A Maddening Myriad of Multiverses: A Crash Course to Multiverse Theories
A layman's introduction to the various kinds of multiverse models - Quilts, Inflation and String
The multiverse is the hypothetical set of infinite or finite possible universes (including the universe we consistently experience) that together comprise everything that exists: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them.
Essentially, any theory stating the existence of a multiverse (many universes) is a prediction based on a theory or hypothesis. Like any and all theories that are formulated, these too are predicated on certain assumptions that one must make as a starting point. However, it is convenient to gloss over the assumptions when contemplating the bizarre ramifications envisioned, such as “an infinite number of duplicate yous living in alternate universes”. The amazing mathematician Max Tegmark has previously covered this concept of “duplicate yous living in alternate universes” in his seminal article on ‘Parallel Universes’.
I. The Exercise
While we will not be delving into the exact specificities of the various multiverse theories that I am about to delve into as part of this article – to be frank, I cannot even summarize them appropriately considering the thousands of pages that cosmologists, physicists, philosophers, astronomers, psychologists and clergymen may have produced on each of these theories – I will be providing a bird’s eye view of the different multiverses that may be possible given what we know about the Universe. In an ideal scenario in my head, this would form the basis for later discussions I may want to delve into with respect to the cosmological multiverse theories.
II. The Bird’s Eye View
The proposed Multiverse Theories represent, on one extreme, an extension of our universe and, on the other extreme, every mathematically possible universe. It would not be wrong to suggest that each of the multiverse theories that this article delves into lies somewhere along the spectrum between these two extreme points.1[1]
There are a few points I think we all ought to keep in mind before we take on these different theories. Firstly, none of the multiverses have all the same characteristics. Secondly, the number of Space dimensions that each of these theories advocate for range anywhere from three to infinity (Yeah, that is a massive margin for error); Thirdly, the Cyclic multiverse is unique in that its multiversal nature stems from a temporal origin rather than a spatial one. In simpler words, this is to say that it imagines (or generates) universes parallel in time rather than space.
The Holographic proposal embodies only two universes, while other proposals range from many to infinite. The Simulated and Ultimate proposals allow universes with different laws of physics.
III. Fine Tuning in the Multiverses through ‘Dimensionless Input Parameters’
On a quick glance at the classification we have provided above, one can notice that eight proposals provide an explanation for ‘fine tuning’ – a noteworthy characteristic. Fine tuning is the improbable coincidence that a random collection of dimensionless input parameters (referred to as “DIPs”) (numbering at around 30, derived from the Standard Models of particle physics and cosmology), would produce a universe like ours. For instance, if the cosmological constant were slightly larger or smaller, galaxies could not have formed2. If force strengths or particle masses were to vary by a few percent, no atoms, stars or planets would exist – our life-friendly universe is, in this sense, a rare combination of input parameters3. Other permutations and combinations of these DIPs might exist in different universes (in those multiverses that allow for differentiation in DIPs and have enough number of universes within them to make it a mathematical possibility), but the vast majority would be devoid of life.
The Input Parameters: The different DIPs that were identified by Max Tegmark, Anthony Aguirre, Martin Rees and Franck Wilczek in their piece on ‘Dimensionless Constants, Cosmology, and Other Dark Matters’ (Click here only if your brain can handle information in excess of an average of 10 equations per page – I know mine struggles with it). While their work is nonetheless noteworthy, it is important to note that their calculations themselves were derived from the work already undertaken by the Particle Data Group under S. Edelmann et. al, R N Mohapatra’s ‘Physics of Neutrino Mass’, and Max Tegmark and Anthony Aguirre’s ‘Multiple Universes, Cosmic Coincidences and Other Dark Matters’.
Physicists have attempted without success to mathematically calculate DIP values from first principals. Even string theory physicists have failed at predicting correct values. If these values defy calculation, how do we explain fine tuning? The multiverse provides an answer – if there are billions or trillions or an infinite number of universes, one or more will have the exact constant values we observe. Envisioning an infinite number of universes to prove the existence of one universe seems a bit excessive but, some proponents use this logic as indirect proof of multiverse proposals.
In a sense we are merely ‘gaming the odds’ by increasing the sample space to such an unimaginably high number that atleast one of the samples will end up exhibiting the DIPs that we are seeking (i.e. the ones that are conducive to the existence of life).
IV. Finite Space or Infinite Space?
An infinite number, when it is reduced by a million or billion or trillions, remains the same: infinite – a truly non-intuitive, incomprehensible notion. There is no experimental evidence of anything infinite in the physical world. When viewed this way, infinity is merely a way of speaking; infinite numbers or infinite anything (for example mass, density, and energy, etc.) have no practical meaning. Furthermore, computers do not acknowledge infinity, dealing only with finite digital numbers. Some mathematicians also reject this concept, noting that mathematics was always about, and always will be about, finite collections, pattern, and algorithms. If one is to accept the notion that these mathematicians propound, it would be noteworthy to accept that all those theories, arguments and daydreams involving “infinite sets” need to be recast into a precise finite framework or relegated to philosophy. Whether infinity is something that is merely a term that is used to refer to a number that may be beyond the limit of our computational ability is something I do not have a conclusive answer to.
