Saturday, April 20, 2013

"Kinds" of evolution, Part Deux: Cosmic nucleosynthesis of elements

The “kinds” of evolution under the last part of "Kind" #1, and "Kinds" #2, and #3 are convoluted in the minds of creationists everywhere. In fact, they are convoluted period. The questions, and answers are so complex because we have to refer back and forth between experimental laboratory results here on earth, and astronomical studies from the deepest reaches of space. And for extra fun, the different elements formed at different times under extremely different conditions. So let's look again at Big Daddy's blackboard.

The Big Bang did make nearly the universal mass of hydrogen as per "Kind 1.2." And the Big Bang also made trillions of tons of helium, and billions of tons of some other stuff (universally just trace amounts) which is the next error by Jack Chick and his science tutor Kent Hovind. Hydrogen and helium under the influence of gravity make stars. The stars make heavier elements by nuclear fusion, and to get the really heavy elements super massive stars need to go bang all by themselves in a supernova. This must happen billions on billions of times before there is even enough heavy elements to make a planet. So, the cosmic comic by Chick and Hovind fails in several ways. First, it denies reality. Second, it confused star formation and stellar nucleosynthesis with super-nova nuclear synthesis. Third, all of those processes cycled mass for billions of years before there was sufficient stuff (heavy elements) to even make a planet like Earth. I'll review this in some detail below.

Part 1.

By knowing the age and background temperature of the Universe today, we can calculate the approximate temperature at the Big Bang. This was an extraordinary 10^30 degrees Kelvin. At that temperature normal matter, baryonic matter, cannot even exist and all the physical forces with the exception of gravity are mashed into a single unified force. As the universe expanded it cooled, and below 10^28 degrees Kelvin the first ordinary matter condensed, and the weak, strong, and electromagnetic forces began to separate. Atoms could not have existed yet, and didn’t emerge until the temperatures dropped to an average of around 10^6 degrees Kelvin, or about the temperature of the stars seen today. The cosmic microwave background radiation is a remnant of this era around 300 to 500 thousand years after the Big Bang. But long before this, neutrons and protons could exist, and to a limited extent interact forming nucleons.

So what were these first atomic nuclei, and how can we study them? In the 1920s, chemical elements were proven to be built from three subatomic particles, neutrons, protons and electrons. Mass is a major property of neutrons and protons, and electric charge is carried by protons (+) and electrons (-). The easiest of these particles to form, and therefore the first, is the neutron. Naked neutrons rapidly decay into a proton, an electron and a neutrino. The simplest atom is Hydrogen with just a proton and an electron. So the early neutron decay generated the resources needed for the formation of hydrogen, and an amazing amount of neutrinos.

Adding a neutron to hydrogen generates Deuterium, an isotope of hydrogen. Smashing two protons together with a combined energy of about 10^7 degrees overcomes the electromagnetic force, and binds then together with the strong nuclear force creating Helium. This process, called nuclear fusion, is what powers nuclear bombs, and stars. It is easier to form helium by fusing two nuclei of deuterium than by bare protons, and makes helium isotopes with mass 3, and 4. Only tiny traces of Lithium (3 protons) could have been formed just after the Big Bang and before the first stars. The next heaviest nuclei are Beryllium (4 protons, with 3 to 6 neutrons). This could not form just by the addition of protons to existing nuclei because the extremely short half life of Be-8 blocks this route, and only extremely small amounts of Be-7, and Be-9 could have formed. So, the Big Bang theory predicts that about 75% of the early mass of the universe was hydrogen, just under 25% was helium, 0.01% deuterium, and less than a millionth of one percent lithium, and beryllium.

How have we tested all this?

The development of space telescopes coupled with spectrographs allowed the direct measurement of the atomic composition of extremely distant stars, and interstellar gases. We find that the proportion of atomic nuclei is exactly as predicted by Big Bang nucleosynthesis.

The graph above shows the theoretical abundances, and the directly observed cosmic abundances of the light elements as discovered by the NASA WMAP project. Click here for more information from NASA.

