If you wish to make an apple pie from scratch, you must first invent the universe.

—Carl Sagan

The science of studying the whole universe is called cosmology. Cosmologists are big-picture scientists. And, as you might have guessed, the universe is pretty big. It also comes with 13.8 billion years of history. This chapter tackles a few of the big questions in cosmology, including the universe's size and how it began.

HOW BIG IS THE UNIVERSE?

The answer: pretty big.

Okay, to start things off, start thinking big…really big, because the observable universe is about ninety-three billion light-years in diameter. A light-year is about six trillion miles. I will let you in on something extra: tomorrow the universe will be bigger.

How much bigger?

The universe is growing at approximately seventy kilometers per second per megaparsec.1 This complicated-sounding rate is known as Hubble's constant. Allow me to attempt to untangle it for you.

Astronomers use parsecs to express stellar distances. A parsec represents 3.26 light-years. A megaparsec is a million parsecs (3,262,000 light-years). By the way, parsec is shorthand for parallax of one arcsecond. If you are planning on writing your own space adventure, I suggest sticking to using parsec.

So if you peer out at the night sky using the powerful telescope NASA loaned you, at a distance of about 3.3 million light-years out into space, the galactic objects will appear to be receding from Earth at about seventy-one kilometers per second.

The deeper into space you set your telescope, the faster the expansion will appear. Look out far enough, and objects will vanish. The reason that distant areas of space appear to (and actually do) move faster than the speed of light (and vanish) relative to our galactic location is due to the geometric nature of the expansion.

The next time you watch Star Wars: A New Hope and Han Solo says, “You've never heard of the Millennium Falcon? It's the ship that made the Kessel run in less than twelve parsecs,” remember that a parsec is a unit of distance, not one of time.2 The only science fiction solution that might make this statement scientifically consistent is if Han meant that he was able to find the shortest route in and out of hyperspace.

Take a moment to appreciate how big the universe is and how fast it is expanding. Traveling at the speed of light, a photon (quantized light) leaving our sun takes a bit more than eight minutes to get here. To reach Pluto, the little guy needs five and a half hours. If the photon is interested in taking a road trip, the journey to the next closest star (Proxima Centuri) is roughly 4.2 light-years.

So, there is a lot of space out there in space. If we could travel at the speed of light (we can't), we would need over four years to get to our next stellar neighbor, Proxima Centuri. So, gallivanting around the Milky Way is time prohibitive. Given current technology, for the foreseeable future, humans will not make it far from Earth.

The news is not all bad. The science (if not the politics or the funding) exists now to create space stations or planetary bases in our solar system. If humanity is more ambitious, a lot of good ideas can be found for terraforming a few of the moons and planets in our solar system. This topic is covered in more detail in chapter 12.

If you imagine escaping the solar system, a sprinkle of science fiction dust might come to your rescue. In the Star Wars universe, ships travel in hyperspace lanes. These are wrinkles in spacetime that allow ships to jump from one point to another without traveling directly toward their destination.

In the Star Trek universe, warp drive technology, powered matter/antimatter annihilation mediated by dilithium crystals, is used. (See the first interlude for a definition of antimatter.) The best technobabble description of the warp engine is that it is a gravimetric field displacement engine powered by matter/antimatter reaction. This is supposed to mean that warp fields are generated around a starship to form a subspace bubble. The bubble distorts local spacetime, allowing the starship to slide through the distortion at velocities (warp factors) exceeding the speed of light. A more scientific description of a warp drive is found in chapter 17.

The speed of a warp factor is never clearly defined in any permutation of Star Trek. The different warp speeds do not make scientific sense to me, so I won't try to explain the exponential nature of the factors.

The best part: no time dilation when zipping around in either the Star Wars or Star Trek universes.

HOW IS THE OBSERVABLE UNIVERSE DIFFERENT FROM THE ACTUAL UNIVERSE?

When describing the universe's size, I deliberately used the term observable. I did this because the universe we live in is different from the universe we can see.

