Tuesday, August 18, 2009

Big Bang Prediction #1: Is The Universe Expanding?

Dear science community,

Albert Einstein proposed the general theory of relativity in 1915. When the Catholic Priest Georges Lemaître discovered the Big Bang was the solution you get when you apply general relativity to cosmology he quickly informed Einstein. He also pointed out this solution predicts the universe is expanding in a fashion now known as Hubble's Law.

When Einstein learned of this he said "Your math is correct, but your physics is abominable." Einstein did not like the idea that the universe was expanding since everybody knew that the universe was static, infinite and eternal. In fact, Einstein then added a constant to his equations to try to force general relativity to predict a static universe. After evidence the universe was expanding emerged, Einstein said the addition of this extra constant was the greatest blunder of his life and removed it. (Actually, removing it may have been the greatest blunder of his life because we know it it is real and drives the acceleration of the universe.)

Two years later, in 1929, Edwin Hubble (on left) published a paper that showed the universe was expanding. He discovered this by red shifts which works just like the doppler shift you can use to tell if a car is coming closer or going farther away. Think of when a car passes you going very fast. It makes a zaaaooommm sound right? Higher pitched when it is approaching and lower pitched when it is going away. The "lower pitch" happens with light too and scientists say the light is red-shifted. (Lower wavelengths are red.)

Hubble used the redshift in the light from the galaxies to determine how fast they are moving away from us. He then used that to discover Hubble's Law: the further away a galaxy is from you, the faster it is moving away from you. The Plot above was Hubble's original plot. 1 Mpc, or Megaparsec is ~3.26 million light years away.

This was a legitimate prediction of the Big Bang confirmed! Nobody had ever guessed the universe is expanding. Lemaître not only predicted the universe was expanding, but that it was expanded in a way that yields the exact law Hubble later discovered. It is a prediction that continues to be confirmed today. The plot to the right is a more modern plot showing just how exactly Hubble's Law, a real prediction of the Big Bang, is followed.

Click on the plots if they are hard to see. Again questions and interesting observations or additions to the story welcome!


  1. One of the best books on this topic is titled, or course, 'Big Bang: The Origin of the Universe'


  2. Thanks Stan. People always ask me for good books on these topics too so I'm glad you mentioned this.

  3. Another good book on this subject is "The Universe at Midnight", which traces the history of modern cosmology.

  4. Nick, the comment you were supposed to make was regarding what a scatter plot Hubble's original plot was and yet how accurate it turned out in the end! I completely set you set you up, a blog "alley oop" if you will. :(

    However, I'm glad you added a book to the list too.

  5. Oh come now, you can clearly see Hubble's law in that plot. Especially considering his value for the Hubble constant was off my an order of magnitude. :)

  6. Nick, what is an order of magnitude among friends?

  7. These are excellent. I am learning a lot, thanks.

  8. Matt, thanks for the compliment, I really appreciate it.

  9. Joseph,
    Isn't Einstein's blunder an excellent example of how scientists should have humility when it comes to their pet theories? One of the greatest scientists was willing to admit he was wrong (when in fact he was kind of right). If Einy were alive today perhaps he would also change his mind on quantum indeterminancy.
    Thanks. Informative post.

  10. Dave, good point, yes this is an excellent example of why scientists need to show humility. Einstein did have a pet theory, general relativity + a static universe, but once the evidence for general relativity + an expanding universe was shown, he was willing to admit his cosmological constant was a blunder.

    Now as I said earlier, this constant is there, just really small. Small enough that it took until 1995 to detect it. It is therefore too small to keep the universe static, but not too small to accelerate it slightly.

    Second, it is true Einstein was against many results coming from quantum mechanics over which he had some very famous debates with Neils Bohr. If he was living today I am sure he would change his mind on many aspects of quantum mechanics.

    Now, humility goes for everyone equally. Scientists with pet theories need to be humble like Einstein once more evidence comes forth putting their theory on shaky ground. Similarly, non-scientists with pet theories that make predictions need to be humble when more evidence comes forward disproving their predictions.

    Although for me, anyone with physical theories that make testable predictions is that much a scientist, degree or not. That means if evidence comes against your theory you, being a good scientist, need to modify your theory, degree or not.

    Everyone is in this together, and I really appreciate your comment.

  11. Does Hubble's law mean that as galaxies move farther away from ours they gain speed? Or is it just saying that some galaxies got a bigger piece of the bang and are further away because they're moving away at a constant speed that is greater than others?

  12. Mephibosheth, good question. The galaxies are not moving any faster, themselves. The space between them is expanding and so it "appears" to observers living on one that the other is moving away. Actually, it may not be moving fast at all, but the space between is expanding so it looks like it is moving.

    If this is confusing don't worry. In a couple days I was going to post on the classic analogy (very physically accurate analogy too) that is very helpful in thinking about this. So stay tuned and I promise this will all make sense.

    Your question provides a good segway for that post so I'm glad you asked it.

  13. Wow --not what I was expecting for an answer. Kind of reminds me of when Spock gives what's-his-name the equation for warp speed in the new Star Trek, something like "Who would have thought that space was the thing that's moving?" Looking forward to the next post!

  14. I wonder about Einstein's blunder. That is, I wonder about the psychology behind it. Do I really believe that one of the smartest guys to ever live couldn't see what was under his nose?

    I wonder if he had religious ideas, or cultural ideas that were preventing him from seeing it. Einstein was very areligious in a typical sense, but was also very spiritual. In the immortal words of Joseph Campbell, Einstein seemed to have found his bliss in science. He felt that there was some greater cosmological force tying us together, responsible for the chaos. He believed in the transcendent god rather than the manifest god.

    What effect then, did this, along with his Jewish roots have on his inability to see what seems so obvious to us now? Or am I reading too much into it and he just screwed up?

  15. jmb275, really good point about Einstein.

    Your right about Einstein's religious nature: he was Jewish, but he didn't believe is a personal/traditional God, and yet he was extremely spiritual and acted as if he felt there was a real majesty behind the laws of nature as if they were "Godlike".

    This definitely effected his approach to science in some respect. He famous critique of quantum mechanics was "God does not play dice". He really seemed to believe there was some divine-like quality to nature and it was probably the fundamental reason he had such a hard time accepting parts of quantum mechanics.

    I'm sure it was similar with the cosmological constant. I'm sure he had spiritual notion of what the world "should" be like that motivated many of his ideas: This is why he demanded all equations in physics to be written in tensors. (He was right here, and I'll discuss why this is so aesthetic later.) I'm sure an infinite and eternal universe (and in his mind therefore static) also seemed like how a majestic universe should be: hence cosmological constant.

    If only he was living today. You would find, not only does modern cosmology predict a big bang, but also an eternal structure, possibly more vast than our wildest imaginations would let us go.

    And for all the nay sayers on the above, read the next post.


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