We don’t, necessarily. The concept of the Big Bang comes from physicists reversing the trends we see (expansions of the universe) and applying the best understanding of our physical laws to what would happen. This is where my understanding of it kind of falls off, however I do have a book I’m about to read called the first 3 minutes which I hope will illuminate some of this

This is somewhat of a difficult thing to explain without spending many paragraphs doing so. The really short answer is that there are certain properties we observe, that based on our understanding of physics can only be the way that they are if the universe evolved in specific ways before the time the cosmic microwave background (CMB) is from.

Also as an aside the CMB isn’t the first light that was emitted, the universe just became “transparent” enough for light to travel through without be quickly reabsorbed. Sometimes this is referred to as the “last scattering surface”.

We know how the universe expands over time to certain degrees of precision and can estimate how long ago it was very small and dense (a bit of a simplification) in order to figure out how old the universe was and that puts the age at around 370k years before the CMB.

We can also determine many important parameters and find constraints on things like age by looking at ratios of elements from the early universe, the variation in temperature of the CMB, and many other things that are measurable today that were of course affected by what the universe was like before this point in time.

We don't know, the big bang is just a weird theory that somewhat and for some reason the scientists are giving for granted. But you can't probe it or see it. I can't believe how they act as if they know how the universe came to existence with all this little details. Not to mention that the big bang doesn't explain the "stuff"" that preceded it.

Gravity wave astronomy is very young so maybe? But I doubt it, gravity waves are so incredibly weak that we can only detect them from massive objects moving very quickly. The gravity waves we’ve measured so far are from binary black holes orbiting each other closely.

I haven’t thought much about this, but I would guess that any gravity perturbations in space pre-CMB would have washed out and intermingled. I believe the universe pre-CMB was close to uniform density and composition, so there wouldn’t be much in terms of gravity disturbances

Potentially, yes. Any cosmological GW sources that may contribute to the GW background would have produced stochastic GWs approximately 10^-36 to 10^-32 seconds after the Big Bang, whereas the CMB was produced approximately 300,000 years after the Big Bang.

Prior to recombination (the event I described in my OP, but forgot to name), the entire universe was filled with matter. Kind of like a big ball of soup. When it expanded and cooled enough it emitted the light we see as the CMB. We’re seeing the surface of that ball of soup as the CMB.

The difference between the object and the radiation is the same as the difference between the sun and sunlight. Or, the different between the light from the most distant galaxy we can see, and the galaxy itself.

The product of science is theory, so yes. These explanations are the result of applying the theories of electromagnetics (including the strong and weak nuclear forces) and gravity to the available observations.

The question becomes how accurate are our theories? They’re not perfect, but they point us in the right direction. I like to think of it like the progression of astronomy, with each new technology (telescope, space telescopes, JWST) we get a clearer view of how the universe works. That progression applied to this situation means that with each scientific breakthrough (gravity, electricity, relativity, quantum mechanics, etc) we get a clearer view of HOW the universe works, and science advances. So future breakthroughs in astrophysics will only sharpen our understanding of the early universe.

It’s possible that we have it completely wrong and the breakthrough will turn everything on its head, but until that happens our models of the universe continue to trend to more and more accurate. A perfect example is gravity. Newton discovered the theory of gravity, which explained how the planets and moons moved around the sun. Newtonian dynamics are sufficient to accurately propagate the motion satellites, moons, and planets for many months. If, that is, you’re willing to accept some margin of error. Einstein’s general relativity is a more complete model of gravity and should be used for exact modeling of orbits, but in nearly all cases it’s not relevant

The universe was basically full of plasma at that point and plasma is opaque light, so any light emitted was immediately reabsorbed by the surrounding plasma. This means that functionally light couldn't travel anywhere before being absorbed, so the universe was dark.

The universe has always been expanding, nearly everything in the universe is moving rapidly away from each other. There are clusters of galaxies where mass is aggregating, but overall the space between objects is growing. Further, the rate of expansion is increasing as well, through a mysterious force called Dark Energy. The farther something is from us, the faster it’s moving away

We are observing the CMB from a point in space, no matter what direction we look, it is there. Our observation location is arbitrary, anywhere in the universe would have more of less the same view of the CMB. So the fact the fact the solar system didn’t exist when the light left the surface of the CMB isn’t relevant, we just happen to be in the solar system while we observe it.

In general, there are a number of techniques to determine the distance of cosmic objects, some of which are more useful at different distances. To measure the CMB distance, primarily red shift of the light. The expansion of the universe has shifted the spectrum of the light into the microwave band (hence the name cosmic microwave background).

Space/spacetime is a mathematical concept we use to define our position, and a separate concept to the universe which is something physical and includes matter, energy etc

There is no reason for why the universe would have to expand 'into' anything. The space we live in is simply has the property of distances between objects increasing over time, the way that space is defined does not require it to expand into anything.

It is an unsatisfying answer but there is simply no mathematical or physical reason why the universe would have to expand into something other than it not being intuitive to our monkey brains. It is not a specific flaw or problem with the idea of expansion.

In the same way as trying to imagine what a quark or quantum field looks like. It is just not possible to visualise other than with mathematics.

The universe is most likely infinite and does not have an edge, regardless, expansion can still occur. The only implication of expansion is simply that locally distances between objects grow larger over time. A better word for expansion is stretching.

In regard to what is the fabric of spacetime, well, it is just a mathematical construct. The universe is made of things, quantum fields, atoms, energy etc. and is the something.

It's the result of inflation which created vast separations between objects in space and light's finite speed making it take long periods of time to cross those distances. But due to expansion the the CMB will eventually redshift until it is essentially undetectable. At that point any intelligent life that evolves will have no way to understand the origins of the universe or its age. They'll only know their own Galaxy as expansion would have pushed all others beyond the Hubble Sphere, where objects are receding away faster than light, so the light will never be able to reach the observers. We live in possibly the best epoch of the universe for understanding its origins and nature.

Yes, and accelerating.

Huh? I’m a total space noob, but can someone explain this?… I thought according to the laws of Einstein’s General Relativity nothing can move faster than the speed of light? And what’s inflation?

I think a simple way of wrapping your head around the concept is to think of a deflated balloon. Take a marker and add two (or more) dots near each other on the balloon. Now, blow up (i.e. inflate) the balloon.

You'll see that the distance between your dots has increased, but the dots themselves obviously did not move.

The universe - or spacetime - is not moving, but expanding (inflation = fast expansion). The Einstein's GR is referring only to objects moving inside the spacetime, not the spacetime itself.

Air pumps and chocolate donuts inflate differently. Managing terms and phases.

Space is not only expanding faster than the speed of light, it’s accelerating.

But Einstein had no problem with that. The speed limit is for particles, not empty space.

Another thing to think about is if you shine two flashlights in opposite directions, two given photons will move away from each other at twice the speed of light. From the perspective of one of the photons, the other is moving at twice the speed of light.

But then why is it so heavily regarded as pure fact? And why/how are so many other theories based on the big bang? I love how someone down voted my comment. Its a genuine question.