Beyond the Galaxy

How Humanity Looked Beyond Our Milky Way and Discovered the Entire Universe

Ethan Siegel

• Publisher: WSPC
• Year: 2015
• eBook ISBN: 9789814667180
• Topic: Economics
• Subtopic: Development Economics
• Index: Economics

A look up at the night sky reveals a treasury of wonders. Even to the naked eye, the Moon, stars, planets, the Milky Way and even a few star clusters and nebulae illuminate the heavens. For millennia, humans struggled to make sense of what's out there in the Universe, from all we can see to that which lies beyond the limits of even our most powerful telescopes. Beyond the Galaxy traces our journey from an ancient, Earth-centered Universe all the way to our modern, 21st century understanding of the cosmos. Touching on not only what we know but also how we know it, Ethan Siegel takes us to the very frontiers of modern astrophysics and cosmology, from the birth of our Universe to its ultimate fate, and everything in between.

A look up at the night sky reveals a treasury of wonders. Even to the naked eye, the Moon, stars, planets, the Milky Way and even a few star clusters and nebulae illuminate the heavens. For millennia, humans struggled to make sense of what's out there in the Universe, from all we can see to that which lies beyond the limits of even our most powerful telescopes. Beyond the Galaxy traces our journey from an ancient, Earth-centered Universe all the way to our modern, 21st century understanding of the cosmos. Touching on not only what we know but also how we know it, Ethan Siegel takes us to the very frontiers of modern astrophysics and cosmology, from the birth of our Universe to its ultimate fate, and everything in between.

Contents:

• So Far, So Good: The Universe at the Start of the 20th Century
• A Relatively Different Story: How Einstein's Relativity Revolutionized Space, Time, and the Universe
• Beyond the Milky Way: A Giant Leap into an Expanding Universe
• The Great Leap Backwards: Theories on Where It All Came From
• An Element-ary Story: How the Stars Gave Life to the Universe
• All the Way Back: It Started with a Bang
• What Does It Matter: Why There is More Matter Than Antimatter in the Universe
• Before the Big Bang: How the Entire Universe Began
• Dancing in the Dark: Dark Matter and the Great Cosmic Web
• The Ultimate End: Dark Energy and the Fate of the Universe
• Past, Present and Future: All We Know About All There Is

Key Features:

• Author of well-known physics blog "Starts with a bang"

Chapter 1

So Far, So Good: The Universe At The Start Of The 20th Century

If you were going to pick one thing in the sky to choose as the most obvious and most important, no matter where on Earth you were, you would likely choose the Sun. Imagine what it must have been like for the first humans who migrated a large distance away (say, north) from the equator. Rather than a terrain that was warm year-round, with the Sun rising in the east, passing high overhead during midday and setting in the west consistently, with only modest variations, things would appear to change dramatically. During late spring and early summer, you would have even more daylight than you had at the equator, with the Sun rising and setting much closer to the North Pole, while its path would still take it high overhead in the skies towards the south at midday. But as the year wore on, the Sun’s path would shorten dramatically. It would both rise and set farther south every day, and would peak just a little lower in the sky than the day prior. As the days got shorter and darker, and the nights grew longer, the world would grow colder, as the onset of winter approached. Someone who had never experienced this before might well worry that the Sun itself would sink lower and lower as the days continued onward, perhaps disappearing below the horizon entirely.

But unless you ventured north of the Arctic Circle, that would never happen. The Sun would slow down in its descent after some time, and reach a minimum height above the horizon, which it would not drop below the next day. It would appear relatively stationary for a few days, which is what we call the solstice: Latin for Sun stands still. And after that, it would begin to rise a little higher once again, signaling that a new year would indeed come, and that the Sun would eventually bring back longer days and another summer (Fig. 1.1).

Figure 1.1On the summer solstice from a significant latitude, the Sun appears to rise closer to the pole than on any other day, pass higher overhead, provide more hours of daylight and then set closer towards that same pole. As the year progresses, its path continues to move farther towards the opposite pole, providing fewer hours of daylight and culminating in the winter solstice, the shortest day of the year. After that, the Sun’s path appears to migrate back towards the first pole again, with days lengthening and the cycle repeating. Image credit: Wikimedia Commons user Tau‘olunga, under a c.c.-by-s.a.-2.5 license.

This story may or may not be factually true, as it is mere anthropological conjecture, but it is illustrative of the very beginnings of astronomy, of what a person would actually see, and of science in general. By taking detailed observations and measurements of the Universe itself, we can learn about the phenomena that occur within it. By repeating these observations and measurements over time, in different locations and by independent observers, we can gain a suite of data describing what occurs. When we understand what has occurred in the past, we can use that information to make accurate predictions about what we can expect to happen in the future.

