Astrophysics for People in a Hurry by Neil de Grasse Tyson

  1. The Greatest Story Ever Told

In the beginning, nearly fourteen billion years ago, all the space and all the matter and all the energy of the known universe was contained in a volume less than one-trillionth the size of the period that ends this sentence.

Einstein’s general theory of relativity, put forth in 1916, gives us our modern understanding of gravity.

In the 1920s, quantum mechanics would be discovered.

But these two understandings of nature are formally incompatible with one another.

The four distinct forces we have come to know and love, with the weak force controlling radioactive decay, the strong force binding the atomic nucleus, the electromagnetic force binding molecules, and gravity binding bulk matter.

The universe was a seething soup of quarks, leptons, and their antimatter siblings, along with bosons.

For another 380,000 years not much will happen to our particle soup. Throughout these millennia the temperature remains hot enough for electrons to roam free among the photons.

For the first billion years, the universe continued to expand and cool as matter gravitated into the massive concentrations we call galaxies.

After nine billion years of such enrichment, in an undistinguished part of the universe ( the outskirts of the Virgo Supercluster ) in an undistinguished galaxy ( the Milky Way ) in an undistinguished region ( the Orion Arm ), an undistinguished star ( the Sun ) was born.

As less and less accretable matter remained in the solar system, planet surfaces began to cool. The one we call Earth formed in a kind of Goldilocks zone around the Sun, where oceans remain largely in liquid form.

Within the chemically rich liquid oceans, by a mechanism yet to be discovered, organic molecules transitioned to self-replicating life.

We owe the remarkable diversity of life on Earth, and we presume elsewhere in the universe, to the cosmic abundance of carbon.

But what if the universe was always there, in a state or condition we have yet to identify — a multiverse, for instance, that continually births universes? Or what if the universe just popped into existence from nothing?

We are stardust brought to life, then empowered by the universe to figure itself out — and we have only just begun.

  1. On Earth as in the Heavens

Newton had figured out that the force of gravity pulling ripe apples from their orchards also guides tossed objects along their curved trajectories and directs the Moon in its orbit around Earth.

This universality of physical laws tells us that if we land on another planet with a thriving alien civilization, they will be running on the same laws that we have discovered and tested here on Earth.

Among all constants, the speed of light is the most famous. No matter how fast you go, you will never overtake a beam of light.

Another class of universal truths is the conservation laws, where the amount of some measured quantity remains unchanged no matter what.

It happens that we cannot see, touch, or taste the source of eighty-five percent of the gravity we measure in the universe. This mysterious dark matter.

In other words, after the laws of physics, everything else is opinion.

  1. Let There Be Light

In this early epoch, photons didn’t travel far before encountering an electron.

Cosmic microwave background or CMB for short.

The first direct observation of the cosmic microwave background was made inadvertently in 1964 by American physicists Arno Penzias and Robert Wilson of Bell Telephone Laboratories.

Because light takes time to reach us from distant places in the universe, if we look out in deep space we actually see eons back in time.

Ordinary matter is what we are all made of. It has gravity and interacts with light. Dark matter is a mysterious substance that has gravity but does not interact with light in any known way.

Dark energy is a mysterious pressure in the vacuum of space that acts in the opposite direction of gravity, forcing the universe to expand faster than it otherwise would.

  1. Between the Galaxies

The universe may nonetheless contain hard-to-detect things between the galaxies.

In any reliably surveyed volume of space, dwarf galaxies outnumber large galaxies by more than ten to one.

Supernovas are stars that have blown themselves to smithereens.

Worse yet, clusters are overrun by dark matter, which happens to contain up to another factor of ten times the mass of everything else.

Quasars are super-luminous galaxy cores whose light has typically been traveling for billions of years across space before reaching our telescopes. As extremely distant sources of light, they make ideal guinea pigs for the detection of intervening junk.

We call them cosmic rays. The highest-energy particles among them have a hundred million times the energy that can be generated in the world’s largest particle accelerators.

  1. Dark Matter

Gravity, the most familiar of nature’s forces, offers us simultaneously the best and the least understood phenomena in nature.

Einstein demonstrated that Newton’s theory requires some modification to describe gravity accurately — to predict, for example, how much light rays will bend when they pass by a massive object.

