I originally had this discussion out with somebody on Facebook, only for the person who created the post we were commenting on to delete it, annihilating everything. All the better though, since I was a fool for writing all of that for free anyhow.
The fellow I was arguing with was challenging everybody in the comments on a science article to "prove the big bang". He contended that if we could not prove it then it was a faith based belief, on no firmer footing than his own.
It was a mostly preventable conflict, had he first understood what the big bang is. It's only the name given to the extrapolation backwards in time of the observation that every galaxy visible to us in the night sky is receding into the distance. Not only that, but accelerating.
Anybody can verify this on their own with the right equipment. When I said this, he objected that I'd not explained how it happened. That's what he was really after, and he behaved as if he felt he'd found a chink in the armor of some grand conspiracy.
When somebody comes at science with an adversarial attitude like that, assuming it's full of shit because they were coached to view it that way in order to keep them fooled, there is little you can do to change their mind. No matter how detailed and well sourced an explanation you give, to them it will all just sound suspiciously convenient.
So, to get it out of the way, the currently most popular idea about where the universe came from is that it was created supernaturally by a magic grandpa who wants women to remain quiet and servile, enjoys the aroma of burnt animal offerings and thinks homos are icky.
However, to everybody not already convinced of this admittedly convincing and airtight explanation of the universe's origins, it seems suspiciously convenient that magic grandpa wants women, gays, children and everybody else to behave in a manner pleasing to adult heterosexual men.
You can point to obvious errors like the Qur'an saying the sun sets on Earth in a muddy puddle and the Muslims become furious, insisting it requires complex interpretation to be understood while Christians laugh. But then you can point to the six day creation in Genesis, with the Earth existing before the sun, birds before land animals and so on and suddenly the Christians who laughed so readily at the prior example now become gravely serious and insist that Genesis requires more complex interpretation to be properly understood, etc. etc.
Sufficed to say, not everybody was satisfied with the magic grandpa explanation because aside from the fact that many of the claims it makes about nature have been disproven since the time of writing, it also didn't really explain anything. The Torah, Bible, Qur'an, Book of Mormon, etc. don't really specify much about how creation is supposed to have been accomplished.
So, people who dedicate their lives to painstakingly investigating questions rather than accepting any old answer somebody makes up set about searching the night sky for clues.
As stated earlier, they discovered every other galaxy is moving away from us, faster and faster. The man credited with this discovery was George LeMaitre, a Catholic priest. This is often held up triumphantly by Catholics, only when asked exactly how it proves Catholicism, they say that isn't the point. When they ask what the point is, you get a lot of quibbling and eventually it boils down to "A guy in my religion discovered an important thing using science" which they try to equate with "therefore science has proven my religion is true".
Actually the pope at the time was very excited by LeMaitre's discovery. He said it looked like the smoking gun. Proof that the universe had not always persisted in its current state, as many astronomers, cosmologists and physicists of that century believed, but instead had a definite and cataclysmic beginning.
LeMaitre cautioned him against saying such things publicly in case some fault in his findings should later be discovered. But also because there is no logical way to go from "the universe began to exist" to "the universe could only have begun to exist if deliberately caused by a being with intelligence and personality."
So far, every time science has investigated the origin of some example of complexity in nature, the cause turned out to be unintelligent natural processes. Snowflakes are a popular example because they look very designed. They are radially symmetrical and geometric. They are extremely intricate and no two are alike.
If we'd been satisfied that this was all adequately explained just because somebody wrote a book that said an invisible sculptor carves all those beautiful shapes into each snowflake as it falls, we never would have investigated the matter closely enough to discover that it's a result of how crystallization occurs.
The fractal, branching paths of the crystal as it grows follow the path of least resistance against the constraining factor of the warmer surrounding air. Because ice is just water molecules locked into a rigid crystalline lattice, the two parts oxygen one part water bonds repetitively in such a way as to form a repeating hexagonal molecular structure.
This is why snowflakes are hexagonal, why they are intricate and why no two are alike, as each snowflake experiences different tempetature fluctuations affecting crystallization as they fall through pockets of warmer and colder air.
So it goes for seemingly designed shapes seen in bismuth, quartz, and a variety of other materials. Likewise with the fractal layout of coastlines, rivers and estuaries. Every example of complexity in nature has so far turned out to be the result of natural processes, when investigated deeply enough.
Why should this not also apply to the big bang? If something as beautiful and complex as a snowflake can just happen as a result of physics, why not the big bang as well? There is no precedent for anything we have so far discovered being inexplicable except as the deliberate creation of a supernatural being, and thus no good reason to assume it.
