Astrophysics and Cosmology

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Astrophysics and Cosmology

• Lecture 26

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Concepts

• Parsec, light year
• Curved space
• Hubbles law
• Big Bang
• Early universe

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Units to measure large distances

• Light second cx1s3x108 m 3x105 km
• Earth circumference 40,000 km 0.13 light
  seconds
• Earth Moon 1.28 light seconds
• Light minute cx60s1.8x1010 m
• Earth Sun 8.3 light minutes
• Earth Pluto 311 light minutes
• Light year cx1y9.46x1015 m

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Scales of the universe

• Distance from Earth
• Proxima Centauri (next door neighbor) 4.3 ly
• Center of our Galaxy (Milky Way) 3x104 ly
• Our galaxy (Milky Way) is a disk
• D100,000 ly
• thickness 2,000ly
• total number of stars in Milky Way 1011
• Nearest galaxy (Andromeda nebula) 2x106 ly
• Farthest galaxies 1010 ly

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How to measure heavenly distances?

• Cannot clock the light, cannot use a ruler
• Parallax apparent motion of a star against the
  background of more distant stars

f90-q Dd/tan(f) d1.5x108km Parallax angle in
seconds distance to the star in Parsec 3.26 ly
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Other information from the sky

• Apparent brightness ? on average related to
  distances
• Spectrum ? temperature
• Red shift related to relative velocity ?
  distances
• High energy radiation
• Neutrinos (m0, weak interaction) propagate
  great distances
• Experiment ? observation
• SLOAN digital sky survey http//skyserver.fnal.go
  v/en/
• Hubble telescope http//www.stsci.edu/ftp/science
  /hdf/hdf.html

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Hubble deep field
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Hertzsprung-Russel (H-R) diagram

• Luminosity increases with stars mass
• Temperature related to the wavelength
  lT2.9x10-3mK
• By measuring l we can find T, then using H-R
  diagram we can predict the absolute brightness
  (L).
• The apparent brightness (l) is related to L and
  the distance to the star

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Evolution of the stars-I

• Stars are born when gaseous clouds (mostly
  hydrogen) contract due to gravity
• Gravity accelerates the particles of the star
  inward ? kinetic energy is increasing, could be
  large enough (1keV?107K) to overcome coulomb
  repulsion and start nuclear fusion H?He (In our
  Sun yellow dwarf)
• Pressure from the energy released in fusion keeps
  the star from collapsing
• When the hydrogen in the core burns out the core
  contracts and T goes up ? the outer envelope
  expands and cools down (Red giant)
• The core continues to heat up and He starts
  burning in fusion and continue to higher Zs
  ending nucleosynthesis at Fe and Ni
• No pressure from fusion gravitational collapse
  white dwarf
• Pauli principle for orbital e keeps the star from
  further collapse
• T goes down white draft becomes black dwarf
  (cloud of ash)

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Evolution of the stars-II

• Heavier stars continue to burn beyond Fe and Ni
  in endoergic reactions
• In addition the following process can occur
• e-p?nn
• Neutrons are formed in abundance neutron star
  (gt1.5 mass of Sun, D10km) Pauli principle for
  neutrons limit the size
• No electrostatic repulsion leads to a
  catastrophic collapse supernova explosion
• If mass of neutron star gt2-3xSolar mass black
  hole not even light can escape

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Gravity and curvature of space

• Einsteins general relativity No observer can
  determine by experiment if he is accelerating or
  is rather in a gravitational field
• Explain gravity (interaction) through curvature
  of space (geometry)
• Establish equivalence between gravitational and
  inertial mass
• Experimental proof Curving light straight line
  becomes curved in gravitational field
• Extreme curvature black hole black because not
  even light can escape it

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Expanding universe

• Redshift spectral lines shifted object is
  moving
• In 1929 Edwin Hubble, measured the redshifts of a
  number of distant galaxies. the redshift of
  distant galaxies increased as a linear function
  of their distance
• Hubbles law
• vHd
• v- velocity of galaxies, d distance
• H80km/s/Mpc
• The universe is expanding.

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Age of the universe

• vHd
• v- velocity of galaxies, d distance
• H80km/s/Mpc 20km/s/million ly
• Farthest galaxy 1010ly
• td/vd/(dH)1/H15x109yr

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Universe evolution
Age of the universe 1010 years Cosmic Microwave
background echo of the Big Bang
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Cosmic microwave background

• Discovered in 1964 by Arno Penzias and Robert
  Wilson as a noise in radio telescope
• Cosmic microwave background at l7.35 cm
• Blackbody radiation at T3K
• Present precise measurement 2.7K
• Echo of the Big Bang, predicted in 1940 by George
  Gamow
• Radiation decoupled from matter when atoms were
  formed and there were no free electrons to
  scatter light (3000K, 0.3 Myears after birth)

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Fate of the Universe

• Gravity slows down the expansion
• Depending on the density the universe might
• Continue to expand infinitely
• Collapse back to a point

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WMAP Launched from cape Canaveral on June 30 2001
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Trajectory
Lunar swingby
Phasing loops
Official arrival date Oct 1, 2001
100 days to L2, 1.5e6 km from Earth.
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COBE 1992
Bennett et al 2003
WMAP 2003
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Facts first, then the conclusions!
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BEYOND LCDM model
FLATNESS
Riess et al. 2001
HST meas. of Ho
de Bernardis et al 2000
Verde et al 2002
(Spergel et al 2003)
After
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We (and all of chemistry) are a small minority in
the Universe.
Compare gravitational rotation of galaxies with
luminous matter