How can we distinguish among the various possible models of the Universe? Are there ways to rule out some of them and thereby converge on the best model by a process of elimination? Observational tests designed to answer questions like these have driven us further into the embrace of evolution as a guiding principle in cosmology—to be sure, evolution writ large as a unifying theme in all of science.
The steady-state model is widely judged untenable for at least two reasons. First, the spread of galaxies is not uniform throughout space. As noted in the next, GALACTIC EPOCH, active (explosive) galaxies at great distances from Earth far outnumber those nearby; most neighboring, normal galaxies (including our Milky Way) are calmer, less active. Had we lived some 10 billion years ago, when active galaxies were presumably the dominant astronomical objects, our view would have been filled with active galaxies—many more than now surround our vantage point on Earth. The perfect cosmological principle is clearly violated: The large-scale view of the Universe was not the same eons ago as it is now; it’s changed.
Second, a serendipitous discovery has become virtually fatal for the steady-state model. Observations made with radio telescopes always yield a signal, regardless of the time of day or night. Unlike optical observations that often show a complete void of light toward dark and obscured regions of space, radio receivers never fail to detect some radiation. Sometimes the radio signal is strong, especially when the telescope is aimed toward an obvious source of radio emission. At other times it’s weaker, particularly in regions devoid of all known radio sources. Yet, whenever the accumulated emissions from all known celestial objects and from all atmospheric and instrumental noise are accounted for, a minute radio signal always remains—a sort of weak hiss like static on a home AM radio or the “snow” on an inactive (non-cable) television channel. Never diminishing or intensifying, this weak signal is detectable at any time of the day, any day of the year, year after year—it’s omnipresent, apparently inundating all of space. What’s more, it’s equally intense in any direction of the sky—that is, it’s isotropic. The whole Universe is apparently awash in this feeble but persistent radiation.
Cosmic Background Radiation The ubiquitous radio signals permeating all of space were accidentally detected several decades ago, in the early years of the Space Age, when scientists sought to improve America’s telephone system. In their data, they unexpectedly noticed a bothersome radio hiss that just wouldn’t go away. Unaware that they had detected a signal of cosmological significance, the researchers tested many different sources for the excess emission, including atmospheric storms, ground interference, equipment short circuits, even pigeon droppings left inside their radio antenna! Later, conversations with theorists enlightened the experimentalists about the static’s most probable source: The fiery origin of the Universe itself.
Figure 1.13 is a map of the entire sky, made by capturing weak, omnipresent radio waves launched from deep space and projected as an oval, much as maps of Earth’s surface are often displayed “flattened” in an oval shape.
This weak, isotropic, radio radiation is widely interpreted as a veritable “fossil” of the primeval event that began the universal expansion long ago. The leftover hiss, often termed the cosmic background radiation (or cosmic microwave background), floods every nook and cranny of space, including that surrounding us presently. Its existence is fully consistent with any of the evolutionary models of the Universe, but there’s no role for it in the now defunct steady-state model.
The cosmic background radiation is presumed to be an ancient remnant of the extremely hot early Universe—a Universe that has greatly cooled during the past 14 billion years or so. Regardless of whether the initial event was a unique big bang producing an open and infinite Universe or a closed and finite one, or even one of several repeated bangs of a cyclic Universe, the primeval, seething, dense matter must have emitted thermal radiation (as elementary particles naturally released energy while interacting with one another). All objects having any heat emit such radiation; a very hot piece of metal (a branding iron, for instance) glows with red- or white-hot brilliance, whereas less-hot metal (such as a home radiator) feels warm to the touch while emitting less-energetic infrared or radio radiation. In its fiery beginnings, the Universe almost certainly launched highly energetic radiation, but with time it expanded, thinned, and cooled, causing its emitted radiation to shift steadily from the lethal, high-energy gamma- and x-ray varieties normally associated with intensely hot matter, down through the less-energetic ultraviolet, visible, and infrared types, eventually becoming the harmless, lowest-energy radio waves usually released by relatively cool matter.
Figure 1.14 illustrates the theoretically expected change of the Planck (blackbody) curve for the average heat of the cooling Universe over the course of time since the big bang. More correctly, this change is interpreted as a Doppler shift of the ancient, fiery radiation near the start of the Universe, now observed by us many billions of years later having been severely red shifted clear across the electromagnetic spectrum.
