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Cosmology

The known part of the Universe - to a distance of ~ 4000 Mpc

• 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

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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=102 to 103)

• Superclusters of galaxies - contain up to thousands of galaxies (N=103 to 104)

• 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 = 105 to 106 x Dstar )

Redshift in spectra of galaxies

Redshift

lightofspeedwavelengthreal

wavelengthrealwavelengthshiftedobjectofspeed ×−

=

wavelengthrealwavelengthrealwavelengthshiftedredshift −

=

0

−=

λλλ 0z

zcv =Late 1920’s:Hubble plots distances versus velocities of galaxies

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1929 1931

Distance and Velocity (away) are related!v = H x D

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”

• What exactly is expanding?– Ordinary things are not expanding – The expansion is noticeable only at fairly large separations

The cosmological redshift means The cosmological redshift means expansion of spaceexpansion of space

Therefore, the redshift of distant galaxies is not because theyspeed up away from us, but because the Universe expands

Cosmological redshift –light waves stretch as space expands 8

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 together - estimation of the expansion age of the Universe

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• Tremendous explosion (filling all of space with all of the particles )

• We can’t see radiation produced during the first 300 000 yrs

• Enormous increase of scale during a very short time in the early universe (inflation model)

• 300 000 yrs after BB universe had cooled to about 3000 billion degrees Kelvin

• The universe became transparent for the radiation – isotropic cosmic background radiation

The Big Bang

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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.

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Galaxy Cluster in the Early Universe The cosmic microwave background (CMB) radiation

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Satellite missions detected fluctuations in the microwave background:

COBE (1989) and WMAP (2001)

2001, NASA’s -- Wilkinson Microwave Anisotropy Probe

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Will The Universe Expand Forever?

• The universe is currently expanding

• 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

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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 energy16

Expansion forever or collapse?

A sketch of how gravity and the energy of expansion determine the behavior of the Universe.

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The fate of the Universe• Not enough information about the parameters of

the Universe• Cosmological models: the fate depends on

average density• Critical density 3H2 / 8πG = 8.3 x 10 -30 g/cm3

• 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

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Cosmic Microwave Background

•Fading light of the cosmic explosion that gave birthto our universe•Radiation we receive now when the universe was one thousandth of its current size

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The makeup of the universe

• Deduced from observations of the brightness variations in the CMB

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The fate of the Universe

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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 expansionand the pull of gravity

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Geometry of the Universe Geometry:

Positively curvedClosed space; finite Universe; Ω > 1 (Big Crunch)

Negatively curvedOpen space; infinite Universe; Ω < 1 (expands forever)

FlatOpen space, infinite Universe; Ω = 1

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Geometry of Universe

2001, NASA’s -- Wilkinson Microwave Anisotropy Probe

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Primordial ripples in density created the large-scale structure that we observe now

What What created created the the galactic galactic clusters?clusters?

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Fig. 25.26

Other SuperclustersSuperclustersVirgoComaCentaurus…

Large Sheets of GalaxiesGreat WallPerseus-Pisces Great Wall…

Voids

“Great Wall”Large single structure Dimensions are about 600x250x30 million light yearsGiant quilt of galaxies across the sky

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The fate of the Universe• Not enough information about the parameters of

the Universe• Cosmological models: the fate depends on

average density• 3H2 / 8πG = 8.3 x 10 -30 g/cm3

• 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

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HST Image of a gravitational lens

Obtaining the mass of clusters of galaxies

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Geometry of Universe

2001, NASA’s -- Wilkinson Microwave Anisotropy Probe

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The fate of the Universe