Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series)

by Neil deGrasse Tyson

That the Sun is one of a hundred billion stars in the Milky Way? Or that the Milky Way is but one of a hundred billion galaxies in the universe?

The most accurate measurements to date reveal dark energy as the most prominent thing in town, currently responsible for 68 percent of all the mass-energy in the universe; dark matter

comprises 27 percent, with regular matter comprising a mere 5 percent.

Since both mass and energy cause space-time to warp, or curve, omega tells us the shape of the cosmos. If omega is less than one, the actual mass-energy falls below the critical value, and the universe expands forever in every direction for all of time, taking on the shape of a saddle, in which initially parallel lines diverge. If omega equals one, the universe expands forever, but only barely so. In that case the shape is flat, preserving all the geometric rules we learned in high school about parallel lines. If omega exceeds one, parallel lines converge, and the universe curves back on itself, ultimately recollapsing into the fireball whence it came.

The first exoplanet was discovered in 1995, and, as of this writing, the tally is rising through three thousand, most found in a small pocket of the Milky Way around the solar system. So there’s plenty more where they came from. After all, our galaxy contains more than a hundred billion stars, and the known universe harbors some hundred billion galaxies.

Take water. It’s common, and vital. There are more molecules of water in a cup of the stuff than there are cups of water in all the world’s oceans. Every cup that passes through a single person and eventually rejoins the world’s water supply holds enough molecules to mix 1,500 of them into every other cup of water in the world. No way around it: some of the water you just drank passed through the kidneys of Socrates, Genghis Khan, and Joan of Arc.

Time to get cosmic. There are more stars in the universe than grains of sand on any beach, more stars than seconds have passed since Earth formed, more stars than words and sounds ever uttered by all the humans who ever lived.