This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Expanding Universe

by Dr Jamie Love Creative Commons Licence 1997 - 2011

Astronomers cannot measure the spectrum of individual stars in distant galaxies because they are too far away. However, they can measure the spectrum of each entire galaxy and that can provide useful information. Indeed, the emission spectrum of a galaxy is really the total emissions of all its stars so it acts as a nice representation of each galaxy's "ingredients".

You will recall from last month's lesson that a star's radial motion (its motion towards or away from us) can be measured by the Doppler shift in its spectrum. There is only a slight amount of motion among nearby stars. The stars in our neighborhood are moving along with us as we orbit the Galaxy. Some are moving slightly more than others but generally speaking we are all moving along together. Even the other galaxies in our Local Group are moving along as a unit, connected together by gravity. Granted, there is some motion among the Local Group, but it is not excessive and not specific in direction. And all the motion is due to gravitational attraction and orbits.

In 1920 a fellow named Slipher, working at the Lowell Observatory in Arizona, discovered that ALL the galaxies outside our Local Group produced a spectrum that was red shifted! Of course a red-shifted spectrum means that the objects are moving away from us or we are moving away from them. Who is actually in motion is relative, but it doesn't really matter just as long as you understand that the distance between the two is increasing and that's what produces the red shift. Slipher' s measurements showed that all the galaxies, outside our Local Group, are moving away from us.

Wow! We (and our Local Group) must be at the center of the Universe!

No, but I can understand why you might think that way.
In fact, those distant galaxies are not only moving away from us (and our Local Group) but they are also moving away from each other. That doesn't make sense until you understand that the universe is expanding - growing larger.
The expansion of the universe is an interesting phenomenon of "space-time" and there is nothing in our normal experience to compare to it, but the following analogy is the best we can do.

Imagine a balloon with speckles of paint on its surface. As the balloon expands its surface stretches and the spots on its surface move away from each other. Each point on the balloon's surface moves away from its immediate neighbor as well as from its more distant spots. If you were an ant on the balloon, sitting on one spot, you would see that all the other spots are moving away from you and you would think that you were on a special spot, but in fact an ant on a different spot would see the same thing and come to the same conclusion. (That his spot was special.) Each spot moves away from the other spots as the balloon expands.

The universe is expanding too, but it's doing so in one extra dimension. The universe doesn't have a surface like a balloon. The universe has a "surface" in three dimensions and as time goes by the distances between the galaxies are increasing as the universe expands. A person living in a distant galaxy (outside our Local Group) would see the Milky Way Galaxy and all other galaxies receding from him.
[Note: Most galaxies are in clusters so that alien may have a "Local Group" of galaxies which are not receding because they are gravitationally connected to his own galaxy. But beyond his "Local Group" he would see an expanding universe.]

You will recall that the distance to a galaxy might be determined using a Cepheid variable in that galaxy. That's how Edwin Hubble worked out the distance to the Andromeda Galaxy in 1923.

He went on to measure the distances to many other galaxies using Cepheids as "standard candles". He also noted how much each galaxy was red shifted. Hubble discovered that the recessional velocity of galaxies (that is the speed that they are moving away from us) is proportional to their distance from us. In other words, the furthest galaxies are the ones moving away from us the fastest.

When Hubble plotted the distance to galaxies against their velocity he found a nice relationship that has come to be called "Hubble's constant". Walter Bade's correction in 1952 of Hubble's work doesn't really change our story, only the exact values. The exact value of Hubble's constant is still a matter of debate but it seems to be around 60 kilometers per second for each million parsecs (when corrected by Bade). That means an object 10 million parsecs away (10 megaparsecs away) will be receding at a velocity of about 600 kilometers per second. An object 100 megaparsecs away recedes from us (or we recede from it) at a speed of 6000 kilometers per second. And so on.

Hubble's constant is one of the most important values in astronomy. It acts as a measuring stick for the universe. Using Hubble's constant we can derive important information about our universe and the galaxies in it.
Let's explore how this is done.

Astronomers use the amount of red shift as a measure of the galaxies recessional velocities and then use Hubble's constant to estimate the distance to it. Red shift (symbol "Z") is defined as the change in wavelength of an absorption spectral line divided by the unshifted wavelength of that line.

For example, assume an astronomer takes a spectrum of a distant galaxy. He sees the same pattern of absorption lines expected of an average star but the lines are shifted by the motion. Regardless, he is able to identify each element, atom and ion in the spectrum by their relative positions.

He can easily identify the hydrogen-alpha line and knows that it should be at a wavelength of 6562 angstroms. But this spectrum has been shifted, due to the motion, and the hydrogen-alpha line is found to be at 7218 angstroms instead.
That is a shift of 656 angstroms.
(That's 7218 - 6562 = 656.)
So this red shift (Z) equals 0.1.
(That's 656 / 6562 = 0.1.)

The motion of that galaxy (and/or ours) causes the wavelengths of its spectrum to be stretched by about 10%. All the bands will be shifted the same amount but we are using only one band in this example. In order to stretch its spectrum that much, the galaxy must be moving at 10% of the speed of light. Light travels very fast, about 300,000 kilometers per second, so this particular galaxy is moving at 10% of that speed or 30,000 kilometers per second.

Now let's put Hubble's constant in this example. For each 60 kilometers per second of recessional speed that galaxy is a megaparsec away. (That's Hubble's constant.) So by dividing the speed of the galaxy (30,000 kilometers per second) by Hubble's constant (60 kilometers per second per megaparsecs) the astronomer estimates that galaxy to be 500 megaparsecs away. That's 500 million parsecs or 1.6 billion light-years away! Not only is that galaxy very far away, but the light he sees, and measures in his spectrophotometer, left that galaxy 1.6 billion years ago.

Observations like that are really observations of the distant past. That leads to a series of important thoughts. The more red shifted an object (galaxy) the farther away it is from us, according to Hubble's constant. The farther away it is the older its light is, having traveled such a long distance to reach us. Therefore, observations of distant galaxies are like looking back in time! Cool, huh?
[Unfortunately, the magnification to see anything interesting is beyond our abilities. Besides, this is the past of some other galaxy or world, not our own. Some people, when they first hear about this technique of "looking into the past", get the wrong impression - they think we should be able to see OUR past! Nope - doesn't work that way. Only an alien in a far off place in space, receiving our "old light", could see into our past. ]

Hubble's constant is based upon values that are difficult to measure accurately, so the value of Hubble's constant and any information derived from it may change as new information becomes available. But the overall idea is sound and it's unlikely that Hubble's constant is off by more than a small fraction. Other "standard candles" besides Cepheids have been used to help us to understand very distant galaxies and they generate data that is more or less in agreement with what you have just learned.

How fast is the fastest moving galaxy?

The very distant galaxies are moving very fast, but nothing can move faster than the speed of light. As an object approaches the speed of light, its mass increases and time within it slows down. These effects are known as relativistic effects and are due to some amazing physics that is beyond the scope of this course. Relativistic effects cause Hubble's constant, spectrums and other data to change in a predictable but complicated manner. At red shifts over 0.2 the relativistic effects become noticeable and Hubble's constant does not follow the simple (linear) relationship of 60 kilometers per seconds per megaparsec.

Regardless, the Doppler effect still occurs and it can be used to estimate Z. At very high speeds the wavelengths are shifted so much they disappear from the visible part of the spectrum but they can still be detected by special instruments designed to "see" those long wavelengths. Consider hydrogen-alpha's normal wavelength of 6562 angstroms. It could be stretched out to 32,810 angstroms by an object moving very fast. It's red shift (Z) would be calculated the same as before. The difference between the moving and stationary wavelengths (32,810 - 6562 = 26,248) would be divided by the stationary wavelength (6562) to give a red shift of 4. (Z = 26,248 / 6562 = 4) That does NOT mean the object is moving 4 times the speed of light. That is impossible! (Sorry Scotty, but warpdrive is not real. )

To figure out its velocity you would have to resort to math that takes into account the relativistic effects. (We won't go into that detail.) If you did you would discover that the object is receding at 92% of the speed of light - that's 276,000 kilometers per second! If you applied Hubble's constant to those observations, and took into account the relativistic effects, you would discover that such an object should be over 10 billion light-years away. Its (stretched out) light would have taken 10 billion years to reach us.

As telescopes become more powerful we get to see further and we have discovered some pretty fast objects. The largest red shift observed (that I know about) has a Z = 4.9. That object is moving very close to the speed of light. While it is difficult to use Hubble's constant with these Z values it is a safe bet that an object with z = 4.9 must be very far away!

Are those distant objects the quasars we learned about last month?

Yes, they are.
But (life would be so nice without "buts" ) new data is coming in every day and quasars are very strange objects. It would be unfair (to your education) if I were to tell you that this stuff about quasars and even Hubble's constant has been "proven". For example, there is the question of whether the "red shift" of these quasars are really an indication of great recessional velocity. If the red shift of quasars is caused by something other than their rapid retreat, then they would not be as far away as we think they are. That would mean that they are not as luminous as we think they are. It would mean they are closer and dimmer.

This kind of thinking (worry) is not new to astronomy. In the early half of the 20th century many astronomers argued over whether the Andromeda "nebula" was a bright distant galaxy or a nearby dim cloud. Hubble showed that Andromeda, and other similar "nebulas", were very far away and, therefore, very bright. He used logic and Cepheid variables to prove his point. But, remember, Hubble's estimates were later updated by Bade. Fortunately, Bade showed that the error Hubble had made was to under estimate the distances so the idea of Hubble (that these points of light are far, far away) was not only confirmed but increased in distance. But (gee, that word comes up a lot ) what if Bade had discovered that the whole idea about Cepheids was silly nonsense? What if the standard candle was all wrong? Face it - Hubble was using as his standard candle a type of star whose physics we didn't understand. Hubble was brilliant but it isn't too far off to say he was taking a bit of a risk by being so bold.

What if the red shift of a quasar has nothing to do with its velocity? We know less about the physics of quasars than Hubble knew about the physics of Cepheids. If we are wrong about the red shift of a quasar then much of what I have told you in this lesson, and in the next lesson, could be wrong!

But what evidence do you have that makes you think those distances could be wrong? What could cause the red shift in a quasar other than velocity?

I don't know. (Hey, I'm just an astronomy teacher! ) Maybe the gravitational pull of the quasar (after all, it has a black hole in it) draws out the spectrum. Maybe the incredible magnetic fields and unimaginable amounts of energy around a quasar are doing strange "physics things" that we do not understand, and have no way of reconstructing in a lab. [I've been assured by professional astrophysicists that those ideas have been considered and taken into account but that they are not likely to cause a "fake" red shift.] Don't get me wrong. I'm a "believer" in distant quasars, but not everyone is.

Some astronomers think they have evidence that you cannot trust the red shift. Remember I told you we have evidence that galaxies can collide. These interactions can leave "trails" of dust or stars connecting the two galaxies for millions of years until their separation is complete. Those who doubt the red shift of quasars point to astrophotographs of galaxies and quasars, with very different red shifts, that appear to be connected by streams of matter! Of course, that's "impossible". If they are so close as to have gravitational interactions then they must be close enough to each other than they should share similar amounts of red shifting. They should be receding at roughly the same velocity.
Most astronomers are skeptical about the evidence of there being real connections between these objects and not skeptical about quasar red shifts. They point out that it is difficult to get a three dimensional understanding from two dimensional photographs. These "connections" may be an illusion - just line of sight effects.

Well, until I learn otherwise, I'm going to trust the red shifts of quasars.

Me too.

Naturally there comes a point at which the recessional velocity is so close to the speed of light that it's practically there. When you work out the numbers from Hubble's constant, taking into account relativistic effects, a galaxy close to the speed of light would be about 12 billion light-years away.
Our observations are that the universe is expanding equally in all directions like a sphere growing ever larger. It is fair to imagine, then, that the universe is shaped as a sphere with a current radius of about 12 billion light-years and getting bigger all the time. We appear to be at the center of that sphere but that's just the effect of the complex geometry of space-time. We cannot observe anything beyond that sphere because its light has not reached us - and it never will. That's because the universe is still expanding and it is fair to say that its expansion creates the edge of our observable universe.

I don't understand! What is beyond the edge?

Nothing!

Nothing!? But that's what most of space is anyway. How can you tell space and nothing apart?

We tend to think of space as empty but there is a little bit of matter in it. Not much matter - about one hydrogen atom per cubic meter is an average density for the universe. Beyond 12 billion light-years that density should drop to zero. It would probably be difficult to tell exactly where the universe ends and nothing begins, but you'll never get there anyway because the speed of light prevents you from ever traveling there! However, powerful telescopes allow us to see almost to the edge of the universe and any image they show us is really what those distant galaxies looked like nearly 12 billion years ago!

It's like a twisted puzzle. The trick is to learn what parts of the puzzle are useful in our understanding of the universe.

Will the universe continue to expand?

That is an excellent cosmological question and demands a proper cosmological answer. Cosmology is our next (and last) lesson.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.