The Big Bang

Before our Universe expanded, everything in it today was condensed into a single point of infinite density known as a singularity.  What would it have looked like?  Pretty much what you see beneath this text.  Because matter, energy, and time didn't exist in any combination of forms we could view, speculate on, or understand.  Consider it a cosmic David Lynch film.

Of course, within a few minutes of the Big Bang, we had hydrogen and helium, since the stellar  medium - space - had cooled enough to let baryons - quarks, electrons, and neutrinos - form, which with further cooling over a period of seconds could join to form heavier particles such as protons and neutrons.  The protons and neutrons then formed the atomic nuclei of our two lightest elements, and it was off to the races.

Sort of.

The universe more or less stayed in this state (although always expanding) for roughly 300,000 years.  But at this time, the universe had cooled enough to let free electrons combine with atomic nuclei to form neutral atoms, and for atoms to form molecules.  Without free electrons to interact with  photons, the universe became transparent to visible light, becoming a relatively blank, black slate for a long, long time - anywhere from 20 million to 200 million years - until the first stars formed.

As the background temperature dropped, clumps of hydrogen and helium became more dense.  Eventually, they became dense enough - and hot enough - to undergo nuclear fusion at the center of these clumps, forming stars.  These stars would have been very different from our Sun - several of them a hundred times as massive, with no heavier elements - only hydrogen and helium (meaning no planets, comets, asteroids, or even dust).  Their life spans were in the range of a few million years (as opposed to the Sun's 15 billion).

At the end of their lives, these stars exploded violently - some so violently that there was no stellar core remaining to form a neutron star or black hole.  These explosions hurled the heavier elements (carbon, oxygen, and iron among them) created by nuclear fusion to be spread out and mixed among loose strands of stellar gas.  This allowed the next generation of stars to burn slower and more efficiently; lasting hundreds of millions to even billions of years.

As this process continued, the stars began to move together, bound by gravity, orbiting each other.  Their combined gravity sucked in more free hydrogen and helium, which formed more stars.  And so, about a billion years after the big bang, galaxies formed.  The massive gravity of these galaxies - combined with the dark matter^[1] they formed within and along side - sucked up the majority of free floating gas and dust that remained, birthing even more stars.

As the second generation of stars - now  collected in galaxies - died and exploded, the shockwaves, dust, and gas emitted pushed and pulled on the gas strands and clouds, causing more clumping, giving birth to the third generation of stars - which includes our own.  And the larger amount of  heavier elements allowed small, rocky bodies to develop - worlds like ours.

So, it's easy to say that without the big bang, and whatever caused it, we wouldn't be here today.  But it's actually a little more complex than that.  It's not just that the universe exploded, it's how.

For instance, had the big bang exploded without tiny wrinkles in it's uniformity^[2], there would have been no initial gas clumps to form the first generation of stars.  Matter and energy would have been created and distributed evenly; gravity would have been to uniform to allow the hydrogen clumps to form before the universe had expanded too much for the needed density for nuclear fusion.

Had the universe expanded just a little faster and you'd end up with roughly the same result - perhaps a few stars here and there, maybe a few small galaxies, but nothing like what we see out there today.   And with the dilution of the heavier elements, it's almost  certain planets would never form, unless you believe in a homeopathic universe.

Had it not been for baryogenesis^[3], there wouldn't have been enough matter to form large scale structures, as it would have been annihilated very early on in the universe by an equal amount of anti-matter.  Instead, we have rocks under our feet to walk on, air to breath, water to drink, and plant and animal matter to eat.

And what if, instead of expanding faster, it had expanded slower?  Chances are, the gravitational force would have overpowered the expansion force, and the universe would have collapsed back in on itself.  Which would have sucked for us, since we wouldn't be  here.

But we are here - so let's move on to the next section - The Galactic Habitable Zone.

1. - Dark matter is matter that only interacts gravitationally with ordinary matter.  We can't see it since it doesn't interact with light or give  off any radiation.  We know it's there, however, because of how the galaxies behave gravitationally.  For instance, the Sun orbits the galactic core at 250 kilometers (155 miles) a second - that's 560,000 miles per hour.  There isn't enough normal matter in the galaxy to keep the Sun from flying off into space at that speed; something else must be pulling on it to keep it in place.

2. - We know the early universe was not completely uniform due to the cosmic microwave background - which shows that there were small variances in the temperature and density of the universe.

3. - Baryogenesis is an unknown process by which during the first few minutes of expansion, matter was formed just slightly more often than anti-matter (for every 10 billion anti-protons and anti-neutrons, there were 10 billion and 1 protons and neutrons).  Not a huge difference, but enough.