A Ray of Light in a Sea of Dark Matter

Charles Keeton

• Publisher: Rutgers University Press
• Year: 2014
• Print ISBN: 9780813565347
• eBook ISBN: 9780813572123
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

 What’s in the dark?  Countless generations have gazed up at the night sky and asked this question—the same question that cosmologists ask themselves as they study the universe.  The answer turns out to be surprising and rich. The space between stars is filled with an exotic substance called “dark matter” that exerts gravity but does not emit, absorb, or reflect light. The space between galaxies is rife with “dark energy” that creates a sort of cosmic antigravity causing the expansion of the universe to accelerate. Together, dark matter and dark energy account for 95 percent of the content of the universe. News reporters and science journalists routinely talk about these findings using terms that they assume we have a working knowledge of, but do you really understand how astronomers arrive at their findings or what it all means? Cosmologists face a conundrum: how can we study substances we cannot see, let alone manipulate? A powerful approach is to observe objects whose motion is influenced by gravity.  Einstein predicted that gravity can act like a lens to bend light. Today we see hundreds of cases of this—instances where the gravity of a distant galaxy distorts our view of a more distant object, creating multiple images or spectacular arcs on the sky. Gravitational lensing is now a key part of the international quest to understand the invisible substance that surrounds us, penetrates us, and binds the universe together.  A Ray of Light in a Sea of Dark Matter offers readers a concise, accessible explanation of how astronomers probe dark matter.  Readers quickly gain an understanding of what might be out there, how scientists arrive at their findings, and why this research is important to us. Engaging and insightful, Charles Keeton gives everyone an opportunity to be an active learner and listener in our ever-expanding universe. Watch a video with Charles Keeton: Watch video now. (http://www.youtube.com/watch?v=Uc3byXNS1G0).

Chapter 1: What’s in the Dark?

Astronomers have long been denizens of the dark, spending nighttime hours on remote mountaintops capturing tiny traces of light from afar. For decades, observers had to ride on telescopes all night, cramped in a cage opposite the mirror, to keep the instrument focused on the target during long-exposure photographs. Then they would carry the photographic plates to the darkroom to be developed. Later, teams of assistants would pore over the plates to measure planets, stars, and galaxies. Observational astronomy was painstaking, and every photon was precious.

A practical change is under way, driven (as in so many other fields) by technology. Today, astronomers guide telescopes by computer from control rooms that, while perhaps not plush, at least offer light, heat, and Internet access. They record data in electronic files that are easily copied and shared. With some telescopes, observers can do everything remotely, working from their offices or even living rooms. (There is ongoing debate about the relative costs and benefits of being on site for observing.) One thing remains the same: photons are still precious, even if they are collected in new ways.

A philosophical shift is also afoot: from working in the dark to working on the dark. Beginning in the 1970s, astronomers realized there is more matter in the universe than meets the eye. Observers started looking for ways to map the dark matter, theorists studied how it would affect the growth of structure in the universe, and particle physicists got excited about the prospect of finding entirely new kinds of particles. Later, in the 1990s, observers found strong evidence for an even more exotic substance dubbed “dark energy.” A plethora of measurements now indicates that dark matter and dark energy together compose 95 percent of the stuff in the universe. While science has always addressed the unknown, cosmology now has to deal with the unseen.

The challenge is acute because astronomy is an observational science. We cannot do experiments on galaxies, let alone the universe; all we can do is look around, search for patterns, and invent ideas that might explain what we see. A good lesson comes from early work on stars. In the late nineteenth and early twentieth centuries, Harvard College Observatory hired a number of women to help process astronomical data. (The women came to be known as “computers.”) Williamina Fleming analyzed measurements known as spectra in which starlight is spread out like a rainbow to show all of the different colors. Dark bands appear where certain wavelengths of light are absorbed by atoms or molecules, and Fleming introduced a classification scheme based on the patterns she saw. Later, Annie Jump Cannon discovered a connection between the absorption pattern and temperature of a star. That relation made it possible for Ejnar Hertzsprung and Henry Norris Russell to plot the luminosities of stars versus their temperatures and learn that most stars fall on a sequence running from bright hot blue stars down to dim cool red stars. The strong pattern in the Hertzsprung-Russell diagram inspired research on the physics of stars and ultimately led to the discovery that the sequence in temperature and brightness is really a sequence in mass: more massive stars have stronger gravity that squeezes the gas to create higher temperatures and luminosities. This example shows that with a critical eye to spot patterns, and a creative mind to interpret them, we can learn a lot about objects we cannot touch.

What about material we cannot even see? Here we turn to Isaac Newton, who discovered a deep connection between motion and mass. When he thought about general forms of motion, Newton deduced three fundamental laws:

1. Left alone, an object will stay at rest or move in a straight line at a constant speed.

2. An object can change speed or direction only if it is acted on by an external force; the change depends on the strength and direction of the applied force.

