Galaxies, Dark Matter and Cosmology

Review for Astronomy 123 Midterm Exam 2

posted May 5, 2015 jeb

Outline

• Galaxies and Dark Matter
• Cosmology

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Galaxies and Dark Matter

Dark Matter in the Universe
• More than 80% of the matter in the universe is dark
• Dark matter is detected in many ways
  • Orbital velocity of stars and gas in the Milky Way
  • Rotation curves of galaxies
  • Orbital motion of binary galaxy systems
  • Motions of galaxies within clusters of galaxies
• Dark matter is probably mostly composed of undiscovered, exotic subatomic particles
• Dark galaxy
  • Possible galaxy composed almost entirely of dark matter, emitting virtually no visible light
  • Existence not yet established

Galaxy Collisions
• Cartwheel galaxy
• Antennae
• Collisions have substantial effects on galaxies
  • Starburst galaxies
  • Star ejection
  • Shape distortion
  • Eventually, galactic cannibalism
• Andromeda is approaching the Milky Way, and will collide in a few billion years

Galaxy Formation and Evolution
• We know of no simple evolutionary path
• Hierarchical merging
  • Galaxies grew through repeated merging of smaller objects
• Galaxies often experience close encounters or collisions
• A galaxy left alone will evolve slowly, in a predictable way
  • Elliptical, lacking interstellar gas, will become fainter and redder as stars burn out
  • Spiral or Irregular, with much gas, will remain bluish as long as new star formation continues
• Galaxy mergers
  • Major mergers turn spirals into ellipticals
    • collisions between galaxies of comparable size destroy galactic disks
  • Minor mergers leave larger galaxy intact
    • collisions of small galaxies with large ones - smaller galaxy is absorbed

Black Holes in Galaxies
• Quasars and active galaxies are powered by supermassive black holes
  • Central engine of million/billion solar mass black hole
    • Black hole is a region of space where the pull of gravity is so great nothing can escape
  • Accretion disk
  • Jets of high-speed particles
    • synchrotron radiation
• Supermassive black holes are common
  • Every bright galaxy, whether active or not, may contain a central black hole
• Quasar epoch
  • early universe - quasars appear, and fade away
• Evolutionary sequence for galaxies
  • Highly luminous quasars
  • Radio galaxies
  • Seyfert galaxies
  • Normal spiral and elliptical galaxies

Universe on Large Scales
• Superclusters
  • Local supercluster is Virgo Supercluster
  • tens of thousands of galaxies
• Galaxies distributed on "bubbles" surrounding unpopulated "voids"
• Largest structure: 200-300 Mpc in size
• Great Wall is one of largest known structures
  • about 100 Mpc away
  • about 200 Mpc across
• Gravitational lenses
  • predicted by Einstein's general theory of relativity
  • Quasar mirages
    • Binary (or double) quasar
    • Einstein cross
  • Blue arcs
    • Images of young, background galaxies, warped by the gravitational field of a foreground galaxy cluster
• Bullet cluster
  • Collision of two galaxy clusters
  • Cluster about one billion pc distant
  • Dark matter passed through collision
    • located by lensing studies of background galaxies
  • Intergalactic gas collided, producing separated X-ray emitting region

Cosmology

Cosmology
• The study of the structure and evolution of the entire universe
• Largest structures of universe
  • 200 - 300 Mpc
• Cosmological Principle
  • On a large scale, universe is homogeneous and isotropic
  • Universe has no edge
  • Universe has no "center"
• Universality of physical laws
  • The same physical laws apply everywhere in the universe

The Expanding Universe and the Big Bang
• Olbers's Paradox - why is the sky dark at night?
  1. red-shift of starlight
  2. finite age of universe
• Big-Bang established by observation
  1. Hubble's Law (1st evidence)
     • v = H × d
     • H ≈ 20 km/s/Mly
     • for d = 100 Mly, v = 2,000 km/s
     • for d = 15,000 Mly, v = 300,000 km/s
       • (Note - in these examples, H is set to 20 km/s/Mly for simplicity. It is actually slightly larger, so d never exceeds 13,800 Mly)
  2. existence of quasars (early universe was different)
  3. cosmic microwave background (CMB)
  4. abundances
     • Helium, deuterium, lithium, beryllium
       • Helium is about 25% of the mass of normal matter in the universe
         • this is about 10% of the number density; hydrogen is about 90% of the number density
• Early on the whole universe was compressed to very high density
  • Since then the entire universe has been expanding
  • Since the universe is expanding everywhere, distant galaxies appear to be moving away from us in every direction

