Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the Universe, part 4: Hubble and the expanding universe

Well, we are finally getting a little closer to our goal, the Big Bang. But there are several things you have to understand before we can get all the way there, I'm afraid....

In my last post, I said that Hubble used Henrietta Leavitt's variable Cepheid stars to prove that the Andromeda galaxy is an independent system of stars, completely separate from the Milky Way. This discovery of Hubble's convinced astronomers that there are huge numbers of other galaxies out there, not just the Andromeda galaxy. Many of the strange faint blotches of light in the sky were now identified as other galaxies:



These are the 110 objects in the famous catalog of Charles Messier. There are very many star clusters belonging to the Milky Way in this catalog, but forty of the objects are other galaxies.

So the existence of a large number of other galaxies quickly became an accepted fact, thanks to the efforts of Hubble. But Hubble wasn't done by far. He wanted to study as many as possible of those other galaxies and learn as much as he could about them.

The other galaxies proved to be too far away for Hubble to detect Cepheids in them in order to determine the distance to them that way. It was, however, possible to study the spectra of those galaxies. Remember the spectra of stars?



The light from individual stars can be divided into its constituent colors, and then dark "absorption lines" will become visible. What absorption lines you see depends primarily on the temperature (and hence "spectral class") of the star.

However, it is also possible to spread the combined light from an entire galaxy into its constituent colors and find absorption lines there. Here you can find the spectrum of an (unnamed) galaxy, whose combined spectrum resembles that of an individual star of cool spectral class K0:



It is no coincidence that many if not most galactic spectra will resemble that of a K0 star. That is because most galaxies are brightest in the middle, so that the combined spectra of galaxies will be dominated by the stars at the galaxies' centers. But the centers of galaxies are generally dominated by cool stars, typically of class K. That is why galactic centers are usually yellow.



NGC 1232, a typical galaxy with a yellow center.

Hubble studied the spectrum of many galaxies, and he made a most unexpected discovery. Almost all galaxies he studied had redshifted spectra! Okay, but what does that mean?

Take a look at these three pictures:



Although the pictures are small, you can see that the topmost light source is stationary, and the light it emits has a spectral line in the green part of the spectrum. In the middle picture, the light source is approaching us. The spectral line has been shifted into the blue part of the spectrum, or in other words, it has been "blueshifted".

In the bottom picture, the light source is moving away from us. Now the spectral line has moved into the red part of the spectrum. The spectrum has been "redshifted".

A spectral line does not have to move clear into the blue part of the spectrum in order to be blueshifted, and it doesn't have to move clear into the red part of the spectrum in order to be redshifted. It is enough that the spectral line has moved from its original position toward a bluer part of the spectrum to be blueshifted, or toward a redder part of the spectrum to be redshifted.

All right. But how can you tell if a single spectral line has been blueshifted or redshifted? How can you tell if a single black line used to be in another part of the spectrum before?

You can't. But spectral lines come in pairs and groups, and you can recognize those pairs and groups. If the pairs or groups have moved from their "proper" place, you can see it. It can look like this:



Here you can see, in the middle picture where the light source is at rest, two prominent spectral lines smack in the middle of the yellow part of the spectrum. To the left of this pair of spectral lines is another pair, even tighter, which sits in the middle of the green part of the spectrum. In the upper picture these two recognizable pairs of spectral lines have moved toward the blue part of the spectrum, and in the bottom picture they have moved toward the red part of the spectrum.

Like I said, Hubble studied the spectra of very many galaxies, and he found, to his amazement, that almost all galaxies have redshifted spectra. What did that tell us about those other galaxies?

If you have seen and heard a police car pass you at full speed in the street while its sirens were going, you may have noticed that the sound of the sirens changed in pitch as the police car passed you, from a higher pitch as the car was approaching to a lower pitch when the car was speeding away from you:



It's really the same thing with galactic spectra. The "pitch" of their spectra changes, too, as the galaxies are approaching you or moving away from you. If the spectrum of the galaxy is redshifted, then the spectrum has changed ot a lower pitch and the galaxy is moving away from us. But what if almost all galaxies have redshifted spectra? Are they all moving away from us? Yes, they are, if their spectra tell us so, Hubble argued. Elementary, my dear Watson!

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So Hubble made the amazing discovery that almost all galaxies are moving away from us. In less than a decade, Hubble went from proving that the Milky Way is not all there is in the universe because the Andromeda galaxy is a huge system of stars which is independent of us, to proving that there are thousands of galaxies out there (today's estimate of the number of existing galaxies is about 40 billion, if I remember correctly), and that these galaxies are almost all rushing away from us. In less than ten years, those who cared about the universe went from believing that they inhabited the only galaxy in the cosmos to seeing a staggering large extra-Milky Way universe out there where the galaxies are fleeing from us, making the universe bigger by the minute. This is a shift in cosmology which constitutes a staggering mental revolution.



Ann