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 gravitat
ional 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 t
here 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.
Blockbuster Science Page 7