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Cosmology and Clusters of Galaxies

Cosmology and Clusters of Galaxies

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Cosmology and Clusters of Galaxies

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  1. Cosmology andClusters of Galaxies Patrick Koch Cosmology: - what is cosmology? - principles - Big Bang: theory and tests - structure in the Universe Clusters: - largest bound objects - X-ray observation - a many-purpose tool - the SZ effect Summer student lecture, july 17, 2007

  2. What is cosmology? • - scientific study of the large scale properties of the • Universe as a whole • tries to understand origin, evolution and ultimate • fate of the Universe • - Big Bang • scientific discipline: • theory  prediction of phenomena observations • century of high-precision cosmology • large scales and small scales and the links in between • interdisciplinary: combining all kind of physics • (plasma, gravity, particle, solid states, chemistry, biology) • tremendous future investements (satellites...) • ....

  3. The Principles (1) The location principle: - It is unlikely that we occupy a special place in the Universe - Universality: the laws of physics are the same everywhere The cosmological principle: • Homogeneity: on the largest scale the Universe has the • same physical properties everywhere • Isotropy: on the largest scale the Universe looks the same • in any direction

  4. The Principles (2) The anthropic principle: • the Universe is fine-tuned to our existence: • strength of gravity • smoothness of Big Bang • strength of strong nuclear force • finally the earth Why??? - chance - luck -‘theory of everything‘ - ...?

  5. Big Bang: theoretical pillars (1) ...expresses the idea that everything started from something very small: why do we need a Big Bang? Experiment } • expansion of the Universe • abundance of light elements • cosmic microwave background observational tests! framework given by: General Relativity (Einstein equations) + cosmological principle + microphysics (particle physics/thermodynamics) Theory

  6. Big Bang: theoretical pillars (2) ... something about General Relativity: • - generalization of Newton‘s law, special relativity • (covariant formalism) • Space (curvature) linked to matter and energy (all kinds) • → equation of state (pressure – energy relation) • missing: the small one (quantum gravity) Basic matter classification: - radiation: photons, neutrinos (positive pressure) - baryonic matter (=‚ordinary matter‘): composed of protons, neutrons, electrons (no pressure) - dark matter: no direct detection yet, but existent (no pressure) - dark energy: the mysterious one (vaccum property?) (negative pressure) Achievements: - correct gravitational bending of light by sun - Mercury perihel - gravitational wave prediction - prediction of black holes as coordinate singularities - Big Bang for a homogeneous expanding Universe

  7. Big Bang: Intermezzo ...some common misconceptions: • Big Band did not occur at a single point in space as an • explosion (space not yet born!) • there is no center of expansion • (both for finite and infinite Universes) • Universe is all space and time we know, thus no • information about the embedding • what gave rise to the Big Bang is beyond the • Big Bang model • (speculations in Pre-Big-Bang models)

  8. Big Bang tests (1): Universe expansion Hubble, 1929 on the very large scale in time and space: Einstein equations+matter distribution give global dynamics galaxies outside of Milky Way systematically moving away from us: v ~ d (Hubble Law), light from galaxies shifted towards red end Redshift z = (ν‘-ν)/ν Hubble law confirms homogeneous expanding Universe ν‘:emitted freq. ν: observed freq.

  9. Expansion: Intermezzo common misconception: redshift (z) = Doppler shift? NO! • it is not galaxies moving through space, but space is expanding, • carrying the galaxies along • the galaxies themselves are not expanding • (gravitationally bound, isolated system) • cosmological redshift (z) is NOT a Doppler shift • (Doppler shift would put us in the center of expansion, • location principle violated) • - redshift to velocity/distance conversion requires specific model

  10. Big Bang tests (2): light elements • Big Bang theory: • early Universe was very hot, 1s after • Big Bang: 10 billion degrees, filled with • neutrons, protons, electrons, positrons, • photons, neutrinos • in 3 minutes (Universe expands and cools): • T and density right for nuclear fusion: • Big Bang nucleosynthesis: • (=light element formation in early Universe) • n → p + e- , n combine with p to deuterium, • deuterium mostly combines to He, some Li •  about 24% of ordinary matter is He, • produced in Big Bang, also observed! •  abundance depends on ordinary matter in • the early Universe • next step: constrain ordinary matter • → WMAP about 8% of critical density

  11. Light elements: Intermezzo Big Bang produces no heavy elements • elements heavier than Li are all synthesized in stars • (bottleneck in BBN: triple collision of He-4-nuclei, takes • 1000 yr for He – C conversion) • late stage of stellar evolution: massive stars burn • He to carbon, oxygen, silicon, sulfur, iron • elements heavier than iron: outer envelopes of • super-giant stars and in supernovae explosion

