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Kindle Notes & Highlights
by
Lisa Randall
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August 17 - September 4, 2020
Dark matter is the elusive stuff in the Universe that interacts through gravity like ordinary matter, but that doesn’t emit or absorb light. Astronomers detect its gravitational influence, but they literally don’t see it.
Paleontologists, geologists, and physicists have shown that 66 million years ago, an object at least ten kilometers wide plummeted to Earth from space and destroyed the terrestrial dinosaurs, along with three-quarters of the other species on the planet. The object might have been a comet from the outer reaches of the Solar System, but no one knows why this comet was perturbed from its weakly bound, but stable, orbit.
Dark matter constitutes 85 percent of the matter in the Universe while ordinary matter—such as that contained in stars, gas, and people—constitutes only 15 percent. Yet people are mainly preoccupied with the existence and relevance of ordinary matter—which, to be fair, interacts far more strongly.
At this point, we know only that dark matter and ordinary matter interact via gravity. Gravity’s consequences are generally so tiny that we register the influence only of enormous masses—such as that of the Earth and the Sun—and even those are pretty feeble. After all, you can pick up a paper clip with a tiny magnet, successfully competing against the gravitational influence of the entire Earth.
Ten times more bacterial cells than human cells live inside us and help with our survival. Yet we are barely aware of these microscopic creatures that live in us, consume nutrients, and aid our digestive systems. Only when bacteria misbehave and make us ill do most of us even acknowledge their existence.
However, when large amounts of dark matter aggregate into concentrated regions, its net gravitational influence is substantial, leading to measurable influences on stars and on nearby galaxies. Dark matter affects the expansion of the Universe, the path of light rays passing to us from distant objects, the orbits of stars around the centers of galaxies, and many other measurable phenomena in ways that convince us of its existence. We know about dark matter—and indeed it does exist—because of these measurable gravitational effects.
Dark matter is not dark—it is transparent. Dark stuff absorbs light. Transparent things, on the other hand, are oblivious to it. Light can hit dark matter, but neither the matter nor the light will change as a result.
I’ll take a brief detour to discuss black holes, which are objects that form when too much matter gets within too small a region of space. Nothing—including light—escapes from their powerful gravitational influence.
Dark energy is not matter—it is just energy. Dark energy exists even if no actual particle or other form of stuff is around. It permeates the Universe, but doesn’t clump like ordinary matter. The density of dark energy is the same everywhere—it can be no denser in one region than another.
Dark energy also remains constant over time. Unlike matter or radiation, dark energy does not become more dilute when the Universe expands. This is in some respects its defining property. The dark energy density—energy not carried by particles or matter—remains the same over time. For this reason, physicists often refer to this type of energy as a cosmological constant.
We don’t yet know dark matter’s true nature, but the measurements I’ll now describe demonstrate that dark matter is a real and essential component of our world. Dark matter, though so far invisible to our eyes or direct observations, doesn’t completely hide.
Based on his measurements of the velocity of the stars, Zwicky calculated that the amount of mass required for the cluster to have sufficient gravitational pull was 400 times greater than the contribution of the measured luminous mass—the matter that emits light. To account for all that extra matter, Zwicky proposed the existence of what he named dunkle Materie, which is German for dark matter and sounds either more ominous or sillier depending on how you pronounce it.
The brilliant and prolific Dutch astronomer Jan Oort came to a similar conclusion about dark matter a year earlier than Zwicky. Oort recognized that the velocities of stars in our local galactic neighborhood were too high for their motion to be attributed solely to the gravitational influence of light-emitting matter. Oort too deduced that something was missing. He didn’t conjecture a new form of matter, however, but merely nonluminous ordinary stuff—a proposal that has since been rejected for several reasons
The percentage of energy in dark matter is about 26 percent, in ordinary matter about 5 percent, and in dark energy about 69 percent.
In other words, dark matter carries five times the energy of ordinary matter, meaning it carries 85 percent of the energy of matter in the Universe.
