Stephen Hawking (30 page)

Just after the Planck time, according to the inflationary scenario, the vacuum itself was in a “false” state, excited and full of energy, like supercooled water. When the false vacuum underwent a transition into its stable, lower-energy state, this energy went into the phenomenal burst of expansion that is known as inflation, creating the smooth Big Bang out of which the Universe as we know it has evolved. But suppose this transition did not happen everywhere at the same time.

Almost as soon as Alan Guth came up with the idea of inflation, researchers such as Alex Starobinsky and Andrei Linde realized that different regions of the primordial false vacuum might have made the transition into the low-energy state independently. The effect would be rather like unscrewing the cap of a bottle of fizzy drink—a myriad of bubbles would
appear throughout the fluid, each corresponding to a stable vacuum expanding in its own way. Unlike the bubbles in your fizzy drink, though, each of these bubbles would carry on expanding, until all the fluid had gone and only bubbles remained.

This possibility raised serious technical problems for early versions of the inflationary scenario because if two or more expanding bubbles were to merge, they would create disturbances that would spread right through both bubbles. If we lived in a universe that had formed in this way, it would not be perfectly uniform, because these disturbances would leave their mark—for example, on the microwave background radiation.

There are ways around this problem. The notion that Hawking himself favors is that of “chaotic inflation,” in which the world beyond our Universe (the infinite “meta-universe,” now usually called the Multiverse) is in a messy state, with some regions expanding, some contracting, some hot and some cold. In such a chaotic meta-universe, there must inevitably be some regions just right for inflation to take place. We just happen, in this picture, to be in a universe produced by a random fluctuation within the chaos.

But you don't have to invoke chaos to explain our existence. Maybe we just happen to live in a bubble that hasn't (yet!) merged with any of its neighbors (if this sounds like an extraordinary coincidence, it may not be, as we shall see later in this chapter). Or perhaps some law of physics prevents bubbles from forming very close together in the “fluid” of the false vacuum. This is where the proposal that Hawking Radiation might be involved comes in.

Hawking Radiation, as we saw in
Chapter 9
, is produced by the interplay of quantum effects and gravity at the horizon surrounding a black hole. But Hawking and his colleague Gary Gibbons, who shared an office with him in Cambridge in the late 1970s, realized that this kind of radiation must be produced wherever there is a horizon of this kind, and that such horizons do not always surround black holes.

Because of the way the Universe expands, the more widely separated two regions are, the faster they recede from each other. So regions of space that are far enough apart can never “communicate” using light beams (or, indeed, anything else) because the space between them expands faster than light can travel. If light cannot travel from one region to another, then in effect there is a horizon which light cannot cross, separating the two regions of space as effectively as the horizon surrounding a black hole separates the inside from the outside.

Hawking and Gibbons showed that this kind of horizon will also produce radiation, just like the radiation at the horizon around a black hole, spreading out from the horizon into both regions of space. In the Universe as it is today, spread thin by expansion, the effect of this radiation is tiny, but it could have played a much bigger role in the early stages of the expanding Universe. The expansion of the Universe is steadily slowing down, as the gravity of all the matter in the Universe tries to pull everything back together in a Big Crunch. So the expansion rate was much faster, and the effect of Hawking Radiation from horizons therefore more pronounced, when the Universe was younger. Long ago, even rapidly separating regions had not had time to move far and were much closer together.

The notion that radiation produced by horizons might affect the expansion of the Universe has been enthusiastically taken up and combined with the idea of inflation by Richard Gott of Princeton University. Andrei Linde has also investigated it, but he has made less noise about the idea than the ebullient Gott.

It turns out that under the right conditions, the Hawking Radiation produced in a volume of space filled with horizons of this kind can provide the energy that drives inflation and makes the Universe (or rather the Multiverse) expand super-fast. The super-fast expansion then creates more horizons, which in turn produce more radiation, driving the super-fast expansion in a self-sustaining continuing process of inflation. The bubbles of ordinary low-energy stable vacuum that form within this infinite sea of inflationary expansion grow at a slower rate; and so even if two bubbles form next to each other, they will be kept apart by the rapid growth of the false vacuum of the Multiverse between them.

The “right” conditions for this process to work are mind-boggling. The temperature of the Hawking Radiation has to be about 10
³¹
K, and the density of mass-energy in the false vacuum has to be an even more staggering 10
⁹³
grams per cubic centimeter. And everywhere throughout this extraordinary, rapidly expanding false vacuum, bubbles of stable vacuum are forming and becoming universes in their own right.

In this scenario, there is not just one Universe but an infinity of universes, forever separated from one another by the impenetrable walls of the super-dense false vacuum. In a sense, such a concept is meaningless. The existence of other
universes which we can never observe, and which can never have any interaction with our Universe, is a matter more suitable for discussion among philosophers than astrophysicists. But it turns out that there are more ways than one to make a universe and that in some scenarios universes can interact with one another, producing consequences of interest to everybody, not just to astrophysicists and philosophers.

With all this talk of superdensity and superenergy, and numbers like 10
⁹³
grams per cubic centimeter being bandied about, it is natural to wonder how much mass-energy our entire bubble Universe contains (assuming, that is, that any of these scenarios have a grain of truth in them). The answer is perhaps even more startling—none at all! Let us leave the discussion of continual inflation to the philosophers and look again at Hawking's no-boundary model of the Universe to see how this can possibly be true.

