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Cosmology Universe everything we can detect Questions: Infinite or finite? Is there an edge, a center? How old is it? Olber s Paradox (actually discussed much ... – PowerPoint PPT presentation

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Title: Cosmology

  • Universe everything we can detect
  • Questions
  • Infinite or finite?
  • Is there an edge, a center?
  • How old is it?
  • Olbers Paradox (actually discussed much earlier,
    by Digges, Kepler, Halley)
  • Why is the sky dark at night?
  • Assumptions static and infinite Universe
  • Universe is uniformly
    filled with stars

  • Then, in any direction, line-of-sight will
    intersect a star ? the sky should be as bright as
    the surface of a star?
  • (of course, L is proportional to 1/distance2, but
    number of stars increases as distance2)
  • Olbers suggested light blocked by intervening
    clouds of matter ? cant work, because clouds
    would heat up and radiate (sky would still be
  • were our initial assumptions wrong?

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  • A solution suggested by E.A. Poe (yes, him!)
  • What if the Universe were created at some time in
    the past?
  • With a long enough look-back time, there would
    be no stars.
  • Basic Assumptions of Cosmology
  • Homogeneity matter uniformly distributed in
  • Isotropy Universe looks the same in all
  • Universality physical laws we know on Earth
    apply everywhere

  • 2) are known as the Cosmological Principle
  • Note the assumptions only apply on the largest
    scales there is ample evidence for local
    inhomogeneity, anisotropy (e.g. galaxies)
  • Also evolutionary changes not considered
  • Expansion of the Universe
  • Hubble detected redshifts of galaxies
  • Velocity proportional to distance (like raisins
    in a rising loaf of bread)
  • But are redshifts really due to motion?

  • General Relativity
  • Expansion of Universe is really an expansion of
    space-time itself
  • Photons are stretched when traveling through
    expanding space-time ? hence, shifted to longer
  • Curvature of Space-Time
  • General Rel. mass curves space-time
  • We experience the curvature as a gravitational
  • The total amount of mass defines the geometry of
    the Universe

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  • How can we comprehend this?
  • Imagine an ant walking on an orange e.g., a
    two-dimensional Universe finite, but unbounded
    (i.e., edge-less)
  • 3 different curvatures (geometries) possible
  • Zero flat Universe in this case, the area
    of circle p r2 same for larger radii
  • Positive closed Universe as radius of a
    circle increases, the area increases at lt p r2
  • Negative open Universe as radius of a circle
    increases, the area increases at gt p r2

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  • Now, in 3-dimensions, we deal with volumes of
    spheres instead of areas of circles.
  • In flat Universe volume 4/3 p r3
  • How can we determine the curvature?
  • Count galaxies if the number increases
    proportional to r3, Universe is flat if the
    number increases more quickly with radius, the
    Universe is open, if more slowly, its closed
  • Also both open and flat cases Universe is
    infinite closed case Universe is finite, but
    edgeless (like the orange)

  • Big Bang Theory
  • Universe began in a high temperature, high
    density state ? not at a single point, the Big
    Bang filled entire volume of the Universe
  • We can look back to the era of the Big Bang
    radiation from that time is highly redshifted (to
    IR, radio)
  • Gamov (1948) predicted that early Big Bang was
    hot, emitted blackbody radiation ? in present
    era, wed see a redshifted blackbody spectrum

  • Penzias Wilson (early 1960s won Nobel Prize,
    1978) radio measurements of the sky found
  • Noise turned out to be Cosmic Microwave
    Background Radiation
  • Butwas it a black-body?
  • Cosmic Background Explorer (COBE) 1989 confirmed
    it was B-body (T 2.7 K, in agreement with
    predictions) z 1000
  • Originally from gas clouds of T 3000K
  • Almost perfectly isotropic some blueshift in
    direction of the Milky Ways motion towards Virgo

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  • A timeline
  • At t 0 matter in state where physics not well
  • At t 10 millionths of a second T gt 1012K,
    density 5 x 1013 gm/cc (same as an atomic
  • At this point, Universe was entirely in the form
    of radiation (but, E M c2)

  • And ?-rays decay ? particles, antiparticles
  • As Universe expands wavelengths increase,
    Energy decreases ? cools off
  • At t 0.0001 sec, T lt 1012 K, ?-ray decay can no
    longer produce protons, neutrons (although
    electron/positron pairs can be produced until t
    4 seconds)
  • Cooling/expanding continue until atomic nuclei