3. If object A exerts a force on object B, then object B exerts an equal and opposite force back on object A.

According to these principles, an apple falls from a tree not because it “wants” to be on the ground but because some unseen force impels it. The Moon orbits the Earth not because it “wants” to move along a circle or ellipse but because an unseen force pulls it away from straight-line motion. That idea was already profound; no one had previously thought about an invisible force transmitted through empty space.

But Newton went further. Using his second law of motion, he computed the amount of force that must act on the apple or the Moon to generate the motion. Newton then asked, What if it is the same force affecting both objects? The numbers worked out if he assumed that gravity scales inversely with the square of the distance between two objects. (If the objects move twice as far apart, gravity weakens by a factor of 2 × 2 = 4.) Even better, applying the inverse square force law to planets orbiting the Sun led to predictions that matched the laws of planetary motion that Johannes Kepler had extracted from observational data. Talk about “Eureka!”: in one fell swoop, Newton explained centuries’ worth of cosmic motion measurements.

These principles can be extended to detect mass that was previously unknown. In 1781, William Herschel discovered the object now known as Uranus. Its motion was initially found to be consistent with Newton’s extrapolation of Kepler’s laws, so it was identified as a planet orbiting the Sun. As the measurements improved, the motion was found to deviate slightly from Newtonian predictions. Urbain Le Verrier and John Couch Adams separately analyzed the deviations and argued that they could be caused by gravity from another planet beyond the orbit of Uranus. Sure enough, in 1846 Johann Galle and Heinrich d’Arrest used Le Verrier’s predictions to discover the planet Neptune.

Similar logic now pervades astrophysics. When we see something move, we can use Newton’s laws of motion to find the force that induces the motion. We can then use Newton’s law of gravity to figure out how much mass is needed to create the required force. This connection between motion and mass is one of the pillars of modern astrophysics. As long ago as the 1930s, Fritz Zwicky noticed that galaxies in collections known as clusters of galaxies move much faster than they should if gravity comes only from the galaxies themselves. His analysis was ahead of its time, however. The idea that fast motions reveal “missing mass” began to gain traction in the 1970s and 1980s when Vera Rubin and others realized that the speeds of stars in spiral galaxies require much more gravity than can be attributed to the visible matter (see chapter 3). Today, astronomers use the motion/mass principle to discover planets orbiting other stars, find monstrous black holes at the centers of galaxies, and much more.

There is a catch. First, we cannot always measure motion in full detail. For example, we can determine how fast stars in a galaxy are moving toward or away from us, but we cannot detect how fast they are moving left/right or up/down in the plane of the sky. Given the uncertainties, we might be able to find multiple configurations of mass that could give rise to the observed motion. It is important to recognize the uncertainties, see if we can find ways to resolve them, and deal with them if we can’t.

Second, we have to guard against being misled. The connection between motion and mass runs through Newton’s laws of motion and gravity. If the laws themselves are incorrect, the conclusions will be erroneous (even if the logic is sound). Urbain Le Verrier’s success with Uranus was actually complemented by failure of this sort with Mercury. Kepler found that planet orbits are ellipses, and Mercury’s is the most elongated of the eight major planets. If Mercury were the only planet orbiting the Sun, it would trace the same ellipse over and over forever. In fact, Mercury’s ellipse precesses, or shifts slightly from one orbit to the next. Most of the precession can be attributed to gravitational influences from the other planets—but not all. Le Verrier argued that the additional, unexplained motion is caused by an unseen planet closer to the Sun, which he named Vulcan. Inspired by this prediction, a number of amateur astronomers claimed to see a new planet crossing the face of the Sun, and Le Verrier died believing his prediction had been confirmed. The observational claims could not be verified, however.

Mercury’s excess precession remained unexplained until Albert Einstein published his general theory of relativity in 1915. According to general relativity, the effects of gravity do not quite match Newton’s inverse square law; there are small deviations close to a massive object like a star. Einstein discovered that those deviations are exactly what is needed to explain Mercury’s orbit. In other words, Mercury is not like Uranus: one case revealed new mass in the solar system, while the other actually indicated changes in the laws of physics.¹

Some skeptics have argued that dark matter is the new Vulcan—a misguided conclusion drawn from a misapplication of Newton’s (and Einstein’s) laws. They suggest that puzzling motions in the cosmos might indicate not new matter but new physics. In 1983, Mordehai Milgrom introduced Modified Newtonian Dynamics (MOND), in which Newton’s second law of motion is adjusted in a way that could reproduce the motions of stars in galaxies (without affecting motions on Earth or in the solar system; see chapter 3). Proponents argue that MOND is simpler than dark matter when it comes to explaining galactic rotation. They also point out that Newton’s laws were thought to be immutable until relativity came along; are we certain that relativity itself can never be modified?

Despite those arguments, most astronomers are firmly convinced that dark matter is real. The reasoning has a number of facets. First, we understand how Newton’s laws break down in the regimes where Einstein’s laws kick in, so we know when it is safe to stick with classical mechanics and when we need to shift to relativity. Second, Einstein’s theory has passed every test to which it has been subjected. Reasoning based on Newton’s and Einstein’s laws therefore seems as if it should be sound. Third, some specific astronomical systems are difficult to understand without dark matter (see the discussion of the Bullet Cluster in chapter 6).

