1. The Big Bang wasn't an explosion in space. It was an explosion *of* space itself. Before the Big Bang, space as we know it didn't exist. There wasn't a "point" in space where it happened, but rather, space and time were born at the Big Bang.
2. We don't know what existed "before" the Big Bang. Our current understanding of physics breaks down at the very beginning of the universe. It's impossible to talk about a size or a state of the universe before the Big Bang because the concept of "before" might not even apply in that context.
3. The "size" of the early universe is tricky. The early universe was incredibly dense and hot. Trying to describe its size with a number like "a pinhead" is misleading. It wasn't a point with a measurable radius. Instead, we can talk about the scale factor, which describes how much the universe has expanded since the Big Bang. The scale factor was very small at the beginning, meaning everything was incredibly close together.
Implications for our understanding of the cosmos:
* The Big Bang singularity: The theory predicts a point of infinite density and temperature at the beginning of the universe. This "singularity" poses problems for physics, and it's likely that our current understanding of gravity breaks down in such extreme conditions.
* The origin of structure: The incredibly small scale factor at the beginning of the universe allowed for tiny fluctuations in density to be magnified by expansion, leading to the formation of galaxies, stars, and planets.
* The "flatness" problem: The fact that the universe appears to be flat (rather than curved) is a puzzle. The early universe should have been curved, but the extremely small scale factor at the beginning explains how it could have expanded to become flat.
* The origin of time: The Big Bang itself is believed to have been the beginning of time, meaning the universe was born at that moment. This leads to deep philosophical implications about the nature of time itself.
In conclusion:
While we can't talk about the "size" of the universe before the Big Bang in the traditional sense, the incredibly small scale factor at the beginning is a crucial element in explaining the formation of our universe. It poses profound questions about the nature of space, time, and the very beginning of existence.