The Universe almost certainly extends beyond the domain we can observe. If indeed the speed of light is finite and if our Universe has been around for only a fixed amount of time, we can access only a finite region of space—no matter how much technology might advance. We can see only those regions that can be reached via a light ray—or something else that travels at light speed—during the lifetime of the Universe. Only from those regions can a signal possibly reach us within the time that the Universe has been around. Anything farther away—beyond what physicists call the cosmic horizon
Antimatter is the stuff with the same mass but opposite charges to ordinary matter. Physical theories tell us that for every matter particle, an antimatter particle must exist. For example, knowing that an electron has charge –1 tells us there must also be an antiparticle—it’s called a positron—with the same mass but opposite charge, +1.
Taking this a step further, if the Universe existed forever and the Big Bang was part of it, either our Universe was all there was or other universes also emerged from their own Big Bangs. The multiverse is the name associated with a cosmos in which, in addition to our own universe, there are many others.
This reasoning leaves us with three choices. Either our universe started with the Big Bang, the Universe has been around forever but eventually went over to the expansion that the Big Bang theory predicts, or we are one of many universes that grew out of a universe/multiverse that has always existed. This covers all the possibilities. The last one seems most likely to me in that it doesn’t assume that our world or even our particular universe is special, which is reasoning that has been invoked since the time of Copernicus. This choice also implies that just as the spatial extent of the
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If nothing travels faster than the speed of light, any region that is too far away—beyond the cosmic horizon—is off-limits to observations.
in all likelihood, we will never know with certainty whether or not we live in a multiverse. Even if other universes exist, they are likely to remain undetectable.
The expansion of the Universe is perhaps an odd concept, given that the Universe has very likely been infinite all along. But it is space itself that is expanding, meaning that the distances between objects like galaxies increases with time. I am frequently asked, “If the Universe is expanding, what is it expanding into?” The answer is that it is not expanding into anything. Space itself grows. If you imagine the universe as the surface of a balloon, the balloon itself stretches.
Today, we sometimes talk about a Hubble constant, which is the rate at which the Universe currently expands. It is a constant in the sense that today, its value everywhere in space is the same. But actually the Hubble parameter is not constant. It changes with time. Earlier in the Universe, when things were denser and gravitational effects were stronger, the Universe expanded far more rapidly than it does today.
Calling it constant but not really a constant. Weird.
Using the Hubble Space Telescope (given the name, it seems only fair), astronomers measured a value of 72 km/sec/Mpc (meaning something at a distance of a megaparsec moves away at 72 km/sec) with an accuracy of 11 percent—a far cry from Hubble’s original and very inaccurate measurement of 500 km/sec/Mpc.
A megaparsec (Mpc) is a million parsecs, and a parsec, like many astronomical units, is a historical relic from the way distances were measured in early times. It is a shortened version of “parallax second” and has to do with the angle subtended by an object on the sky, which is why it has an angular unit in
The age of the Universe is now known to within a couple of hundred million years, and measurements have continued to improve. When I wrote my first book it was 13.7 billion years old but we now believe it to be a bit older—13.8 billion years from the so-called Big Bang. Note that it is not only the changing Hubble parameter, but the discovery of the dark energy that I mentioned in Chapter 1, that led to this more refined result, since the age of the Universe depends on both.
According to the Big Bang theory, the very early Universe originated 13.8 billion years ago as a hot, dense fireball consisting of many interacting particles with temperature higher than a trillion trillion degrees.
Only fundamental particles were present in this Universe, and not, for example, atoms, which are made of nuclei bound together with electrons—or protons—that are made from the more fundamental particles called quarks. Nothing could remain trapped in a bound object in the face of so much heat and energy.
But perhaps the most significant milestone in the Universe’s evolution, at least in terms of detailed testing of cosmological predictions, occurred somewhat later on—about 380,000 years after the Big Bang. The Universe was originally filled with both charged and uncharged particles. But at this later time, the Universe had cooled sufficiently that positively charged nuclei combined with negatively charged electrons to form neutral atoms. From that time forward, the Universe consisted of neutral matter, which is matter that carries no electric charge.
The visible structure of the Universe lies in gas and in stellar systems. These congregations of stars come in a wide variety of sizes and in several different shapes. Binary stars, with one star rotating around another, constitute a stellar system, as do galaxies, which range in size from one hundred thousand to a trillion stars. Clusters of galaxies with a thousand times as many stars as that are stellar systems too.
The center of the Milky Way also contains a black hole of about four million solar masses—known sometimes as Sagittarius A*.
