Farewell to Reality (36 page)

The kind of model developed originally by Vilenkin and Linde is now more commonly referred to as
eternal inflation.
In this model, our universe is merely one of countless ‘bubbles' of inflated spacetime,
triggered by quantum fluctuations in a vast inflaton field (or fields) driven by competition between the decay of the field's energy and the exponential growth of energy in pockets of inflation. In certain models the competition is unequal, and the bubbles proliferate like a virus, or like the bubbles in a bottle of champagne when the cork is extracted.

The bubbles form a multiverse much like holes in an ever-inflating piece of Swiss cheese. This is, of course, an imperfect analogy, as it is the holes or bubbles themselves (rather than the cheese) that account for much of the spacetime volume. Such an ‘inflationary multiverse' could be essentially eternal, with no beginning or end.

In the inflationary multiverse, anything is possible. The essential randomness of the quantum fluctuations that trigger bubbles of inflation imply a continuum of universes with different physical laws (different cosmological constants, for example).

Let's reserve judgement on this idea for now, and simply note that we're dealing here with another assumption.

We need to be clear that the multiverse of the many worlds interpretation and the multiverse of eternal inflation are necessarily different. They originate from within different theoretical structures and have been proposed for very different reasons. But, as the saying goes: in for a penny, in for a pound. If we're going so far as to invoke the idea of a multiverse, why not simplify things by assuming that the bubble universes demanded by eternal inflation are, in fact, the many worlds demanded by quantum theory?

In May 2011, American theorists Raphael Bousso and Leonard Susskind posted a paper on the arXiv pre-print archive in which they state:

In both the many-worlds interpretation of quantum mechanics and the multiverse of eternal inflation the world is viewed as an
unbounded collection of parallel universes. A view that has been expressed in the past by both of us is that there is no need to add an additional layer of parallelism to the multiverse in order to interpret quantum mechanics. To put it succinctly, the many-worlds and the multiverse are the same thing.
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Breathtaking.

But we're still not quite there. Joining the many worlds interpretation to the inflationary multiverse cannot explain the current fascination with multiverse theories. To understand this fascination, we must return once again to superstring theory. As Alan Guth explained in 2007:

Until recently, the idea of eternal inflation was viewed by most physicists as an oddity, of interest only to a small subset of cosmologists who were afraid to deal with concepts that make real contact with observation. The role of eternal inflation in scientific thinking, however, was greatly boosted by the realization that string theory has no preferred vacuum …
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The cosmic landscape

In
The Hidden Reality,
Brian Greene wrote of his early experiences with Calabi—Yau shapes, the manifolds which are used to roll up and hide away the six extra spatial dimensions demanded by superstring theory:

When I started working on string theory, back in the mid-1980s, there were only a handful of known Calabi—Yau shapes, so one could imagine studying each, looking for a match to known physics. My doctoral dissertation was one of the earliest steps in this direction. A few years later, when I was a postdoctoral fellow (working for the Yau of Calabi—Yau), the number of Calabi—Yau shapes had grown to a few thousand, which presented more of a challenge to exhaustive analysis — but that's what graduate students are for. As time passed, however, the pages of the Calabi—Yau catalog continued to multiply … they have now grown more numerous than grains of sand on a beach. Every beach.
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Greene wasn't kidding. In 2003, theorists Shamir Kachru, Renata Kallosh, Andrei Linde and Sandip Trivedi worked out the number of different Calabi—Yau shapes that are theoretically possible. This number is determined by the number of ‘holes' each shape can possess, up to a theoretical maximum of about five hundred. There are ten different possible configurations for each hole. This gives a maximum of 10
⁵⁰⁰
different possible Calabi—Yau shapes.

The precise shape of the Calabi—Yau manifold determines the nature of the superstring vibrations that are possible. It thus determines the physical constants, the laws of physics and the spectrum of particles that will prevail. In other words, the shape determines the type of universe that will result. The figure 10500 therefore refers to the number of different possible
types
of universe, not the total number of possible universes. This is what Guth meant when he talked about string theory having no preferred vacuum.

I believe there was a time in the history of physics when this kind of result would have been taken as evidence that a theoretical programme had failed. We could conclude that 10
⁵⁰⁰
different possible Calabi—Yau shapes with no compelling physical reason to select the one shape that uniquely describes our universe — and hence describes the laws and the particles that we actually observe — leave us with nowhere to go. Time to go back to the drawing board.

Except, of course, we now have eternal inflation and the inflationary multiverse.

Far from this vast multiplicity of possible Calabi—Yau shapes being seen as evidence for the failure of the superstring programme, it is instead used to bolster the idea that what the theory is describing is actually a multiverse. Greene again:

The idea is that when inflationary cosmology and string theory are melded, the process of eternal inflation sprinkles string theory's 10 possible forms for the extra dimensions per bubble universe — providing a cosmological framework that realizes all possibilities. By this reasoning, we live in that bubble whose extra dimensions yield a universe, cosmological constant and all, that's hospitable to our form of life and whose properties agree with observations.
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Each bubble in the inflationary multiverse represents a universe that may be much like the one we inhabit, or it might be subtly different, or it might be vastly different. It doesn't take a great leap of the imagination to suggest that each bubble is characterized by the Calabi—Yau shape that governs its extra spatial dimensions.

Leonard Susskind calls it the cosmic landscape. He is at pains to explain that the landscape of possibilities afforded by the 10 Calabi— Yau shapes is not ‘real'. It is rather a list of all the different possible designs that universes could possess. However, he is unequivocal on the reality of the multiverse: ‘The pocket [i.e. bubble] universes that fill it are actual existing places, not hypothetical possibilities.'
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Let's log it as another in what is proving to be a long series of interconnected assumptions.