We are used to thinking of mass-energy chiefly in terms of lumps of matter: stars, planets, and so on. Each of them contributes its own amount of
mc
²
to the total mass-energy of the Universe. But there is another, equally important contribution (
exactly
equally important, if Hawking's ideas are correct): it comes from gravity. And there is a strange thing about gravitational energy—it is negative.

To understand what this means, physicists talk in terms of the gravitational energy of a hypothetical collection of particles. This is zero if the particles are dispersed to infinity, spread apart from one another as far as possible. But if the
collection of particles falls together under the influence of gravity, perhaps eventually to make a star, it loses gravitational energy. Since the particles start with zero energy, this means that by the time they have collected together to form a star or a planet they have negative energy. And if all the matter in the entire Universe could be collected together at a single point, its negative gravitational energy (–
mc
²
) would exactly cancel out all the positive mass-energy (+
mc
²
) of all the matter.

But that is exactly how we think the Universe did start out: with all its mass-energy concentrated in a point. The closed Universe scenarios actually describe a situation in which a point of zero energy becomes separated into matter (with positive energy) and (gravity with negative energy), expands out to a certain size, and then collapses back into a point of zero energy again. At first, the idea seems ridiculous. However, this is not some crackpot, lunatic-fringe theory, but a respectable cosmological idea, backed up by the equations of relativity.

The Universe, it seems, is the ultimate free lunch. And if the Universe contains zero energy, how much energy does it take to make a universe? Not a lot—certainly not very much compared with the amount of
mc
²
contained in your body or the pages of this book. For according to Alan Guth and his colleague Edward Fahri, all you need is enough energy to squeeze some matter into forming a black hole. Then the new universe comes free—one universe free with every black hole. In a
tour de force
to rank with the great conjuring tricks, Guth and Fahri have shown that the two great threads of Hawking's life's work are really one and the same: black holes are big bangs.

In principle, the seeds of entire universes could be produced out of nothing at all, in a manner reminiscent of the way pairs of virtual particles can be produced out of nothing at all by quantum uncertainty (as we saw in
Chapter 9
). Such a baby universe would be in the form of a super-dense concentration of mass, smaller than a proton but containing no energy because the mass is balanced by negative gravitational energy. Of course, according to the ideas of the 1970s and before, such tiny super-dense seeds would immediately collapse back into nothing under their own weight. But inflation provides a way to blast out such a seed to form an expanding universe before gravity can make it collapse. It would then take many billions of years for gravity first to halt the expansion and finally to make the universe disappear into a Big Crunch.

So do we really need the continually inflating false vacuum to make bubble universes pop up in infinite numbers? At first sight, this raises a worrying possibility. If a bubble universe can pop into existence out of the ordinary vacuum, what would happen if one burst into existence near us? Would we be overwhelmed by the expanding fireball of a Big Bang going on right next door? Fahri and Guth think there is nothing to worry about. If such baby universes pop into existence spontaneously, or if they were created artificially, they would have no further interaction with our Universe once they had been born.

Remember that the seed of such a bubble universe must be self-contained, destined ultimately to collapse back in on itself; in other words, it must be a black hole. Fahri and Guth found that you could trigger this process of universe creation artificially, by squeezing a small amount of matter into a black
hole at a temperature of about 10
²⁴
K (quite modest compared with conditions in the false vacuum).

But they gave their scientific paper on the subject the tongue-in-cheek title “An Obstacle to Creating a Universe in the Laboratory,”
¹
pointing out that although we have the technology (hydrogen bombs) to do half the job, releasing the energy required, we don't yet have the ability to confine the energy released by hydrogen bombs within a black hole.

But it is not beyond the bounds of possibility that a civilization more advanced than our own might be able to confine the required energy in a small enough volume. What would happen then? To the people who created this energetic minihole, very little. The black hole would simply form, spend billions of years evaporating through Hawking Radiation, and then disappear. But within the horizon of the hole, things would be very different.

According to the calculations by the American team, conditions inside such an energetic minihole will sometimes be such as to trigger inflation. But when such a baby universe begins to expand, it does so not by bursting out of the minihole to engulf its surroundings in the spacetime in which it was created, but by expanding in a set of directions which are
all
at right angles to
each
of the dimensions of the parent universe. And exactly the same thing will happen to baby universes that are produced by natural quantum fluctuations.

Because all the sets of dimensions are at right angles, the different universes never interact with one another once they have formed. But there is a crucial difference with the continual-inflation idea, where the bubbles never interact at
all: in the scenario sketched by Fahri and Guth (and studied by others, including Linde), one universe is created
from
another. In this picture, our Universe is the progeny of a previous universe; and it is even possible that our expanding bubble of spacetime was created artificially in the equivalent of a laboratory in that parent universe. Science fiction writer David Brin is already working on the implications in a linked series of stories; we will leave further speculations along these lines to Brin and his colleagues while we try to explain the implications in terms of the spontaneous creation of baby universes.

It is hard to get a mental grip on the proliferation of dimensions that this implies. Every baby universe will contain its own vacuum, within which other quantum fluctuations can occur, producing yet more baby universes each with their own set of dimensions, with every set of dimensions at right angles to every other set. As usual, we have to fall back on an analogy in two dimensions, bent around a third, to get a picture of what is going on.