Like the other hundred billion stars in the Milky Way disk, the Sun circles around the galaxy at a speed of about 220 kilometers per second. At this speed, it takes about 240 million years for the Sun to orbit the galactic center.
About 50 tons of extraterrestrial material enters the Earth’s atmosphere every day, carried by millions of small meteoroids. And none of us are affected in any noticeable way.
Planets—as we now understand the term—were created after the birth of the Sun, when dust grains collected increasingly large amounts of material that then collided, growing more or less into their current state over a time span of perhaps a few million to a few tens of million years—a very brief time interval from an astronomical perspective.
Together, the four gas giant planets, as they are known—Jupiter, Saturn, Uranus, Neptune—contain 99 percent of the Solar System’s mass (aside from the Sun itself),
So now a planet is classified as an object that is round due to its own gravity and has “cleared its neighborhood” of smaller objects that would otherwise orbit the Sun in its vicinity.
Asteroids—like planets—don’t emit visible light. Planets, asteroids, and meteoroids are illuminated only by the light they reflect from the Sun. Finding asteroids is more difficult, however, since they are so much smaller and therefore dimmer and harder to see. Comets have bright trailing tails and shooting stars are relatively nearby and bright. Asteroids, on the other hand, have no readily apparent features so discovering them was (and is) a challenge.
The first standardized definition of meteoroid, devised only in 1961 by the International Astronomical Union, was a solid object moving in interplanetary space that was considerably smaller than an asteroid and considerably larger than an atom.
Like asteroids, meteoroids differ dramatically in their nature, probably as a result of their wildly varying origins in the Solar System. Some are snowball-like objects with densities only a quarter that of ice, while others are dense, nickel- and iron-rich rocks, while still others have more abundant carbon.
Most meteors arise from dust or pebble-sized objects. Millions of them enter our atmosphere daily. Since most meteoroids fall apart above 50 kilometers in altitude, meteors typically occur between about 75 and 100 kilometers above sea level in what is called the mesosphere.
The meteoroids that make it through the atmosphere and hit the Earth can lead to meteorites. Meteorites are the rocks left on Earth after an extraterrestrial object has hit, decomposed, melted, and partially vaporized. Meteorites are yet another tangible reminder that the Earth is intrinsically part of a cosmic environment.
An AU, or astronomical unit, is about 150 million kilometers—the approximate distance between the Earth and the Sun.
The Oort cloud in the distant region of the Solar System, which extends from perhaps 1,000 AU to beyond 50,000 AU, lies outside the domain of the planets and the Kuiper belt.
The proposed distance of the Oort cloud is enormous. The Earth’s distance from the Sun is 1 AU and that of Neptune—the most distant planet—is 30 AU. Astronomers think the Oort cloud extends from perhaps as close as 1,000 AU from the Sun to distances farther than 50,000 AU—significantly farther than anything we have so far considered.
The website of the International Astronomical Union’s Minor Planet Center of the Harvard Smithsonian Center for Astrophysics, http://www.minorplanetcenter.net/, reports the latest numbers of minor planets, comets, and near approaches that have been found.
Objects that form impact craters usually hit the ground at speeds up to eight times the Earth’s escape velocity, which is 11 km/sec, with roughly 20–25 km/sec most typical. For larger objects, this speed—many times the speed of sound—guarantees that an enormous amount of kinetic energy gets released, since kinetic energy grows not only with the mass but with the square of the speed. An impact on solid rock, which can be comparable to a nuclear blast, produces shock waves that compress both the object from space and the surface on Earth. The shock released on impact heats up the material it
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Carbon dating is perhaps the best-known example of this method. It is used to determine the age of older organic materials and is indeed very precise. However, given the half-life of carbon isotopes, it is effective only for stuff less than 50,000 years old,
their research, the Chicago paleontologists identified five major mass extinctions (see Figure 29) and about twenty lesser ones, in which approximately 20 percent of life-forms died out.
The oldest major event they identified is the Ordovician-Silurian extinction, which occurred somewhere between 450 and 400 million years ago. Essentially all life was in the ocean back then so most of the species lost were marine-based. This mass extinction—the second most deadly, in which about 85 percent of all species went extinct—occurred in two stages over a period of about 3.5 million years. The cause seems to have initially been lower temperatures and massive glaciations that caused dramatic drops in sea level.
