Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series)

by Neil deGrasse Tyson

But light turns out to be quite happy traveling through the vacuum of space, devoid of any medium to carry it. Unlike sound waves, which consist of air vibrations, light waves were found to be self-propagating packets of energy requiring no assistance at all.

Dark matter is just as real as the many exoplanets discovered in orbit around stars other than the Sun, discovered solely through their gravitational influence on their host stars and not from direct measurement of their light.

Science is not just about seeing, it’s about measuring, preferably with something that’s not your own eyes, which are inextricably conjoined with the baggage of your brain.

Either dark matter particles must wait for us to discover and to control a new force or class of forces through which their particles interact, or else dark matter particles interact via normal forces, but with staggering weakness.

the universe in recent decades was discovered to wield a mysterious pressure that issues forth from the vacuum of space and that acts opposite cosmic gravity. Not only that, this “negative gravity” will ultimately win the tug-of-war, as it forces the cosmic expansion to accelerate exponentially into the future.

GR outlines the relevant mathematical details of how everything in the universe moves under the influence of gravity.

when gravitational waves were discovered by a specially designed observatory tuned for just this purpose.† These waves, predicted by Einstein, are ripples moving at the speed of light across the fabric of space-time, and are generated by severe gravitational disturbances, such as the collision of two black holes.

Yet, in the case of Einstein’s relativity, the founding principles of the entire theory require that everything must happen exactly as predicted.

So lambda’s sole job was to oppose gravity within Einstein’s model, keeping the universe in balance, resisting the natural tendency for gravity to pull the whole universe into one giant mass. In this way, Einstein invented a universe that neither expands nor contracts, consistent with everybody’s expectations at the time.

Instead of settling for Sir Isaac Newton’s view of gravity as spooky action-at-a-distance (a conclusion that made Newton himself uncomfortable), GR regards gravity as the response of a mass to the local curvature of space and time caused by some other mass or field of energy.

“Matter tells space how to curve; space tells matter how to move.”

To invoke an unstable condition as the natural state of a physical system violates scientific credo. You cannot assert that the entire universe is a special case that happens to be balanced forever and ever. Nothing ever seen, measured, or imagined has behaved this way in the history of science, which makes for powerful precedent.

the more distant a galaxy, the faster the galaxy recedes from the Milky Way. In other words, the universe is expanding.

Standard candles simplify calculations immensely: since the supernovas all have the same wattage, the dim ones are far away and the bright ones are close by. After measuring their brightness (a simple task), you can tell exactly how far they are from you and from one another.

But there’s a second way to measure the distance to galaxies: their speed of recession from our Milky Way—recession that’s part and parcel of the overall cosmic expansion.

astrophysicists were left with a universe that had expanded faster than we thought, placing galaxies farther away than their recession speed would have otherwise indicated. And there was no easy way to explain the extra expansion without invoking lambda, Einstein’s cosmological constant.

Lambda suddenly acquired a physical reality that needed a name, and so “dark energy” took center stage in the cosmic drama,

The most accurate measurements to date reveal dark energy as the most prominent thing in town, currently responsible for 68 percent of all the mass-energy in the universe; dark matter comprises 27 percent, with regular matter comprising a mere 5 percent.

If omega is less than one, the actual mass-energy falls below the critical value, and the universe expands forever in every direction for all of time, taking on the shape of a saddle, in which initially parallel lines diverge. If omega equals one, the universe expands forever, but only barely so. In that case the shape is flat, preserving all the geometric rules we learned in high school about parallel lines. If omega exceeds one, parallel lines converge, and the universe curves back on itself, ultimately recollapsing into the fireball whence it came.

Omega does equal one, just as the theorists demanded of the universe, even though you can’t get there by adding up all the matter—dark or otherwise—as they had naively presumed.

So what is the stuff? Nobody knows. The closest anybody has come is to presume dark energy is a quantum effect—where the vacuum of space, instead of being empty, actually seethes with particles and their antimatter counterparts.

Dark energy inhabits one of the safest harbors we can imagine: Einstein’s equations of general relativity. It’s the cosmological constant. It’s lambda.

Without a doubt, Einstein’s greatest blunder was having declared that lambda was his greatest blunder.

anything not gravitationally bound to the neighborhood of the Milky Way galaxy will recede at ever-increasing speed, as part of the accelerating expansion of the fabric of space-time. Distant galaxies now visible in the night sky will ultimately disappear beyond an unreachable horizon, receding from us faster than the speed of light.

Only three of the naturally occurring elements were manufactured in the big bang. The rest were forged in the high-temperature hearts and explosive remains of dying stars, enabling subsequent generations of star systems to incorporate this enrichment, forming planets and, in our case, people.

With only one proton in its nucleus, hydrogen is the lightest and simplest element, made entirely during the big bang. Out of the ninety-four naturally occurring elements, hydrogen lays claim to more than two-thirds of all the atoms in the human body, and more than ninety percent of all atoms in the cosmos, on all scales, right on down to the solar system.

