Authors: Natalie Angier
To get a grip on why redshifting occurs, it might help to briefly train your sights on the sound of one train passing. As vigorously as the United States Congress has sought to eviscerate America's passenger rail system, you undoubtedly have had the pleasure of listening to the prolonged, poignant wail of a train whistle. If so, you surely noticed that the sound changed pitch as it rushed by. On approaching, the whistle had the high, grating pitch of a piccolo. At the moment of alignment between you and the whistling engine car, the sound dropped down to the midlevel root-a-toot-hoot of a conventional train whistle. And as
the train left you in its rust-flecked dust, the whistle pitch dipped even deeper, finally assuming the mournful lowing of so long, take care, good-night to noises ... everywhere.
To a pair of station-bound ears, the whistle's sonic profile changes so dramatically from processional to recessional that it's easy to forget how the whistle pitch sounds if you're on the train rather than missing it: pretty much the same from beginning to end. Only to a target in relative motion compared to the source of the sound does the famed Doppler effect come into play. The effect, named for the nineteenth-century Austrian mathematician and physicist who formulated it, says the waves generated by a moving object will shift in size depending on whether the object is headed toward or away from you. If the object is a noisemaker, the sound waves will compress into a higher pitch on approach and relax into a lower pitch on retreat. If the object is a floating leaf, the ripples spreading on water will appear more closely spaced if the leaf is drifting toward you than away from you. The redshifting of light detected in the study of distant galaxies, then, is just another instance of the Doppler effect at work.
Importantly, however, the light from distant galaxies is always displaced in one direction. In our immediate Local Cluster of galaxies, there is some give and take. The Andromeda galaxy, for example, is blueshifted, a persuasive sign that it is headed toward us and we toward it, and that in about 6 billion years the two galaxies will, as a result of our mutual gravitational attraction, merge into one. But step outside the neighborhood, and you'll see only red. As revealed by the unwavering augmentation of their light waves, the remote galaxies must all be racing away from us. What's more, the greater the distance of the galaxy, the more extreme its redshifting, and in a roughly proportional manner. That is, if Galaxy Wibbleton lies at twice the distance from Earth as Galaxy Wobbleton, the light from Wibbleton will be twice as redshifted, twice as elongated, as the light from Wobbleton; if three times as far, it will look three times as red. How to explain this link between galactic distance and redshift radicalism? According to Doppler's equations, the velocity of any wave-making body affects the relative spreading or narrowing of its waves. The whistle of a fast train will, to an outside listener, squeal higher as it approaches and moan lower as it recedes than that of a slow train. In fact, the correlation between velocity and degree of Doppler shiftiness is precisely what allows a police officer to gauge your driving speed by bouncing radar signals off your vehicle and seeing how severely the car's speed distorts the wavelengths of the incident radiation; the greater the Doppler shift observed, the bigger the ticket
you'll be served. Those distant galaxies, in other words, must be retreating from us at a greater velocity than are the closer galaxies.
Or maybe we're just paranoid. As it turns out, the sense of being uniquely repulsive to almost everything in the universe is an illusion. If we were located in the Andromeda or Sombrero or M63 galaxy, the redshift profile of the cosmos would look the same as it does here in the Milky Way: as though other galaxies are all moving away from us, and at a velocity more or less proportional to their distance. How can this be, that each of us is cast in the role of mortician at a bridal shower? To understand the phenomenon, try this simple experiment, which requires nothing more than a balloon, a felt-tipped pen, and a second pair of lips. First, decorate the flaccid balloon with polka dots, spacing them out as evenly as you can. Then, ask your assistant lips to take up the newly freckled balloon and slowly blow. Put your finger on any of the dots, and study the dots that surround it. Note that as the balloon expands, the neighboring dots are all moving away from your finger. Note as well that the dots nearest your finger are retreating from you at a slower pace than are those farther away. The reason is that there is less expanding rubber between you and a neighboring dot than you and a far dot, less relative surface area to tug adjacent points from your vicinity. Now place your digit on one of those far-off dots and again look at the points around it. Same thing. The inflation of the landscape is pushing all spots out and away, out and away, and the far dots are receding from your finger more briskly than those close at hand. OK, that's enough blowing for now, Grandma, can I get you a cup of tea, seltzer, oxygen tent?
