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

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 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 new observations, theories New Era for Cosmology as a Science! → < 2% total energy is known!

Understanding the Dark Universe 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」 Summary

I. Dark Matter

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! Unless Newton’s Gravitation Law is wrong!

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

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

1. Hot Gas in Galaxy Clusters Large amount of X-ray emitting hot gas (T~10 8 K) surrounding galaxy clusters, with total mass >> stars Visible light Radio X-ray Photo Courtesy NASA Eg.: 3C295 Hot gas mass ~9 times stellar mass Photo credit: NASA

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 Typical speed? Enclosed mass?

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? Zwicky 2. galaxies’ speeds

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

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

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

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

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? …..

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!

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 10 6 interacts (trapped) after passing through the entire Earth Produced in Big Bang: ~300/cc left over 朱明中, ＜中微子與中微子天文物理＞,

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

Gravitational Lens Illustration credit: NASA/STScI

Gravitational Lens Photo credit: NASA/STScI

A galaxy 13 billion l.y.s away, red shift ~ 7, farthest seen so far; first generation galaxy Credit: ESA, NASA, J.-P. Kneib (Caltech/Observatoire Midi-Pyrénées) and R. Ellis (Caltech)ESANASA g/newscenter/news desk/archive/releas es/2004/08/ Gravitational Lens

Tune matter distribution to fit lensing effects From T. Tyson

Distribution of dark matter Galaxy Cluster CL galaxy dark matter From T. Tyson

II. Dark Energy

Hubble’s Law v =Hr

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

Type IA Supernovae 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. d) to ~10 billion l.y.s M. Chu, ‘ 量子星 ’

S. Perlmutter Science Magazine: Breakthrough of the year 1998

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 ！ What is dark energy? CUHK PHY

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

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

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 observations, theories New Era for Cosmology as a Science!

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

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

Neutrino Star ( 中微子星 ) Can massive neutrinos form a stable ‘ star ’ ？ Yes. Hydrostatic equilibrium: gravity balanced by degenerate pressure (Pauli Exclusion Principle) r (kpc) m -3 ~1/r 2 M.H.Chan 1kpc~3.3kly radius of visible Milkyway distance from center neutrino density

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

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

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

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!

1+3+n Cosmology spacetime curvature energy-momentum

Generalized Friedmann Equations Effects of extra dimensions (geometric)

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

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

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

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!

Understanding the Dark Universe 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」 Summary