It’s Professor Dave, let’s check some evidence. Way back at the beginning of this series, we talked about how the universe was born from a singularity 13. 8 billion years ago, which we call the Big Bang, and we went over everything we know regarding this event, and what must have happened since then to produce the universe we see today.
We weren’t ready yet to talk about the overwhelming evidence that supports this model, but after having learned about galaxies and many other things, now we are. Let’s talk a little bit about the models that were competing with the Big Bang around the time that it was proposed, and all of the separate threads of evidence that have cropped up to firmly support our current conception of the origin of the universe. Let’s start with the first primitive demonstrations of a universe with a finite age.
This began with something called Olber’s paradox. This is named after a German astronomer, who in 1823, wondered how the night sky could be dark if the universe is infinite. He reasoned that if the universe were infinite in both space and time, full of stars in all directions forever, and with no beginning in time, then every line of sight that is possible should eventually arrive at a star.
If every line of sight were to reach a star, then the whole sky should be as bright as a star, and the night sky should be as light as daytime. Olbers himself concluded that since the night sky is dark, perhaps space is not completely transparent. But other thinkers that followed, most notably the poet Edgar Allen Poe, interpreted the paradox as evidence that the universe must be finite.
They hypothesized that not every line of sight ends at a star, because there are not infinite stars, or infinite space to contain them. Of course, this isn’t the most rigorously scientific argument of all time, as there are variables it doesn’t take into account, but it did set the stage for more substantial evidence for a finite universe in the following century. In the beginning of the 20th century, modern cosmology began, thanks to Einstein’s general relativity as a consistent mathematical description of the universe.
At this time, the notion of an infinite universe was still quite prevalent, with such prominent supporters as even Einstein himself. He made the assumption, on next to no evidence, that the universe as a whole was quite smooth, with all of its galaxies distributed more or less evenly. This simplified universe was referred to as homogeneous, or roughly the same everywhere, and isotropic, or appearing the same in every direction.
These two assumptions together form the Cosmological Principle. But his own general theory of relativity required that spacetime be dynamic and changing, and therefore either expanding or contracting. At the time, astronomers reported that stars were neither approaching nor receding from our solar system, and Einstein was so convinced that the universe should be static, that he introduced a modification that would reconcile this static universe with general relativity.
This was called the cosmological constant, which bestowed space itself with the ability to expand or contract in such a way that precisely cancels out the expansion or contraction of the universe, allowing for the static universe he envisioned. Just a decade later, Hubble published the results that were used to demonstrate that the universe is indeed expanding, with redshift values arising as the result of the stretching of spacetime, and Einstein called his cosmological constant the biggest mistake of his career, although current studies show it may not have been totally off the mark, for other reasons we will get to later. Once it was accepted that the universe is expanding, the Big Bang model was proposed, but it had a competitor in the Steady State model.
This proposed that the universe was expanding, but has the same properties at all times. In order for this to be true, the model postulated something called a C-field, which continuously creates new matter as the universe expands, so as to maintain the same overall density for the universe. This seems absurd now, but at the time, it was more popular than the Big Bang model, which postulated that the properties of the universe change dramatically over time, beginning from an extremely hot and infinitely dense point.
So what was the evidence that cropped up to result in the discarding of steady state in favor of the big bang? This began in the 1960s, with Arno Penzias and Robert Wilson. They were using a microwave antenna to study the emission produced by earth’s atmosphere, looking for sources of interference that would cause problems for satellite communication systems.
To their surprise, they found a uniform background of noise in every single direction, no matter where they pointed it. After trying everything they could think of, including removing pigeons from the apparatus, this signal just would not go away. Completely by accident, they had discovered a smoking gun, the leftover heat from an event just after the big bang itself, which we call the cosmic microwave background radiation.
This radiation, an emission of blackbody thermal energy, has a temperature of around 2. 7 Kelvin, just barely above absolute zero. Because of the extreme isotropy of this radiation, detectable in every direction and not associated with any particular source, its origin must have been from a time of thermal equilibrium, when the entire universe was one opaque ball of plasma.
It was the era of recombination that we discussed in our overview of cosmology that produced this radiation, just 300 thousand years after the big bang. When electrons combined with nuclei for the first time, they immediately relaxed to lower-energy states, and in doing so, they emitted electromagnetic radiation. The radiation was then stretched out as the universe expanded over billions of years, leaving it in the microwave portion of the spectrum today.
Proponents of the big bang model had predicted this cosmic microwave background prior to this discovery, and had estimated its temperature at around five kelvin. The confirmation of this phenomenon was the first huge victory for the big bang model, as the steady state model could not account for such a blackbody spectrum. But it wasn’t the only stunning piece of evidence that would be compiled.
We can use the cosmic microwave background, and the recession velocities of the universe, and turn back the clock, so to speak. We just use math to play the movie backwards until we get to the beginning. The math, in any of these cases, agrees with a figure of about 13.
8 billion years for the age of the universe, and that’s the essence of the big bang model, which goes on to make a tremendous amount of predictions. For one thing, the model predicts that given the expected rate of cooling, there must have been a period where the universe was just cool enough for subatomic particles to exist and just hot enough for them to fuse. This was the period of nucleosynthesis we described when we first examined cosmology.
This period should have been long enough such that about one fourth of the primordial hydrogen fused to become helium, and when we look around, we do indeed see a universe that is about 3 to 1 hydrogen to helium. A similar prediction can be made for the baryon to photon ratio in the universe, and this also matches up with observation. The model also predicts when galaxies ought to form, about a half a billion years into the lifetime of the universe, and when we look as far out into the universe as we can, we can see these early galaxies forming 13.
4 billion light years away, their light only now reaching our eyes, just as the model predicts. The calculations associated with these kinds of predictions are too complicated to be shown here, but if you go on to study astrophysics and cosmology, they will be examined in detail. For the extremely early epochs, the ones where symmetry breaking occurred to separate some unified force into the four we know today, we have particle accelerators to test predictions.
We’ve described the electroweak epoch, where the electromagnetic force and weak nuclear force were combined as one, and particle physicists can make predictions about what kinds of particles should exist at such high temperatures to mediate this force. We can predict their mass, charge, and other parameters. Then, when we do experiments by smashing particles together at nearly the speed of light, the collision converts these particles to pure energy, by Einstein’s E = mc^2.
For the tiny region encapsulated by this collision, temperatures and energies resembling the very early universe are achieved, and the particles that existed in those early times have a brief chance to exist again. We use bubble chambers to measure their properties, and when they precisely match predictions, that is a huge victory for the standard model of particle physics, which is intimately intertwined with early-universe cosmology. As we build more and more powerful particle accelerators, we can generate collisions that yield more and more energy, thus probing earlier and earlier towards the initial singularity.
This will help us produce theories that describe the unification of the electroweak force with the strong nuclear force, and maybe one day, all four forces, as they were in the very first epoch of the universe. And there you have the evidence for the big bang model. The power of the model lies in its predictions, which although quite disparate, have been largely confirmed by observation.
The temperature of the cosmic microwave background. The distribution of hydrogen and helium in the universe. The results of experiments in particle accelerators.
We make quantitative predictions, make some observations, and see that we were correct. This makes the big bang so much more than some creation myth, because the model fits the data. All the data, of every kind.
That’s empiricism at its finest, and it’s the best that science can hope to do. For this reason, cosmologists are about as sure that the big bang happened as they are that the earth goes around the sun. But of course, there is still more to learn.
Not just about the first few instants after the big bang, but other aspects of the universe. Let’s move forward now and take a look at the frontier of astronomy.
