Monday, September 7, 2009

Big Bang Prediction #2: How Do We Determine The Elemental Abundances?

Dear science community,

The Big Bang predicts that the percentages of the various elements in the universe have to be very strict ratios. (Ie. how much hydrogen versus helium versus Lithium, versus etc... has to be exact.)

This calculation is straight forward.  This is how you do it:

1. Realize You Start With A Plasma.  Using basic thermodynamics you show that the universe cools as it expands.  Therefore our past was much hotter.  If you go back far enough in time, you come to situation where the universe is so hot that the energy literally rips the atoms apart into their fundamental components.  This is called a plasma.

2. Run The Tape Forward.  This nearly homogeneous and isotropic plasma cools as the universe expands.  The fundamental quarks that form protons and neutrons begin doing so.  However, they must  conform to the laws of physics.  If you use some highfalutin math, like relativistic Boltzmann equations, you find that a quark soup cooling in an expanding universe would produce ~12% neutrons and ~88% protons.

3. Elemental Production Gone Wild.  At this stage, armed with protons, neutrons and a cooling universe, elemental production goes crazy.  It turns out elements want to be at the top of this chart to the left (click to see) for physical reasons I don't want to get into now.  Hydrogen quickly forms and two neutrons grab two protons and form Helium, and then they try to start Lithium and then....

4. Universe's Expansion Immediately Ruins The Party.  As soon as these elements begin forming, the quick expansion of the universe pulls these atoms too far apart to form the heavy elements at the top.  Sure the helium and hydrogen atoms want to further combine to form such atoms but the universe's expansion pulls them too far apart to do so.

5. What We Are Left With?  I mentioned ~12% of the mass was neutrons and that the first thing they do is pull in two other protons to form Helium before the party is crashed.  Since protons are about the same mass as neutrons, this means ~24% of the mass of the universe should be Helium.  The rest should essentially all hydrogen since the next elements on the list didn't have time to really form before expansion took over. 

 So this is the prediction: ~24% Helium, ~76% Hydrogen and trace amounts of other things such as Lithium.

The chart to the right highlights all this even better.  The red strip is the prediction by the Big Bang Threory.  The boxes show the measured values with experimental error factored in.

As you can see, this prediction of the Big Bang is confirmed every well by experiment.

Moreover, a hot cooling plasma in an expanding universe is the only known mechanism to produce such abundances!  First, If we didn't start with a hot cooling plasma we wouldn't get 12% neutron production by mass.  Second, if these reactions were not going on in an expanding universe then all the elements at the top of the first chart would have been eventually produced.

To explain what is observed you need both a hot dense plasma and an expanding container/universe: the Big Bang.

Nick can post later how this works in stars and you will find that, since there isn't an expansion inside stars ruining the party, the heavy elements above are able to form. (See picture at top.)


  1. Math + Reasoning = Observation => truth

    I love it!

  2. Thanks Stan. Yes, the Big Bang is a very wonderful theory.

  3. Gotta plug the book.

    'Big Bang: The Origin of the Universe'

    Very easy reading


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