Evolutionary models predict that some 14 billion years after the start of all things, the average temperature of the Universe—the relic of the big bang—should now be quite cold, in fact no more than about -270 degrees Celsius. That’s far below the zero degree Celsius temperature at which water freezes and only a few degrees above the absolutely coldest value at which all atomic and molecular motions virtually cease. On the scientific scale, -270 degrees Celsius equals a mere 3 kelvins (see Figure 1.15).
To confirm the big-bang theory, astronomers have carefully measured the intensity of this weak isotropic signal at a variety of frequencies up and down the radio band. All the data collected during the past few decades, especially those acquired by the Cosmic Background Explorer (COBE) satellite in the early 1990s (Figure 1.13), as well as more recently the Wilkinson Microwave Anisotropy Probe (WMAP) are consistent with a universal temperature of ~3 kelvins. Furthermore, this oldest fossil really does seem to pervade the whole Universe, including the Earth, the building, or wherever you are now reading this. The amount of cosmic radiation present at any one time, however, is miniscule, totaling about a billionth of the power shone by a hundred-watt light bulb.
Existence of the cosmic background radiation, together with the spread of galaxies in space, discredits the steady-state idea as a viable model of the Universe. Clearly, the Universe has changed with time; it has not been steady at all. The choice of correct Universe type must then be made from among the various evolutionary models. Other data must be obtained to sift through each of them.
Critical Density The most straightforward way to distinguish between the open and closed Universe models requires an estimate of the average density of matter in the cosmos. More than anything else, density is what differentiates the closed model, which has enough matter to halt the expansion before it reaches infinity, from the open model, wherein there simply isn’t enough to bring it all back.
We would be foolish to try to inventory all the matter in the Universe. Authors don’t try to count by hand all the words in a written manuscript; rather, they make an estimate by counting the words on a single page and then multiply by the number of pages (or nowadays let computer-based word processors count all the words for us). Likewise, astronomers measure the amount of matter within a certain volume of space and then extrapolate that amount to include the observable Universe. This is tantamount to estimating the mass density, for density is nothing more than mass per unit volume.
The precise density of matter—known as the “critical density ”—needed to halt the expansion just as the outer limits of the Universe reach infinity can be computed theoretically. For today’s thinned-out Universe, the answer is ~10-30 g/cm3, or some million million million million million times less than one gram per cubic centimeter. (A cubic centimeter is just about the volume contained within a small sewing thimble, and a gram is ~0.002 pound; such a thimbleful of water has a mass of about 1 gram, the density of that liquid being 1 g/cm3.) This extraordinarily small density amounts to a few hydrogen atoms within a volume the size of a typical household closet, namely ~10-6 hydrogen atoms/cm3, or equivalently 1 atom/cubic meter. That’s extremely tenuous; in fact, many orders of magnitude thinner than the best vacuum attainable in laboratories on Earth. But remember, this is an average density of the entire Universe—calculated by lumping groups of galaxies, where the matter is most concentrated, together with intergalactic space, where little if any of it exists.
We can then ask: What is the actual density of matter in the Universe? Is it more or less than this critical value? Cosmologists quantify these questions by letting the symbol Ω (omega) denote the following ratio,
Ω = actual density / 10-30 .
If Ω = 1 precisely, then the actual density equals this theoretically computed density and the Universe obeys the parabolic model; it's open and will continue to expand forever. This model dictates that the Universe has no net curvature; it's a flat Universe governed largely by Euclidean geometry. Localized regions of spacetime, especially those near massive astronomical objects, are surely curved, but on the whole the accumulated curvature of spacetime for the Universe en massse is zero.
Should Ω < 1, the Universe's matter is not dense enough to ever stop its expansion. This type of Universe is destined to expand forever, conforming to the open, infinite, hyperbolic model. On the other hand, if Ω > 1, the closed, finite, elliptical Universe prevails and it will someday start contracting.
Galaxy Counting Theory aside, how can we determine the actual density of matter in the Universe? At first, it would seem simple. Just measure the total mass of all the visible galaxies residing within some large parcel of space, estimate the volume of that space, and compute the average density. Having done this many times for many pieces of cosmic real estate, astronomers usually find about 10 times less density than the amount needed to halt the expansion of the Universe, that is ~10-31 g/cm3, or therefore Ω = 0.1. As best we can tell, this calculation is independent of whether the chosen region contains only a few galaxies or a rich cluster of them; the resulting density is roughly the same, within a factor of two or three. Galaxy-counting exercises of this sort therefore imply that the Universe is open, meaning that it originated from a unique big bang and will expand forever. Such a Universe has no end, though it definitely had a beginning.