  12. Big Bang tests (3): CMB BBN • surfaces of last scattering • = snapshots of the Universe • different opacities: • z ~ 1032: graviton opacity • z ~ 1010: neutrino opacity • z ~ 103: photon opacity: • CMB initial conditions primary fluctuations + time evolution + secondary fluctuations + foreground = today‘s CMB (primary fluctuations: intrinsic temperature change due to compression, Doppler shift, Sachs-Wolf effect)  size, matter content, age, geometry of the Universe tool: linear perturbation of Einstein equations

  13. Big Bang tests (3): ...CMB ΔT ~ 10-4 immense progress: - 30 x better resolution - 45 x higher sensitivity integration: 2 yr WMAP=4500 yr COBE

  14. CMB: Intermezzo ... the story of a Nobel Prize: The well known thing: 1963: Penzias and Wilson: radio emission from Cassiopeia A SN remnant, uniform noise source → NP in 1978 1965: Dicke et al.: explain cosmological significance of measurement The unknown things in theory: 1934: Tolman: TD in expanding Universe (no relic predicted) 1946: Gamov: Hot Big Bang model 1948: Alpher and Herman: Nucleosynthesis in early Universe, predict T ~ 10 K 1949: Alpher and Herman: T ~ 5 K 1964: Doroshkevich and Novikov suggest CMB detectability as test if Gamov’s model 1964: Dicke: search for radiation The unknown things in observations: 1940: Andrew McKellar: interstellar absorption lines from rotationally excited CN molecules, consistent with thermal equilibrium of T ~ 2.3 K (remark in footnote of PhD thesis!) 1941: Walter Adams: similar data 1955: Emile Le Roux: all sky survey at 33cm wavelength: finds isotropic emission from T ~ 3K. 1957: Shmaonov: signal at 3.2cm corresponding to T ~ 4K, independent of direction

  15. Big Bang tests (3): CMB – angular power spectrum • Decomposingpatterns of structuresin spherical harmonics (“Fourier transform” of anisotropy on the sphere) • Rotational invariance (average over m) Angular power spectrum Cl is the variance of structures on scale l, on angular scales DT/T map ΔT ~ 10-4 q

  16. Big Bang tests (3): CMB - fundamental physics geometry, age Acoustic oscillations: Interaction photon/baryon fluid with gravity Baryon density DM density detailed physics worked out in cosmological perturbation theory

  17. In summary: Universe = 95% unknown!

  18. Structure in the Universe - observed structures: from stars to galaxies to cluster to supercluster - Big Bang does NOT explain the existence of structure: structure formation scenario must be built in Big Bang framework SDSS: galaxy survey Idea: - Structure grown from gravitational pull of small fluctuations (where do the fluctuations come from?) - hierarchical formation

  19. Clusters of galaxies • largest bound objects in the universe: hot plasma • virialized, hydrostatic equilibrium • M ~ 1015 MS, T ~ 107-108K, n ~ 10-2 - 10-4cm -3, r ~1-2 Mpc • high luminosity Lbol 1041-1046 ergs/s • observations: X-rays, radio, optical Merger tree major mergers Non-linear processes: different morphologies! Linear perturbation theory Clusters of galaxies: present at least since z ~ 1.5 back in redshift z

  20. Clusters of galaxies:coma diffuse radio (Effelsberg) optical core: turbulences? Merger example: a lot of microphysics! X-ray (Rosat) X-ray (XMM) X-ray (Chandra)

  21. Clusters of galaxies same scale: very different characteristics/emission extension! high energy emission: ~ keV mechanism: thermal bremsstrahlung source: intracluster medium How can it be so hot? → deep grav. potential What is the gravitational potential? stars, galaxies the ‚invisible‘ DM

  22. Clusters of galaxies: DM (1) ... or how do we know about DM? • 1930: Fritz Zwicky discovered that the galaxies in the Coma cluster were moving too fast to remain bound in the cluster according to the Virial Theorem: • Stable galaxies should obey this law: 2K = -U • where K=½mV2 is the Kinetic Energy • U = -aGMm/r is the Potential Energy (a is usually 0.5 - 2, and depends on the mass distribution) • Putting these together, we have M=V2r/aG. • Measure M= Σi mi, r and V2 from observations of the galaxies; then use M and r to calculate Vvirial • Compare Vmeasured to Vvirial • Vmeasured > Vvirial which implies M was too small Evidence for DM  90 % DM • Mass/Luminosity comparison:

  23. Clusters of galaxies: DM (2) ... or how to measure the weight of clusters? Crude diagram of cluster atmosphere ICM in (most) clusters close to gravitational equilibrium: outward gas pressure ≈ inward gravitational pull Hydrostatic Equilibrium Spherical Symmetry Poisson’s Equation luminous matter: temp., density total mass! (including DM) Pressure reaches maximum in cluster core X-ray obs. Outward pressure force balances inward gravity at every location within the cluster • Cluster gas pressure profile: • - Separable problem: • measure gas density profile • measure gas temperature profile • - Need X-ray observations!