We don’t know who’s next in the genius sequence, but we’ve now been waiting nearly a century for somebody to tell us 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.

Today, we’ve settled on the moniker “dark matter,” which makes no assertion that anything is missing, yet nonetheless implies that some new kind of matter must exist, waiting to be discovered.

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.

Dark matter exerts gravity according to the same rules that ordinary matter follows, but it does little else that might allow us to detect it.

More likely, dark matter consists of matter whose nature we have yet to divine.

Particle physicists are confident that dark matter consists of a ghostly class of undiscovered particles that interact with matter via gravity, but otherwise interact with matter or light only weakly or not at all.

For now, we must remain content to carry dark matter along as a strange, invisible friend, invoking it where and when the universe requires it of us .

  1. Dark Energy

As if you didn’t have enough to worry about, 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.

One of the most powerful and far-reaching theoretical models ever devised, already introduced in these pages, is Einstein’s general theory of relativity — but you can call it GR after you get to know it better. Published in 1916, GR outlines the relevant mathematical details of how everything in the universe moves under the influence of gravity.

Einstein’s concept as, “Matter tells space how to curve; space tells matter how to move.”

Einstein discarded lambda entirely, calling it his life’s “greatest blunder.”

Sixty-nine years later, in 1998, science exhumed lambda one last time.

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.

The closest anybody has come is to presume dark energy is a quantum effect.

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. Whatever dark energy turns out to be, we already know how to measure it.

Einstein’s greatest blunder was having declared that lambda was his greatest blunder.

  1. The Cosmos on the Table

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.

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.

Although a distant second to hydrogen in abundance, there’s fifty times more of it than all other elements.

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

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

Aluminum occupies nearly ten percent of Earth’s crust.

Titanium, the ninth most abundant element in Earth’s crust, has become a modern darling for many applications.

In most cosmic places, the number of oxygen atoms exceeds that of carbon.

By many measures, iron ranks as the most important element in the universe.

Unstable weapons-grade plutonium was the active ingredient in the atomic bomb that the United States exploded over the Japanese city of Nagasaki.

  1. On Being Round

Of all shapes, spheres are favored by the action of simple physical laws.

For large cosmic objects, energy and gravity conspire to turn objects into spheres.

So, contrary to what it looks like to teeny humans crawling on its surface, Earth, as a cosmic object, is remarkably smooth. If you had a super-duper, jumbo-gigantic finger, and you dragged it across Earth’s surface (oceans and all), Earth would feel as smooth as a cue ball.

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.

For rich clusters of galaxies, the overall shape can offer deep astrophysical insight. Some are raggedy. Others are stretched thin in filaments. Yet others form vast sheets. None of these have settled into a stable—spherical—gravitational shape.

The sphere to end all spheres — the largest and most perfect of them all — is the entire observable universe.

The universe beyond this spherical “edge” is thus rendered invisible and, as far as we know, unknowable.

There’s a variation of the ever-popular multiverse idea in which the multiple universes that comprise it are not separate universes entirely, but isolated, non-interacting pockets of space within one continuous fabric of space-time.

  1. Invisible Light

Filling out the entire electromagnetic spectrum, in order of low-energy and low-frequency to high-energy and high-frequency, we have : radio waves, microwaves, infrared, ROYGBIV, ultraviolet, X-rays, and gamma rays .

Radio telescopes, the earliest non-visible-light telescopes ever built, are an amazing subspecies of observatory .

The world’s largest radio telescope, completed in 2016, is called the Five-hundred-meter Aperture Spherical radio Telescope, or “ FAST ” for short. It was built by China in their Guizhou Province, and is larger in area than thirty football fields .

Another variety of radio telescope is the interferometer, comprising arrays of identical dish antennas, spread across swaths of countryside and electronically linked to work in concert .

Intermittent, distant, titanic stellar explosions across the universe, signaling the birth of gamma ray astrophysics.

Today, telescopes operate in every invisible part of the spectrum, some from the ground but most from space.

  1. Between the Planets

Hundreds of tons of meteors per day — most of them no larger than a grain of sand. Nearly all of them burn in Earth’s upper atmosphere.

Nearby interplanetary space also contains rocks of all sizes that were jettisoned from Mars, the Moon, and Earth by the ground’s recoil from high-speed impacts.