The universe next door

The inflationary multiverse provides a mathematical metaphor for a cosmos in which countless bubbles of spacetime are constantly inflating like balloons. Any region of spacetime devoid of content but for the inflaton field is potentially unstable and susceptible to quantum fluctuations which may trigger inflation. When combined with superstring theory's demand for extra ‘hidden' dimensions, we conclude that the bubbles are characterized by different Calabi—Yau shapes, giving rise to universes with different physical constants, laws and particles.

Of course, we have already encountered something very similar to this. The D-branes of M-theory represent extended string-like objects that can accommodate whole universes with different physical constants, laws and particles. Indeed, the Ho
ř
ava—Witten and Randall—Sundrum braneworld scenarios are examples of cosmological models involving parallel universes. In the latter, the two branes describe
universes in which gravity acts very differently. In one universe it is strong. The warped spacetime of the bulk then dilutes the gravitational force so that when we experience it in our braneworld, is it considerably weakened.

It doesn't require a great leap of imagination to propose that there may be many more than just two braneworlds ‘out there'. If the structure of each brane is governed by the Calabi—Yau shape that hides its extra dimensions, then M-theory would suggest that the multiverse is actually one of parallel braneworlds.

On the surface, this is yet another take on the multiverse idea. But the brane multiverse is rather different. The many worlds interpretation and the inflationary multiverse leave us with a rather static picture. The parallel universes are almost by definition detached one from another and they do not interact. But in M-theory, branes are dynamic objects. They move around (in the bulk). And they can interact.

What happens when brane universes collide? They don't just pass through each other like ghostly ships in the night. M-theory suggests that a collision between braneworlds could be very violent.

In 2001, Princeton theorists Paul Steinhardt and Justin Khoury, working with Burt Ovrut at the University of Pennsylvania and Neil Turok, then at Cambridge University, used heterotic M-theory to study the effects of colliding braneworlds. They concluded that, under the right circumstances, the collision of a ‘visible' 3-brane containing four large spacetime dimensions and standard model particles with an ‘invisible' 3-brane moving slowly through the bulk could in principle trigger a cataclysmic release of energy. A small proportion of the kinetic energy (the energy of motion) of the invisible brane is converted into hot radiation, which bathes the matter in the visible brane.

The hot radiation is later recognized as a hot big bang. It accelerates the expansion of spacetime without requiring the mechanism of inflation as used in the ACDM model and eternal inflation. The collision resets the visible brane's time clock, and, to all intents and purposes, the universe we observe today is born. The theorists wrote:

Instead of starting from a cosmic singularity with infinite temperature, as in conventional big bang cosmology, the hot, expanding universe in our scenario starts its cosmic evolution at a finite temperature. We refer to our proposal as the ‘ekpyrotic
universe', a term drawn from the Stoic model of cosmic evolution in which the universe is consumed by fire at regular intervals and reconstituted out of this fire, a conflagration called ekpyrosis. Here, the universe as we know it is made (and, perhaps, has been remade) through a conflagration ignited by collisions between branes along a hidden fifth dimension.
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As far as I can tell, the term ‘ekpyrotic universe' didn't catch on. Some have referred to the scenario instead as the
big splat,
which is something of a misnomer, as the branes don't so much splat as bounce off each other.

I should make clear that Steinhardt and Turok and their colleagues do not promote their work as an argument in favour of a brane multiverse. Rather, they have developed a cosmology which seeks to explain our universe in terms of an eternal cycle: two branes collide and bounce apart, but never separate far enough to escape their mutual gravitational influence (which is manifested in our universe as dark energy). Some trillions of years later the branes collide again, the visible universe is ‘reset' and the cycle repeats.

In this colliding braneworld scenario, the pattern of temperature variations observed in the CMB radiation is the result of quantum fluctuations in both braneworlds at the point of collision. The branes are not perfectly ‘flat'. Quantum uncertainty means that the braneworlds come into contact first in places where the amplitudes of fluctuations in the direction of the bulk dimension are high. These cause ‘hotspots' — places in the visible brane where the radiation temperature is higher than the average.

Steinhardt, Turok and their colleagues have continued to develop this cyclic cosmology, which they now refer to as a ‘phoenix universe', championing it as a viable alternative to cosmologies based on inflation. In 2008, Steinhardt and Princeton theorist Jean-Luc Lehners adapted the model to prevent spacetime collapsing into an amalgam of black holes. In this revised model, much of the universe is destroyed but a small volume (of the order of a cubic centimetre) survives the ekpyrotic phase (hence the term ‘phoenix'). ‘As tiny as a cubic centimetre may seem, it is enough to produce a flat, smooth region a cycle from now at least as large as the region we currently observe. In this way, dark energy, the big crunch and the big bang all work together so that the
phoenix forever arises from the ashes, crunch after crunch after crunch.'
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The reality check (II)

There are other kinds of multiverse theory that unfortunately we don't have the space to consider here. In
The Hidden Reality,
Greene identifies nine different types, including many worlds, the inflationary multiverse, the brane and cyclic multiverses and the cosmic landscape.

This is surely an extraordinary situation. At great risk of repeating myself, I want to summarize briefly the steps that have brought us here.

The current authorized version of reality consists of a collection of partially connected theoretical structures — quantum theory, the standard model of particle physics, the special and general theories of relativity and the ΛCDM model of big bang cosmology. There remain many significant problems with these structures and we know this can't be the final story. But there is little guidance available from experiment and observation. There are no big signs pointing us towards possible solutions.