  24. Clusters of galaxies: DM (3) X-ray obs. spectrum  temperture Measure T in Each Annulus [millions of light years] Remark Line: Best Fit heavy elements: Hints for stellar nucleosynthesis, Supernovae, galaxy-galaxy interaction Points: Observations brightness  density

  25. Clusters of galaxies: DM (4) ... or how to use clusters as cosmological probes • Recall Primordial nucleosynthesis: • Light elements Deuterium, Helium and Lithium were produced when the universe was minutes old • Observations of the abundances of D, He and Li tightly constrain the mean baryon density WB of the universe: • Consider the universal baryon fraction fU? • ratio of mean baryon density ΩB to mean matter density ΩM • Can we use clusters as testbed for universal baryon fraction? • Galaxy clusters are the largest collapsed structures in the Universe. Next best thing to taking a census of the entire observable universe! • We can measure the fraction of galaxy cluster mass which is baryonic • Cluster baryon fraction fB • three baryon reservoirs: gas fgas, galaxies fgal and possibly even dark baryons fdb

  26. Clusters of galaxies: DM (5) • Measuring the mean matter density parameter WM • Measure cluster baryon fraction fB and calculate WM • Gas constitutes ~20% of the total mass in the most massive clusters • This provides lower limit on cluster baryon fraction, and hard, upper limit on cosmological density parameter ΩM Mohr, Mathiesen & Evrard ApJ 1999 , • Combined with measurements of the galaxy contribution to the cluster mass we get a best estimate of the density parameter

  27. Clusters of Galaxies and the Sunyaev-Zeldovich effect (SZE) What is it? • Inverse Compton scattering • (CMB photons – hot cluster e- • Spectral distortion of CMB • Up-scattering of photons • (Rayleigh-Jeans to Wien) • Secondary anisotropy radio signal, very weak ~ 100 mJy Spectral (x) function Scattering process Cluster properties / state  Effect depending on frequencyandcluster!

  28. Clusters of Galaxies and the SZE(2): power of SZE 10' 10' 6' OVRO/BIMA SZE vs. X-ray (insets) • X-ray emission brightness falls off sharply with distance • SZE brightness independent of distance (hν/kTcmb~ const.) • only depends on profile (potential well growth with frequency shift) with z and slight z dependence inTcmb. • can locate very distant clusters, if they exist… X-ray X-ray X-ray (Carlstrom et al.)

  29. Clusters of Galaxies and the SZE(3): science • (1) Cluster Survey via the Sunyaev-Zel’dovich Effect (SZE) • Evolution of number counts of galaxy clusters, N(z)  Dark Energy Equation-of-State, cosmological parameters • scaling relations: SZ(y0) – M(r500), SZ(y0) – Tx, SZ(y0) – Lx, Sν/fν – y0 (SZ only)  information of large-scale structure formation, entropy floor (non-gravitational processes) • Probing high-z universe (z>1) (2) Targeted cluster SZ observations: suitable target at right z • complementary to other SZ observations at lower frequency: e.g. additional data point for spectrum of thermal SZ fit, evolution of CMB temperature with selected clusters in z-range

  30. Clusters of Galaxies and the SZE(4): cluster microphysics multifrequency thermal SZ spectral fit: cluster microphysics • additional AMiBA data point @ 90 GHz: • further constraint on cross-over • frequency • → relativistic e- population, • cluster l.o.s velocity • further constraint on maximum • SZ decrement • → relativistic, non-thermal e- SuZIE 140,210,270 GHz BIMA/OVRO 30 GHz AMiBA 90 GHz → AMiBA science goals: next lecture by Keiichi Umetsu

  31. Clusters of galaxies: what else? • X-ray: the ‚classical‘ way • zoomed-in substructures: a lot of microphysics! • (mergers, relativistic particles, magnetic fields, • turbulences, ....) • X-ray with SZ observations: e.g. Hubble constant • clusters (huge mass!) as gravitational lenses • for background objects • (cosmological parameters, including DE)

  32. Appendix: The Beyond Einstein Program WMAP LIGO Hubble Science and Technology Precursors Chandra GLAST