In layman's terms, what was before the Big Bang, and how did the Big Bang start?

In laymen terms, the correct answer is "We don't know".

The Big Bang theory describes the behavior of a universe with a constant energy content, where the energy density decreases as space expands. As such, the theoretical universe described by this theory is "darn close" to what we observe in our universe.

It is when we use the observations of our present (real) universe (for example, temperature, energy density, matter density...) and apply it to the theoretical universe, and then we run the calculations backwards (to see what could our universe look like in the past), that we come up against two walls (one would have been enough).

1. We come to a time in the past (called the Planck Time) when the energy density -- same as "temperature" at these levels -- where we can't understand how things work. In fact, we can't understand how time itself could have flowed at that energy density. To put it in layman's terms: we do not know how to apply the word "before" to that moment.

2. If we extend the calculations even further back, we come to a time when the output of the equations grow without bound (what some people call "infinity"). That moment constitutes a singularity (in mathematics, the singularity is a place in the domain; in this case it is a moment in time -- it may have nothing to do with the size, shape or property of the universe itself).

That moment is what we have decided (for now) to call "zero".

Since the singularity occurs "before" the Planck Time, then we do not know what it really means.

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The Big Bang theory can only be applied after the Planck Time. At that moment, the initial energy already exists and space is already expanding. The theory works very well to explain what happens AFTER.

When we use the theoretical universe of the theory, we can make "predictions". For example, if you begin with a universe that is very hot (gazillion degrees) and let it cool -- by expansion -- until it reaches a temperature of less than 3 K, then you can "predict" that at some moment, the temperature passes from "more than 4000 K" to "less than 4000 K".

Something happens to hydrogen around that temperature. Hotter than that, electrons cannot stay in orbit around protons and the hydrogen is completely ionized. Ionized hydrogen is opaque (not transparent).

At some point, once the temperature gets cool enough, the hydrogen becomes neutral and this occurs "suddenly" (well, at least on a cosmological scale). Since the universe is mostly hydrogen in volume, the whole universe suddenly becomes transparent and all the trapped light is suddenly liberated form everywhere at once, and in all directions.

Some scientists used the theory to predict what this light should look like today IF the Big Bang theory was a good description of our real universe (at the time, in the late 1940s, the Big Bang theory was NOT the favored theory and the name Big Bang had been given to it somewhat in derision). I suspect that the prediction had been calculated in order to provide a way to prove that the theory was wrong.

Then in 1964, the Cosmological Microwave Background radiation was discovered. Almost a perfect fit for the prediction made in 1948-49.

That (and other evidence) tells us that the Big bang theory is a very good tool to understand the behavior of our universe, but we also know that it is useless (for now) for anything before the Planck Time.

The Big Bang started as an infinitely hot and dense ball of mass. Then it blew up and expanded at an infinite speed creating the universe we have today. When scientist say the universe is expanding they mean it is intrinsically expanding. Space itself isn't getting bigger or growing into a predetermined or pre-existing amount of space; the objects in space are growing further apart. We will never know what "happened" before the big bang, we won't know why it blew up or how that ball of mass got there. Even if we do figure out how our universe started we won't be able to understand what was going on before that because it will be outside of our universe and beyond our realm of understanding and norther how far back we can go the answer will have to be it started from nothing. For example say it was "God" who created our universe, the next question would be how did god come into existence, then how did his creator and then his creator and so on, it'll go back to the point of it just happened. So the answer to what was before the big bang is nothing, and something came out of nothing.

there is neither theory nor evidence to support any speculation as to what may have caused the big bang nor the events if any that transpired before it. The currently accepted big bang theory is based on the observable evidence of an expanding universe.

Evidence of a cause and evidence of conditions "before" that cause are REALLY unlikely to have survived. You'd have a MUCH better chance of determining the fingerprints of the grandfather of the technician who screwed shut the wiring case on an atom bomb by studying the ground two miles from the site of a nuclear explosion, then you would of determining the cause of the big bang. we dont know wehre to look or what to look for... al we know is that "stuff" is still expanding... it doesnt take a physicist to know that an expansion had to have a starting point and place

You really should Google it and also read the Wikipedia article.

In a nutshell, the Big Bang explains the expansion of the Universe, NOT its beginning.

In other words, it doesn't start with:" There was this big explosion ....."

The "start" is in the realm of Quantum Mechanics and all we have presently are guesses as Quantum Mechanics and General Relativity have not been "fused" together and that (we think ...) is where the answer to the beginning will come from.

So, we have no physics for the beginning and, as a consequence, there is NO "before the Big Bang".

The physical laws that we are aware of break at the beginning.

Time started with the Big Bang.

You may want to look up what "Quantum Fluctuations" are, but let me warn you that Quantum Mechanics is truly odd to untrained people, as it is completely out of anybody's everyday experiences.

It does work and extremely well, but it is bizarre ....

the hot big bang theory does not and can not speculate on what came before the big bang. Either nothing and matter was created in blatant violation of the laws of physics or the universe has always existed and is constantly expanding to a threshold then crunching to a threshold and doing the funky accordion, other thoeries dodge the question by saying the universe existed in a form other than matter such as some form of energy, but that begs the question what came before that. The atoms/anti atoms theory your thinking about happens after the big bang not before it. The theistic theory says the supernatural omniscient being independant from the universe existed before it and doesn't require a creator so that ties up the loose ends on that one. Another good one is that the universe existed on another dimension and just moved itself into this one but again the question is dodged, take you pick. (The non-lamen version of any of those given theories involve lots of math)

You really don't have to be able to explain it. It is a common misconception that the BB theory explains where and how everything started. It does not. The BB theory starts off with matter in a highly condensed state and then describes what happens as the "Bang" develops into the present day universe.

To describe what happened earlier you need the "Big Cause" theory. We don't have that yet. Try asking again in a few decades time.

Cheers!

Before the big-bang some cosmologist believe that two 5-dimensional membranes collided and that interaction created a 10 dimensional space-time which froze into the big-bang.

At the end of our universe, when space time become 'meaningless' the conditions will be reset so the original two membranes can again collide again.

http://en.wikipedia.org/wiki/Cyclic_model

http://en.wikipedia.org/wiki/Ekpyrotic

This is probably the latest and best theory that science has put forward to explain the origin and fate of the universe IMHO.

Science is a matter of accepting evidence or not, not belief. Belief is for things that have no evidence. There's plenty of evidence that the big bang happened (the universe is expanding and cooling, therefore it was once smaller and hotter - there's a lot more you can read about on the wikipedia article). However, we don't know what happened before it. That's outside our universe by definition, and we can't see outside our universe. We have some ideas about how it could have happened, but they aren't currently testable.

There is no observable facts from before the big bang, so we don't know what was before, nor how the big bang started. To read more about this, check out Stephen Hawkings' "A Brief History of Time".