This is a big deal in cosmology. To astronomers, observable refers to the ability to see the light emanating from distant regions of the universe. However, some regions of the universe are so distant that the light from their stars has not had time to reach us yet. When it finally does, I suspect we will be long gone, but our descendants might get an eyeful.

Then there are other regions that are expanding away from us faster than the speed of light (according to general relativity, anything containing mass might not exceed the galactic speed limit while traveling through space, but this does not apply to space itself), so their light will never reach our pale blue dot. As the universe expands, the horizon will get smaller. The universe we can interact with will also get smaller.

The shrinking horizon isn't the only consequence of the galactic speed limit. It also means that nothing we see is current. The greater the distance, the greater the time differential. Time differentials are surprisingly commonplace and accepted in our perception of reality. I will not know what is going on in the world at the moment you read this sentence. Your now is my future.

When you look out at the stars, you see yesterday. Actually, what you see is yesterday's great-great-great-grandmother. The Milky Way is about 100,000 light-years wide, and our sun is pretty far out from the galactic center, so any light emitted from a star at the opposite end of the Milky Way would need 100,000 years to travel here. If you see it, then it is old light, a long-gone yesterday. Here's the kick in the pants: as far as the universe is concerned, this is more or less a recent snapshot. Any light you see from Andromeda, the next closest spiral galaxy, is about three million years old.3

Yes, it is sad that there are parts of the universe we'll never know about, but this isn't necessarily bad news for science fiction.

HOW ARE GALACTIC DISTANCES CALCULATED?

So, how do we know the distance between stars or between galaxies? Astronomers use cosmic yardsticks, also called standard candles, to measure distances. They also use a lot of geometry. For galaxies that are very far away, the yardstick is a supernova. Supernovas are ridiculously bright stellar explosions. Their observed brightness and a measurement of red-shifting due to expansion can be used to determine distance. More on red-shifting in a moment.

Closer to home, Cepheid stars are used to gauge distances. These are very luminous stars that pulsate (changing diameter, temperature, and brightness) in a predictable pattern. Henrietta Leavitt discovered the period-luminosity relationship in 1912.⁴ Astronomers are able to measure distance because of the relationship between the brightness (seen by a telescope) and the pulsation period. Bonus 4 of this chapter provides a more detailed description of how Cepheid stars are used to measure distance.

AND IN THE (OR RATHER A) BEGINNING…

And now for an origin story…our origin story. I'm talking about the beginning of the observable universe, the big bang. The name is a misnomer because the event was neither big nor bangy.

The name started out as an insult by English astronomer Fred Hoyle made on BBC radio. He believed there was no such thing as a spectacular beginning. Instead, he believed in the competing theory called steady state. In that theory, the universe doesn't change over time, but the stuff within it, like galaxies, can move around. He was wrong, but he still gets credited for coming up with the name.5

So, what exactly is the big bang? It was an amazing event where all the matter that ever was and ever will be arose from a very (very) small point called a singularity. Recall from chapter 1 that a singularity is an infinitely small and dense point in spacetime. And 13.8 billion years ago, everything we observe spewed from one. Oh, and the big bang also created time.

You want evidence for a big bang? Good! You are thinking like a scientist. Below are three lines of evidence. There are more, but this is a good start.

1.Observation

Through observation, Edwin Hubble discovered evidence for an expanding universe from the red-shifting of galaxies. When light from an object moving away from an observer shifts to the red end of the electromagnetic spectrum with respect to the observer, redshift has occurred. The color of the visible portion of the electromagnetic spectrum, in order of least to most energetic (frequency), is red, orange, yellow, green, blue, indigo, and violet. These are the colors of a rainbow.

That shift of light to the red end of the spectrum is similar to the Doppler Effect. The sound of an object changes with its movement relative to an observer. As an ambulance rushes toward you, for example, the sound's frequency increases and pitch rises. After it has dashed past you, the frequency decreases and the pitch becomes lower.

By studying the redshift of moving galaxies, Hubble proved they were moving away from the Milky Way.6 He also showed that the farther away the galaxy, the faster it appeared to be traveling and the younger it appeared. The really distant galaxies appear to have been formed recently or as glowing gas that has yet to develop into stars. This evidence, plus general relativity, allowed cosmologists to rewind cosmic history. They showed that the farther back in time you go, the smaller the universe was.

