Ch. 5: Dark Matter
Posted: Thu Oct 12, 2017 2:38 pm
Astrophysics for People in a Hurry
by Neil deGrasse Tyson
Ch. 5: Dark Matter
Please use this thread to discuss this chapter.by Neil deGrasse Tyson
Ch. 5: Dark Matter
We're getting into some loopy stuff now! We know very little about the most common substance in the universe.Thus, as best we can figure, the dark matter doesn’t simply consist of matter that happens to be dark. Instead, it’s something else altogether. Dark matter exerts gravity according to the same rules that ordinary matter follows, but it does little else that might allow us to detect it. Of course, we are hamstrung in this analysis by not knowing what the dark matter is in the first place. If all mass has gravity, does all gravity have mass? We don’t know. Maybe there’s nothing wrong with the matter, and it’s the gravity we don’t understand.
The counter-forces between coagulation and dilution become even more complicated in the next chapter.What we know is that the matter we have come to love in the universe—the stuff of stars, planets, and life—is only a light frosting on the cosmic cake, modest buoys afloat in a vast cosmic ocean of something that looks like nothing.
During the first half million years after the big bang, a mere eyeblink in the fourteen-billion-year sweep of cosmic history, matter in the universe had already begun to coalesce into the blobs that would become clusters and superclusters of galaxies. But the cosmos would double in size during its next half million years, and continue growing after that. At odds in the universe were two competing effects: gravity wants to make stuff coagulate, but the expansion wants to dilute it. If you do the math, you rapidly deduce that the gravity from ordinary matter could not win this battle by itself. It needed the help of dark matter, without which we would be living—actually not living—in a universe with no structures: no clusters, no galaxies, no stars, no planets, no people.
How much gravity from dark matter did it need? Six times as much as that provided by ordinary matter itself. Just the amount we measure in the universe. This analysis doesn’t tell us what dark matter is, only that dark matter’s effects are real and that, try as you might, you cannot credit ordinary matter for it.
That is so cool. Sorry about your head, though.Litwitlou wrote: They proved the correctness of the bizarre properties of quantum mechanics, i.e. that electrons can be two places at the same time.
Then I take a pill for my migraine.