There is only one more major step required to transform this initial, primitive set of information into what we consider today to be science: The construction of a physical framework that accounts for what we see. In science, it is not enough to merely describe what happens; we want to uncover how it happens, and what its causes are. For that, we need an explanation of what causes our observations. We need a physical theory that underlies the reasons these phenomena occur. And we need for that theory to go out and make new predictions that we can test, either via observations or by experiment, to validate or falsify it. For a single object like the Sun, seen moving through the sky, the explanations are simply too numerous to be of any use. But if we turn our attention towards what we see when the Sun goes down, a whole new Universe opens up.

* * *

As the skies darken with the onset of twilight, a clear, cloudless night will bring out hundreds of stars easily visible to the naked eye, with that number rising well into the thousands if the night is moonless as well. No matter where you are on Earth, those stars will move throughout the night, as it appears the entire canopy of the sky rotates about a single point, focused either on the North or South celestial pole. Night after night, the stars appear with the same patterns, in the same relative positions, with the same brightness and always making the same motions: rotating counterclockwise about the North Pole. (Or, from the Southern hemisphere, clockwise about the South Pole.) Just like the Sun, stars appear to rise in the eastern half of the sky, move to a position high above the horizon, and set along the western side. And on nights where the Moon is visible, it, too, rises in the east, reaches a maximum position over the horizon, and sets in the west.

Why would all of this occur? The explanation that most of the early scientists defaulted to made an awful lot of sense: that all the objects in the sky were a part of a fixed sphere high above the Earth’s surface, and that sphere rotated about its axis once every 24 hours, giving rise to the motion of the Sun, Moon and stars on a daily basis. This is a great start to a scientific theory, as it accounts for the full suite of observations available — of all the celestial objects — with one single explanation. The only things missing, preventing this from becoming a full-blown scientific theory, are the mechanism of how this happens and a new, testable prediction that should arise from this description of our Universe (Fig. 1.2).

Figure 1.2The stars appear to be located on a sphere distant from Earth, with only the constellations visible that appear opposite the Sun (i.e., on the night side). It is unclear, from this observation alone, whether the stars and Sun rotate around the Earth on a daily basis, or whether the stars and Sun are relatively stationary and the Earth rotates on its axis once per day. Image credit: E. Siegel, based on the original by Wikimedia Commons user Tau‘olunga.

Despite its appeal, it is not the only conceivable explanation for these phenomena. From our vantage point here on Earth, it certainly appears that the Sun, Moon and stars — the objects that appear in the skies — all move on an invisible sphere rotating around our world. But they could, just as easily, be fixed in the skies, and it could be our world that rotated instead. Certainly, from these simple observations of the objects in the sky alone, there would not be a way to tell these two scenarios apart, and there were many scientists and philosophers as early as Pythagoras and his disciples who favored this latter approach. But without a way to test one idea against the other, without differing predictions that the two ideas make, we would not yet have a scientific theory. Still, keeping these phenomena and their possible explanations in mind will help inform us about the Universe, and our place in it, as we continue to gather superior observations and information in our quest to make sense of the world.

* * *

While the fixed stars returned to the same position, night after night, relative to one another, the Sun and the Moon did no such thing. The Moon’s shifts were the most notable, even more severe than the Sun’s! If you were to measure the Moon’s position one night at a specific time, and then look for it at the same exact time on the next day, you would find that its position would have shifted by twelve degrees (12°), or roughly the amount of space between your index and little finger if you throw heavy metal horns with your arm extended (Fig. 1.3). Similarly, the Sun’s position shifts a little bit each day, which is why the positions of the visible stars shift ever so slightly from night-to-night. It appears, on average, that the sky is off by just one degree from one night to the next if you mark positions at the same time on successive nights. This is why different constellations are visible during different parts of the year, as the Sun’s position relative to the background of stars appears to shift through the sky.

Figure 1.3The Moon is the brightest object in the picture that appears to shift by the greatest amount, with two planets — Venus (brighter) and Jupiter (slightly fainter) — that shift by much smaller amounts relative to the background of static stars. Image credit: ESO/Y. Beletsky, with the author’s hand superimposed to show 12° on the sky, the amount the Moon appears to shift its location in a 24 hour timespan.

In addition to this, the Sun and Moon appear to be related in a particularly interesting way: through the Moon’s phases. When the Moon appears very close to the Sun in the sky, it appears as a very thin crescent. With each passing day, it falls farther and farther behind the Sun, with progressively more of it becoming illuminated over time. It transitions from a crescent phase to halfway filled, then becomes a gibbous and, eventually, completely full, as it appears to be a complete circle. This process, as the Moon appears to fill in over a roughly two-week time period, is known as waxing. But once the Moon becomes full, it does not remain that way for long. Practically immediately, it begins waning, or emptying out, from its west side to its east side, the same way it waxed earlier. Over the same timeframe that it took to reach its full phase, it becomes a gibbous, then half-full, then a crescent once again, all the while getting closer and closer to the Sun in the sky. Finally, the Moon becomes new once again, a crescent so thin and close to the Sun it is usually invisible, and the lunar cycle repeats, beginning once again.

