The Milky Way

Hubble's Discovery of Galaxy Redshifts

In 1912, Vesto Slipher made the remarkable discovery that the spectral lines of most galaxies are shifted to the red away from their rest wavelengths, implying that they are moving away from the Milky Way. Edwin Hubble, working with the new 100-inch telescope on Mt. Wilson ten years later, made an even more stunning discovery. He plotted the recession velocities, V (determined from redshifted spectral lines), of galaxies against their estimated distances, D (determined by estimating the distance to Cepheid variable stars within each galaxy), and found a tight linear relationship. The recession velocity of the galaxies tended to increase with their distance away from us (all galaxies were moving away from us, and the more distant ones were moving away more rapidly than those nearby)!
The constant of proportionality, H_o, is now called the Hubble constant. The speed of a galaxy is typically measured in kilometers per second (km/sec), while the most common unit of for measuring the distance to nearby galaxies is the megaparsec (Mpc, one Mpc = 3.26 million light years). The units of the Hubble constant are thus km/sec/Mpc, and it has a current value of 72.
We can summarize Hubble conclusions as follows:
- There exist many galaxies beyond the Milky Way.
- They are receding, moving away from us.
- Their velocity of recession is proportional to their distance.
The correlation between velocity (redshift) and distance was fairly clear even in Hubble's initial data (left), and modern observations (right) extend far further and have strengthened the conclusion.

Hubble's data, drawn from the nearby Universe Modern data, covering a much larger region

[NMSU, N. Vogt]

Hubble's data indicated that galaxies which were located farther away from Earth were receding from Earth more quickly than those nearby. Because all galaxies are receding away from the Milky Way, it is tempting to assume that we occupy a preferred position at the center of the Universe. A simple thought experiment will show us that this need not be the case.

Let us imagine the Universe expanding in size, uniformly.; every part of the Universe expands in the same way. The fabric of space itself is expanding, and the galaxies are carried along like dandelions in the wind. What will this look like from Earth? We will pretend that the Universe is a straight line, with a galaxies spread out all along the line. If the Universe were to expand uniformly, then the space between each pair of galaxies would eventually double. What would this look like?

The top panel of the figure shows eight galaxies in a row, with roughly equal amounts of space between them. The bottom panel shows the same row of galaxies, now spaced twice as far apart from each other. A small blue-green figure is shown next to the fifth galaxy in the row, indicating that this is our Milky Way galaxy. This galaxy is shown at the same horizontal position in both panels, as it is the reference position where the observer is located. Because the other galaxies are now much more spaced out, the ones furthest to the left and right now fall off of the edges of the figure. The nearest neighbors which were formerly one space away from us are now two spaces away (an increase of one space), the next set have moved from two spaces away to four (an increase of two), the next set have moved from three spaces away to six (an increase of three), and so on.

The distance between each galaxy increases, while the Milky Way appears to stay in one place. We see nearby galaxies moving only a little way (at a slower velocity), while more distant galaxies appear to travel a great distance (at a higher velocity). [NMSU, N. Vogt & NASA/HST]

The galaxies close to us did not move very far, so they didn't move very fast. But galaxies that are much further away went a very great distance in the same amount of time. To do this, they must be moving very fast. This is exactly what the Hubble Law shows! Furthermore, aliens living in other galaxies observed the exact same pattern from their galaxies. No matter where you stand, the rest of the Universe appears to be moving smoothly away from your position.

The top panel of the figure shows the same eight galaxies in a row, still with roughly equal amounts of space between them. The bottom panel shows the same row of galaxies, now spaced twice as far apart from each other. A small orange-red eight-legged figure is shown next to the third galaxy in the row, indicating that this is a friendly alien astronomer living in another galaxy. (The blue-green figure is still located next to the fifth galaxy.) This other galaxy is now shown at the same horizontal position in both panels, as it is the new reference position where the new observer is located. Because the other galaxies are now much more spaced out, the ones furthest to the left and right again fall off of the edges of the figure. The nearest neighbors to the alien's galaxy which were formerly one space away from them are now two spaces away (an increase of one space), the next set, including the Milky Way galaxy, have moved from two spaces away to four (an increase of two), the next set have moved from three spaces away to six (an increase of three), and so on.

The distance between each galaxy increases, while the home of our bold, red, eight-legged alien astronomer appears to stay in one place. She sees nearby galaxies moving only a little way (at a slower velocity), while more distant galaxies appear to travel a great distance (at a higher velocity). [NMSU, N. Vogt & NASA/HST]

Hubble's data, drawn from the nearby Universe	Modern data, covering a much larger region

[NMSU, N. Vogt]