Discovery of Cosmic Microwave Background
• Predicted by Big Bang
  • Universe's distant past was very dense and hot
• First detected by Penzias and Wilson with radio telescope
• Very sensitive measurement achieved after eliminating sources of extraneous noise
  • Fixed loose joints
  • Repaired faulty connections
  • Removed pigeon droppings that created noise in detector
• Found 2.7 K background
  • Uniformly coming from all directions
  • Uniform throughout the day
  • Uniform through the seasons
• Peak wavelength about 1 mm
• Matches blackbody spectrum
• Emerged from big bang fireball when the universe was 400,000 years old and had a temperature of 3000 K
• Following big bang, as universe expands, background radiation gets cooler
  • from background radiation we can determine motion of earth through universe
• COBE
  • small asymmetry due to the Earth's motion through space

Steady State Theory
• Alternative to Big Bang
• Consistent with Hubble expansion
• Could not explain CMB - therefore rejected

Prediction of Big Bang
• Expansion of the universe shifted the initially high energy radiation in the universe to longer and longer wavelengths
  • Cosmological redshift
• The Big Bang predicted that the universe should be "hissing" (static)
  • Coming from all directions
  • Follows "black body" curve
  • Temperature about 3 K (we now know it is 2.7 K)

Age since Big Bang
• 13.8 billion years measured from CMB fluctuations
  • Time of expansion depends on the density of matter
  • and the cosmological constant (or "dark energy")

Space-Time Geometry
• 3 possibilities in Einstein's General Theory of Relativity
  • open
  • flat
  • closed
• Open, flat, or closed depends on density of matter (and "dark energy")
  • Astronomers define a ratio of densities, Ω₀("Omega nought") = actual density / critical density
    • the amount of mass needed to close the universe is call the critical density
    • critical density = 9 × 10 ^-27 kg/m ³
      • Earth's atmosphere contains about 1.2 kg/m ³
• We know Ω₀ is close to 1
• Luminous matter explains Ω₀ = 0.02-0.05 ("a few percent" of critical density)
• Dark matter is much more, bringing Ω₀ to about 0.3
• Dark energy completes closure of the universe, bringing Ω₀ close to 1.00
  
  
• Einstein found the General Theory of Relativity required the universe to be expanding of contracting, not static
  • Einstein invented "cosmological constant", a kind of "fudge factor" to add to his theory
    • Allowed the universe to be static, neither expanding nor contracting
    • Or it could still be expanding or contracting, depending on the densities of matter, and the cosmological constant
    • When Hubble discovered the universe was expanding, and it didn't need to be static, Einstein considered the "cosmological constant" his "greatest blunder"
    • Cosmological constant is similar to "dark energy" or the energy of the vacuum
  
  

  density of matter	nature of universe	geometry of universe	future
  less matter than critical	OPEN	infinite and unbounded	expand forever
  critical density	FLAT	infinite and unbounded	expand forever
  more matter than critical	CLOSED	finite and unbounded	collapses in BIG CRUNCH

  
  
Future of the universe
• Future of expansion also depends on the matter density and the dark energy density ("the cosmological constant")
• The expansion of the universe is accelerating
  • Supernovae at large redshift are fainter than they would be if the universe were expanding at a constant speed
• This acceleration must be caused by an unknown cosmic force field
  • Called "dark energy"
  • Appears like "cosmological constant"
    • Perhaps Einstein did not "blunder"
  • Could come from "vacuum energy"

Planck satellite
• A European Space Agency Explorer mission that launched May 2009
• satellite which measured the tiny fluctuations in the CMB in different directions
• CMB fluctuations tell us many things
  • Age of the universe
  • Total energy-mass density (Ω₀)
  • Fractions of ordinary matter, dark matter, and dark energy
• Planck results as of 2015
  • Age of the universe = 13.8 billion years (with less than 0.1% uncertainty)
  • Ω₀ = 1.00 (less than 1% uncertainty)
    • in other words, the universe is flat
  • Ordinary matter makes up about 4.9% of the universe
  • Dark matter makes up about 27%
  • Dark energy makes up about 68%
• Note- dark matter represents more than 80% of the matter
  • dark matter / all matter = 27%/(27%+4.9%) = 85%