People in the "magic grandpa" camp lean heavily towards making that assumption, because it's a big part of how they cope with fear of their own eventual death, and with the trauma of having watched loved ones grow old and die. They have a very strong emotional motive to hear only evidence that fits with the magic grandpa hypothesis, and reject whatever doesn't.
So, what has science found? So far there is no complete answer, only clues. So really, nobody knows. But after discovering that every galaxy appears to be accelerating away from us, they thought "why us"? Are we really the center of the universe, which all expansion in spacetime occurs from?
Improbable, as every other time in history we assumed a special position for Earth in the cosmos, we turned out to be wrong about it. Instead, imagine drawing dots all over a balloon with a black marker.
As you inflate the balloon, the dots all grow more distant from each other. However, from the perspective of any one of those dots, all the other dots look like they're moving away from it. Like it's the center of the universe that everything is expanding from.
Aha! So the point from which spacetime is expanding is not within the universe itself. It is not anywhere in three dimensional space, just like the point the balloon's surface is expanding from isn't anywhere on the 2D surface of the balloon, but in a 3D position relative to the surface in the balloon's interior.
Carry that all up one dimension and it will make sense of why every galaxy seems to be moving away from ours, and why there is no specific point in our universe we can point to and say that's where the big bang happened. Everything is expanding at once, in all directions, and appears equally so regardless of where in the universe you examine the process from.
So, how did this start? How do you even go about investigating something that far back in time? After all, for the first instant of the big bang, matter was moving faster than light, which it isn't supposed to be able to. In fact galaxies are still technically moving away from each other faster than light, but it's the spacetime they exist in that's expanding faster than light, no movement of any particle of matter relative to any other within causal range ever exceeded C because otherwise it would violate causality.
Sufficed to say, physics unavoidably gets weird and stops being able to make useful descriptions or predictions as you approach the moment of the big bang. This is why, even if some technological means existed of examining what came before the big bang, the science of physics itself would still not be up to the task of modeling exactly what happened.
What we have done is go looking for any examples in nature of something coming from nothing. In fact, they found plenty. It happens all around us, all the time, at the quantum scale. Particles and their anti-particle twins just split out of nothingness, so far as we can tell, then recombine and annihilate one another in the process.
It's believed that the annihilation repays the debt, as spontaneous appearance of persistent matter and energy from nowhere would otherwise violate the law of conservation. Yet that law must be possible to violate in a big way for our universe to be here, surely?
What they found is that if there exists some way to keep the particle and its antimatter twin separated so that they do not annihilate each other, the cumulative effect of these constant particle/antiparticle separation events happening everywhere, all the time would be to generate a massive amount of matter and energy.
But if that were true, the antiparticles would have to go somewhere. They would have to turn into some benign form that doesn't react violently with matter and energy, otherwise all matter and energy in the universe would have destroyed itself straight away.
Because, by various processes, matter and energy are interchangeable and transmutable, it occurred to these physicists that the antimatter could have decayed into negative gravitational energy, a form which is non-reactive with matter and energy.
But that isn't a good enough explanation. In science, it's not enough to just propose something that sounds good, you actually have to show that it's true, or at least that it appears true according to the best available evidence. If all of the matter and energy in our universe originated by this weird effect, splitting out of nothing into a particle and antiparticle pair, only for the antiparticle to decay into negative gravitational energy, then there should be a quantity of negative gravitational energy in the universe that exactly equals the amount of matter and energy.
That's what is called a predictive proof. If a hypothesis is really true, we should be able to extrapolate further truths about reality from it which we have not observed yet. Then when we go to investigate that stuff, if it's exactly as predicted by the hypothesis, it's achieved predictive confirmation, part of how a hypothesis becomes a theory.
Of course we cannot examine the whole universe. We can examine only the modest region observable with our telescopes. But because the distribution of matter and energy appears to be consistent throughout the space we're able to observe, and the distribution of microwave background radiation appears consistent, there's thought to be good reason to suppose that the distribution of negative gravitational energy in the region of space we can examine is typical of the universe at large.
That is an assumption of course. We have to assume the laws of physics in our part of the universe are the same laws that apply everywhere else in the universe, as another example. But no claims to absolute truth are being made, only observation and inference.
If indeed the distribution and quantity of negative gravitational energy observed in the region of space observable by humans is typical of the universe at large, then it should precisely equal the quantity of matter and energy, based on similar observations concerning their distribution in the region of space observable by us.
That's a lot of "ifs". Still, exciting results. It didn't have to be that way. Nothing guaranteed we'd discover that about the distribution of negative gravitational energy. It "just happened" to precisely equal the amount of matter and energy in the universe, provided the distribution of matter, energy and negative gravitational energy we observe around us is typical for the rest of the universe.
Can you see why that's such an exciting result? We observed a mechanism which could explain the buildup of matter and energy from nowhere, providing the equal amount of antiparticles also generated turned into something non-reactive. And that's exactly what they found, in exactly the quantity predicted.
So, that's what is known about the mechanism responsible for producing the matter and energy we're made of, and which surrounds us. It appears to naturally occur and can at least temporarily circumvent the law of conservation by the decay of antiparticles into a nonreactive form of energy.
But how come spacetime just naturally produces particles and antiparticles? Why is it like that? Physicists think that simple entropy causes occasional, unpredictable fluctuations in vacuum energy that cause it to briefly turn into matter and energy as we know them.
Do these particle and anti-particle pairs really come out of nowhere, like 1 and -1 being divided out of zero? Is there a "nowhere" to come out of? After all, even if you remove all the air, dust, and every last particle of matter or energy from a given volume of space, it still exists. It still at least contains spacetime.
So, was the big bang really the beginning of all existence or just the abrupt collapse of a stable prior state into a less stable one we call the universe? Many physicists now suspect this, but so far there's no way to investigate. What's being done is a sort of cosmological forensics.
Forensics is of course the science of reconstructing past events from the aftermath. One thing physicists have done is to examine the distribution of microwave background radiation and extrapolate backwards to see what it can tell us about the initial conditions of the big bang, and the possible curvature of spacetime.
By running simulations of the big bang with variables consistent with the physical constants in our own universe, and continual refinement for accuracy, they eventually generated the same distribution of microwave background radiation in their simulations that we observe in the actual night sky.
What were they able to discern from this? It would seem, for one, that the universe is non-repeating. That is to say, if it were a four dimensional or higher sphere, we could send a spaceship far enough in one direction that it would come back to where it started, despite having traveled in a straight line.
Think of this as similar to how, in old arcade games, you could go off the edge of the screen and re-appear at the other side, still traveling in the same direction. The notion that perhaps space folds back on itself like this was the basis for the "big crunch", the hypothesis that the expansion of all galaxies away from each other would eventually loop back and bring all matter and energy back together, far exceeding critical mass, imploding into a new singularity which might produce a new universe.
Like the world's biggest perpetual motion machine. Bang, crunch, bang, crunch, and so on forever. Very elegant, but science does not care what is elegant. Only what stands up to the test of observation. The tests carried out by the COBE and WMAP probes confirm there is no larger, objective curvature to spacetime.
Instead it would appear the universe extends in all directions, forever so far as we can tell, and does not fold back on itself eventually (or "repeat" if you prefer). What does this mean in relation to the origin of the universe? Nobody knows, it's just what we've been able to piece together so far. It does offer some hope, however, that we'll keep discovering new clues in places that nobody's thought to look yet.
Anyway that's all that is presently known about why galaxies are moving away from each other, why they all appear to specifically be moving away from ours (from our perspective), what that implies about the distant history of our universe when spacetime hadn't yet begun to expand, and where all the matter and energy came from so that we could be made out of it and build computers out of it too, for reading stuff like this on. :)
Is there still some possibility that magic grandpa is behind it all? If not the specific character from one of the human religions, then some kind of abstract being which has never been described by any human religion? It could be! It's good to retain some humility. After all it wasn't until about a century ago when mass spectrometry was invented that we could even determine what our own sun is made out of.
That means that the very scope of what it is possible to know doesn't remain the same, but continues to expand over time, just like our universe. Questions which scholars long ago assumed would be forever impossible to answer, have since become possible to answer thanks to improving technology they never could have foreseen.
So, even if the science of our day falls apart when trying to actually model a singularity, we might some day be able to pierce the veil and see what was there before it. Maybe it will be magic grandpa after all, in spite of everything! Or maybe a giant turtle god named Maturin who vomited out our universe. Or maybe nothing at all, and the universe only exists because it couldn't not exist, or something.
Until then, nobody has the final answer, and anybody pretending to probably wants something from you.
Citations:
https://www.physicsoftheuniverse.com/scientists_lemaitre.html
https://www.e-education.psu.edu/astro801/content/l10_p4.html
https://www.universetoday.com/13810/why-are-distant-galaxies-moving-away-faster/
https://phys.org/news/2015-10-galaxies-faster.html
https://www.nature.com/articles/d41586-018-05095-z
https://www.quora.com/Virtual-particles-dont-exist-but-under-certain-conditions-they-can-be-separated-and-be-real-When-this-happens-do-they-become-real
https://physics.stackexchange.com/questions/19973/what-causes-virtual-particle-pair-production-to-not-occur-in-the-space-occupied
http://iopscience.iop.org/article/10.1088/0143-0807/11/4/008/meta
https://www.astrosociety.org/publication/a-universe-from-nothing/
https://www.space.com/24309-shape-of-the-universe.html
Stay Curious!