The expanding universe is not that different from an expanding balloon, except that the universe is bigger, colder, and darker, and it won't pop even if you put it in a cage with a pair of mating ferrets. Still, the balloon analogy shows how every vantage point on its swelling terrain appears to be the center of the universe without really being the center of the universe, and how distant objects will recede from one's vantage point at a faster velocity than near points not because they're sailing faster in any "real" or absolute sense but merely relative to the recession rate of sites nearby. The galaxies at greatest distance from Earth are not Olympian sprinters, not Hermes of Praxiteles to the local Yertles of Sala-ma-Sond. Their spectacular velocity is spectacular only to us, while to one another they are cruising along at unremarkable speeds. As Albert Einstein demonstrated in his special theory of relativity, it is meaningless to speak of an object's absolute speed or motion through space, for there is no final arbiter, no unchanging, eternal grid against
which that speed can be clocked. All you can ask is "fast compared to what?" From our perspective, we and our nearby galaxies are moving through space at about 370 miles, or 590 kilometers, per second, which is only slightly faster than a tractor-trailer headed down a Montana freeway at 2:00 in the morning. By contrast, the most distant galaxies seem to be receding from us at velocities of thousands or tens of thousands of miles per second, uncomfortably close to the speed of light and illegal even on the Autobahn. To their local highway patrolmen, however, those far-off galaxies are tooling along at disappointingly licit speeds of roughly 590 kilometers per second.
Another way in which the balloon exercise can illuminate the nature of our expanding universe is this: the dots are not really taking the initiative and moving away from one another, as they might if they were ants on the surface rather than pen markings embedded in the surface. Rather, the skin between the dots is stretching wider. Similarly, the galaxies of our universe are not really rushing away from one another. They're not traveling through space, they're traveling with space. They are more or less staying put, while the space between them just keeps expanding. This distinguishes large-scale galactic motions from other celestial pilgrimages. Under the spur of gravity, the Earth and its sibling planets orbit the sun. Our solar system in turn is gradually making its way around the dense and gravitationally convincing hub of the Milky Way, completing a galactic circumgyration once every 230 million years. But while there are some regional exceptions (like the gravitational attraction that is slowly drawing us and Andromeda closer together), galaxies are distributed through the cosmos with sufficient homogeneity that they end up being gravitationally neutral with respect to one another. The galaxies themselves are neither wandering nor widening. It is the space in between that can't stop loosening its belt.
I fear that, on a visceral level, this idea is almost impossible to accept, no matter how many packages of balloons your loyal, rapidly blue-shifting Nana inflatesâthat the expansion of the universe is not a tangible matter of galaxies exploding outward into space like shrapnel from a bomb, but of space itself exploding outward, the shrapnel trapped in its hide. For one thing, space is not supposed to do anything except sit there waiting to be crossed or filled. For another, what is it expanding
into?
More space? If so, why doesn't all the space just smear together in the first place? How could we have an expanding universe if space is expanding into space? Wouldn't that be like trying to blow up a balloon full of holes? Well, you may derive some comfort in knowing that astronomers don't have an intuitive grasp of the subject either.
"The expansion of space is a concept that I understand mathematically, but on a personal level, no, I can't do it," said Mario Mateo, a professor of astronomy at the University of Michigan.
In fact, when Raman Sundrum, a professor of physics and astronomy at Johns Hopkins, gives public talks about universal expansion "and where all the galaxies are going," he has his audience do a modest mathematical exercise. Think about simple, whole numbers, he saysâone, two, three, four, five, and so on. How far apart are these numbers from one another? One, his listeners reply. The distance between them is one. And how many such numbers are there? Well, they keep going, the audience responds. There's an infinite number. "Now I tell people to double every number, so that one becomes two, two becomes four, three becomes six, and on and on," said Sundrum. "For every original number, I've given you a new number, but the distance between them has gotten bigger. Now I tell you to double the numbers again, to four, eight, twelve, sixteen, twenty. And again, to eight, sixteen, twenty-four, thirty-two. I'm always giving you a new number for the old number, so that it's the same number of numbers, but the distance between the numbers is getting further and further apart. In a sense, the numbers are receding from each other faster and faster, and what we're seeing with galaxies is something like that. As far as we know, the cosmos can extend forever, just like the numbers can go on forever. What are the cosmos expanding into? Well, what are our numbers expanding into? There's no running out of space, and there's no running up against an edge, and yet, the distance between the galaxies, as with our integers, keeps increasing."