  • At t 2 minutes p n ? deuterium
  • At t 3 minutes deuterium ? helium
  • But, nothing heavier (no stable atomic nuclei
    with atomic weights 5 8)
  • At t 30 minutes nuclear reactions mstop 25
    of mass in He nuclei rest is in H nuclei (i.e.
    protons) same as oldest stars
  • Up until t 106 years dominated by radiation
    gas ionized

  • Photons cant travel far (interact with
    electrons) matter and radiation are coupled
  • At t 106 years cool enough to form atoms gas
    becomes transparent to radiation (Recombination)
    T 3000 K this is the point where the CMB
    radiation originates
  • Matter now dominates

  • Future of the Universe
  • Question of density ? hence, geometry
  • critical density 4 x 10-30 g/cc results in a
    flat Universe
  • If density lt critical density ? open
  • If density gt critical density ? closed
  • If the Universe is open or flat ? will expand
    forever gravity may slow expansion, but will
    never stop it

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  • If Universe is closed expansion stops, followed
    by contraction and collapse to high density state
    (Big Crunch)
  • Oscillating Universe? Probably not.
  • How do we measure the density?

  • Galaxy counts (but, hard to really determine the
    mass) and what about dwarf galaxies?
  • Best estimate 5 x 10-31 g/cc
  • But gt 90 may be in form of Dark Matter from
    study of lensing, mass 10 times greater than
    estimated from luminosity halos 10 to 20 times
    larger than visible parts of galaxies ? so, 10 x
    more mass

  • Deuterium
  • Amount depends on density of early Universe
  • Can be destroyed, but no longer created
  • In gas near Quasars, 25 D per every 106 H atom
    tells us that the Universe was not so dense that
    more D ? He
  • Taking all into account, maybe 5 of mass
    required for a flat Universe

  • Regarding Dark Matter
  • Exotic non-interacting particles (WIMPs)
  • Neutrinos (not enough of them) also, cant
    account for galaxy formation (too hot)
  • MACHOs massive compact halo objects should act
    as gravitational lenses a few detected, but not

  • Quantum Universe
  • flatness why a flat Universe (note, we are
    within a factor of 10 of critical density)
  • horizon problem isotropy of CMB (same to 1
    part in 1000)
  • At time of recombination, gas was transparent to
    radiation but not enough time for signals to
    travel from one region to another

  • Inflationary Universe
  • Sudden expansion when Universe very young
  • 4 fundamental forces Gravity, Electromagnetism,
    Strong, Weak Unification Electromagnetism and
    Weak force same at high energies
  • Grand Unification Theories in early Universe,
    all 4 forces unified
  • At t 10-35 sec, fundamental forces separated
    huge amounts of energy released, Universe
    inflated by factor of 1020 to 1030

  • Solves problems inflation forces curvature to
    zero (Flatness) and temperatures equalized
    before inflation (Horizon)
  • Quantum Mechanics predicts spontaneous creation
    of matter/anti-matter -- maybe how Universe
    appeared in the first place

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  • Age of Universe
  • Distance/velocity time, but v H x d
  • So, T (1 / H) x 1012 years
  • Doesnt account for gravity
  • If H 80, Universe is younger than oldest
    globular clusters!
  • Best current values age of globulars 11 x 109
    years, H 70 Universe is 13.5 x 109 years, so
    OK, but a concern

  • Cosmological Constant
  • Einstein (1916) didnt know Universe was
    expanding --- need force to keep it from
    collapsing ? Cosmological Constant
  • Hubble saw redshifted galaxies, so ? 0,
    everything Ok, right?
  • By looking at Type Ia Supernovae in very distant
    galaxies, get distances expected to see
    expansion slow down ? But saw just the

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  • Thus, ? is not 0 what is going on? Energy from
    Q.M. processes in empty space? Maybe.
  • Anyhow, if ? gt 0, Universe could be older than
  • 1 / H (so, no worries about stars older than the
  • Also critical mass normal matter 5 dark
    matter 25 rest in form of empty space (E Mc2)

  • Structure of Universe
  • Superclusters groups in filaments/walls
  • CMB is uniform
  • How did the structure form out of the hot gas in
    the Big Bang?
  • Also must have occurred early (Quasars at
    look-back time 93 of the age of the Universe)
  • Cold dark matter galaxies
  • Hot dark matter filaments
  • But, what are the galaxy seeds QM
    fluctuations, magnified by Inflation consistent
    with small CMB fluctuations detected by COBE