A fourth facet involves a conceptual principle known as Ockham’s razor. William of Ockham was a medieval theologian and philosopher who sought to make scholarly reasoning more efficient. Today many people interpret his thinking in terms of simplicity, formulating a principle that might be stated as, “The simplest explanation is probably the correct one.” A more nuanced interpretation allows that ideas may be intrinsically complex but still advocates parsimony: “When faced with competing hypotheses that are otherwise equal, pick the one with the fewest new assumptions.” Ockham’s razor, in other words, should be used to trim away ideas that are needlessly complex. This is not a rigorous scientific principle; experimental evidence remains the arbiter of truth in science. But it is a rule of thumb that can be helpful when developing new hypotheses or models and performing an initial assessment.

Dark matter survives Ockham’s razor because it represents a single concept that explains a myriad of astronomical observations. It explains the motions of stars in galaxies, the properties of galaxy clusters, the distribution of galaxies in the universe, the overall expansion of the universe, and much more. Without dark matter, in fact, it is hard to make sense of the vast array of cosmological data now available. There remain some puzzles, to be sure, but they are considered to be fodder for ongoing research rather than reasons to discard the idea of dark matter altogether. As a competing hypothesis, MOND does well with galaxies but has some trouble with other systems. There are enough ambiguities that we cannot say for certain whether MOND fails to explain clusters of galaxies and the universe as a whole, but even proponents admit that MOND alone is not enough; some mass beyond the visible stuff is needed (although it might be something we know about, such as neutrinos).

A fifth facet of the belief in dark matter comes from particle physics. While the Standard Model of particle physics describes all the particles we know now, theorists have long suspected that other particles could exist. In the 1960s, for example, work by Peter Higgs led to predictions of a new particle that was ultimately discovered at the Large Hadron Collider in 2012–13. (The predictions were honored with the 2013 Nobel Prize in Physics.) That is not the end of the story, however, by a long shot. Many other types of particles have been hypothesized for a variety of reasons, and some of them could have the right properties to be cosmic dark matter (see chapter 4).

Such arguments have convinced the vast majority of astronomers and physicists that what’s in the dark is a new form of matter. Assuming that to be true, what do we know about the substance? A little or a lot, depending on your perspective. Although we do not actually know what it is, we have begun to figure out what it is like; we know, in other words, few nouns but many adjectives. First, dark matter is, well, dark; it does not emit, absorb, or reflect light. That suggests it is electrically neutral, because charged particles interact with light through the electromagnetic force. Dark matter is abundant; in the universe as a whole, it outweighs normal matter by a factor of five. Dark matter is pervasive; it is found in many different types of galaxies, in clusters of galaxies, and throughout the universe. But dark matter is not distributed uniformly; rather, it is clustered so the density is higher in some places than in others. This is caused by gravity; any region of the universe that has a little more than the average amount of matter will have a little extra gravity, so it will tend to draw in even more matter from surrounding regions.

The fact that dark matter is clustered suggests that it is “cold,” a term physicists use to indicate that the material is slow compared with the speed of light. If the particles were fast, they would fly away instead of collecting in halos around galaxies and clusters of galaxies. Finally, dark matter appears to be exotic—some fundamental particle that is different from all of the particles we have seen in physics experiments to date.

We might view all of these adjectives as pieces of a jigsaw puzzle that, when complete, will show what dark matter is. Unfortunately, we do not know how the pieces fit together, or how many remain to be found. So we do our best to assemble a plausible picture and then look for gaps we can try to fill in. The conventional picture today suggests that dark matter particles weigh a few tens or hundreds of times as much as a proton. If so, the particles are all around you; there are a few dozen in every liter of volume throughout the solar system and even on Earth. The particles are zipping through the room in which you are sitting at several hundred thousand miles per hour. They interact so weakly that they rarely hit anything, but every once in a while a piece of dark matter might collide with a piece of normal matter on Earth. Physicists are now trying to use such collisions to catch the particles and measure their properties (see chapter 4). Particles of dark matter might also be able to interact with one another, annihilating into a cascade of normal particles.

These two types of collisions provide opportunities to probe dark matter in new ways. They have some limitations, however. It goes without saying that collisions with particles on Earth probe dark matter right here. We can assume that dark matter here is the same as dark matter everywhere, but experiments on Earth cannot test this assumption. Collisions among dark matter particles might occur throughout ...

Table of contents

• Title Page
• Copyright Page
• Contents
• Preface
• 1. What’s in the Dark?
• 2. When Mass Is Like Glass
• 3. How Do You Weigh a Galaxy?
• 4. Is Dark Matter MACHO or WIMPy?
• 5. Finding What’s Missing
• 6. “A Long Time Ago in a Galaxy Far, Far Away”
• Glossary
• Notes
• Notes on Sources