How did the universe begin?

Some people wonder if our universe originated on the "other side" of a black hole.

It's widely accepted that the universe began with what is called the big bang. Roughly 13.8 billion years ago the universe expanded from an infinitesimally small and extremely dense and hot state. It has continued to expand, cool, and evolve to form the stars, galaxies, and large-scale structures of the cosmos we see today.

The theory has a lot of parts and involves an in-depth chronology of important events for the laws of physics. In the first tiniest fraction of a second, the four fundamental forces of nature—the strong, weak, electromagnetic, and gravitational forces—"froze out" of the unimaginably hot, chaotic universe. That unleashed a momentary period of "inflation," when the universe expanded from the size of a proton to the size of a grapefruit almost instantaneously before continuing to grow at a more regular pace.

As the universe cooled, matter froze out of energy, and heavier particles such as protons solidified from smaller particles such as quarks. Three minutes after the big bang, protons and neutrons smashed together to form heavier elements such as helium and lithium. Some 300,000 years later, the universe cooled down enough for electrons to settle in around protons and form atoms. Even later, stars and galaxies coalesced from the ubiquitous sea of matter in the cosmos, and billions of years after that, here we are to contemplate it all.

So how do you get a big bang to hap-pen? How do you get something from nothing? As we try to reverse the clock and engineer the universe back to the big bang, our laws of physics start to break down. And if time as we know it started with the big bang, it doesn't make a whole lot of sense to think about a time before the big bang. But of course that hasn't stopped people from speculating.

Some people wonder if our universe originated as the "other side" of a black hole in another universe. Others theorize the big bang was sparked by collisions between higher-dimensional structures of space. Still others hypothesize that because different types of energies cancel each other out and there is no preferred direction for objects to spin (and because additional conservation laws apply), the universe is evenly balanced in every way—one big "free lunch."

From National Geographic's "Are We Alone?" special magazine: