There’s a big, big difference between a “scientific” theory and all other types. It should make you more confident, not less.
One of the most important things any expert hoping to communicate their passion and knowledge for their field with the rest of humanity must do is interact with a varied audience. Sometimes, that variety can cause problems for a portion of the audience themselves. As part of the questions and suggestions you can submit for our Ask Ethan series, we have this relatable inquiry from Chris Shaw, who asks:
If someone asked you and you had to answer your true feelings would you say that the universe expanding from a hot dense state is fact beyond theory? If you had to bet your life on it, what would you say?
There are a whole lot of important nuances to this issue, so let’s start with the specific one you brought up: the Big Bang.
Contrary to what you might have heard, the Big Bang doesn’t necessarily mean that the Universe began from a singularity, but rather that it started from a hot, dense, expanding state and has been cooling ever since. There was a key observation and a key theoretical development that led to this theory being formulated:
- On the observational side, it was noticed that the spiral nebulae — discovered to be galaxies unto themselves in the 1920s — were moving away from us at tremendous speeds, and that the farther away their distances were determined to be, the faster (on average) they appeared to be receding from us.
- At right around the same time, it was realized that Einstein’s theory of General Relativity, verified in 1919 by the observation of starlight bent by gravity during a total solar eclipse, forbade a static Universe, and that the fabric of space itself must either be expanding or contracting over time.
By combining the observational results with what the best, most successful theory predicted, there was a new conclusion that couldn’t be avoided: the fabric of the Universe itself was expanding, and the reason for the apparent recession was the fact that the light from distant sources was being stretched by the expansion of space itself. The Big Bang took this one step farther, and said that the expanding Universe today could be extrapolated backwards in time to a state that was hotter, denser, and expanding more rapidly in the past.
But that combination of observation and theory, by itself, doesn’t make the Big Bang a necessity. Rather, the Big Bang was merely one of many new hypotheses that came to the forefront, along with the tired-light scenario, the steady state model, and (later) the plasma cosmology/electric universe model. What made the Big Bang stand apart from these other alternatives were its unique predictions:
- The Universe was once so hot and dense that atomic nuclei couldn’t form without immediately being blasted apart. This predicted a very specific abundance of the light elements in the early Universe, including the amounts of deuterium, helium-3, helium-4 and lithium, which should have been unchanged until the first stars in the Universe formed.
- The Universe would then cool through a state where neutral atoms could form, resulting in the Universe becoming transparent to all the radiation that was present then. This light should then be stretched (and hence, cooled) by the expansion of the Universe, giving us a leftover glow from the Big Bang that would just be a few degrees above absolute zero, and would have a blackbody spectrum.
- And in addition, this predicts a Universe where structure — stars, galaxies and clusters — would emerge in progressively more evolved forms over time, and that the Universe would appear less evolved the farther away we looked.
The observation of the leftover glow from the Big Bang, known as the Primeval Fireball at the time of its discovery in the 1960s and known today as the Cosmic Microwave Background (CMB), vindicated the Big Bang model and disfavored all the others. Subsequently, the observations of its imperfections, the measurement of its temperature spectrum and other properties have completely ruled out all the other alternatives. The Big Bang, as the scientific theory of where our Universe came from, is here to stay.
Now, that doesn’t mean it’s a fact, in the sense that this is by no means the final answer that explains everything. This is a step along the journey, explaining many properties concerning the Universe today successfully and unambiguously; there is no other explanation consistent with all we’ve seen. But there is more to explain than the Big Bang alone can handle, including such questions as:
- Where do the seeds of structure — the overdense regions that gave rise to stars, galaxies, clusters, etc. — come from?
- Why does the Universe have precisely the same temperatures, densities and properties in all directions, equally, in space?
- Why is the spatial curvature of the Universe completely flat, as opposed to positively or negatively curved?
- Why does the expansion rate behave as it does over time, and not in some other way?
- Why are there no ultra-high-energy relics, like magnetic monopoles, populating the Universe today?
- And why does the formation of structure indicate that there’s a component of matter, the majority of matter, that doesn’t interact with radiation?’
This is why we have subsequent improvements on the Big Bang as initially proposed, including the existence of cosmic inflation (which precedes and sets up the Big Bang), dark matter (whose nature still has yet to be determined), and dark energy (whose properties are still being discovered). This is why we expect new, cosmic discoveries to explain the masses of neutrinos, the preponderance of matter (and not antimatter) in the Universe, and the nature of dark matter itself.
The point of this is to say that there is more to learn, and that while much of this may supersede the Big Bang, none of it will invalidate it. Newtonian gravity didn’t stop being true when General Relativity was put forth; it was merely recognized to have a limited range of validity: to slow speeds and weak gravitational fields. Similarly, the Big Bang has a limited range of validity: to “only” the past 13.8 billion years, but not before that.
It’s worth noting that this is true of all scientific theories! The theory of evolution explains the diversity of life on Earth today as well as the mechanism of how it came to be this way, but it doesn’t explain the origin of life itself. The theory of gravitation explains the attraction that matter and energy experiences and how it affects space and time, but it doesn’t explain the other fundamental forces, nor the nature of space and time itself. Each theory has a range of validity, and beyond that, a better scientific theory will need to come along, extending that range of validity further.
As far as whether I’d bet my life on it? In a sense, I already have. This is what I study for a living; the entire enterprise of science — this self-correcting method of gathering data, comparing with observations, and revising based on what we discover — is how we gain this sort of knowledge and understanding in the first place. My life is “bet” on the assumption that this is a good, valid and useful way to approach learning about the reality we all inhabit. And yet, I’m fully aware that a single observation could alter everything we think we know, and that our most sacrosanct conclusions and theories could be falsified and invalidated if the data contradicts our theory’s predictions. It won’t invalidate what we already know, though; it will merely provide a superior way forward to better understand the Universe around us. And whenever that happens, that’s the greatest scientific joy of all: the joy of finding things out.