Perhaps easier on the mind's eye than the unseemly uppitiness of expanding space is to do as cosmologists have and run the tape in reverse. If you systematically rein in all those receding galaxies, at speeds counterproportional to the velocities with which they are disappearing into the distance, you eventually reach a point where they're piled one on top of the other: the equivalent of some 100 billion galaxies of hundreds of billions of stars apiece, all cohabiting the same pre-place, ante-space patch of protoâreal estate. The lights, and the plasma, and the inferno come free.
After discovering galactic redshifts and connecting and reconverging the dots backward toward an event that, for all the hubristic ring of it, could be thought of only as the birth date of the cosmos, scientists began sketching out what conditions must have been like when the universe was new. Their computational conjurings proved materially as well as aesthetically fruitful, for they eventually yielded the second major piece of evidence in support of the Big Bang, as we'll discuss in a moment. So what might our big fat bouncing pistol of a neonate have looked like? First of all, cosmologists plead, bear in mind that the birth of the universe didn't take place in a specific location in space because the space and matter of our universe came into being simultaneously, essentially bubbled up out of ... well, we don't know what. The void? Another bubble in a larger burbling crockpot of cosmic chowder, of universes within universes? We don't know, and we may never know, for what is beyond our universe may remain forever inaccessible to any sensors or instruments within our universe; and without evidence, we are trading not in the science of astrophysics, but in idle metaphysics, sophomoric philosophistry, and a few too many boxes of Milk Duds.
In any event, what we do have evidence for is this: In the beginning, there was light all right, an overwhelmingly bright, hot light like nothing you've seen or felt or could see or feel, for, as Alan Guth so gaily put it, "The photosensors in your eyes would evaporate instantly." The light! the light! exploded into being, a radiant seed of pure energy, tinier than the proton of an atom yet almost infinitely dense, with a temperature in the trillions of degrees, and it instantaneously began swelling outward. Almost immediately after the expansion had begun, some of the energy managed to condense into matter, into elementary particles, like electrons and the subparts of protons and neutrons, the quarks; and their countercharged, counterspinning antimatter counterparts, the positrons and the antiquarks.
Still the mat of matter and energy expanded mightily. In a fraction of a trillionth of a second, the universe had inflated from its subatomic birth girth to something on par with a cantaloupe; before a thousandth of a second had passed, the cosmos had cast itself across two-thirds of a mile. It grew and it glowed with a light not only blindingly, deretinizingly bright, but of a purity and uniformity not seen in our quotidian lights, our lamps or our suns or our bombs. There were, in fact, tiny ripples in this dawn's early light, minute irregularities in the radiant paste of the universe that eventually would prove our salvation, but these gradations, these flickering slubs, were of quantum amplitudes, and so the light at first seemed unimpeachably pure, fittingly ethereal.
For the firstborn particles, the swelling belly of the baby was pure hellion, and they were smashed and rattled and ashed into radiation and reparticulated back out again, over and over. Nevertheless, with the expansion came a deruffling of tempers, a sufficient cooling for the condensation of matter to continue beyond the most elementary phase.
Quarks teamed up as triplets into reasonably sturdy protons and neutrons, while nearly equal numbers of triumvirates of antiquarks formed into antiprotons; and through the archaeostew flew electrons and positrons, too. Still the material surl was not through, for matter and antimatter cannot share the same province and survive. Protons and antiprotons sprang into being, only to collide into mutual annihilation; electrons and positrons were tossed together and lost together. Luckily, for reasons that remain mysterious, the early universe had been salted with a slight excess of matter over antimatter: for every billion or so antiprotons and positrons aswirl in the starter stew, there were a billion and one protons and electrons. The result? When the great matter-antimatter hissing match had finally played itself out, there were just enough protons and electrons left to start building atoms, stars, galaxies, cats, hats, pianos, piano tuners, physicists, and atom smashers to recapitulate conditions of the early universe.