Cosmology. Clusters of galaxies. Redshift. Late 1920 s: Hubble plots distances versus velocities of galaxies. λ λ. redshift =

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1 Cosmology Study of the structure and origin of the universe Observational science The large-scale distribution of galaxies Looking out to extremely large distances The motions of galaxies Clusters of galaxies Galaxy - huge group of stars, dust, gas, and other celestial bodies bound together by gravitational forces. Single galaxies (field galaxies): less than 8 % of total number of galaxies Groups of galaxies - the smallest aggregates of galaxies (N=10 to 100 members in a diameter of 2 Mpc) Cluster of galaxies - contain up to one thousand galaxies (N=10 2 to 10 3 ) Superclusters of galaxies - contain up to thousands of galaxies (N=10 3 to 10 4 ) Enormous clouds of extremely hot intergalactic gas and Dark matter Average distance between clusters r ~(10 x Cluster s Diameter ) -- much larger spatial concentration in comparison to the concentration of stars in a given galaxy ( r between stars = 10 5 to 10 6 x D star ) The known part of the Universe - to a distance of ~ 4000 Mpc 1 2 Redshift in spectra of galaxies Redshift shifted wavelength real wavelength speed of object = speed of light real wavelength shifted wavelength real wavelength redshift = real wavelength λ λ 0 z = λ v = zc 0 Late 1920 s: Hubble plots distances versus velocities of galaxies 1

2 Implications of the Hubble s Law Space between galaxies is expanding uniformly - the universe itself is expanding - the further away a galaxy is from us the higher the recession velocity Even though the galaxies appear to be moving away from us, we are not at the centre of the universe an observer in a distant galaxy would see the same effect Cosmological principle: the universe is homogeneous and isotropic no preferred places and directions Distance and Velocity (away) are related! v = H x D What exactly is expanding? Ordinary things are not expanding The expansion is noticeable only at fairly large separations The cosmological redshift means expansion of space Therefore, the redshift of distant galaxies is not because they speed up away from us, but because the Universe expands The Big Bang and the expansion age of the Universe Everything was located very close together in the past, Universe was very dense and hot about 14 billion years ago The expansion began with the Big Bang expansion of space the universe is expanding everywhere we look: there is no special location in the universe where the Big Bang originated the Universe has no centre How long a galaxy has been traveling: t=d / v t=d/(hd) t=1/h Hubble time one Hubble time ago all of the matter of the Universe was located Cosmological redshift together - estimation of the expansion age of the Universe light waves stretch as space expands 8 2

3 The Big Bang Tremendous explosion (filling all of space with all of the particles ) We can t see radiation produced during the first yrs Enormous increase of scale during a very short time in the early universe (inflation model) yrs after BB universe had cooled to about 3000 billion degrees Kelvin The universe became transparent for the radiation isotropic cosmic background radiation 9 Tests of Big Bang Theory The Universe at high redshifts galaxies look more irregular, different star-formation rate; galaxy interactions; quasars The expansion of the universe Galaxies are generally receding from each other The abundance of the light elements H and He observations agree with amounts of H and He from the Big Bang theory The cosmic microwave background (CMB) radiation The cosmic microwave background radiation is the remnant heat leftover from the early universe. 10 Galaxy Cluster in the Early Universe The cosmic microwave background (CMB) radiation 11 3

4 Satellite missions detected fluctuations in the microwave background: COBE (1989) and WMAP (2001) Will The Universe Expand Forever? The universe is currently expanding 2001, NASA s -- Wilkinson Microwave Anisotropy Probe Universe will expand forever: unbound universe Stars will consume their hydrogen and die. Universe will become a black, cold, empty space Universe might eventually stop expanding and begin to collapse: bound universe All objects and atoms will be compressed to higher and higher densities: Big Crunch Any alternatives? The expansion will stop after time = infinity 14 Expansion forever or collapse? Expansion forever or collapse? Factors affecting the expansion or collapse The Big Bang began the expansion of the Universe The gravitational force between galaxies has slowed the expansion Relative strength of these two effects expansion energy vs. gravitational energy Total energy = positive energy of expansion +negative gravitational binding energy 15 A sketch of how gravity and the energy of expansion determine the behavior of the Universe. 16 4

5 The fate of the Universe Not enough information about the parameters of the Universe Cosmological models: the fate depends on average density Critical density 3H 2 / 8πG = 8.3 x g/cm 3 Ω = actual density / critical density Ω = 0, the universe is empty, will expand forever Ω > 1, the universe will recollapse, bound universe Ω < 1, the universe will expand forever, unbound universe Ω = 1, still unbound universe 17 Fading light of the cosmic explosion that gave birth to our universe Radiation we receive now when the universe was one thousandth of its current size Cosmic Microwave Background 18 The makeup of the universe The fate of the Universe Deduced from observations of the brightness variations in the CMB

6 The Universe Flat or Curved? (Geometry) Bound or Unbound? Finite or Infinite? Closed or Open? (Shape or Evolution) Density of matter geometry of space The shape of the universe is determined by a struggle between the momentum of expansion and the pull of gravity Geometry of the Universe Geometry: Positively curved Closed space; finite Universe; Ω > 1 (Big Crunch) Negatively curved Open space; infinite Universe; Ω < 1 (expands forever) Flat Open space, infinite Universe; Ω = Geometry of Universe 2001, NASA s -- Wilkinson Microwave Anisotropy Probe What created the galactic clusters? Primordial ripples in density created the large-scale structure that we observe now

7 Fig Other Superclusters Superclusters Virgo Coma Centaurus Large Sheets of Galaxies Great Wall Perseus-Pisces Great Wall Voids Great Wall Large single structure Dimensions are about 600x250x30 million light years Giant quilt of galaxies across the sky The fate of the Universe Not enough information about the parameters of the Universe Cosmological models: the fate depends on average density 3H 2 / 8πG = 8.3 x g/cm 3 Ω = actual density / critical density Ω = 0, the universe is empty, will expand forever Ω > 1, the universe will recollapse, bound universe Ω < 1, the universe will expand forever, unbound universe Ω = 1, still unbound universe 26 Obtaining the mass of clusters of galaxies HST Image of a gravitational lens Geometry of Universe 2001, NASA s -- Wilkinson Microwave Anisotropy Probe

8 The fate of the Universe 29 8