Big Bang for beginners-3: The basic story

(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released and is now available through the usual outlets. You can order it from Amazon, Barnes and Noble, the publishers Rowman & Littlefield, and also through your local bookstores. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)

For previous posts in this series, see here.

The starting point of the Big Bang story is a cosmic event that started out small and expanded rapidly (like an explosion). This event brought into being the universe we now inhabit and produced all the matter that our universe is presently composed of, though not in its present form. The time at the beginning is arbitrarily set to zero.

We do not know what happened right at the very beginning (at time zero by our convention) because our known theories are believed to not apply right at the beginning. So our story begins very shortly after the Big Bang occurred. It is believed that what existed then were quarks, gluons, electrons, and photons that were moving freely around in a hot dense gas called a plasma. (There were also a few other exotic particles that I will ignore as they are not central to a basic understanding of the evolution of the universe). As the universe expanded over time, these quarks and gluons and electrons and photons eventually became the ordinary matter that we now have. No new matter was created after the Big Bang, but the form that the matter took did change dramatically.

To understand how this process of evolution occurred, we have to understand two basic relationships.

  1. The ‘temperature’ of the universe is related to the average energy of the photons that the universe contains.
  2. As the universe expands and gets bigger, the same amount of matter now occupies a larger amount of space and so the density of matter gets smaller and the temperature of the universe drops.

So right at the beginning the universe was highly dense because all the matter that now exists in the visible universe was compressed into a region that was smaller than a golf ball, if you can imagine that. As a result, the early universe was extremely hot. Because the density of photons (the number occupying a given region of space) was huge and they were so highly energetic, they could (and did) blast apart every composite object they encountered, so that the only things that could exist in the very early universe were those that could not be further broken up into smaller bits, which is what we think that quarks and gluons and electrons are.

As the universe expanded, it became cooler and the photons became less energetic and after some time their average energy became so low that if some quarks and gluons happened to combine to form protons and neutrons, the photons could not break them apart anymore, so the protons and neutrons remained intact. So now the universe consisted of protons, neutrons, electrons, and (lower energy) photons, with all the quarks and gluons being trapped inside the protons and neutrons and no longer free to move around independently.

As the universe expanded and cooled even more, the photons became even less energetic and they could not break apart any nuclei that happened to be formed by protons and neutrons combining. So now the universe consisted of nuclei as well, along with protons, neutrons, and electrons. The only nuclei that formed during the Big Bang were those of the three lightest atoms: hydrogen, helium, and lithium in decreasing order of abundance, with hydrogen forming about 75%, helium about 25%, and just trace amounts of lithium. (The nuclei of all the hundred or so heavier elements that currently exist were formed in the interior of heavy stars, and so came along much later.)

As time went by, the temperature of the universe became even less, so that the photons could not break apart any atoms that formed by nuclei and electrons combining, so the universe now included atoms as well.

And finally, the temperature of the universe became so low that the photons could not even break apart any molecules that formed.

After that, as the universe expanded and cooled even further, the primordial photons in the universe (i.e., those that were created right at the beginning) had such low energies that they ceased to have any effect on anything else. They became, in effect, disconnected from the rest of the matter in the universe. But these photons still exist, getting steadily cooler, and occupy all of space, surrounding us. They give us important information about the origin of the universe, its evolution, and its eventual fate. It is these photons that are referred to as the ‘cosmic microwave background radiation’.

The term ‘microwave’ is used because the average energy of these primordial photons now is roughly equal to that of the photons produced by your microwave oven. Fortunately for us, the density of these cosmic photons is now extremely low (only about 400 per cubic centimeter) because the universe has become so large. Otherwise we would all be cooked in this cosmic microwave oven. The temperature of the universe at the present time is about -270 degrees Celsius, very close to the absolute minimum temperature that is possible which is -273 degrees Celsius.

Next: How rapidly did this cosmic evolution happen?

POST SCRIPT: Goodbye, Peter Graves

Actor Peter Graves, actor and star of the long running TV series Mission: Impossible, has died at the age of 83. Like Leslie Nielsen, Lloyd Bridges, and Robert Stack, he was better known for serious roles but their deadpan delivery helped make the spoof Airplane! one of the funniest films of all time, one that I watch every few years and it still makes me laugh.

Here is a compilation of the scenes with Graves (as Captain Clarence Oveur), though there was a funny opening segment with the Hare Krishnas that did not make this cut.

And here is the trailer:


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