Raisin Bread is the new universe: Hubble’s Law and the expanding universe

https://en.wikipedia.org/wiki/Edwin_Hubble

Raisin Bread is the New Universe:

Hubble’s Law and the Expanding Universe

Edwin Hubble was a famous astronomer who discovered that the Milky Way is just one of many galaxies in an enormous universe. In 1924 he determined that the Andromeda Galaxy lies beyond the Milky Way using Cepheids. Before this great discovery however, there was great debate amongst astronomers. Some argued that the spiral shaped objects were clouds of gas within the Milky Way while others thought perhaps these objects were distant and distinct galaxies.

http://www.dailykos.com/story/2011/8/30/1004549/-

The diagram to the left is a perfect analogy to describe our expanding universe. As time passes, the raisins (galaxies) are moving away from each other. In 1929 Hubble concluded that the more distant a galaxy, the greater its redshift (redshift is a result of the Doppler Effect, the effect that shifts the wavelengths of spectral features in objects that are moving toward or away from an observer. Redshift indicates motion away from the observer. A higher red shift, the longest wavelength, results in faster motion.); therefore the farther apart the raisins (galaxies) are the faster they are moving away. Thus the UNIVERSE IS EXPANDING! Today astronomers use Hubble’s Law to show that more distant galaxies are moving away from us at greater speeds. Additionally, based on observation, no matter where you are in the universe you can see galaxies move away from wherever you are uniformly because we do not have a center or edges. This is called the cosmological principle.

http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/hubble.html                   

*r is sometimes written as d*

*Today we estimate that Hubble’s constant is between 21-23 kilometers per second per million light years or 70 kilometers per second per megapar. These values were gathered using the Hubble telescope.

http://www.astro.cornell.edu/academics/courses/astro201/hubbles_law.htm

Hubble’s law represents the relationship between galaxy speed and distance. Recessional velocity (speed) is measured using the Doppler shift. Astronomers take the spectrum of a distant object and study the shift in the lines of its spectrum. We determine velocity from this shift. Using the equation d = v/H0 we can determine how far away the galaxy is because we were able to determine the recessional velocity by looking at the galaxies redshift.

Research has discovered two practical difficulties with Hubble’s Law when we try to measure galactic distances:

1. Gravitational tugs alter the speed derived from Hubble’s law. This issue is most prominent in nearby galaxies. For example the Local Group is gravitationally bound by the Milky Way and is thus NOT moving away from us. Despite the problems nearby galaxies create, Hubble’s law continues to work better for more distant galaxies because recession velocities are so great that the gravitational tugs have a smaller effect. However, gravitational tugs can still be problematic for distant galaxies because they will maintain some motion with respect to their cluster, peculiar motion, and some motion as a result of the universe expanding. A galaxy orbiting around a cluster is moving slightly towards us. At the same time, universal expansion is moving the entire cluster away at a faster velocity. The motion towards us causes the motion away from us to lesson by a small amount. The farther away a galaxy is, the less it is affected by peculiar motion. 
2. Hubble's constant is not a constant at all! Observations of distant White Dwarf Supernovae indicate the expansion rate was much slower in the early universe. In fact, right after the Big Bang, the universe was not accelerating at all. In this case gravity was strong enough slow the expansion of the universe for the first few billion years. Then dark energy, a repulsive force that causes the expansion of the universe to accelerate, became dominant and thus the expansion rate accelerated.