Beyond the Galaxy

The following sections are included:

• Preface
• Contents

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…

With the serendipitous discovery of Uranus in 1781, we not only realized that our Solar System was larger than we had previously suspected, extending out twice the distance from the Sun to Saturn, but we also gained a new opportunity to test our most cherished physical law: Newton's law of universal gravitation. With centuries of observations of the other planets behind us, we had extraordinary confirmation that they all matched Kepler's three laws of motion:

• They all moved in ellipses with the Sun as one focus.
• They all swept out equal amounts of area in equivalent intervals of time.
• They all orbited with periods related to their semi-major axes (in proportion to the $\genfrac{}{}{0.1ex}{}{3}{2}$ power)

As our understanding of the Universe progressed forward in great leaps from Copernicus to Newton to Einstein, so did the instruments with which we viewed the curiosities of the night sky. We went from using our naked eye to small refracting telescopes to larger and larger reflecting telescopes. The maximum diameter of your eye's pupil — when fully dilated — is up to 9 millimeters, which sets the limit to what an unaided human (with otherwise perfect vision) can see. Early telescopes, such as Galileo's, improved this only slightly; Galileo's first telescope was just 15 millimeters in diameter. But in time, this improved significantly, and the increase in aperture brought along with it an increase in the amount of light that could be gathered. By the late 1600s, both refracting and reflecting telescopes had reached the size of an outstretched human hand, and became widespread among astronomers. And as the light-gathering power of these telescopes increased, they became capable of seeing progressively fainter, dimmer and more distant objects. This led to the identification of what appeared to be many permanent, fixed, deep-sky objects that appeared as extended smudges on the sky. They were classified, broadly, as nebulae…

With the revolution of General Relativity now firmly entrenched, along with the observation that the Universe was filled with galaxies that were expanding away from one another on the largest scales, we had developed a clear picture of what our Universe looked like today. On small scales, matter was clumped together to form individual stars and planets, star clusters, globular clusters, nebulae, dwarf galaxies and individual spiral and elliptical galaxies. On intermediate scales, galaxies were clumped together in groups and, in some larger cases, clusters, containing as many as a few thousand times the mass of our Milky Way. These were the systems that were gravitationally bound, and so, over time, these individual systems would not expand away from any other system that they were bound to. And on the largest scales, the Universe was mostly uniform. Any galaxy, group, or cluster that lay beyond an individual, gravitationally bound system would inevitably find itself caught up in the Hubble expansion of the Universe (Fig. 4.1)…

One of the most hotly contested points of the two main competing theories of the cosmos — the Big Bang and the Steady-State models — was about the origin of the heavier elements that we find not only here on Earth, but across the Universe as well. We know that atoms come in around 90 naturally occurring types here on Earth, with the number of protons in each nucleus determining what type of atom you have. We also have neutrons and electrons along with the protons, where the neutrons and protons are bound together in the atom's nucleus, and the electrons orbit the nuclei. The early stages of the Big Bang predicted that only free protons, neutrons and electrons should have existed, with heavier elements forming through nuclear reactions, while the matter creation hypothesis of the Steady-State theory only adds individual particles to the Universe. Were the heavy elements that are now so abundant created in the early stages of the hot Big Bang, as Gamow contended? Or were they created in stars, as favored by Hoyle and his contemporaries? To answer this, let us first take a look at how we know what elements actually exist in the Universe (Fig. 5.1).

By the late 1950s, astronomers and physicists alike were sharply divided as to which model of the Universe was more likely to be correct. Of the two main competing theories, proponents of the Steady-State framework — governed by the “Perfect” Cosmological Principle — were able to successfully show how the heavy elements in the Universe originated in the hearts of prior generations of stars. Proponents of the Big Bang were forced to accept that if their model was correct, it could only explain the abundance of the lightest elements: hydrogen, helium and their isotopes, along with lithium-7 and nothing heavier than that. But there was another test to be performed, one that would prove far more discriminating between the two options…

The greatest successes of the Big Bang, of its predictions for what we would see in the Universe today, came from extrapolating the known laws of physics using this model extraordinarily far back into the distant past. A cooling, expanding Universe that has billions of years to form stars, galaxies and clusters of galaxies on the largest scales also has the extraordinary property that as we look farther away at greater distances in the Universe, we also see back in time. When we look at galaxies and clusters that are farther away, we not only see that they appear to be speeding away from us more quickly due to the Hubble expansion of the Universe, we also see them when the Universe was around for less time, and hence when these objects were less evolved. This should mean a huge variety of things, a great number of which have been tested observationally (Fig. 7.1)…