But—and this is a crucial but—an important caveat deserves mention. All the matter in the Universe is not likely housed exclusively within the brightly visible galaxies. Observations imply that invisible matter exists beyond each of them—“dark matter” sensed only indirectly by means of its gravitational effects mostly outside galaxies. The extent and amount of this dark matter is presently unclear, but if much additional matter resides outside the galaxies as within them, then the universal density would correspondingly increase. Reservoirs of as-yet unseen matter skirting the galaxies could reverse the solution to this first cosmological test, forecasting a closed Universe possibly having an end as well as a beginning. Whether such a Universe originated from a unique big bang prior to which nothing at all existed, or whether such a Universe ends for all time without bouncing, cannot be addressed by this test.
Frankly, that astronomers are deeply puzzled about the nature of this dark, or hidden, matter is an understatement. We don’t know what it is, only that it almost surely exists. Nor do we know much about how it’s distributed in space, but there are some clues. We can only infer its effects indirectly by two methods, each of which measures the dynamical behavior of individual galaxies: First, the outer parts of galaxies rotate faster than expected for the visible matter seen, implying that invisible (or dark) matter must be present to gravitationally prevent those outer parts from dispersing, like mud flung away from a spinning bicycle wheel. Second, within much larger clusters of galaxies, some galaxies have motions so large that they should have escaped from their group long ago—unless, again, some sort of dark matter is gravitationally binding the clusters together.
What is this dark stuff and where is it hiding? For the past few decades, astronomers have sought unconventional forms of normal, or “baryonic,” matter, suspecting that they may have overlooked an important part of their cosmic inventory. (Baryons include the atoms of which all stars, planets, and life forms are made—mostly the protons and neutrons comprising our tangible world, namely, the basic ingredients in chemistry's’s periodic table of the elements.) One possibility is that cold, tenuous matter might be lurking in and amongst the galaxies, but radio astronomers, whose equipment is most sensitive to this kind of low-energy gas, have found little of it. Hot, tenuous matter is another possibility, but x-ray astronomers who are best equipped to detect such intensely glowing, high-energy gas have also found hardly enough of it to account for the hidden matter. Dwarf stars that are not only small but also very dim, especially among the rich globular star clusters in the large, spherical halos of galaxies, are yet another candidate for locales where matter might have gone unseen; but recent, direct, telescopic observations of such clusters have found surprisingly few dwarf stars. Wandering blobs of compressed matter, either clumps of gas that never achieved official stardom or burned-out cores of erstwhile stars—collectively called MAssive Compact Halo Objects, or MACHOs for short—were once a leading possibility, but few of them have been spotted in the halo of our Milky Way and none at all in distant galaxies. Even black holes, as noted later in the STELLAR EPOCH, are not found in great abundance, making them unlikely places to trap lots of matter that cannot be seen. And so on, down the list of many candidates for normal matter, none of which has panned out in recent years, despite direct, exhaustive observational searches for them.
In contrast, most astronomers are now agreed that the bulk of the suspected dark matter is probably made of material that is abnormal. Dark matter is more likely “non-baryonic,” that is, composed of matter completely different from that comprising atoms as we know them. This type of dark matter probably exists as exotic subatomic particles, formed in the early Universe and now moving around sluggishly and elusively. Known as “cold dark matter,” it also collectively goes by the acronym WIMPs, which stands for Weakly Interacting Massive Particles—weakly interacting since these elementary particles ostensibly remain aloof from normal matter, yet massive since they still exert gravity even if they are “dark” and emit no light. To solve the dark-matter problem, the putative particles would have had to survive to the present day in gargantuan quantities and to pervade virtually every part of the cosmos. Alas, no one has ever seen such particles directly, or even evidenced them indirectly, and no telescope is built to detect such peculiar stuff. Our best bet to do so is in the high-energy accelerators where elementary particles can be created from packets of energy, as noted later in this PARTICLE EPOCH, but thus far no WIMPs have definitively emerged. Whatever the dark matter is, its reality seems unambiguous. Until its nature and composition are resolved, the issue of dark matter remains one of the thorniest challenges for astronomy today.
