Expansion of the Universe

• The apparent brightness of a white dwarf supernova tells us the distance to its galaxy.
• Redshift: moving away from us
• The redshift of a galaxy tells us its distance through Hubble’s law.
• Astronomers use Cepheid variable stars as standard candles to measure the distances to nearby galaxies.
• Cosmological Principle: The universe is expanding and is therefore getting older.
• Hubble’s constant tells us the age of the universe because it relates the velocities and distances of all galaxies.
• Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time.
• Cosmological redshift tells you:– the relative distance between any two galaxies when the light was emitted vs. the distance between them today
• Modelling collisions on a computer shows that two spiral galaxies can merge to make an elliptical.
• If the centre of a galaxy is unusually bright, we call it an active galactic nucleus (AGN).
• Quasars are the most luminous examples of Active Glactic Nucleus AGN.
• Galaxies around quasars sometimes appear disturbed by collisions.
• Radio galaxies contain active galactic nuclei shooting out vast jets of plasma that emit radio waves coming from electrons that move at near-light speed.
• An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light.
• Black Holes in Galaxies:

  • Many nearby galaxies—perhaps all of them—have supermassive black holes at their centers.• These black holes seem to be dormant active galactic nuclei.• All galaxies may have passed through a quasar-like stage earlier in time.
• cepheid variable stars with longer periods have greater luminosities.