فصل 04

4.

Between the Galaxies

In the grand tally of cosmic constituents, galaxies are what typically get counted. Latest estimates show that the observable universe may contain a hundred billion of them. Bright and beautiful and packed with stars, galaxies decorate the dark voids of space like cities across a country at night. But just how voidy is the void of space? (How empty is the countryside between cities?) Just because galaxies are in your face, and just because they would have us believe that nothing else matters, the universe may nonetheless contain hard-to-detect things between the galaxies. Maybe those things are more interesting, or more important to the evolution of the universe, than the galaxies themselves.

Our own spiral-shaped galaxy, the Milky Way, is named for its spilled-milk appearance to the unaided eye across Earth’s nighttime sky. Indeed, the very word “galaxy” derives from the Greek galaxias, “milky.” Our pair of nearest-neighbor galaxies, 600,000 light-years distant, are both small and irregularly shaped. Ferdinand Magellan’s ship’s log identified these cosmic objects during his famous round-the-world voyage of 1519. In his honor, we call them the Large and Small Magellanic Clouds, and they are visible primarily from the Southern Hemisphere as a pair of cloudlike splotches on the sky, parked beyond the stars. The nearest galaxy larger than our own is two million light-years away, beyond the stars that trace the constellation Andromeda. This spiral galaxy, historically dubbed the Great Nebula in Andromeda, is a somewhat more massive and luminous twin of the Milky Way. Notice that the name for each system lacks reference to the existence of stars: Milky Way, Magellanic Clouds, Andromeda Nebula. All three were named before telescopes were invented, so they could not yet be resolved into their stellar constituencies.

As detailed in chapter 9, without the benefit of telescopes operating in multiple bands of light we might still declare the space between the galaxies to be empty. Aided by modern detectors, and modern theories, we have probed our cosmic countryside and revealed all manner of hard-to-detect things: dwarf galaxies, runaway stars, runaway stars that explode, million-degree X-ray-emitting gas, dark matter, faint blue galaxies, ubiquitous gas clouds, super-duper high-energy charged particles, and the mysterious quantum vacuum energy. With a list like that, one could argue that all the fun in the universe happens between the galaxies rather than within them.

In any reliably surveyed volume of space, dwarf galaxies outnumber large galaxies by more than ten to one. The first essay I ever wrote on the universe, in the early 1980s, was titled “The Galaxy and the Seven Dwarfs,” referring to the Milky Way’s diminutive nearby family. Since then, the tally of local dwarf galaxies has been counted in the dozens. While full-blooded galaxies contain hundreds of billions of stars, dwarf galaxies can have as few as a million, which renders them a hundred thousand times harder to detect. No wonder they are still being discovered in front of our noses.

Images of dwarf galaxies that no longer manufacture stars tend to look like tiny, boring smudges. Those dwarfs that do form stars are all irregularly shaped and, quite frankly, are a sorry-looking lot. Dwarf galaxies have three things working against their detection: They are small, and so are easily passed over when seductive spiral galaxies vie for your attention. They are dim, and so are missed in many surveys of galaxies that cut off below a prespecified brightness level. And they have a low density of stars within them, so they offer poor contrast above the glow of surrounding light from Earth’s nighttime atmosphere and from other sources. All this is true. But since dwarfs far outnumber “normal” galaxies, perhaps our definition of what is normal needs revision.

You will find most (known) dwarf galaxies hanging out near bigger galaxies, in orbit around them like satellites. The two Magellanic Clouds are part of the Milky Way’s dwarf family. But the lives of satellite galaxies can be quite hazardous. Most computer models of their orbits show a slow decay that ultimately results in the hapless dwarfs getting ripped apart, and then eaten, by the main galaxy. The Milky Way engaged in at least one act of cannibalism in the last billion years, when it consumed a dwarf galaxy whose flayed remains can be seen as a stream of stars orbiting the galactic center, beyond the stars of the constellation Sagittarius. The system is called the Sagittarius Dwarf, but should probably have been named Lunch.

In the high-density environment of clusters, two or more large galaxies routinely collide and leave behind a titanic mess: spiral structures warped beyond all recognition, newly induced bursts of star-forming regions spawned from the violent collision of gas clouds, and hundreds of millions of stars strewn hither and yon having freshly escaped the gravity of both galaxies. Some stars reassemble to form blobs that could be called dwarf galaxies. Other stars remain adrift. About ten percent of all large galaxies show evidence of a major gravitational encounter with another large galaxy—and that rate may be five times higher among galaxies in clusters.

With all this mayhem, how much galactic flotsam permeates intergalactic space, especially within clusters? Nobody knows for sure. The measurement is difficult because isolated stars are too dim to detect individually. We must rely on detecting a faint glow produced by the light of all stars combined. In fact, observations of clusters detect just such a glow between the galaxies, suggesting that there may be as many vagabond, homeless stars as there are stars within the galaxies themselves.

Adding ammo to the discussion, we have found (without looking for them) more than a dozen supernovas that exploded far away from what we presume to be their “host” galaxies. In ordinary galaxies, for every star that explodes in this way, a hundred thousand to a million do not, so isolated supernovas may betray entire populations of undetected stars. Supernovas are stars that have blown themselves to smithereens and, in the process, have temporarily (over several weeks) increased their luminosity a billion-fold, making them visible across the universe. While a dozen homeless supernovas is a relatively small number, many more may await discovery, since most supernova searches systematically monitor known galaxies and not empty space.