As far-fetched as dark matter might sound, do note that the history of astronomy is replete with dark objects that were later identified by other means. These include the planet Neptune and the small, dim companion star to Sirius A. Both of these (now) well-known objects were first detected by their gravitational effects alone, and thus so may dark matter be identified.
How much dark matter are we talking about? Some observations imply that each galaxy could conceivably contain as much as 10 times more dark matter than its luminous material, and the figure for groups of galaxies might even be higher; astonishingly, perhaps as much as 95% of the total mass in the huge galaxy clusters is invisible. Even so, based on the best data currently available, most astronomers now reason that dark matter probably raises the overall cosmic density to no more than about a third of that critical value needed to collapse the Universe in some far-future time.
The observed universal density determined by this galaxy-counting method is thus quite uncertain at present. This test cannot clearly distinguish between the open and closed models, though at face value it favors an open Universe destined to expand forevermore.
An Accelerating Universe Another observational test seeks to determine the ultimate fate of the Universe, and here a new and unexpected result has been recently reported. Apparently, the real Universe might be a great deal stranger—and more complicated—than the simple models outlined earlier. The newest data suggest that the Universe may be not merely changing, nor merely expanding, but actually receding at ever-faster rates—a shocking development that has profound implications for cosmology.
Like the first destiny test, this second test also seeks to estimate the average mass density of the Universe. And it again relies on the fact that each and every piece of matter gravitationally pulls on all other pieces of matter. This second test addresses the question: How fast is gravity applying the brakes, which would ordinarily cause cosmic expansion to decelerate? Put another way, what is the rate of change of evolutionary change?
Given that the Universe began in a titanic bang, it must have expanded rapidly at first, thereafter gradually growing more sluggish. The expansion of anything violent—the debris of a bomb, the sound of a thunderclap, whatever—is always greater at the moment of ignition than at some later time. Hence, since looking out into space is equivalent to probing back into time, the recessional motions of the galaxies should be larger for the distant galaxies and somewhat smaller for those nearby.
Observers therefore try to detect any change in the Doppler-shifted velocities of our neighboring galaxies compared to those far away. This change is presumed greater for the finite, closed model of the Universe, since a large amount of matter needed to stop and then contract the Universe would have well slowed its expansion over the course of 14 billion years. The infinite, open model is predicted to show smaller changes in the galaxy recessional velocities, so in this case the deceleration of the Universe would be less.
Surprisingly, current data acquired in the late 1990s show none of this expected deceleration. Instead, observations of supernovae (exploded stars, to be discussed in the third, STELLAR EPOCH) in distant galaxies imply that the Universe is speeding up—in short, accelerating! Basically, the brightnesses of the supernovae are fainter than expected, meaning that they are probably farther away and somehow they had to get out to their greater distances. If it were not for the fact that two independent groups of astronomers found the same startling result, no one would believe it. But science is not a matter of belief and the data do clearly imply that the galaxies at large distances (hence those seen far back in time) are receding less rapidly than expected. These data are forcing a major revision in our Universe models.
Not that astronomers need to return to the drawing boards; that would be too drastic. Contrary to many hyped news reports, this surprising finding doesn’t mean that big-bang cosmology has been overthrown. The new results mandate a revision, but not a revolution, in our previous thinking. Some “wiggle-room” still exists in the analysis, and some astronomers prefer to treat the new results as tentative, pending more knowledge about how distant supernovae actually behave. Others argue that faraway supernovae might be dimmed because of tainted radiation from ancient stars or attenuating dust along our line of sight to them—yet none of these and other alternative interpretations have panned out. Most researchers have reluctantly accepted the new observational results and, for now at least, the speeding Universe seems to be real.
