This is the second post in my series on “Defending the Big Bang.” The focus of this one is to explain the four main pieces of evidence that support the modern Big Bang theory. You will notice that I am using the prefix, “modern,” to describe this theory, and that is because – like almost any scientific theory – it has been modified over time to address new information. However, the core essence of the idea – the universe started as an infinitesimal point/plane/field/brane of energy that grew to become our universe – has remained the same since it was first proposed.
Other posts in this series:
Pillar #1 – Everybody Hates Us
Okay, not really, but that’s a good way to think about the first piece of evidence that supports the Big Bang – most far-away objects are moving away from us in every direction that we look. Unless we inhabit a very special place in the universe (which modern scientific thought does not allow unless all other explanations are ruled out), then this implies not that everything is moving away from us, but that everything is moving away from everything else.
This supports a Big Bang model because it fits very well with the idea the universe is expanding. The very hackneyed thought experiment is to picture a loaf of raisin bread rising in the oven. The bread is the fabric of space and the raisins are different observers (such as us on Earth in our galaxy). As the bread rises and expands, each raisin will move away from every other raisin. Raisins that are farther away will appear to move away faster. And this is precisely what is observed.
This phenomena was first discovered by Edwin Hubble in 1929, and hence we have termed the expansion rate of the universe the “Hubble Constant,” H0.
So if we’re now at a point where the universe is expanding, then logically if you run time backwards, it would be shrinking. And if you follow it back, say, 13.7 billion years, then the entire universe would be compacted into an infinitesimally small “object.”
Pillar #2 – The Cosmic Microwave Background (CMB) Radiation
The CMB was discovered in 1965 by Arno Penzias and Robert Wilson from Bell Labs (who later received a Nobel Prize for their discovery). In every direction they pointed their microwave receiver, they “heard” a noise. This noise came from the first photons that could stream freely through the universe that have become so stretched out over time (due to the universe’s expansion) that they now exist in the microwave range.
These photons are an “echo” left over from when the universe was approximately 380,000 years old. Before this time, the universe was completely opaque to light. This is because the electrons that existed at the time were too energetic – too hot – to be bound to atomic nuclei, and therefore were able to roam freely about. This means that photons could not move about freely because they kept being absorbed and re-emitted by the electrons.
After the universe had aged to around 380,000 years, it had cooled to approximately 3000 K. Electrons no longer had enough energy to overcome the attractive force of atomic nuclei, and they became bound. Light could now stream forth unimpeded. This process is called “recombination,” and this “first light” is what we now see as the CMB Radiation.
Pillar #3 – Abundance of Light Elements
Before the CMB, when the universe had only been in existence for about 1 second, individual protons and neutrons had already been created (along with all the quarks and leptons and other subatomic particles). It was no longer hot nor dense enough to create protons nor neutrons, and so the ratio of the number of protons to neutrons was frozen in (at about 0.223).
After the universe was a few minutes old, the temperature had dropped more and the era of nucleosynthesis could begin. This is when the light atomic nuclei of deuterium, tritium, and helium could form. But, after about 4 minutes, the universe had again cooled to a point where neutrons and protons could not combine, and so the primordial ratios of these elements were frozen.
Despite the universe having been around for another 13.7 billion years (minus 4 minutes), very little of the primordial material has been used to create stars. So astronomers can go to telescopes and measure these abundances. The Big Bang predicts about 72% of the material out there is hydrogen, and 28% is helium. Astronomers have found that about 24% is helium and 76% is hydrogen, and with error bars, this is in good agreement with the theory.
Pillar #4 – Growth of Structure
The nitty gritty parts of this one require a lot of computer modeling and telescope observations, but the basic idea that the Big Bang – the expansion of the universe from something that was smaller – and the way that it unfolded is the only scientific theory that explains how the large-scale structure that we observe today on the scale of gigantic super clusters of galaxies and the voids between them.
Any theory has to explain the available data. If it doesn’t, then it must be modified to account for the data, or it must be abandoned for one that can. I don’t and won’t pretend that the Big Bang theory that exists today has been the exact same one that existed decades ago. That’s not how science works, and that’s not how the history of the theory has played out. But, the Big Bang theory is the only scientific theory that explains all available observations, and propping it up are four fundamental observations.