(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.

Why has the Big Bang theory become the standard model for understanding the origins of the universe? In the 15th century and earlier, most people thought that the Earth was the center of the universe and that the stars were embedded in a celestial sphere beyond the outer planets and that the size of the universe was not much larger than the Solar System. The Copernican revolution (with the publication of his book in 1543) displaced the Earth from the center of the universe. This led to suspicions that the universe could be very large, possibly even infinite, but there were at that time no good theories to explain its origins and structure.

Einstein’s General Theory of Relativity (published in 1915) provided a framework for building more systematic models of the universe and various theories began to be put forth. The initial ones argued for a static universe in which everything had a fixed and unchanging location. But some early data suggested that some galaxies were moving away from us and around 1922 models of an expanding universe were proposed, with some early suggestions that perhaps galaxies were moving away from us at speeds proportional to their distance from us. Soon after, observational data supporting that theory started coming in, most famously that of Edwin Hubble in 1929 that, while somewhat scattered, seemed to support that general idea.

If this steady movement away from us had been the case throughout all of time (a reasonable enough assumption in the absence of contradictory evidence), people inferred that if we looked back in time, then everything must have been closer to each other than they are now. And if we go back in time far enough, everything would have all converged to a single point. Thus was born the idea of a Big Bang, the basic idea of which was floated around as early as 1927 by Georges Lemaitre (a Belgian physicist who was interestingly enough also a Roman Catholic priest) and made concrete by George Gamow in 1948, along with the prediction that if this theory were true, the present temperature of the universe (as measured by the primordial photons left over from that initial state) would be around -268 degrees Celsius (5K).

At around the same time another theory called the Steady State was also proposed. This theory also assumed that the universe was expanding but that new matter was also being produced continuously to keep the density of the universe constant. The underlying idea behind this was something called the Perfect Cosmological Principle which said that the universe should look the same everywhere, in every direction, and at all times. This meant that the density of the universe should not change with time either. The amount of new matter that was needed to keep the density constant as the universe expanded was really small (about one hydrogen atom per cubic meter per billion years) but the key idea that the total matter in the universe was not constant made it radically different from the standard Big Bang model.

In 1964, the temperature of the universe was accidentally measured by scientists who had been looking for something else and was found to be -270 degrees Celsius (2.7K). This gave a huge boost to the Big Bang theory.

Another early prediction of the Big Bang theory was the relative abundance of light nuclei (hydrogen, helium, lithium), all of which depended on just one parameter, the total density of protons and neutrons at the time the nuclei were created. The measured values of the light nuclei are in good agreement with the predictions.

These successes added to the credibility of the Big Bang theory and pretty much eliminated the appeal of any competitors. The theory has since moved from strength to strength as scientists have used this basic model to make new predictions that can be tested. These later evidences include the large-scale structure of the universe and the evolution of galaxies, all of which are in reasonable, though not perfect, agreement with expectations.

There is one item about the evidence that I listed in favor of the Big Bang that might have puzzled some readers. I said that Edwin Hubble’s initial data and those that came later seemed to confirm early speculations that all the objects in the universe are moving away with speeds that are directly proportional to their distance from us. i.e., if galaxy A is moving away from us with some speed, then galaxy B that is twice as far away will be moving with twice that speed.

The question is why are they all moving away from us? Don’t they like us? Oddly enough, such an issue would not have been a problem to someone living in pre-Copernican times when it was thought that the Earth was the center of the cosmos. But with the Copernican revolution, it has become common to think that Earth does not occupy any special place in the universe. So wouldn’t you expect at least a few galaxies to be moving towards us since we are not located at a special place in the universe? How do we explain this?

Then explanation goes back to the crucial idea that it is space that is expanding as a result of the Big Bang. We need to go back to the raisin bread analogy from yesterday. If we view the dough as space and the raisins as the matter that is dragged along with the dough (space), then as the dough expands uniformly everywhere, it is easy to show that every raisin will be moving away from every other raisin and its speed will be proportional to its distance from that raisin. This is true irrespective of which raisin we choose as the vantage point from which to make measurements of velocity and distance.

The idea that no particular point in the universe has any special significance has been extended to what is called the Cosmological Principle, which asserts that when viewed on a large enough scale, the observed universe will look the same irrespective of where the observer might be situated. This implies that the laws of science will also be the same everywhere in the universe and underlies our belief that we can apply the same laws of physics that we have discovered to work so well in our neighborhood of the universe even to the most distant reaches of it.

Next: What about the edge of the universe?

POST SCRIPT: The Galaxy Song

From Monty Python and the Meaning of Life. It captures the sense of wonder at the amazing universe we live in.