Nov 15 2017

Astrophysics for People in a Hurry by Neil deGrasse Tyson

In just over 200 pages, Neil deGrasse Tyson takes his readers, who are presumably in a bit of a rush, on a grand tour of the cosmos, with a refreshing emphasis on what scientists don’t know. He’s bumptious, conversational, unafraid of including personal opinions about people in the field and commendably clear even when describing mind-expanding notions. He’s also a bit cheeky, titling his first three chapters “The Greatest Story Ever Told,” “On Earth as in the Heavens,” and “Let There Be Light.” Apparently he’s always been that way. The first essay he wrote about the wider universe was about diminutive galaxies that are companions to the Milky Way. He titled it “The Galaxy and the Seven Dwarfs.”

He mentions that essay in a chapter on intergalactic space, which concludes that it “is, and forever will be, where the action is.” (p. 74) Not only is there far more intergalactic space than the other kind, there’s a lot more in it than one might think, “dwarf galaxies, runaway stars, runaway stars that explode, million-degree X-ray-emitting gas, dark matter, faint blue galaxies, ubiquitous gas clouds, super-duper high-energy charged particles [cosmic rays], and the mysterious quantum vacuum energy.” (p. 64) How much mass does it all add up to? “Nobody knows for sure. The measurement is difficult because the stars are too dim to detect individually. We must rely on detecting a faint glow produced by the light of all stars combined. In fact, observations of [galactic] clusters detect just such a glow between the galaxies, suggesting that there may be as many vagabond, homeless stars as there are stars within the galaxies themselves.” (p. 67) Astronomers have also seen more than a dozen supernovas far away from presumed galaxies. Tyson notes that in ordinary galaxies, for every supernova there are one hundred thousand to one million stars that do not explode in that fashion. The isolated supernovas may point to “entire populations of undetected stars.” (p. 67) They may be even more numerous, because to date systematic supernova searches have monitored known galaxies, rather than intergalactic space.

The last item on the list of what’s in intergalactic space points to one of the great riddles of modern cosmology: “why the bulk of all the gravitational force that we’ve measured in the universe — about eighty-five percent of it — arises from substances that do not otherwise interact with ‘our’ matter or energy.” That is, observers can see the effects of gravity, but cannot find or infer the mass that would ordinarily be necessary for that gravity to be present.

In any case, we are essentially clueless. We find ourselves no closer to an answer today than we were when this “missing mass” problem was first fully analyzed in 1937 by the Swiss-American astrophysicist Fritz Zwicky. He taught at the California Institute of Technology for more than forty years, combining his far-ranging insights into the cosmos with a colorful means of expression and an impressive ability to antagonize his colleagues. (p. 77)

Zwicky observed a cluster of galaxies about 300 million light years distant and found that “its member galaxies are all moving more rapidly than the escape velocity for the cluster.” (p. 79) The cluster should have flown apart after a few hundred million years, yet the cluster is more than ten billion years old. Since then, scientists have observed the same phenomenon in other clusters. In the 1970s, Vera Rubin discovered a similar discrepancy within individual spiral galaxies. The speeds at which stars were observed to be orbiting galactic centers did not match what they should be for the observed masses involved. “Across the universe, the discrepancy averages to a factor of six: cosmic dark matter has about six times the total gravity of all the visible matter.” (p. 82) Tyson follows with a discussion of how observations have ruled out various notions of what dark matter might be like, and he adds historical context about how scientists in previous eras tried to explain observations that the theories of their times could not account for. It’s a bracing balance among showing clearly what people do not know, how they are eliminating possibilities, and how similar problems have been dealt with in the past. The end result is excitement about the opportunities for discovery of how the universe works on its grandest scales.

Which is good, because the next chapter, on dark energy, shows the scope of some of the problems that cosmologists wrestle with, even when they have good theoretical frameworks. In a discussion about how much mass and energy in total are required to produce a universe with the distribution that scientists observe, Tyson describes how one aspect of quantum mechanics could account for a pressure against gravity.

Unfortunately, when you estimate the amount of repulsive “vacuum pressure” that arises from the abbreviated lives of virtual particles, the result is more than 10120 times larger than the experimentally determined value of the cosmological constant. This is a stupidly large factor, leading to the biggest mismatch between theory and observation in the history of science. (p. 111)

I like the use of the technical term “stupidly large.”

Not all of the mysteries are quite as stupendous. A later chapter in the book zips through the periodic table of the elements, tying many of them to their cosmic origins. And then there’s technetium.

… it’s found nowhere on Earth except in particle accelerators, where we make it on demand. Technetium carries this distinction in its name, which derives from the Greek technetos, meaning “artificial.” For reasons not yet fully understood, technetium lives in the atmospheres of a select subset of red stars. This alone would not be cause for alarm except that technetium has a half-life of a mere two million years, which is much, much shorter than the age and life expectancy of the stars in which it is found. In other words, the star cannot have been born with the stuff, for it it were, there would be none left by now. There is also no known mechanism to create technetium in a star’s core and have it dredge itself up to the surface where it is observed… (pp. 125–26)

Tyson wraps up his main text with a chapter on one of my favorite topics, exoplanets, a field that was wholly theoretical when I took college-level astronomy back in the late 1980s. The first one 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.” (p. 192) There may be as many as forty billion Earth-like planets in the Milky Way. Lots to discover at every scale, even for people in a hurry.

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