Universe Is Expanding

Edwin Hubble and Milton Humason discovered in the 1920's that the universe is not static---it is expanding. This is enough to resolve the Olber's paradox. As the universe expands, the light waves are stretched out and the energy is reduced. Also, the time to receive the light is also lengthened over the time it took to emit the photon. Because the luminosity = the energy/time, the apparent brightness will be reduced enough by the expansion to make the sky dark.

The expansion of the universe means that galaxies were much closer together long ago. This implies that there is a finite age to the universe, it is not eternal. Even if the universe is infinite, the light from places very far away will not have had enough time to reach us. This will make the sky dark.

The age of the universe can be easily (this is a pretty funny word here, since the equations that are used are exotic) estimated from the simple relation of Time = Distance/Speed. The Hubble law, Speed = Ho × Distance, can be rewritten 1/Ho = Distance/Speed, where Ho is the Hubble constant. Notice that the expansion time interval = 1/Ho. The Hubble constant tells us the age of the universe, i.e., how long the galaxies have been expanding away from each other: Age = 1/Ho. This value for the age is an upper limit since the expansion has been slowing down due to gravity. Taking the expansion slowdown into account, we get an age closer to 2/(3 Ho). Still, the age looks like a number × (1/Ho), so if the Hubble constant is large, the age of the universe will be small.

How did Hubble figure out that the Universe is expanding?

Famed Austrian scientist, Doppler (a mentor to a young monk named Gregor Mendel who discovered the genetic inheritance patterns of traits in peas) was the first to explain the cause of changes in sound pitch as a train approaches or recedes. In a little series of powerpoint slides, I hope to give you a simple explanation of the Doppler Shift as it pertains to sound, and then as it pertains to light. Since Ragnar the Viking mascot is a close personal friend, I have no trouble poking fun of him in these slides :)


Ragnar has been my friend for many years, and while he no longer is the Vikings mascot, nor does he work at Hopkins High School any longer, we stay in touch. I am happy to tell you that he is no longer a fan of the Minnesota Vikings, and that we will occasionally cheer for the Paclkers :)

Saul Perlmutter's Type Ia page might be of interest to you.

Slipher discovers that nebulae in the Milky Way are not clouds, but collections of stars!

In 1913 V.M. Slipher vesto slipher at the Lowell Observatory (same place where Lowell saw "canals" on Mars, and where Clyde Tombaugh later discovered Pluto) reported on the spectra of faint, nebulous objects in the sky. These were some of the Messier Deep Sky objects that you will look at on the next page. They nebulae showed a mixture of stellar spectra, meaning that they were composed of more than a single star ionizing the gas. In fact, some of the nebulae had Doppler shifts that suggested rotation of the nebulae. Even stranger, most had red shifts as if they were receding. The red-shift is discussed in the slides above, and seen in stars within our Galaxy. The nebulae that Slipher was photographing with his spectroscope showed a shift in location of the absorption lines of Hydrogen, and the faintest of the nebulae had large red shifts. His conclusion was that the nebulae might be objects OUTSIDE the confines of our Galaxy, and that they were receding from us.

Slipher and Edwin Hubble collaborated after this date and began a serious search of the so-called nebulae.

In 1929 Edwin Hubble published an extensive study of galaxies, and this is the subject of the following page, or go back to Olber's Paradox, or even to the Syllabus .

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