Thousand tons of Martian rocks rain down on Earth each year.

Most of the solar system’s asteroids live and work in the main asteroid belt, a roughly flat zone between the orbits of Mars and Jupiter.

Asteroids are not the only space objects that pose a risk to life on Earth. The Kuiper belt is a comet-strewn swath of circular real estate that begins just beyond the orbit of Neptune.

Far beyond the Kuiper belt, extending halfway to the nearest stars, lives a spherical reservoir of comets called the Oort cloud.

If we had eyes that could see magnetic fields, Jupiter would look five times larger than the full Moon in the sky.

Earth’s Moon is about 1 / 400th the diameter of the Sun, but it is also 1 / 400th as far from us, making the Sun and the Moon the same size in the sky — a coincidence not shared by any other planet – moon combination in the solar system.

Jupiter’s system of moons is replete with oddballs.

Pluto’s largest moon, Charon, is so big and close to Pluto that Pluto and Charon have each tidally locked the other.

The Sun loses material from its surface at a rate of more than a million tons per second. We call this the “ solar wind, ” which takes the form of high-energy charged particles.

Earth’s atmosphere is commonly described as extending dozens of miles above Earth’s surface.

Orbiting high above this level, twenty-three thousand miles up ( one-tenth of the distance to the Moon ) are the communications satellites.

  1. Exoplanet Earth

A celebrated photograph taken in 1990 from just beyond Neptune’s orbit by the Voyager 1 spacecraft shows just how underwhelming Earth looks from deep space : a “ pale blue dot, ” as the American astrophysicist Carl Sagan called it.

Water covers more than two-thirds of Earth’s surface ; the Pacific Ocean alone spans nearly an entire side of the planet.

Earth’s distinctive polar ice caps, which grow and shrink from the seasonal temperature variations, could also be seen using visible light.

The nearest exoplanet — the nearest planet in orbit around a star that is not the Sun — can be found in our neighbor star system Alpha Centauri, about four light-years from us and visible mostly from our southern hemisphere.

NASA’s Kepler telescope, designed and tuned to discover Earth-like planets around Sun-like stars, invoked.

Yet another method of detection, mightily adding to the exoplanet catalog.

So if those alien eavesdroppers turn their own version of a radio telescope in our direction, they might infer that our planet hosts technology.

Bell and her associates realized they’d discovered a new class of cosmic object — a star made entirely of neutrons that pulses with radio waves for every rotation it executes. Hewish and Bell sensibly named them “pulsars.”

There’s also cosmochemistry. The chemical analysis of planetary atmospheres has become a lively field of modern astrophysics.

If the aliens track our nighttime side while we orbit our host star, they might notice a surge of sodium from the widespread use of sodium-vapor streetlights that switch on at dusk in urban and suburban municipalities.

If the aliens decide that Earth’s chemical features are sure evidence of life, maybe they’ll wonder if the life is intelligent.

Looking more closely at Earth’s atmospheric fingerprints, human biomarkers will also include sulfuric, carbonic, and nitric acids, and other components of smog from the burning of fossil fuels.

The first exoplanet 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.

As many as forty billion Earth-like planets in the Milky Way alone.

  1. Reflections on the Cosmic Perspective

However big the world is — in our hearts, our minds, and our outsized digital maps — the universe is even bigger.

More bacteria live and work in one centimeter of my colon than the number of people who have ever existed in the world.

I began to think of people not as the masters of space and time but as participants in a great cosmic chain of being, with a direct genetic link across species both living and extinct, extending back nearly four billion years to the earliest single-celled organisms on Earth.

Imagine a life-form whose brainpower is to ours as ours is to a chimpanzee’s. To such a species, our highest mental achievements would be trivial.

Some of the water you just drank passed through the kidneys of Socrates, Genghis Khan, and Joan of Arc.

Some of the air you just breathed passed through the lungs of Napoleon, Beethoven, Lincoln, and Billy the Kid.

There are more stars in the universe than grains of sand on any beach, more stars than seconds have passed since Earth formed, more stars than words and sounds ever uttered by all the humans who ever lived.

The four most common, chemically active elements in the universe — hydrogen, oxygen, carbon, and nitrogen — are the four most common elements of life on Earth.

It’s conceivable that life began on Mars and later seeded life on Earth, a process known as panspermia.

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