(CNN) -- While Christians celebrate the birth of Jesus at Christmas, atheists may wonder if there is another birth they might be able to commemorate.
One possibility is to give thanks for the arrival of Isaac Newton, who was born on Christmas Day 1642 according to the Julian calendar, which was still in use in England at the time.
Another possibility, and probably my preference, is to use Christmas Day as an excuse to celebrate the biggest birth of all, namely, the creation of the entire universe.
For tens of thousands of years, humans have stared up into the heavens and wondered about the origin of the universe. Up until now every culture, society, and religion has had nothing else to turn to except its creation myths, fables, or religious scriptures.
Today, by contrast, we have the extraordinary privilege of being the first generation of our species to have access to a scientific theory of the universe that explains its origin and evolution.
The Big Bang model is elegant, magnificent, rational, and (most importantly of all) verifiable. It explains how roughly 13.7 billion years ago matter exploded into being and was blown out into an expanding universe. Over time this matter gradually coalesced and evolved into the galaxies, stars, and planets we see today.
Before explaining how you might celebrate the birth of the universe, let me quickly explain why we are convinced that there was a Big Bang. First of all, telescope observations made back in the 1920s seemed to show that all the distant galaxies in the universe were redder than they should have been.
Red light has a longer wavelength than all the other colors, so it was as if the light from the galaxies was being stretched. One way to explain this stretching of galactic light (otherwise known as the "red shift") was to assume that space itself was expanding. Expanding space is a bizarre concept, but it is exactly how we would expect space to behave in the aftermath of a Big Bang explosion.
However, this single piece of evidence was not enough to convince the scientific establishment that the Big Bang had really happened, particularly as the observations were open to interpretation. For example, the Bulgarian-born astrophysicist Fritz Zwicky pointed out the redness of the galaxies was merely an illusion caused by the scattering of light by dust and gas as it passed through the cosmos.
By the way, as well as being a critic of the Big Bang and the data that seemed to support it, Zwicky was also responsible for inventing a beautiful insult. If a colleague annoyed him, Zwicky would scream out, "Spherical bastard." Just as a sphere looks the same from every direction, a spherical bastard was someone who was a bastard whatever way you looked at him.
A second pillar was needed to support the Big Bang model, and this time the crucial evidence relied on measuring the ingredients of the universe, most importantly hydrogen and helium. These are smallest atoms in the periodic table and the most common in the universe, accounting for 74 percent and 24 percent of all atoms, respectively. Crucially, the only way to create such large amounts of hydrogen and helium is in the wake of the Big Bang. In particular, the pressure, density, and temperature of the early universe would have cooked exactly the right amount of hydrogen and fused it into exactly the right amount of helium.
In other words, the Big Bang is the best (and probably the only) way to explain the abundances of these light elements. Nearly all the other elements were made later in collapsing stars. These stars provided the perfect environment for the nuclear reactions that give rise to the heavier elements that are essential for life. CNN
