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1 COSMOLOGY PHYS OBSERVING THE UNIVERSE Part I Giampaolo Pisano - Jodrell Bank Centre for Astrophysics The University of Manchester - January

2 OBSERVING THE UNIVERSE Darkness at Night Universe at different Wavelengths Large Scale Structures of the Universe The Cosmic Microwave Background Homogeneity and Isotropy The Expansion of the Universe Particles in the Universe References: Harrison, Cosmology Chap. 24 Ryden, Introduction to Cosmology Par. 2.1

3 Darkness at Night (1/6) Harrison Figs Dark Night Sky Paradox Digges (1576), Kepler (1610), Olbers (1826) Why the night sky is dark at visible wavelengths instead of being bright? If the Universe has infinite extent, populated everywhere with bright stars, the entire sky should be covered by stars with no dark gaps. A line of sight must eventually intercept the surface of a distant star. Forest analogy

4 Darkness at Night (2/6) Harrison Figs Interpretation A Interpretation B Sky covered by stars with no gaps but most stars cannot be seen Sky not covered by stars and dark gaps are real

5 Darkness at Night (3/6) dr Halley s approach (1720) - Let s assume an infinite Universe with: r n: average # density of stars L: average stellar luminosity - Flux received on Earth from a star at a distance r : f ( r) = L 4π r 2 - Brightness from a shell of stars: db( r) = 2 L n 4π r nl dr = dr 2 4π r 4π 4π Power Brightness = Unit area Solid (it doesn' t depend on - Total brightness from all the stars in the Universe: nl B = db = 4 dr π The night sky should be infinitely bright Absurd r) 0 0 Angle =

6 Darkness at Night (4/6) - We have implicitely made different assumptions: Assumption 1 Unobstructed line of sight (not true) Nearby stars, with their finite angular size, hide distant stars: Finite sky brightness equal to that of a typical star Absorption by Inter-Stellar Medium (ISM) might hide stars (Olbers): ISM would heat up and emit at the same temperature (Kelvin) Assumption 2 n L constant throughout the Universe (might not be true) Distant stars might be less numerous and less luminous than nearby stars: Hierarchical clustering Universe would be anisotropic Cosmic island (Stoic): Not supported by observations

7 Darkness at Night (5/6) Assumption 3 Universe infinitely large (might not be true) Finite Universe with r max Aristotelian Cosmic edge: nl Finite brightness: B max 4π r Assumption 4 Light flux distant source follows the inverse square law Universe might not be Euclidean: Einstein: the Universe can have different geometries Sources non stationary: Light can be blue- or red-shifted (might not be true) Assumption 5 Universe infinitely old (might not be true) Finite speed of light: We look back in time nl Universe with finite age with t 0 : Finite brightness: B 0 4π ct

8 Darkness at Night (6/6) Edge of observable Universe Paradox solutions Poe (1848), Kelvin (1901), et al.. The Universe has a finite age Speed of light is finite Stars beyond the horizon distance are invisible to us Number of visible stars too few to cover the entire sky However: Stars have a finite luminous age (~10 10 y) Even in an infinitely old Universe, the stars would not contain enough energy to shine and fill the space with starlight radiation Note: The sky is not dark at mm-wavelengths CMB...

9 OBSERVING THE UNIVERSE Darkness at Night Universe at different Wavelengths Large Scale Structures of the Universe The Cosmic Microwave Background Homogeneity and Isotropy The Expansion of the Universe Particles in the Universe References: Liddle, Introduction to Modern Cosmology Par. 2.1, 2.2

10 The sky at different wavelengths ν Initially, the Universe was observed only at optical wavelengths

11 The sky at different wavelengths and beyond.. Beyond visible light Radio waves: high resolution maps of very distant galaxies, many furthest galaxies detected this way Microwaves: CMB as BB at K, astonishingly uniform, best evidence of cosmological principle, tiny anisotropies at 10-5 level Infrared: surveys spotting different galaxy population: young galaxies; useful to look through dust close to galactic plane X-ray: cluster of galaxies hot gas emission at tens of millions K γ-ray: gamma-ray bursts Beyond e.m. spectrum Neutrinos (supernova explosions) High energy cosmic rays Gravitational waves (colliding stars)

12 OBSERVING THE UNIVERSE Darkness at Night Universe at different Wavelengths Large Scale Structures of the Universe The Cosmic Microwave Background Homogeneity and Isotropy The Expansion of the Universe Particles in the Universe References: Liddle, Introduction to Modern Cosmology Par. 2.1, 2.2 Ryden, Introduction to Cosmology Par. 2.2 Serjeant, Observational Cosmology Par. 3.11