Rather than moving along with the rest of the stars, the Sun and the Moon’s motions must be independent of them, as their positions change relative to all the other bodies, fixed in the sky. We can learn that the Moon must be closer to Earth than the stars are, since when a crescent moon passes into the same location as a star, the Moon passes in front of the star, blocking its light, a phenomenon known as occultation. We can also conclude, quite remarkably, that the Moon’s phases are caused by reflected light from the Sun! When the Moon’s phase appears full, it appears on the opposite side of the Earth from where the Sun appears, indicating that the lunar hemisphere illuminated by the Sun is completely visible to our eyes. On the other hand, when the Moon appears in a new phase, it is on the same side of Earth as the Sun, indicating that not only is the hemisphere facing away from our planet the one that is illuminated, but also suggesting that the Moon is closer to our world than the Sun is (Fig. 1.4).

You can imagine a very strong light source coming from a single bulb down at one end of a hallway as the Sun. You can imagine that your head is the Earth, and you can imagine that a ball — held at arm’s length — is the Moon. If you face towards the light source, just a tiny bit to your left, and hold out the ball, what do you see when you look at it? You know that half of your “Moon” is lit up by the Sun, but you can only see a tiny sliver illuminated; the rest of the side of the Moon that faces you is in shadow. Now, rotate your arm farther to the left, and watch how more and more of the ball appears illuminated to you. Eventually, the crescent fills in to become halfway full (known as a “quarter” phase), which occurs when you make a 90° angle with respect to both the ball and the light source. As you move the ball to the opposite side of your head, the entire illuminated side is visible to you, provided you keep your head’s shadow out of the way. Finally, you can complete the revolution, coming back so that the ball is once again between you and the light source. An entire trip of the Moon around the Earth, just like that, is what causes not only the phases that we see, but is where the idea (and the name) of a “month” comes from, with a full lunar revolution about Earth taking 29.5 days on average (Fig. 1.5).

Figure 1.4Relative to the Earth–Sun distance, the Moon orbits the Earth at a vastly smaller scale, with the Moon being the closest celestial object to our world. The reflected sunlight off of the Moon is what causes its phases as seen from Earth, as the portion of the Moon that’s illuminated is what changes over the course of a month from our vantage point. Note how the full disc of the Moon always obscures the objects behind it, even when that disc is not visible itself. Image credit: E. Siegel, based on an original by Wikimedia Commons user Orion 8, under c.c.-by-s.a.-3.0.

You will notice, if you do this demonstration yourself, that you will have to take care at two different times to avoid blocking out the lights! Once when the Moon passes in between the Earth and the Sun, otherwise the sunlight will be blocked from reaching Earth (represented by your eyes), and a second time when the Moon lines up so that it is in the Earth’s shadow, as the Sun’s light can be blocked (by your head) from reaching it. In reality, these alignments do happen occasionally — about twice a year each, on average — and are known as eclipses. Most months see no eclipses happen, as the Moon’s orbit around Earth is inclined at about 5° (about the width of your three middle fingers at arm’s length) to the Earth–Sun plane, while the Moon and Sun take up only about 0.5° (or half the width of your littlest finger at arm’s length) in angular diameter apiece. During the majority of months, the Moon passes either above or below the line connecting the Sun and Earth during its new and full phases, so no eclipses occur. But twice a year, the Moon comes close enough to crossing that imaginary line that, when it passes between the Sun and the Earth, some or all of the sunlight that should fall on our planet is blocked by the Moon instead. This is known as a solar eclipse (or an eclipse of the Sun), and it comes in three varieties, depending on both how good the alignment

Figure 1.5If you take a light source and place it far away in a room while holding out a ball with your left hand and rotating, you can simulate the phases of the Moon that you would see from Earth, including when it is in the new, crescent, quarter, gibbous and full phases. Image credit: E. Siegel. between the Sun and Moon is and also on their relative angular sizes. The three varieties are as follows:

1)	Partial Sola...

Table of contents

• Cover Page
• Title
• Copyright
• Contents
• Preface
• Chapter 1 So Far, So Good: The Universe At The Start Of The 20th Century
• Chapter 2 A Relatively Different Story: How Einstein’s Relativity Revolutionized Space, Time, And The Universe
• Chapter 3 Beyond The Milky Way: A Giant Leap Into An Expanding Universe
• Chapter 4 The Great Leap Backwards: Theories On Where It All Came From
• Chapter 5 An Element-ary Story: How The Stars Gave Life To The Universe
• Chapter 6 All The Way Back: It Started With A Bang
• Chapter 7 What Does It Matter: Why There Is More Matter Than Antimatter In The Universe
• Chapter 8 Before The Big Bang: How The Entire Universe Began
• Chapter 9 Dancing In The Dark: Dark Matter And The Great Cosmic Web
• Chapter 10 The Ultimate End: Dark Energy And The Fate Of The Universe
• Chapter 11 Past, Present And Future: All We Know About All There Is
• Index