Most multiverses assume an infinite space where reality takes on surprising new meanings – for example, the prediction of duplicate yous (or infinite number of yous) living in an identical world somewhere in another universe. Einstein’s equations for general relativity allow for either a finite or an infinite spatial extent. All cosmological models involve an expanding universe, but they differ in the overall shape of space and whether space is finite or infinite. Although finite space is considered in the inflation framework, infinite space is the favored assumption, which of course permits an infinite number of universes. Also, space is not “nothing” but is governed by laws and contains fields and virtual particles.
V. Theory #1: A Finite Quilted Multiverse
The first theory propounds that a ‘Finite Quilted multiverse’ exists within a finite expanse of space, simply more of what we observe, each universe like a patch replicated on a quilt. Some argue that there may exist hundreds or thousands of ‘Hubble volumes’. These parallel universes embody similar initial conditions, the same constants of nature, the same laws of physics, and three space dimensions. However, the Finite Quilted multiverse adds no insight to fine tuning. In the 1920’s, Alexander Friedmann showed that a variety of expanding, contracting, or oscillating universes were compatible with Einstein’s General Theory of Relativity.4 At around the same time, George Lemaitre predicted that our universe began with a big bang – everything we observe originated in a hot, dense, and rapidly expanding space that was encapsulated in something smaller than the size of an atom.
Subsequently, in the 1940s, as part of the Big Bang theory, Friedmann’s student George Gamow calculated that the expanding space, after cooling to about 3000 K, would allow atoms to form and thus free photons to stream through space. This theory predicted that photons from the creation event should be visible in the sky, although at a vastly shifted wave length. When this microwave background radiation was observed in 1964, the theory was validated. The observed universe, sometimes called the “Hubble sphere”, is the farthest distance we can “see”, around 41 billion light years or 4 x 10^26 meters (on a very fundamental level, the light has been traveling for about 13 billion years, but because space is expanding, the current distance is greater). The Finite Quilted multiverse is crafted on the assumptions of extended space, finite universe but larger than the observed universe, expanding universe and approximately flat space.
The posited ‘Finite Quilted multiverse’ has a radius that is at least 100 times larger than the Hubble radius based on the observational precision of Cosmic Microwave Background (CMB) data, which did not detect curvature to within a precision of one percent.
VI. Theory #2: An Infinite Quilted Multiverse
The second theory proposition, of the ‘Infinite Quilted multiverse’, is similar to the Finite Quilted multiverse except for different initial conditions and the number of parallel universes – instead of thousands of versions, an infinite number of universes populate space and therefore fine tuning can be explained through the ‘gaming the odds’ as we spoke about previously. The assumptions made to account for the Infinite Quilted multiverse include the fact that first, space has Infinite extent with no edge and no boundary and that space is something.
VII. Theory #3: An Inflationary Multiverse
As part of the Inflationary multiverse proposition, the eternal (chaotic) inflation theory produces different constants of nature in separate universes. Each universe in such a multiverse, sometimes referred to as “bubbles” (original, I know) is separate by expanding space. Inflation produces an infinite number of universes, and thus provides an explanation for the presence of fine tuning. Inflationary cosmology is not one unique theory, rather it provides a framework containing many versions differing in details, such as differences in the number of ‘inflaton fields’ (this is not a typo, it is spelled differently than inflation) and their potential-energy curves.
However, in general, the basic inflation concept modifies the Big Bang theory by inserting an extremely brief burst of astoundingly rapid expansion during the universe’s earliest existence. This stupendous growth explains the uniformity of the CMB temperature, predicts that the universe is flat, and addresses several other problems associated with the Big Bang theory as we know it. The source of the brief burst is a tiny speck of space existing in a hypothetical “scalar inflaton field,” a field similar to the scalar Higgs field.5 The inflaton field contains a high amount of potential energy, exists uniformly throughout space, and does not dilute as space expands. The field is subject to random quantum fluctuations that can cause the energy value to vary, and if the energy drops too far, the overall superfast expansion of space stops and inflation starts.6
In one popular version, the inflation process is astonishingly short, lasting about 10^(-35) seconds.7 During this brief period, space expands by a mind-boggling amount of at least 10^30 times (or about a nonillion) times. As expansion ends, the field’s energy is converted into particles, thus creating a universe. Subsequent expansion was considerably slower (now doubling every 8 billion years) as gravity went to work forming galaxies, stars, and planets.
