Lecture 22
Hubble's law and the expanding universe
Wednesday August 6, 2008
Slides on Angel
Concepts:
- Olbers' paradox: Inverse square decrease in flux is compensated for by increase in number of objects at larger distances, so why is night sky dark?
- Hubble's law: Linear relationship between distance and recession velocity. A galaxy twice as far away from us would be receding twice as fast
- Expanding universe: Galaxies are embedded in expanding universe, like raisins in bread or coins taped to balloon. Naturally gives rise to Hubble's law
- Distance ladder: Can use Hubble's law to determine distance to galaxies by measuring their redshift, converting to velocity, and then matching to distance
Vocabulary:
- Hubble constant: H0 is slope of line governing Hubble's law; modern value is 70 km/s/Mpc. 1/H0 gives rough approximation for age of universe (14 billion years)
- Cosmological redshift: Expansion of universe stretches out light traveling through it; explanation for redshift of distant galaxies
- Big bang: Event that created the universe; we can understand (most of) what immediately followed it, but not event itself
Activity:
Used data to plot Hubble's law, then measured slope (Hubble's constant). Commented on scatter in plot, which as it turns out
arises from two different sources: measurement error, and intrinsic random motions of galaxies. Did a worksheet exploring
perspective in the universe, and concluded that any observer in any galaxy would see same Hubble's law, with other galaxies
receding from them with speeds proportional to distance.
Random link:
Excerpt from Chandra image description titled "M87: A Nearby Galaxy Metropolis"
M87 lies at a distance of 60 million light years and is the largest galaxy in the Virgo cluster of galaxies. Bright jets moving at close to the speed of light are seen at all wavelengths coming from the massive black hole at the center of the galaxy. It has also been identified with the strong radio source, Virgo A, and is a powerful source of X-rays as it resides near the center of a hot, X-ray emitting cloud that extends over much of the Virgo cluster. The extended radio emission consists of plumes of fast-moving gas from the jets rising into the X-ray emitting cluster medium.