There’s more to clusters than their constituent galaxies and their wayward stars. Measurements made with X-ray-sensitive telescopes reveal a space-filling, intra-cluster gas at tens of millions of degrees. The gas is so hot that it glows strongly in the X-ray part of the spectrum. The very movement of gas-rich galaxies through this medium eventually strips them of their own gas, forcing them to forfeit their capacity to make new stars. That could explain it. But when you calculate the total mass present in this heated gas, for most clusters it exceeds the mass of all galaxies in the cluster by as much as a factor of ten. Worse yet, clusters are overrun by dark matter, which happens to contain up to another factor of ten times the mass of everything else. In other words, if telescopes observed mass rather than light, then our cherished galaxies in clusters would appear as insignificant blips amid a giant spherical blob of gravitational forces.

In the rest of space, outside of clusters, there is a population of galaxies that thrived long ago. As already noted, looking out into the cosmos is analogous to a geologist looking across sedimentary strata, where the history of rock formation is laid out in full view. Cosmic distances are so vast that the travel time for light to reach us can be millions or even billions of years. When the universe was one half its current age, a very blue and very faint species of intermediate-sized galaxy thrived. We see them. They hail from a long time ago, representing galaxies far, far away. Their blue comes from the glow of freshly formed, short-lived, high-mass, high-temperature, high-luminosity stars. The galaxies are faint not only because they are distant but because the population of luminous stars within them was thin. Like the dinosaurs that came and went, leaving birds as their only modern descendant, the faint blue galaxies no longer exist, but presumably have a counterpart in today’s universe. Did all their stars burn out? Have they become invisible corpses strewn throughout the universe? Did they evolve into the familiar dwarf galaxies of today? Or were they all eaten by larger galaxies? We do not know, but their place in the timeline of cosmic history is certain.

With all this stuff between the big galaxies, we might expect some of it to obscure our view of what lies beyond. This could be a problem for the most distant objects in the universe, such as quasars. Quasars are super-luminous galaxy cores whose light has typically been traveling for billions of years across space before reaching our telescopes. As extremely distant sources of light, they make ideal guinea pigs for the detection of intervening junk.

Sure enough, when you separate quasar light into its component colors, revealing a spectrum, it’s riddled with the absorbing presence of intervening gas clouds. Every known quasar, no matter where on the sky it’s found, shows features from dozens of isolated hydrogen clouds scattered across time and space. This unique class of intergalactic object was first identified in the 1980s, and continues to be an active area of astrophysical research. Where did they come from? How much mass do they all contain?

Every known quasar reveals these hydrogen features, so we conclude that the hydrogen clouds are everywhere in the universe. And, as expected, the farther the quasar, the more clouds are present in the spectrum. Some of the hydrogen clouds (less than one percent) are simply the consequence of our line of sight passing through the gas contained in an ordinary spiral or irregular galaxy. You would, of course, expect at least some quasars to fall behind the light of ordinary galaxies that are too distant to detect. But the rest of the absorbers are unmistakable as a class of cosmic object.

Meanwhile, quasar light commonly passes through regions of space that contain monstrous sources of gravity, which wreak havoc on the quasar’s image. These are often hard to detect because they may be composed of ordinary matter that is simply too dim and distant, or they may be zones of dark matter, such as what occupies the centers and surrounding regions of galaxy clusters. In either case, where there is mass there is gravity. And where there is gravity there is curved space, according to Einstein’s general theory of relativity. And where space is curved it can mimic the curvature of an ordinary glass lens and alter the pathways of light that pass through. Indeed, distant quasars and whole galaxies have been “lensed” by objects that happen to fall along the line of sight to Earth’s telescopes. Depending on the mass of the lens itself and the geometry of the line-of-sight alignments, the lensing action can magnify, distort, or even split the background source of light into multiple images, just like fun-house mirrors at arcades.

One of the most distant (known) objects in the universe is not a quasar but an ordinary galaxy, whose feeble light has been magnified significantly by the action of an intervening gravitational lens. We may henceforth need to rely upon these “intergalactic” telescopes to peer where (and when) ordinary telescopes cannot reach, and thus reveal the future holders of the cosmic distance record.

Nobody doesn’t like intergalactic space, but it can be hazardous to your health if you choose to go there. Let’s ignore the fact that you would freeze to death as your warm body tried to reach equilibrium with the 3-degree temperature of the universe. And let’s ignore the fact that your blood cells would burst while you suffocated from the lack of atmospheric pressure. These are ordinary dangers. From the department of exotic happenings, intergalactic space is regularly pierced by super-duper high-energy, fast-moving, charged, subatomic particles. We call them cosmic rays. The highest-energy particles among them have a hundred million times the energy that can be generated in the world’s largest particle accelerators. Their origin continues to be a mystery, but most of these charged particles are protons, the nuclei of hydrogen atoms, and are moving at 99.9999999999999999999 percent of the speed of light. Remarkably, these single subatomic particles carry enough energy to knock a golf ball from anywhere on a putting green into the cup.

Perhaps the most exotic happenings between (and among) the galaxies in the vacuum of space and time is the seething ocean of virtual particles—undetectable matter and antimatter pairs, popping in and out of existence. This peculiar prediction of quantum physics has been dubbed the “vacuum energy,” which manifests as an outward pressure, acting counter to gravity, that thrives in the total absence of matter. The accelerating universe, dark energy incarnate, may be driven by the action of this vacuum energy.

Yes, intergalactic space is, and will forever be, where the action is.