Bottom line, astronomers are able to see the observable universe spreading out. It should not be difficult to imagine that a year ago, it was a little smaller than it is today. If we wind the clock backward 13.8 billion years, the universe must have been only a primordial point in space.

2.Cosmic microwave background

The cosmic microwave background (CMB) is a baby picture of the universe. It is what the universe looked like when it was only 380,000 years old, and it represents the furthest back in time that we can peer.⁷ The CMB is a thermal energy leftover of the big bang, a remnant, an afterglow.

What is so marvelous about the CMB is that its existence is predicted from the big bang theory and (wait for it) CMB is detectable. From every direction, when we look out at the galaxy, we see evidence of the universe's earliest light. Also, the CMB appears the same in all directions, meaning there is no up for the universe. It is isotropic.

Fig. 4.1. Illustration of cosmic microwaves. (NASA/JPL-Caltech.)

3. The elements (big bang nucleosynthesis)

Our universe contains a lot of elements. For example, there is the carbon-based you reading a carbon-based book (or the composite material of an electronic device). These elements did not exist in the early period of the universe. When it all began, there was nothing more than hydrogen nuclei compressed into a tiny volume. About ten seconds to twenty minutes after the big event, the universe's hydrogen nuclei began fusing into helium.⁸ Over hundreds of millions of years and a lot more nuclear fusion, the heavier elements were created.

Something to ponder: the universe began mostly as hydrogen that, after quite a lot of time (and by fusion and evolution), became people to think about what hydrogen actually is.

Based on the elements of the early universe, the big bang standard theory successfully predicts how much of the elements we should (and do) observe today. For example, our sun contains hydrogen and some helium, and yet we have heavier elements here on Earth. These came from previous generations of stars.

WHAT CAUSED THE BIG BANG?

Fair question, but I don't have a fair answer because the question itself might be meaningless. Consider how cause comes before effect. In the standard theory, there was no before (i.e., time) before the big bang. The question of time before the big bang is like asking what is north of the North Pole. As you read in chapter 2 (on quantum mechanics), some things happen randomly for no particular reason; they have no particular cause. This is a capricious property of the universe. Yes, there might have been some cause, but our scientific understanding of the universe doesn't require one.

A good book that blends fiction and nonfiction on this subject is George and the Big Bang.⁹ It is the third book in a young adult science adventure series written by the daughter-father team of Lucy and Stephen Hawking. In the story, there is a scheme to destroy the large hadron collider (LHC) before it can conduct an experiment to recreate the initial conditions of the big bang. Yes, mayhem and science ensue. The book also includes essays by Professor Hawking and other scientists about the origins of the universe.

A more speculative origin story proposed that the big bang occurred, but it wasn't the beginning of everything. I am talking about brane theory, one of the string theories described in chapter 3. It hypothesized that our three-dimensional universe is a stretched mem(brane) floating through a higher dimensional background called the bulk.¹⁰

Now imagine that our brane is not the only one floating through the bulk. Occasionally two branes collide, like two hands clapped together, releasing a lot of kinetic energy. To viewers inside the clapping branes, it would look like a big explosion, a big bang. The kinetic energy would create matter (you do remember the energy-matter equivalency from chapter 1, right?); matter would give rise to a universe, a universe might give rise to life, and life-forms could include creatures such as us science fiction geeks.

Fig. 4.2. Illustration of two branes colliding.

WHAT HAPPENED NEXT?

Let me guess: you've heard enough about the big bang. You want to know what happened next. I'm here to serve.

