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One alternative that was popular what the steady state theory.

It assumed that the universe had always been more or less as it was now and would always be that way in the future.

This fell out of favor as we realized that the world and the stars above had not always been as they were now, and that things were changing over time.

There also were troubles with the fact that the night sky looked awfully black for a universe where stars were in every direction and had always been shining forever.

As far as we can tell there was no "before" the big bang. The big bang was the event in which space and time (known as spacetime) came into existence. There was no before this, because time didn't exist as we understand it.

The only theory consistent with the Big Bang is that there is no "before", as the word "before" refers to a point lower than X on the time axis, but time itself was only created with the Big Bang, so the axis came into existence "after" the Big Bang, hence there can't be a "before".

It's like asking "what lies to the North of the North Pole?"

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As far as we know absolutely no information could have been preserved from “before” the Big Bang to afterwards. Therefore there will never be a testable theory. We could hypothetically discover in the future the existence of additional universes and may then develop a theory of how ours occurred and if something existed before it. But we’ll likely never know even in that scenario whether our universe was the first or if it came into existence at the same time as those hypothetical additional universes.

For all intents and purposes nothing existed before the Big Bang including time.

But isn't time just a human construct used to measure duration? So wouldn't that imply that time didn't begin with the big bang, but with humans recording time?

You're conflating time units and temporal vocabulary, with the part of the universe described by the part of the mathematical model that we call time.

Humanity not existing didn't prevent galaxies to form and recombination to happen. However, our current models cannot even talk about "before the big bang"

That's like saying distance is a human construct used to measure length?

The units to measure them might be manmade - minutes and meters. But time and distance are real things.

Weirdly, both space and time as we know them began from the big bang. So neither distance nor duration exist when the whole universe was compressed into one small thing. The correct answer is that there is no "before".

Personally I have always imagined the universe in a bit of bouncy equilibrium - the big bang followed by expansion, which peters out eventually, to be followed by the big squeeze and compression, which leads to overcompression and another big bang, so on and so forth.

we don't know, yet, and that's OK. Maybe we'll find out someday, maybe we won't. That's science, baby.

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It’s also possible that events still happened before the Big Bang. Just not in any kind of cause > effect order. What started existing at the Big Bang was the arrow of time.

There could be time, just not space time with any meaningful relationships we would recognize constituting a recognizable “before” or “after” relationship. There could still be change as in your bubbling equilibrium theory.

That nothing is called God..

Not to go down the incomprehensible road, but if they say the universe is expanding relative to origin. What physically is it expanding into? A void? And would that void have existed before the beginning of time?

Best analogy I’ve heard is to imagine our universe was flat and existed on the surface of a balloon. As the balloon expands all 2 dimensional creatures living on the balloon see other galaxies moving away from them. But what is it expanding into? Nothing. Their universe is made of a flat ball of expanding rubber. It’s not expanding into more rubber. There is absolutely nothing that exists within the laws of nature in that 2 dimensional universe that can possibly describe what it is expanding into. In fact it’s really only expanding in time. The balloon was smaller in the past and will be larger in the future. But it isn’t expanding into anything.

Now imagine three dimensions of space and one of time expanding (if you have a better imagination than me) in the same way that balloon expands. It’s not expanding into anything, it is everything and everything is expanding. Also as far as we can tell our universe does not curve into itself like a balloon. Or if it does it must be at least 23 trillion light years in diameter based on the accuracy of our measurements.

Yes, it's expanding, but on the inside (like a balloon). Unless you can already look outside, you're not getting any closer to the outside. (Assuming an "outside")

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This is the way to ELI5 thanks

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The actual energy/mass that shot out from the big bang event. We don't know the actual formation of that stuff but maybe it has always existed in some form until it all collected into one point with the entire mass of the universe existing in a small point. The mass flowered into the universe in a "big bang" a quaint term for the sheer amount of power released into, everywhere all at once.

Quantum Fields can just exist on their own and create interesting interactions. Look them up if you want, really neat stuff. It's an interesting phenomena that can create energy by itself I think.

You need two fixed points separate from each other in order to measure time. Since all things existed in the same location before the mass/energy erupted away from this fixed single point, time was irrelevant.

Time itself is not a construct but the language of mathematics is as we can say "two" or "dos" and mean the same thing. Aliens for instance on another planet could use math just fine to measure things but the would have different language to explain it. Maybe the communicate in with smell and produce specific smells to indicate math language.