What could be the cause of such cosmic acceleration whose effects (as for any accelerated object) are likely minimal in the past yet more dramatic in the future? Frankly, it’s unknown, but one ironic possibility is that the culprit is the same “cosmological constant” invented (largely out of thin air, on a philosophical hunch) by Einstein decades ago to act as a repulsive force to counter gravity and thus keep his Universe models from collapsing. This factor acts only on the largest scales, thus potentially explaining its dormancy for the first many billions of years of universal history; only today would it be emerging as a major factor in cosmic expansion. What’s more, it’s thought to arise from “vacuum energy” associated with empty space itself, thus potentially accounting for a negative, or outward pushing and repulsive, pressure that might increasingly challenge gravity on the largest scales. In other words, according to quantum theory, any region of “empty” space—traditionally called a vacuum—actually seethes with energy as subatomic particles burst in and out of existence for extraordinarily short periods of time. Not to do so would be a violation of physical law, specifically of Heisenberg’s uncertainty principle that cannot bear the certainty of true emptiness. Vacuum energy thereby gives even to empty space a push at every point.
That said, astronomers have no clear understanding of the cosmological constant, and physicists can’t even define it. At best, we know that it must be related to a new kind of force whose strength, quite unlike gravity, must increase with distance. It would therefore grow stronger over the course of time, thereby escalating to runaway expansion on large scales, yet remaining negligible on small scales so as to avoid interfering seriously with Einstein’s gravity that has been so well tested locally in the Solar System. Said another way, vacuum energy is proportional to space itself, and as space increases with the Universe's expansion, so does the energy and its associated force. This wholly new force, whose physical significance is mostly a mystery and numerical value largely unknown, is neither required nor explained by any currently known law of physics.
Quintessence is the fanciful, generic name of another candidate phenomenon that might force the Universe outward, ever faster with time. Beyond the Aristotelian notion that all terrestrial things are made of four interchangeable elements—air, earth, fire, and water—the “fifth essence” of ancient Greek philosophy was responsible for celestial phenomena. Hence, the return of an ancient term (in name only) to describe an omnipresent property of spacetime that has the dual, and most peculiar, effect of both positive mass to gravitationally clump matter locally and negative pressure to accelerate the expansion of the Universe globally. But if quintessence does exist, where did it come from and is it any less ad hoc than the cosmological constant?
Whatever it is and however it works, the mysterious force that might cause the Universe to accelerate apparently derives from neither conventional matter nor ordinary radiation. For now, and partly as a pun—both on the nature of the substance and the extent of our ignorance—astronomers have given it the name “dark energy.”
Dark matter and dark energy have become embarrassing for astronomers struggling to inventory the Universe. Dark matter itself—whatever it is—seems to outnumber by a factor of 6 the normal (baryonic) matter of which galaxies, stars, planets, and life are made. And now, dark energy—whatever that is—dominates them all. Numerically, normal matter probably comprises only ~4% of the Universe, dark matter ~24%, and dark energy the rest—implying that more than 95% of the Universe is unaccounted for! Having so much of the Universe on “the dark side” is highly disconcerting and most scientists are more than a little uneasy about it.
Do note that an outside chance remains for dark matter and dark energy to be hardly more than theoretical artifacts devoid of reality. Both quantities are merely inferred to keep the Universe “balanced”—that is, to theoretically grant it that precise critical density demanded by the nearly equally peculiar concept of “cosmic inflation,” thereby making the Universe globally flat and of zero net energy (all of which to be described shortly in this first, PARTICLE EPOCH). It has often been said that this troubling state of affairs sits like a bone in the collective throat of many astrophysicists, and someday it could conceivably be shown to be incorrect. All the current uncertainty about dark matter and dark energy makes the astronomical community feel insecure, as though the mounting complications may well bring down our intricately constructed and increasingly complicated house of cards—the standard model of modern cosmology—as the next generation of scientists seeks to reinfuse simplicity into one of humankind’s greatest intellectual adventures.
Luckily for us studying cosmic evolution, the recent findings of possible cosmic acceleration do not strongly affect our story from big bang to humankind. Dark energy (if it really exists) was likely a negligible factor for the first many billions of years, only recently becoming more relevant as the cosmos grew larger. Nor does our present ignorance of dark matter much affect the recent story of natural history, for it too obeys gravity that dominates on large scales and doesn’t care much about particulars locally. Just as our cosmic-evolutionary scenario’s validity holds whether the Universe is as young as 10 or as old as 20 billion years—provided the relative ages are consistently and chronologically sequenced along the arrow of time—cosmic evolutionists are prepared to revise this grand narrative to incorporate the latest data. As for the future, an accelerating Universe portends an even more dramatic rise in energy flows, novel environments, and ordered structures—the likely result being ever-greater richness and diversity among all types of complex systems, including life, intelligence, and whatever comes next.