During the first few trillionths of a second after the big bang, the universe was so hot and compressed that spacetime boiled with energy. After that brief hotfoot, the universe made a phase transition from that high energy state to a lower one. A lot of energy fell out of the vacuum, accelerating the already expanding universe. This brief period of cosmic inflation was first proposed by Alan Guth in 1981.11

After 0.0001 of a second after the big bang, quarks got together to form protons and neutrons. It took between ten seconds and twenty minutes before the first atomic nuclei made an appearance and 300,000 years before electrons are captured into orbits of the atomic nuclei and the first complete atoms arise.¹²

Based on microwave background radiation emitted about 380,000 years after the big bang, cosmologists are able to piece together what happened next. And it is a tale of the Dark Ages. As the universe continued its expansion, it grew cold and dark. In the otherwise uniform distribution of the CMB is evidence of pockets of small-scale clumps.

This clumping would become the first small protogalaxies, masses of gas that form galaxies. These early protogalaxies are different from the ones astronomers see today. Back then, they were mostly only hydrogen and helium. Today they include heavier elements that are created from stars.

The first clumps capable of forming stars would arise between 100 million to 250 million years after the big bang.¹³ The scaffolding on which these first galaxies would be built was dark matter. Let's face it: there wasn't enough ordinary matter in the universe to create the gravity needed for these gas clouds to form suns. In the Dark Age, dark matter and ordinary matter hung out together in the protogalaxies. Over time they must have had a feud because today, ordinary matter hangs out with us in the galaxy's inner region while dark matter hangs out in the galaxy's outer halo.

Let's go back to when they did get along.

The gas clouds began to compress and rotate under the weight of gravity from the dark and ordinary matter. The pressure caused heat to build up, and hydrogen atoms combined into hydrogen molecules. These molecules cooled the densest parts of the gas. As the rotating gas flattened and cooled, ordinary matter separated from the dark matter. The densest clumps of gas continued contracting until some of these clumps collapsed into stars. Yes! Let there be light. The end of the Dark Age.

Fear not, this isn't the end of the story for dark matter. The third interlude will try to light it up for you.

After the universal Dark Age, first-generation stars burned very hot because they lacked the heavier elements. It is more difficult to produce nuclear energy without the heavier elements, so a first-generation star had to be hotter to produce enough energy to counteract gravity.

These stars were a wild bunch. They burned bright and lived short lives that lasted only a few million years. By comparison, our sun is middle-aged at 4.5 billion years old. Some of the early stars eventually exploded and became supernovae that fused the heavier elements into existence. This is the birth of metal.

WHAT ARE GRAVITATIONAL WAVES?

This was already answered in chapter 1, but it is very much worth mentioning here because gravitational waves might tell us more about our history.

The math of general relativity shows that gravitational waves are disruptions in spacetime that cause ripples where time speeds up, slows down, and speeds up again. They might be caused by violent high-energy processes such as the big bang, or by massive objects like black holes or neutron stars moving through spacetime.

They carry information about their cause at the speed of light. These types of waves are believed to be unaltered as they ripple through space, meaning they don't fade away like a wave on a lake. This “unalterable” property might help scientists learn what happened before the earliest light (the cosmic microwave background).

As I've said before and will say again, science loves evidence. The theory of cosmic inflation suggests that there was a rapid expansion of spacetime after the first fraction of a second after the big bang. This sudden “push” might have produced detectable ripples that prove the theory. This wave may carry with it new information about this earliest moment.

HAVE GRAVITATIONAL WAVES EVER BEEN DETECTED?

Yes! Colliding black holes 1.3 billion light-years away gave researchers the perfect chance to confirm the existence of gravitational waves. The colliding black holes created a violent storm in the fabric of space and time. It swept across space until it reached the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in 2015.14 After confirming the results, the LIGO team formally announced the spacetime ripples in 2016—around the one hundredth anniversary of Einstein's theory of general relativity, which first predicted their existence.

The LIGO study used laser beams projected along pipes; the beams were reflected in mirrors at each end. Researchers monitored the distance between the mirrors for fluctuations as space expanded and contracted. The amplitude of the gravitational wave was smaller than the size of an atom. Nonetheless, the experiment further confirmed general relativity.

