Before the Big Bang: The Prehistory of Our Universe by Brian Clegg

Author:Brian Clegg [Clegg, Brian]
Language: eng
Format: epub
ISBN: 9780312385477
Amazon: 0312680287
Publisher: St. Martin's Press
Published: 2009-08-04T00:00:00+00:00

MISMATCHED LITHIUM

There are also other problems in matching the way the universe has behaved to the theory. One comes up in the amount of the element lithium made in the initial preuniversal soup. As do many elements, lithium comes in two “flavors” or isotopes. If you remember your periodic table from school, chemical elements are identified by two numbers, the atomic number and the atomic weight. The atomic number tells you the number of protons in the nucleus, or electrons in orbit around the atom (both the same). The atomic weight tells you the number of heavy nuclear particles (protons and neutrons) present.

When it first became clear what atomic weight was, it was thought there was something horribly wrong with the idea, as it is a measure of the number of particles, and whereas for some atoms this is fairly straightforward, for others the value doesn’t seem to make sense. Nitrogen, for example, has atomic number 7, and atomic weight very slightly over 14, so it’s easy enough to say it has 7 protons and 7 neutrons in the nucleus. Chlorine, however, has atomic number of 17 and atomic weight of 35.45. It seems to have around 18½ neutrons, which just can’t be right.

The solution to this problem is the realization that atoms can come with different numbers of neutrons in the nucleus. In the case of chlorine there are stable versions with both 18 and 20 neutrons. As there are more with 18 than 20, they average out around the 18½ mark. These different versions are called isotopes. Lithium, too, comes in two stable isotopes, Lithium-6 with three neutrons, and Lithium-7 with four.

According to conventional Big Bang theory, a lot of the lithium in the universe was made before the stars were formed, although as with helium, a proportion is produced in stars. However, a big problem has recently emerged. The theory doesn’t match observation. There seems to have been only one-third of the amount of Lithium-7 in the early universe that theory predicts, whereas there was 1,000 times as much Lithium-6 as there should have been.

As with all measurements reaching back to the very early days, we are reliant on some indirect observation and calculation. The actual values are produced by looking at very early stars. By peering as far as we can into the distant universe and using spectroscopic analysis (here) it is possible to get an idea of the proportions of the different elements in very early stars. Without a chance to have been cooked up in the stars themselves, this balance should roughly reflect what things were like as the stars were formed.

To find what theory predicts involves a more subtle measurement, relying as so often is the case on the cosmic microwave background radiation. If the Big Bang theory is correct about how helium, lithium, and beryllium came into being, the amounts produced would depend on the ratio of particles such as neutrons and protons in the original mix (collectively known as baryons) to the number of photons.

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