The nuclear physics is tested in accelerators. The most recent big result was from the European CERN experiments (we Americans have dropped out of “big science,” in favor of massive wars in tiny places. Many American physicists have maintained active collaborations with CERN). The CERN Large Hadron Collider smashed pairs of lead nucleons, each with an energy of 5.75 X 10^14 electron volts. The collision energies were well over 10^20 eV. This is still well below the energies of the Big Bang, but it generated quark-gluon storms similar to the post-inflation period just before the emergence of the first neutrons.

Part 2.

The Big Bang creation of atoms was limited to 75% hydrogen, just under 25% helium, 0.01% deuterium, and tiny traces of lithium, and beryllium. The physical forces were all fixed, and the weakest and most profound force was gravity. It was gravity that controlled the next stage in atom building. Hydrogen, and helium were swept together by gravity, and the density, or pressure of the gas increased. This heats the gases. The deeper the gravity well, the greater the pressure, and the greater the heating. Stars are masses of hydrogen and helium that have heated by gravitational collapse to temperatures high enough to trigger fusion. This starts at about 15 million degrees, or 1.3 thousand electron volts per nucleon resulting in the simplest fusion path of 3 hydrogen nuclei fusing into one helium. The larger the mass of gasses condensing, the faster the temperature raises and the sooner fusion begins. The heat released by hydrogen fusion is what powers a thermonuclear bomb. What keeps stars from exploding is their deep gravity well. But, the heat of fusion does act against gravity, stopping the increasing pressure from gravitational collapse.

The maximum temperature in the interior of a star depends on how much gas was available to collapse to build it. Basically, the larger the star the hotter the maximum core temperature. As the size and core temperature of a star increases, different fusion pathways from hydrogen to helium take over, and release even more energy as heat, and as neutrinos. But, there is no way to go simply from hydrogen to the heavy elements. I mentioned this earlier. What stops the reaction is the extreme instability of Berylium-8 which decays into helium in less than a trillionth of a second. This problem was solved theoretically by Fred Hoyle in the 1950s. He proposed that three helium nuclei (alpha particles) could fuse essentially instantly in the core of a large enough, and hot enough star to form the stable carbon-12 nucleon (6 protons and 6 neutrons). This “triple alpha” process was shown observationally/experimentally to occur in 1957 by William A. Fowler who received the 1983 Nobel Prize for this work. In spite of the fact that this reaction is rare even under the best of conditions, nearly all the carbon, and oxygen in the universe formed this way.

© Copyright CSIRO Australia

For an excellent overview of stellar nucleosynthesis, visit the the Australia Telescope
Outreach and Education pages.

Once carbon, and oxygen are formed, other higher elements follow by the addition of more alpha particles (heliun nuclei), or protons (beta particles). There are other pathways also opened once the Be-8 barrier is overcome, particularly the formation of elements by Beta decay which removes one proton from a heavier element converting it into a lighter one. As the concentration of heavier elements increased, they also undergo fusion reactions, for example two carbon-12 nuclei fuse to make either a Neon-20 + Helium, or a Sodium-23 + Hydrogen. These are called s-reactions because they are all slow.

But another roadblock appears due to the extreme stability of the iron nucleus. Elements formed with atomic mass higher than iron are less stable, and the end result is that iron cores begin to form in massive active stars. (There are still stars today a million times larger than the sun, and these were much more common in the early universe). There is no further nuclear synthesis in the iron core, and the star acquires a stratified structure with most of the active nuclear fusion happening away from the core. This is leading to the collapse of the star, and a Super Nova.

Experimental evidence:

C. W. Cook, W. A. Fowler, C. C. Lauritsen, and T. Lauritsen
1957 “B12, C12, and the Red Giants” Phys. Rev. 107, 508–515

PS: On a personal note I received my first Isotope Producer License when I was about 20 years old and under the instruction of Dr. George Miller at the University of California, Irvine. I used thermal neutrons generated in a TRIGA Mark I nuclear reactor to produce radioactive isotopes. Beta decay is accompanied by the emission of a gamma ray, and the energy of the radiation indicates the mass of the nucleus. This kind of research is called Neutron Activation Analysis. It was quite new back then. Dr. Miller and I have maintained a professional relationship, and friendship for over 40 years.

Suggested reading;

Dickin, Alan P.
2000 “Radiogenic Isotope Geology” Cambridge University Press

Helpful websites;

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