13 Solar System A.Z. Colvin - Wikipedia ~ 0.25x 10-3 pc Ø (~50 AU) 1 AU = 150x 10 6 km 1pc = l.y. = km - Astronomical Unit: Average distance Earth-Sun -Parsec (pc): Distance from the Sun corresponding to a parallax of 1 arcsec (1 AU at1pc distance looks with an angular separation of 1 arcsec)

14 Solar Interstellar Neighborhood ~ 40 pc Ø A.Z. Colvin - Wikipedia - Nearest star: Proxima Centauri 4.2 l.y. -Main source of light: Nuclear fusion within stars - Different type of stars: Less or more massive than Sun

15 Milky Way Galaxy ~ 30 kpc Ø A.Z. Colvin - Wikipedia -Milky Way: ~100x10 9 stars ( M ʘ ) -Bulge + disc: r= 12.5 kpc t= 300 pc Solar Interstellar Neighborhood - Solar system: 8 kpc off-centre Rotation period ~200x10 6 yr -Disc: slow differential rotation ESO - Globular clusters: Symmetric distribution: 5-30 kpc ~10 6 stars per cluster - Galactic Halo: Spherical, larger than disc Spiral Galaxy M100

16 Local Galactic Group ~ 2 Mpc Ø A.Z. Colvin - Wikipedia - Local Group: ~40 galaxies -MW and M31: largest galaxies -MW accreting LMC & SMC -M31 at 770 kpc -LMC at 50 kpc -Canis Major dwarf galaxy: At 7.6 kpc Closer to us than MW centre! -Sagittarius dwarf galaxy at 25 kpc -MW falling toward M31: Future merger HST Andromeda Galaxy M31

17 Local Group: Predicted Merger between Milky Way and Andromeda Galaxies Today 2 x10 9 yr 3.75 x10 9 yr M31 bigger angular size 3.85 x10 9 yr New star formation 3.9 x10 9 yr Star formation 4 x10 9 yr MW warped M31 tydally stretched 5.1 x10 9 yr Galaxy cores as pair of bright lobes 7 x10 9 yr Core of huge elliptical galaxy

18 Virgo Supercluster ~ 50 Mpc Ø A.Z. Colvin - Wikipedia - Galaxy groups: ~ few Mpc 3 Galaxies separation ~ 1 Mpc - Galaxy clusters: M > M ʘ Largest bounded structures Universe - Nearest cluster: Virgo cluster Local group will be accreted by it - Local group: Interacts with Maffei I, Sculptor, M81, M83 Groups - Local (Virgo) supercluster: Contains all these structures

19 CfA Galaxy Redshift Survey ( ) ~ 220 Mpc deep Stick man & Great Wall -Survey: spectra / redshifts -Narrow slice: 6 x 130 deg - Discovery: - Galaxy distribution not random Filamentary structure - Great Wall: Cluster of galaxies surrounded by voids

20 2dF Galaxy Redshift Survey ( ) ( 2 degree field ) Z < 0.3 Robert Smith Survey regions Milky Way -Survey: spectra / redshifts -Range 0 < z < 0.3 -First large scale map: - Superclusters -Walls - Giant Voids - Very large scales: Begin of Universe homogeneity

21 Local Superclusters A.Z. Colvin - Wikipedia ~ 600 Mpc Ø COMA Supercluster: at 100 Mpc ~10000 galaxies -Superclusters: joined by filaments and walls of galaxies Longest dimension ~100 Mpc -Foam-like structures: with large voids (up to Mpc) - Superclusters: Largest structures in the Universe

22 2dF Quasar Redshift Survey -Survey: redshifts -Range: z < 3 Galaxy survey Billions of light years Z< 3 Milky Way -Quasars: due to supermassive black hole in the centre of galaxies AGNs Redshift Quasars Extremely luminous, visible at large distances -Distribution of quasars almost homogeneous at these scales

23 Observable Universe Z< 3 A.Z. Colvin - Wikipedia -2dF survey & Sloan Digital Sky: Hundred thousands galaxies No structures greater than Superclusters and Voids - Large Scale Smoothness At hundreds of Mpc the Universe begins to appear smooth Cosmological Principle Observational demonstration

24 OBSERVING THE UNIVERSE Darkness at Night Universe at different Wavelengths Large Scale Structures of the Universe The Cosmic Microwave Background Homogeneity and Isotropy The Expansion of the Universe Particles in the Universe