SOME GENERAL FACTS ABOUT OUR UNIVERSE (AS WE KNOW IT)

• 13.82 billion years ago (BYA), the big bang went bang.
• 300,000 years after the big bang, the first complete (with electrons and protons) atoms form.
• 380,000 years after the big bang the cosmic microwave background radiation is emitted.
• Two hundred million years after the big bang, the first stars ignited and there was light.
• The universe is ninety-three billion light-years wide and growing.
• A light-year is about six trillion miles.
• At the latest census, at least two trillion galaxies existed in the universe.15
• Our galaxy is called the Milky Way. It is 100,000 light-years wide and contains at least two hundred billion stars. It is about 13.6 billion years old.
• The Milky Way contains between seven hundred billion and one trillion solar masses. This includes all the stars, planets, moons, and just about anything that has mass. A solar mass is equal to the mass of our sun. The sun's mass is about two nonillion kilograms (that is two followed by thirty zeros).¹⁶

  To visualize a nonillion, imagine that you've counted every grain of sand on all the beaches and in all the deserts of Earth. You'd come to a nice round number of about seven quintillion, five hundred quadrillion. (You would have to set aside a very long day from your schedule for this task.) If you then added that number to the estimated number of stars in the Milky Way (two hundred billion), your count would still be less than half the value of a nonillion.¹⁷

• Up to about one hundred years ago, it was believed that the Milky Way was the entire universe. It is not.
• Our sun is about twenty-six thousand light-years from the Milky Way's center.
• The Milky Way moves through space at a velocity of about 550 kilometers per second. It rotates around its center at a rate of about 220 kilometers per second.¹⁸
• The Milky Way is a spiral galaxy dragging four major arms as it spins. Older stars tend to occupy the center hub while the newbies take up residency in the spiral arms. Earth is in the Orion Arm, which possibly is a bridge between the Sagittarius and Perseus arms.
• At the center of the Milky Way is a black hole with the mass of around 2.6 million suns.¹⁹ This supermassive black hole is named Sagittarius A*.
• The Milky Way is part of a local cluster group of fifty-four galaxies that are held together by mutual gravitational attraction. This Local Group is about one hundred million light-years across.
• Eventually the Milky Way will collide with its neighbor, the Andromeda galaxy. Don't worry. This won't be a problem for about four billion years.

OUR SOLAR SYSTEM, HOME OF SOL, OUR SUN

• Our solar system has eight planets and ten confirmed dwarf planets (the actual number of dwarf planets in our solar system is probably closer to four hundred). A dwarf planet is similar to a planet, but it is not large enough to gravitationally clear its orbit around the sun. Pluto is a dwarf planet because, with its partner Charon, it is part of a cohabiting binary system. They are tidally locked, meaning they always present the same face to each other as they circle each other. This union orbits the sun.

  To date, the farthest dwarf planet found on the outskirts of the solar system is called Sedna. The second-most distant dwarf has the cute name Deedee (for distant dwarf). The closest is Ceres, where evidence of water was recently discovered.20

• The sun makes up 99.9 percent of the mass of the solar system. It can hold around one million Earths.²¹ Someday the sun will shrink to the size of the earth. After this slimming down (while retaining mass), our sun will be a white dwarf. This won't be for a while, because the sun is currently only middle-aged (at 4.5 billion years). Its core is about fifteen million degrees Celsius.
• It takes eight minutes and twenty seconds for the light of the sun to reach the earth. It takes 1.3 seconds for light from the moon to reach the earth.²²
• An astronomical unit (AU) is defined as the average distance between the center of the sun and the center of the earth. This is about 150 million kilometers.
• Traveling outward within our solar system from the sun, we first pass the four smaller rocky inner planets (Mercury, Venus, Earth, and Mars). Next, we travel through an asteroid belt that includes a dwarf planet rock named Ceres. After exiting the belt, we come to the four gas giants (Jupiter, Saturn, Uranus, and Neptune).
• Jupiter's gravity helps protect Earth from meteors. Think of it as a giant vacuum sucking up space debris. But I'm sure the dinosaurs will tell you that Jupiter doesn't get them all.
• The Kuiper (rhymes with diaper) Belt begins just past Neptune. It is disk-shaped and circles the sun at thirty AU out to about fifty-five AU.23 It is an area of icy objects and short-period comets. Short period is defined as orbiting the sun in less than two hundred years. Pluto is part of the Kuiper Belt. For the record, Pluto is not a comet.
• Speaking of Pluto, did you know it has liquid water? About one hundred miles beneath its surface there is (probably) a slushy ocean that might be sixty miles deep. The dwarf planet has retained enough radioactive heat from its formation to keep the water in a semi-liquid state.²⁴
• The Oort cloud is a shell of about two trillion icy objects in the outermost region of the solar system. Its innermost edge may be as close as two thousand AU from the sun and the outer edge as far as 200,000 AU.²⁵ Long-period comets might have their origins here. Long period is defined as having orbital periods longer than two hundred years.