If you ask the scientific question of where the Universe came from, “the Big Bang” is likely to be the answer you get from almost anyone you can ask. But it is actually a relatively new idea, scientifically speaking; just a few decades ago, the Big Bang would have been hotly contested, and a few decades before that, it hadn't even been considered. As far as we came to confirm it, and for all of the Big Bang's successes, there are a number of points where it does not provide a satisfactory answer to the question of where our Universe and everything in it comes from. Remember that the whole concept was born from two simple facts: (1) Einstein's description of gravitation as a changeable spacetime fabric whose curvature is determined by the matter and energy in it, and (2) the observed relationship between the measured distance of galaxies beyond our own and their redshift. When taken together, these two pieces of evidence — both verified to incredible degrees of accuracy through multiple lines of observation — lead us to conclude that we live in a Universe whose spacetime fabric is expanding over time…

With everything we know about the Universe from its early stages up through the present day, you might imagine that starting off with the right initial conditions and applying the laws of physics would be sufficient to reproduce the Universe as we know it. Any small discrepancies would merely be a matter of filling in the details. If we can start off from expanding spacetime itself, create the hot-and-dense state of the Big Bang, construct a matter–antimatter asymmetry, have the Universe cool to annihilate the leftover antimatter away, produce the first atomic nuclei and then form neutral atoms, it seems like gravitation acting on matter to clump it together to form stars, galaxies and clusters would get us the rest of the way there. In fact, we would be absolutely crazy if we did not try that first! After all, that is what our best theories predict ought to happen in the Universe after those first neutral atoms form…

The story of where our Universe came from is a remarkable one, and the fact that we have reached the point where we understand as much of it as we do is perhaps equally remarkable. But up until very recently, the fate of our Universe — how it would continue to evolve in the future — was very much unknown. We have already explored (back in Chapters 4 and 8; see Fig. 4.2) the idea that there are three possible fates for the Universe, dependent on the relationship between the amount of matter-and-energy present and the rate of the initial expansion. These fates are:

• The Big Crunch: an initially large expansion rate causes the young Universe to grow incredibly quickly, but the density of matter and energy is also tremendously large, slowing the expansion rate down over time. Eventually, there is just enough gravitation to overcome the expansion completely, causing the Universe to reach a maximum size and cease expanding altogether. Once it reaches this turning point, the gravitation from all the matter and energy present causes the Universe to begin contracting. After the same amount of time passes again that it took the Universe to expand from the Big Bang until reaching maximum size, it will recollapse into an arbitrarily hot, dense state again: the Big Crunch. A Universe that ends in a Big Crunch will have a positive (closed) spatial curvature.
• The Big Freeze: the initial conditions of the Universe — the initial expansion rate and the matter-and-energy density — are practically 322 Beyond The Galaxy indistinguishable from the Big Crunch conditions early on. The Universe expands incredibly rapidly and slows down to a minuscule fraction of its initial value. But rather than there being just enough gravitation to stop and reverse the initial expansion, it is the other way around: the initial expansion is just slightly too great for the amount of matter and energy in our Universe. As a result, the expansion rate remains positive at all times, and the galaxies that are not gravitationally bound to ours recede farther and farther away as time goes on. Inevitably, the Universe keeps expanding and cooling forever and ever. A Universe that ends in a Big Freeze will have a negative (open) spatial curvature.
• A Critical Universe: at most, the departure from a perfect balance between the early matter-and-energy density of the Universe and the initial expansion rate must be less than one part in 10²⁵, so why couldn't the balance be exactly perfect? Perhaps the Universe really is right on the border that separates an eternal expansion from a recollapse, where only one more proton in the Universe would change our fate. A critical Universe will have the expansion rate drop, asymptotically, towards zero, but the expansion rate will never reverse itself. A Universe that possesses this exact critical density of matter-and-energy will have zero (flat) spatial curvature.

In the span of the past 100 years, or a rather long human lifetime, our conception of the Universe has changed forever. A Universe assumed to be static, ruled by Newton gravity and spanning thousands of light years, within which all the stars — past, present and future — are contained, has been superseded in every way. The Universe is not static, but is rather expanding, cooling and evolving. Newtonian gravity is only an approximation of Einstein's General Relativity, which brings with it a whole host of observable consequences, nearly all of which have been validated. Our galaxy alone has been determined to be around 100,000 light years in diameter, and yet is only one out of hundreds of billions populating our observable Universe, which extends for some 46 billion light years in all directions (Fig. 11.1)…

The following sections is included:

• Index