We simply don’t know.
Our laws of physics stop working during the big bang, and some people think the laws got created by the big bang.
But it’s all speculative. There could have been time before just like we know it. There could have been time that worked completely differently (for example 2D time where travelling backwards in time is very easy), and there could be an entirely different kind of physics that doesn’t need time. Or existence itself wasn’t a thing.
The only the we know is that our current understanding of physics did NOT apply
Any question regarding something prior to the big bang is inherently speculation. It can’t be answered, as all laws of physics breakdown as we approach the first nanoseconds of the big bang. There is no way to tell if there was anything prior to it. We have no way to tell if time worked differently, if physics worked differently.
From a purely logistical standpoint, having the big bang as the start of time also makes a lot of sense. Since the singularity serves as an information event horizon, we can’t measure anything prior (if there is a prior). So we have the start of time be from the big bang, 13.8 billion years ago, and everything counts forward from there and we dont have consider things like “negative time”.
Technically not the most accurate analogy but this may help you visualise it a bit easier (please add any other details if you have any corrections or to clarify better).
If you’re familiar with audio waveforms -like shown on Soundcloud for example- start a song, watch the cursor move as the song progresses and pause it at any point. The waveform takes different shapes as the song progresses and the exact shape when you pause it at a specific time shows you what’s happening in that exact moment. It changes over time as there’s different stuff going on. Now go back to the beginning of the song. There’s no waveform before the song starts because there’s nothing there right?
Time is just what we use to determine what’s going on in a certain instance. As far as we know there was nothing going on before the big bang so there’s no time.
Because time seems to be a function of our universe, like gravity, magnetism, etc. but not a force outside of it.
We measure time by measuring change -for example, the amount of vibrations a caesium atom makes. Since our universe has began, this rate of change has been remarkably consistent, which is good. But if there’s nothing for that change to act upon, then there’s no “time”, just like there was probably no gravity or no electricity before the Big Bang.
We don’t know.
All we know is that the universe, as in the space (and time) we exist in is expanding. That means if you go back in time, the universe used to be denser than what it is now.
Imagine you’re standing in a room that expands out to the horizon, infinitely. Your only reference point are the tiles on the ground inclufing the one you stand on. You can see other objects around you, very far away, and as time passes the tiles in which everything sits grow. Do it backwards now; despite the room being infinite, the objects in the room are now getting closer and closer to you as the tiles they rest on shrink. Eventually, they are within reach.
These tiles aren’t just space, but also time. So time itself was “smaller” in the early universe. Go far back enough, and both time and space are so small that the universe has infinite density, kind of like a black hole. It’s also infinitely hot, because everything is packed together so tightly there’s no room for particles to move without crashing with one another.
At this point, physics as we understand them no longer work, so no point in trying to go further to the past. It’s kind of like trying to divide by 0, you can get close to it by trying 0.01, then 0.0001, then 0.00001 and so on. Our physics can get very close to T=0, but not to the exact moment the universe “began” (if that’s even a thing)
Imagine you have a note you don’t want me to read so you burn it, but I can still look at the ashes under a microscope and maybe make out the letters, so you use a hotter fire that vapourises the paper, but then I can maybe analyse the gas residue and work out what kind of paper you used and how much paper and ink there was. The big bang is like a fire that burns information, there’s theoretically no way to know how the matter and energy before the big bang was organised, how much of it there was or even if it, or time itself existed, and it would make no difference to ‘now’ if it did.
The idea is that the current laws of physics, time being one of them, were created with the big bang.
The truth is, don’t fully understand all this stuff, and most of it is theoretical math and whatnot. But space, time, and gravity are all interconnected, and before the big bang, there was no space or gravity, so why would there be time?
Just a note: nobody says there wasn’t matter before the Big Bang. (No scientists, anyway. That’s a creationist talking point.) The Big Big model is that all of the matter and energy currently in the universe was concentrated in a single point of infinite heat and infinite density. Then it rapidly expanded, becoming the current presentation of the universe.
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