Astrophysics for People in a Hurry

The universe began as an extremely small dot that underwent a “big bang” and expanded across 14 billion years into the cosmos of today. During the first moment of that explosion, the laws of physics simply didn’t apply, but a trillionth of a second later, the four forces of physics appeared, “with the weak force controlling radioactive decay, the strong force binding the atomic nucleus, the electromagnetic force binding molecules, and gravity binding bulk matter” (20).

At first, the densely packed matter and energy freely changed back and forth into each other in accordance with Einstein’s famous equation E=MC^2. Energy is made up of submicroscopic particles called bosons, and matter includes equally tiny particles that include quarks. Quarks make up protons, neutrons, and leptons, which include electrons and neutrinos. When all this was a millionth of a second old, it was already as large as our current solar system.

Matter also contains antiparticles, like matter but with opposite charge; for example, electrons have twins called positrons. When these particles touch, they annihilate each other in a burst of energy. The early universe had a tiny imbalance, though, so that matter outnumbered antimatter by one in a billion. All the rest of the matter and antimatter got converted into photons of energy, leaving that last one-billionth to become the matter we see today in the universe.

One second had passed, and the universe was as large as the distance from the Sun to its nearest star neighbor. By two minutes, the universe was made up largely of simple hydrogen atoms, some helium atoms, and a lot of energy. It took 380,000 years for this thick soup to become thin enough to liberate photons so they could deliver visible light.

Over the next billion years, matter gathered into galaxies, 100 million of them, each of which further condensed into hundreds of billions of stars. The largest stars were dense enough to fuse simple atoms into larger varieties, then explode and spread those heavier atoms across space to form new suns and their planets. Our Sun and its planets, including Earth, along with billions of comets formed out of the interstellar dust about nine billion years into the universe’s history.

Earth formed in the “Goldilocks zone” around the Sun—not too hot and not too cold—and liquid oceans formed where biochemicals began to reproduce themselves and create life forms that grew in complexity over billions of years. A large asteroid struck the Earth 65 million years ago and extinguished most life on Earth, including the dinosaurs. This made room for mammals to take over new territory, and today human beings thrive, do science, and figure out how the universe got started.

It’s not known what came before the universe. Perhaps it arose from a multiverse, or it’s a simulation, or it comes from God. Science doesn’t assume it knows those answers; instead, it searches for them. 

Newton’s laws of gravity were the first to make uniform the rules governing both Earth and space. In the 19th century, prisms split sunlight and showed the Sun’s elements display the same light spectrum as materials on Earth. Further experiments found a solar element not seen before; it was named helium, for Helios, the Sun, and was later found on Earth.

The laws applied elsewhere: Light spectra showed that faraway stars also contain chemicals like those in the Sun, and those stars obey the same laws of gravity and rules governing nuclear fusion as in our solar system. Some distant objects show properties unknown here, but even those extremes obey the laws of physics. Light from stars billions of years old shows that, way back in time, the laws of physics were the same as today.

If alien civilizations exist, they probably have different social and political rules, but they might understand basic science. In the 1970s, four spacecraft—Voyager 1 and 2, and Pioneer 10 and 11—were sent deep into space with metal plaques containing scientific diagrams of our solar system and the makeup of hydrogen atoms. They also contained recordings of our sounds and music.

From subatomic particles to the structure of the universe, physics obeys conservation laws “where the amount of some measured quantity remains unchanged no matter what” (43). Matter can be turned into energy, for example, but both together can’t be created or destroyed.

Mysteries remain. Roughly 85% of all gravitational pull comes from “dark matter,” which has effects on the matter around it but otherwise remains invisible to our detectors. It’s possible that a small adjustment to the known laws of physics will explain away dark matter, but only further experiments will decide the issue.

Though scientists often disagree vigorously, this happens “on the bleeding frontier of our knowledge” (45), where new discoveries are made. The basic, well-established laws are agreed on.

Early in the universe’s expansion, matter spread out enough that electrons could join up with protons to form the first atoms. In doing so, they released energy in the form of photons that began to streak across the expanding cosmos. Photons from the early universe lost energy as space expanded; today, the universe is 1,000 times more spread out, and the remaining photons from that era are 1,000 times weaker. Their frequencies are much slower, so detectors pick them up mainly as microwaves.

Mid-20th-century scientists, relying on discoveries in cosmology and nuclear physics, predicted the precise temperature of today’s cosmic microwave background (CMB) is five degrees Kelvin. Researchers at AT&T’s Bell Labs, trying to develop a good microwave receiver in 1964, accidentally recorded the CMB and found it was 2.7 degrees Kelvin. The scientists weren’t right, but they were close: “That their prediction even remotely approximated the right answer is a stunning triumph of human insight” (53).

By noting the slight temperature differences in the CMB from different regions of space, scientists can diagram how the matter in the early universe was shaped into clusters of galaxies. This also shows that dark matter exists because it exerts a large gravitational influence, and a mysterious form of energy—”dark energy”—causes the universe to expand faster than it should. In fact, “most of the universe is made of stuff about which we are clueless” (60).

The data from the CMB turned cosmology, the study of the universe, from a bunch of untested theories into a science.

A hundred billion galaxies of stars light up the universe, but the dark spaces between them contain stuff that’s also important, such as dark matter, dark energy, gas clouds, and runaway stars. Our own galaxy, the Milky Way, presides over dozens of small neighboring “dwarf” galaxies. The Milky Way’s bigger twin galaxy, Andromeda, lies two million light-years away.

Many galaxies collide and blend together into chaotic shapes and randomly strewn stars, so “there may be as many vagabond, homeless stars as there are stars within the galaxies themselves” (67). The gas between galaxies is usually several times more massive than the galaxies themselves, and equally large is the quantity of dark matter in those regions.

Several billion years ago, small, faint, blue galaxies thrived; today, such blue star clusters aren’t seen, though perhaps some matured into dwarf galaxies within today’s large galactic clusters. The light from quasars, the intensely brilliant cores of ancient galaxies, reaches us after being filtered through dozens of intergalactic gas clouds; the light gets warped, or “lensed,” by the gas and other massive objects.

Also in space are high-energy particles, mainly protons, called “cosmic rays” that travel at nearly the speed of light and have enough energy to knock a golf ball across a putting green. Intergalactic space is an interesting, if dangerous, place.