Tommy Evans asked:

So the Big Bang theory is only a theory, and when another theory comes along, that will become another ‘theory’. In other words, no one knows or prove exactly how how the universe came into being.

Answer by Shaun Williamson

You are being deceived by the different senses of the word theory. In ordinary life we often describe something as just a theory meaning it is a proposed explanation for things which has not yet been supported by the evidence.

However in science the word theory is used to mean an explanation for the facts which is supported by all the evidence. In science a hypothesis is a proposed explanation. When we get good solid evidence for a hypothesis it becomes a scientific theory.

So the Big Bang is a scientific theory. It is misleading to describe it as ‘only’ a theory. It will only ever be replaced if someone comes up with a stronger theory that is also supported by all the evidence.

Evolution is a scientific theory. Relativity is a scientific theory. Quantum mechanics is a scientific theory. No one who understands science doubts that these theories are true because there is just so much evidence in favour of them that it makes no sense to doubt them. We may modify these theories in the light of further evidence but we will never completely discard any of them.

The Big Bang Theory is not as strong a theory as the previous three but it is still strongly supported by all the available evidence. It isn’t just a wild idea that will be easily replaced.

To summarise ‘just a theory’ and ‘a scientific theory’ mean two completely different things. Don’t confuse one with the other. It is unfortunate that we use the word theory in these two completely different senses.

Answer by Craig Skinner

You don’t exactly ask a question. But you do express an important truth about science. Science does not prove things. It probes them. Proof is the business of logic and maths, consisting of deductions from axioms, but tells us nothing about the world. To learn that we need to investigate the world. Science does this systematically by the method of conjecture and testing. We start with an intelligent guess (hypothesis or theory) about how the world works, then test it by seeing whether things predicted by the theory are found (corroborating the theory so that we hold on to it for now) or not (refuting the theory so that we think up a new or amended one and test that). We aim to arrive at true theories, and probably do sometimes, but we can never be sure we have done so. A theory is just the current best-supported conjecture — a future observation or experiment may require us to change it.

Of course in practice we accept many theories as true. For example, aerodynamics — we fly off on holiday in machines designed as per this theory, and never doubt that they will fly; I compose this email, confident of electronic theory that says you will receive it; I am careful not to drop my beer glass although it’s ‘only’ a theory (Einstein’s theory of gravitation) that says it will fall to the floor.

There are good theories and bad ones. A good one needn’t be true. For example Newton’s theory of gravity served us well for 250 years, it’s calculations good enough to get men to the moon and back (but not good enough for accurate urban car sat navs). But it was superseded by Einstein;s theory when different predictions flowed from the two theories and observations supported Einstein.

For a theory to be scientific at all, it must be testable. In particular there must be some observation or experiment which would refute it. As in the example described above where Einstein’s theory would have been refuted had no bending of starlight been found.

So what are the features of a good theory? Here they are:

1. Consistency — the theory must account for what is already known

2. Scope — the theory explains more, often by uniting theories dealing with seemingly different things eg Newton’s laws of motion accounted for movements of stars, planets, Earth, moon, comets, cannon balls, falling rocks

3. Novel predictions — Einstein’s theory predicted bending of starlight in the vicinity of the sun: observations at the time of the 1919 solar eclipse found this.

4. Fertility — the theory stimulates research leading to new understanding.

5. Elegance or simplicity — prized by some scientists, difficult to measure.

So, although theories are ‘only’ theories, they are not all on a par, and good ones are rarely dreamed up by punters in their armchairs (such as me) by by scientists familiar with existing theory and practice of science, and with good intuition and judgment.

So what of the Big Bang theory? Until the 20th Century, the universe was considered static: after all, the patterns in the stars do not change over a human lifetime or over historical periods. Einstein’s theory predicted an expanding universe, but this was too much for the great man (or anybody else) to accept. So he added a fudge factor (cosmological constant) to his equations so that they implied a static universe. But then observations found clear evidence of expansion of the universe (‘red shift’ of light from distant galaxies). Some held on to the static universe idea despite expansion by suggesting that matter was constantly created from nothing in small amounts everywhere, thus counteracting the expansion’s effect of making the universe ever less dense, even if expansion went on for ever. Hoyle was a main proponent of this ‘steady state’ theory. The alternative view extrapolated backward, reasoning that, if expanding, the universe was smaller, denser and hotter in the past, and would have started in a very tiny very hot state about 13 billion years ago. In a radio programme Hoyle ridiculed this theory as the ‘Big Bang’, and the name stuck.

The Big Bang theory is a good theory. It accounts nicely for the expansion of the universe. It makes a novel prediction, different from what the steady state theory predicts. Thus, if the universe started off very very hot and tiny 13 billion years ago, it has since expanded cooled down gradually to leave a background very low temperature radiation (3 degrees absolute) everywhere in the sky. Sure enough, this Cosmic Background Radiation was discovered in the 1960s. The Big Bang theory has also stimulated much other theory and experiment.

But, of course, it’s not the last word. As you say another theory comes along (importantly, though, this often adds to rather than replacing the old theory, and this is the case here). Thus the Big Bang didn’t account for the homogeneity or the flatness of the universe (I wont go into the technicalities), and the Inflationary Big Bang theory replaced it. But plenty of uncertainties remain. Such as what happened before the BB ( was there a ‘before’), why did it happen, was it unique or just one of many BBs (serially or in parallel).