PARTING COMMENTS

The universe has a history, and its structure is explained by the theories of relativity and quantum mechanics. The big bang theory is the leading theory of its origin, but it doesn't tell what came before or why it even had to begin. Quantum theory or string theory, on the other hand, might have something to say on this subject. The universe might come from quantum entanglement of quantized spacetime, or it might have been the result of branes clapping together in a higher dimension, or both, or neither. Cosmology is exciting that way.

CHAPTER 4 BONUS MATERIALS

BONUS 1: OLBERS’S PARADOX

Named after Heinrich Wilhelm Olbers, this paradox asks this question: if there are billions and billions of stars, then why isn't the night sky completely lit up? If the universe is infinite and eternal, the night sky should be uniformly bright.

An answer consistent with a big bang event is that the light from the more distant stars has not reached us yet. The ones we see are close enough that their light has taken less than 13.8 billion years to reach us.

BONUS 2: YOUR SUNTAN

When you are on a beach working on your suntan, about 0.001 percent comes from photons originating from the big bang. Another 0.000000001 percent comes from stars not in our solar system. A full 77 percent of your skin darkens due to direct sunlight, and the remainder is triggered by light reflected off the sky.26 This is known as the greenhouse effect, a huge topic of chapter 11.

BONUS 3: THE BRIEF HISTORY OF OUR SUN

In the beginning, roughly 4.5 billion years ago, a cloud of interstellar hydrogen gas filled the universe. Hydrogen is the most abundant element in the known universe. As the cloud cooled (thanks to obeying the laws of thermodynamics, described in detail in chapter 21), it rotated and contracted due to gravity. As the cloud continued to condense, more and more pressure was applied to its hydrogen core.

The pressure caused its temperature to rise. The hydrogen nuclei were pushed together with such tremendous force that they fused, creating helium nuclei. This nuclear fusion produced a force that prevented the sun from collapsing further, and—presto!—there was light.

This is about where we are now in the sun's history. When the sun runs out of hydrogen to fuse, the outward energy will eventually fail. When this happens, our star will collapse once again, applying tremendous pressure to its helium core. This pressure will cause the helium nuclei to fuse into heavier elements until carbon is achieved. But all of that is in the future, the topic of chapter 21.

BONUS 4: THE ON AND OFF LIGHTING SYSTEM OF A CEPHEID STAR

Here is how astrophysicists use Cepheid stars to calculate distances. A Cepheid politely increases its temperature, size, and brightness at regular intervals that scientists can observe. Even more accommodating, the amount of brightness is proportional to its period. So if an astrophysicist knows how often it pulsates then, with a bit of math, she knows how bright it is. Now all she has to do is aim her telescope at the Cepheid and compare her results with its observed visual brightness. By comparing the two, she will discover how far away it is.

We can go deeper and ask why a Cepheid pulsates. It is caught in a feedback loop called the Eddington valve.

1. As the star compresses, it heats up. The helium in its outer layer becomes ionized. This is fancy talk that means the helium loses its electrons.
2. The star becomes more opaque, which dims the star. Temperature begins to increase, and the star becomes unstable.
3. The outer layer pushes out against the compression, and the star expands.
4. As the star expands, the helium becomes less ionized.
5. The less ionized the star is, the more transparent it becomes.
6. The brighter the star is, the cooler it becomes.
7. As it expands, gravity kicks in, forcing the star to contract again.