Key Concepts:

1. What is Hubble's Law?
2. How is the Hubble's Law used in astronomy?
3. How does one construct the map of the universe?
4. What is "hierarchical large scale structure"?
5. Where does it come from? and how does it related to the Big Bang?
6. What are the "Great Walls" and "voids"?

Charting the Solar Neighborhood

• Herschel's Milky Way: W. Herschel (1738-1822) sketch based on number of stars seen in different directions. Correctly deduced that it is a flattened structure, but the Sun was thought to be in the center.
  
• H. Shapley (1885-1972): we live on the edge of the MW -- from the asymmetric distribution of globular clusters (used as a standard candle)


• 2MASS: By taking photographs of the sky using TWO specialized telescopes (one in Arizona and one in Chile) one square degree at a time and putting it together like a giant jigsaw puzzle, astronomers at UMass and NASA put together the following portrait of the Milky Way Galaxy.

Question: how would you find out what your neighborhood is like?

Hierarchical Large Scale Structure: How Dark and Visible Matter are Distributed in the Universe

The Local Group

The Milky Way Galaxy is part of a small group of galaxies that includes a handful of other large and small galaxies. This "Local Group" includes:

The Milky Way Galaxy M31 (Andromeda Galaxy; how to find it (chart)) M33 (Triangulum Galaxy; how to find it (chart)) LMC & SMC many dwarf satellites (new ones are still being discovered, including Canis Major Dwarf and Sagittarius Dwarf Tidal Stream)

The Local Cluster and Supercluster

The "Local Group" is part of a larger ensemble of galaxy groups. The nearest other groups include the "M81 Group" and "Sculptor Group", which are all part of the Local Cluster and Local Supercluster.

Doppler Shift (Review)

• Spectral line from a light source shifts in wavelength with motion:
• High frequency (shorter wavelength) if moving toward you
• Lower frequency (longer wavelength) if moving away from you
• A measure of the line-of-sight speed (e.g. radar gun)


• A measure of the line-of-sight speed in astronomical sources as well.

Cosmological Principle: The universe is homogeneous and isotropic when average over large scales, and no special location exists.

Quiz 19A: Center is everywhere...

Hubble's Law

In 1920, Edwin Hubble discovered that more distant galaxies have larger recession velocities, following the relation The constant H that relates the recession speed V and distance D is refereed to as the Hubble constant. Cosmological Principle says EVERYONE in the universe sees everyone else moving away the same way (the more distant galaxies are receding faster)! ==> UNIVERSE IS EXPANDING! Turning this around, one can infer the distance of a galaxy with measured recession speed V as D = V/H. For the recession velocity in km/s, H = 65 gives D in megaparsecs (Mpc; million parsecs). A crude age estimate of the universe can be derived as t = D/V = 1/H. Such a calculation suggests the age of universe between 12 and 15 billion years. Quiz 19B: calculate Hubble distance

Large Scale Structures: The Stick Man, the Great Wall, and the Fingers of God

Click on the picture below to read much more about the large scale structures made of galaxies that surround the Milky Way Galaxy and its cosmic neighborhood.

Hierarchical Structure Formation

• Dense clumps of matter condense out of uniform environment


• Cosmic ripples on the pond that is the young Universe: high density regions (groups and clusters) grow denser by accumulating more matter(dark matter, stars)

The top picture shows the ripples in the last visible surface of the "Big Bang". The amplitude of these ripples are only about one part in 100,000 compared with the average brightness of the "glow". The bottom picture shows the large scale structure mapped in the local universe by plotted the density of galaxies detected by the 2MASS project (done by UMass astronomers). It is thought that those tiny ripples seen on the top panel has grown by many orders of magnitudes to form the structures seen on the bottom panel by gravity over the cosmic history.

Scientific Results from the WMAP

The above picture shows structures in a computer generated universe using initial conditions that mimic those of the real observed universe. Stringy, filamentary structures and "voids" are the most prominent characteristics, created by gravity, as seen in the real universe.

Reading assignment for next lecture: Unit 75