Universe is Expanding
This article introduces the idea of an expanding Universe. When we left the Cosmic Times in 1919, Albert Einstein had added the Cosmological Constant to his equations of General Relativity to prevent the Universe from expanding. The idea of an expanding Universe was against his philosophical viewpoint, and nothing was violated by adding this Cosmological Constant to his equations.
One key concept to understanding Hubble's results is understanding the idea of redshift. Often this idea is introduced alongside the Doppler effect for sound because students are much more familiar with this it is what causes the pitch of a siren to appear to drop as it passes by a listener. For light, when a source is emitting light while it is moving away from us, the wavelength of the light as we see it will be shifted toward the red, or toward longer wavelengths.
When astronomers look at light from distant galaxies, they can identify signatures of certain elements in the form of "spectral lines". By looking at how much these signatures are offset from their "at rest" wavelengths, astronomers can tell how fast the object that emitted them was moving. If the object is moving away from the observer, the light is shifted toward the red, or toward longer wavelengths, and this is often called "redshift". If the object is moving toward the observer, the light is shifted toward the blue, which is often called "blueshift".
In 1917, an astronomer named Vesto Slipher, discovered that the light from several "nebulae" (later found to be galaxies) was "redshifted". It is commonly thought that Hubble discovered the redshift of galaxies, but this is not true. Hubble found the relationship between redshift and distance that is discussed in this Cosmic Times article, but he was not the first to see the redshift itself.
The thing that Hubble added to the picture was determining both the distance and redshift of galaxies. The distance determination was made using a standard candle, called Cepheid variables. The importance of these stars and how they are used in distance determinations are discussed in more detail in the notes for the "Andromeda Nebula Lies Outside Milky Way Galaxy" article.
When Hubble plotted up the distance of the galaxies in his sample versus the redshift of those galaxies, he found that there was a trend the further a galaxy was, the faster it was moving away. This is exactly what would be expected if the Universe was expanding.
This is one concept that your students might struggle with why does an expanding Universe imply that the further a galaxy is from us, the fast it would be moving away from us. To help illustrate this, imagine that this picture shows several galaxies in the Universe as they were in the past:
Let's say that our galaxy is the blue star near the middle of the picture. Then let the Universe expand for a while and look at what's happened:
We are the blue galaxy, so we haven't moved. However, all of the other galaxies appear to have moved away from us. And, if you examine the size of the arrows from the original galaxy positions to the new ones, it is clear that the further a galaxy was away from us, the faster it appears to have moved, in the same amount of time. That means that the further away the galaxy was, the faster it appears to move away from us as the Universe expands.
One misconception that your students may have is that because all of the galaxies appear to be moving away from us, that means that we must be the center of the Universe. This is not true. Any galaxy would actually perceive the same thing. Let's say that instead of being the blue galaxy in that image above that we are the yellow galaxy. The picture we would see after the expansion would then be this:
The yellow galaxy also sees all of the other galaxies moving away from it, and the further the galaxy started from the yellow galaxy, the more it moved.
The fact that we see the galaxies all moving away from us is indicative of the expanding Universe, but not that we are the center of the Universe. In fact, the expansion happens everywhere.
Einstein still didn't believe
If you read the 1919 edition of the Cosmic Times, you know that Einstein did not like the idea of an expanding Universe. In fact, even though his equations of General Relatively suggested an expanding Universe, Einstein added his "Cosmological Constant" to those equations to prevent expansion.
As of the publication date of this edition of the Cosmic Times, Einstein is still not convinced of Hubble's results. However, in 1930, Einstein travels to Mount Wilson to talk with Hubble and see his work first-hand. After meeting with Hubble, Einstein does become convinced that the Universe is, indeed, expanding.
Hubble's work holds up
Hubble's results have proved to hold over much greater distances than Hubble himself could probe. As we've developed larger and more sensitive telescopes and as we've determined more ways to pinpoint distances in further and further objects, we have continued to find that the further a galaxy is from us, the faster it is moving away, as this image shows for just a sample of the currently available data:
The relationship between a galaxy's distance and its speed holds far beyond what Hubble originally measured. This diagram uses modern measurements to show the same relationship. The gray box shows the region that Hubble probed. Data from the Hubble Space Telescope Key project, courtesy Prof. John Huchra.