44 pages • 1 hour read
Neil Degrasse TysonAstrophysics for People in a Hurry
Nonfiction | Book | Adult | Published in 2017A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Summary
Background
Chapter Summaries & Analyses
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Index of Terms
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BetaChapters 5-6Chapter Summaries & Analyses
Chapter 5 Summary: “Dark Matter”
In the 1920s, astrophysicist Fritz Zwicky discovered that the galaxies of the Coma supercluster, 300 million light-years away, were traveling around the cluster’s gravitational center at speeds much too fast to keep them from flying away. He realized that some sort of matter, unseen by scientific detectors, might be hiding within the cluster. He called it “dark matter.”
In 1976, another astrophysicist, Vera Rubin, found that outlying stars orbiting galaxies also traveled too fast for the amount of visible mass within those galaxies. Thus, the dark matter problem exists not only within large groups of galaxies but inside the galaxies themselves. Overall, there’s about six times as much dark matter as visible matter in the universe.
None of the usual suspects—non-luminous clouds, rogue planets, even black holes—can come close to accounting for the excess matter. If dark matter participated in nuclear fusion, there’d be much more helium in the cosmos because, during the first few minutes of the universe, hydrogen was converted into helium so quickly that 10% of all atoms became helium. This isn’t nearly enough to account for all the mass in the universe, including dark matter. Dark matter exerts gravity but otherwise doesn’t interact in any detectable way with the mass around it.
On the other hand, “[m]aybe there’s nothing wrong with the matter, and it’s the gravity we don’t understand” (84). Moons and planets don’t suffer from the dark matter discrepancy, and we don’t detect, say, comet-sized chunks of dark matter. It only happens in and between galaxies. Still, calculations show that without dark matter our universe would be expanding so quickly that planets, stars, and galaxies would never form. The cosmos needs dark matter to shape up the way it does.
Dark matter isn’t a “placeholder” to explain a contradiction in our beliefs. It’s an actual thing with mass we derive from observation but don’t understand. Something similar happens with neutrinos: 100 billion of these nearly massless particles pass through a single square inch every second, yet they almost never interact with anything. Aside from its gravitational effect on other matter, either dark matter itself interacts extremely weakly with other objects, as do neutrinos, or it expresses an unknown force of nature.
Chapter 6 Summary: “Dark Energy”
Einstein’s 1916 theory of general relativity describes “how everything in the universe moves under the influence of gravity” (96). It says that gravity acts by warping space, especially around large objects, so that objects traveling nearby tend to curve around the objects. The theory predicts gravity can move across space in waves. It has withstood every test in the decades that followed its publication. In 2016, a special observatory, LIGO, recorded gravity waves as predicted by Einstein.
Einstein added to his equations for gravity a “cosmological constant” that prevented the universe from collapsing. In 1929, Edwin Hubble found the universe is expanding; this made the constant unnecessary; Einstein was embarrassed.
In 1998, though, scientists reported the light from distant exploding stars called supernovas was dimmer than expected, suggesting the expansion of the universe is speeding up. Researchers added Einstein’s cosmological constant back in and found his equations now perfectly predict the accelerating rate of expansion. His constant describes a force that repels gravity. Called “dark energy,” this force makes up about two-thirds of all matter and energy; dark matter supplies another 27%, while normal matter makes up only 5%.
Over the eons, the universe will spread out faster and faster until the expansion of space will push galaxies apart faster than light can travel between them. In the distant future, observers in the Milky Way will see only blackness in the space around their galaxy; they’ll be unable to observe other galaxies, much less that they’re hurtling away at great speed. It will seem then as if the Milky Way is all there is.
Chapters 5-6 Analysis
These chapters discuss two of the great mysteries that scientific observation has revealed, the dark matter and dark energy that together make up most of the universe. The chapters also touch on how these puzzles have challenged and tested science’s ability to overcome our ignorance about reality.
Dark matter and dark energy have gravitational effects but otherwise can’t be “seen”—hence the term “dark.” But “dark” also serves as a fill-in-the-blank for whatever the phenomenon might turn out to be. It’s possible that slight tweaks in the laws of nature will explain the problem, or maybe there’s an entirely new principle of physics that explains why galaxies behave as if they’re heavier than they appear or why our universe is expanding faster than expected. It is more likely dark matter is made of some kind of particle with mass: Without all the extra mass, our universe would fly apart too quickly for stars and galaxies to form.
The standard model of the universe contains a glaring contradiction: Its theory of relativity and its theory of quantum mechanics, each of which perform beautifully in their realms but clash with each other in fundamental ways (see the Contextual Analysis section below for a detailed discussion of this concept). It’s possible that a solution to that dilemma in the form of a new standard model will also resolve the mysteries of dark matter and energy.
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