Henry Cavendish discovered hydrogen in 1766 during his experiments with H2O (hydro-genes is Greek for “water-forming”),

Helium is the second simplest and second most abundant element in the universe. Although a distant second to hydrogen in abundance, there’s fifty times more of it than all other elements in the universe combined.

And with 92 percent of hydrogen’s buoyancy in air, but without its explosive characteristics, helium is the gas of choice for the outsized balloon characters of the Macy’s Thanksgiving Day parade, making the department store second only to the U.S. military as the nation’s top user of the element.

Lithium is the third simplest element in the universe, with three protons in its nucleus. Like hydrogen and helium, lithium was made in the big bang, but unlike helium, which can be manufactured in stellar cores, lithium is destroyed by every known nuclear reaction.

The element carbon can be found in more kinds of molecules than the sum of all other kinds of molecules combined.

Just edging out carbon in abundance rank, oxygen is common, too, forged and released in the remains of exploded stars. Both oxygen and carbon are major ingredients of life as we know it.

Silicon sits directly below carbon on the Periodic Table, which means, in principle, it can create the same portfolio of molecules that carbon does.

sodium is the most common glowing gas in municipal street lamps across the nation.

Aluminum occupies nearly ten percent of Earth’s crust yet was unknown to the ancients and unfamiliar to our great-grandparents. The element was not isolated and identified until 1827 and did not enter common household use until the late 1960s, when tin cans and tin foil yielded to aluminum cans and, of course, aluminum foil.

Titanium is 1.7 times denser than aluminum, but it’s more than twice as strong. So titanium, the ninth most abundant element in Earth’s crust, has become a modern darling for many applications,

By many measures, iron ranks as the most important element in the universe. Massive stars manufacture elements in their core, in sequence from helium to carbon to oxygen to nitrogen, and so forth, all the way up the Periodic Table to iron.

And if you combine iron atoms via fusion, they will also absorb energy. Stars, however, are in the business of making energy. As high-mass stars manufacture and accumulate iron in their cores, they are nearing death.

Along with osmium and platinum, iridium is one of the three heaviest (densest) elements on the Table

an unknown element was discovered in the debris of the first hydrogen bomb test in the Eniwetok atoll in the South Pacific, on November 1, 1952, and was named einsteinium in his honor.

With ninety-two protons packed in its nucleus, uranium is widely described as the “largest” naturally occurring element, although trace amounts of larger elements can be found naturally where uranium ore is mined.

Unstable weapons-grade plutonium was the active ingredient in the atomic bomb that the United States exploded over the Japanese city of Nagasaki, just three days after Hiroshima, bringing a swift end to World War

Apart from crystals and broken rocks, not much else in the cosmos naturally comes with sharp angles. While many objects have peculiar shapes, the list of round things is practically endless and ranges from simple soap bubbles to the entire observable universe. Of all shapes, spheres are favored by the action of simple physical laws. So prevalent is this tendency that often

Using freshman-level calculus you can show that the one and only shape that has the smallest surface area for an enclosed volume is a perfect sphere. In fact, billions of dollars could be saved annually on packaging materials if all shipping boxes and all packages of food in the supermarket were spheres.

For large cosmic objects, energy and gravity conspire to turn objects into spheres. Gravity is the force that serves to collapse matter in all directions, but gravity does not always win—chemical bonds of solid objects are strong.

Earth’s mountains are also puny when compared with some other mountains in the solar system. The largest on Mars, Olympus Mons, is 65,000 feet tall and nearly 300 miles wide at its base. It makes the highest of Earth's mountains look like molehills. The cosmic mountain-building recipe is simple: the weaker the gravity on the surface of an object, the higher its mountains can reach. Mount Everest is about as tall as a mountain on Earth can grow before the lower rock layers succumb to their own plasticity under the mountain’s weight.

The stars of the Milky Way galaxy trace a big, flat circle. With a diameter-to-thickness ratio of one hundred to one, our galaxy is flatter than the flattest flapjacks ever made. In fact, its proportions are better represented by a crépe or a tortilla. No, the Milky Way’s disk is not a sphere, but it probably began as one.

This general flattening of objects that rotate is why Earth’s pole-to-pole diameter is smaller than its diameter at the equator. Not by much: three-tenths of one percent—about twenty-six miles. But Earth is small, mostly solid, and doesn’t rotate all that fast. At twenty-four hours per day, Earth carries anything on its equator at a mere 1,000 miles per hour.

To reach this density, you must compress all the empty space that atoms enjoy around their nucleus and among their orbiting electrons. Doing so will crush nearly all (negatively charged) electrons into (positively charged) protons, creating a ball of (neutrally charged) neutrons with a crazy-high surface gravity.

The sphere to end all spheres—the largest and most perfect of them all—is the entire observable universe. In every direction we look, galaxies recede from us at speeds proportional to their distance. As we saw in the first few chapters, this is the famous signature of an expanding universe,