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How can we have so much confidence in something like the Big Bang theory when we haven’t observed it?

It is true that we can’t observe air but we detect it in the present. It is true that we can’t observe historical events but we can detect similar events in the present.

The beginning of the universe seems to be a special event in its own right even if it began. How can we have confidence in the way this event supposedly happened?

  • The same way science is confident about everything else. A lot of modern science does not rely on immediate observation by the senses anyway. – Frank Mar 08 '23 at 21:03
  • We do not have as much confidence in it as in ordinary events we observe on Earth, but we have enough. We observe bits and pieces and routinely combine them into compound events theoretically. That is how we model what goes on inside of stars or, indeed, what went on in the early universe around the time of the Big Bang. Those models produce predictions (like the Cosmic Microwave Background radiation) that are tested and give us confidence. – Conifold Mar 08 '23 at 21:18
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    AFAIK there is no scientific consensus about the origin of the universe, or how it came to be. All we have is fairly confident models about the very early state of the universe. Hubble observed it is expanding, so logically if we think backward from now it must have been smaller. At some point it would have been so small that we have no data about how matter behaves in this situation, so the honest answer is we don't know. People can of course push hypothesis, but anybody who says they know what happens before that and does not get a Nobel is a charlatan with a quack theory. – armand Mar 08 '23 at 23:23
  • By and large, this may not really be a question for this Stack Exchange, if the answer needs to provide details about how astrophysics works (see Niels Nielsen's answer). The question of "confidence" is about the same here as for any other question in astrophysics, but it's a scientific rather than philosophical question. – Frank Mar 09 '23 at 00:06
  • If x then y; if y then z; z; ??? – Agent Smith Mar 09 '23 at 07:47

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The well-known, well-tested and well-understood laws of physics can be used to take the universe in its present state and answer the question: what did the universe have to have looked like 1 million, ten million, one hundred million, one billion, and finally 10 billion years ago in order for it to have evolved into exactly what we see today?

Those laws place extremely stringent constraints upon what the so-called "initial conditions" of the universe could possibly have been in order to eventually furnish us with today's universe- and predict for us what evidence if any was left behind at each epoch in the universe's evolution through time. The best example of this is the Cosmic Microwave Background, which is a relic- a fossil- left behind after the Big Bang had gone to completion and launched the universe onto its evolutionary course. This fossil radiation was predicted to exist years before the instruments that eventually found it (see below) had been built.

Then we train our astronomical instruments upon the heavens and search for that evidence. Note that without those instruments, we humans could not possibly see those signals because they are far too faint for our eyes, or they consist of signals that our eyes are not capable of detecting (radio waves, infrared, ultraviolet, or x-ray light, etc.). Remember: just because out eyes can't detect radio waves does not mean that they are not "real".

Note also that when we look at far-away objects, we are seeing them not as they are today but as they were hundreds of millions or even billions of years ago when the light we see today first departed those objects. This means that a powerful telescope is in fact a time machine which lets us look into the past and see just what the universe was like then.

Dozens of excellent books have been written on this topic by the experts in the field. My favorite is The First Three Minutes by Stephen Weinberg.

niels nielsen
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