positions and brightness of 100 million celestial objects Will measure distances to 10 6 galaxies, 10 5 quasars →

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 repulsion) the universe is flat (k = 0) Cosmic Concordance: independent measurements all agree on a common set of cosmological parameters

 Einstein (after knowing Hubble’s result):  = 0 Quantum Mechanics (vacuum energy):  = Everybody but Peebles (pre-1998):  = 0 Almost everybody (2005):  = 0.7 How come??????

III. Our Crazy Ideas

The CUHK Models 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

Neutrinos Elementary particles – no structure 3 kinds ﹕ neutral Only weak and gravity forces, no strong or EM forces Penetrating: only 1 in 10 6 interacts (trapped) after passing through the entire Earth Produced in Big Bang: ~ 300/cc left over Produced in the sun: 2x10 38 neutrinos/s

Neutrino Star Can massive neutrinos form a stable ‘ star ’ ？ Yes. Hydrostatic equilibrium: gravity balanced by degenerate pressure (Pauli ’ s Exclusion Principle) General Relativistic, Quantum Mechanical ‘star’!

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

Formation of a Neutrino Star: Hydrodynamics Always form (t ~ 6 Gyrs) a stable star at hydrostatic equilibrium with some oscillations t = 0 t = 4.9Gyr t = 5.5Gyr 1000 r (kpc) 1000 (g/cc) Provides just the right gravity to hold the hot gas in galactic clusters with the correct density distribution neutrino star model Density profile of hot gas r (kpc) (g/cc)

Galaxy Cluster CL Neutrino Star? Doesn’t it look like a …

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 models (1990’s) – our universe is in only one 4-d brane of the multi-dimensional 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? Dark Energy as a Signature of Extra Dimensions?!

1+3+n Cosmology spacetime curvature energy-momentum Generalized Robertson- Walker metric

Generalized Friedmann Equations Effects of extra dimensions (geometric) n = no. of extra dimensions Note: no added in by hand. Instead, are generated automatically → Space-time-matter approach!

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

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

Dark Energy EOS Alam et al., MNRAS 354, 275 (2004). Cosmological constant reconstructed from SN data in Alam et al. CUHK model z w (z)

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!

Rapid compactification of extra dimensions in early universe Li et al., CUHK Preprint, b(t)b(t) a(t)a(t) n = 7 a(t)a(t) b(t)b(t) t (GYr) today

Dark Matter – need them for the structures in the universe! Numerous proposals for what they are, but mostly particles yet undiscovered. Dark Energy – accelerating expansion of the universe; Cosmic Concordance → new force? New physics? Our Crazy Ideas: - neutrino stars → rotation curves, hot gas profiles, galaxy cluster formation; perhaps the largest ‘star’ in the universe! - extra dimensions → cosmic acceleration, cosmic age, dark energy EOS; signatures in early universe? Dark force is a part of gravity, signature of extra dimensions? Summary Preprints downloadable from

The Gravity of Darkness and the Dark Side of Gravity 朱明中 Chu Ming-chung Department of Physics The Chinese University of Hong Kong