As proposed by Andrei Linde8, eternal or chaotic inflation enhanced an existing version of the theory introduced by Alan Guth and Paul Steinhardt by including numerous inflaton fields with random values rather than one field with a constant value. According to eternal inflation, ‘space expands until the scalar field releases both its energy and a uniform negative pressure’9.
This expansion creates a universe with characteristics like our universe. Since eternal implies continuing forever, space must expand faster than it “decays”. In order to conceptualize something like this, we can visualize an expanding block of cheese. The holes, sometime referred to as bubbles or pockets, are where inflation stopped; the cheese represents the immense expanse of space surrounding the holes (universes). To explain the full scale of this process, it would be more accurate to state that the size of the universe increased by a factor larger than a million trillion trillion in less than a millionth of a trillionth of a trillionth of a second10. Take that for growth, you capitalists!
Inflationary cosmology may be mathematically sound but it does sound incredulous from a human perspective. Although the brief discussion above assumes that negative pressure in the scalar field creates repulsive gravity (negative or borrowed gravity), this idea is rather unsettling. In the words of Max Tegmark, it is hard to ‘shake the uneasy feeling that we are living in a Ponzi scheme of cosmic proportions’. Thus, even though inflation is based on theoretical physics, due to the sheer incredible absurdity of the process, it leaves room for doubt. The assumptions that its proponents make for justifying the possibility of an Inflationary multiverse are the ‘scalar non-diluting inflaton fields’ with high potential energy and negative pressure, space doubling its size every 10^(-38) seconds, random quantum fluctuations establishing initial conditions, and space expanding faster than it decays.
VIII. Theory #4: The 9D String Theory Multiverse
The 9D String Theory multiverse combines two immensely complicated concepts of string theory and eternal inflation to construct an even more complex structure that can predict an infinite number of universes. The significant difference from the Inflationary proposal is that space consists of nine space dimensions instead of the three space dimensions. These extra dimensions provide numerous “shapes” for space, each shape a different variety of universe – a bit of an overkill with respect to explaining the fine tuning problem. In the first version of superstring theory, propounded sometime around the 1980s, now referred to as string theory, tiny strings or vibrating filaments, replace electrons and quarks as nature’s building blocks.
Strings are so minute they may never be observed (Planck distance of 10^[-33] cm). The string vibration pattern dictates intrinsic features that may represent an electron or a quark or more importantly a graviton (massless, chargeless, and having a spin-2 quantum property). Thus, without contradicting previous theories, string theory bridged the gap between general relativity and quantum mechanics. However, the mathematics, as defined in five unique theories, required nine rather than three dimensions for space. The extra dimensions are curled up into Calabi-Yau shapes, which dictate particle properties.11 (Although there are simpler examples of six-dimensional compact manifolds which include shapes like the six-sphere and the six-dimensional torus). This is done by compactifying the extra six dimensions into the three dimensional space that we are used to (to bring the 9 spatial dimensions to 3). However, distinct compactifications correspond to different results and possess different amounts of energy.12
Unfortunately, there are “many” possible shapes (~10^500) for the extra dimensions and there is no way to select the one that matches existing particles. If this version of the multiverse exists, then all these options may be realized. The assumptions for the Landscape multiverse include the existence of tiny vibrating strings, nine spatial dimensions and one string vibration pattern for the graviton particle.
IX. Theory #5: The ‘Brane’ Multiverse
The ‘Brane multiverse’ evolves from the current version of string theory, which has ten space dimensions. The theory predicts multi-dimensional branes. In this environment, our universe might exist on a three-dimensional brane along with possibly an infinite number of similar branes. If so, fine tuning is explained.
Sidebar 1: What is a 'Brane'? The easiest way to conceptualize this may be to think of the term 'x-brane' meaning that a particular thing occupies 'x' number of spatial dimensions. Therefore, a '0-brane' would mean a point particle, whereas '1-brane' would be a string. It was conceptualized in 1995, when physicist Ed Witten started what is called the 'second revolution of string theory' by refining his calculations [Source]. He would go to show that the five previously unique string theories were encompassed in one overriding theory, 'M-theory'. The old calculations missed one dimension; there were actually ten space dimensions. M-theory generates “n-dimensional” braneworlds, where n has values from one to nine. A “one” brane corresponds to a one-dimensional string and a “three” brane corresponds to a three-dimensional space. Thus, our universe could exist on a 3-brane (one of many) with large or infinite extent. Different branes reside in different dimensions, not necessarily separated by vast distances in space, and possibly hovering in close proximity to each other. All strings are attached to the brane except for gravitons, which are unattached loops that can leave and re-enter the 3-brane.
The assumptions for the Brane multiverse include the existence of ten space dimensions, branes with one to nine space dimensions, strings attached to branes except for graviton strings, and the observed universe residing on a 3-brane.
In the next part, the other six multiverse models (Cyclic, Big Bounce, Quantum, Holographic, Simulated and the ‘Ultimate’) will be discussed in addition to the five that we already have looked at. Including analyzing what it will take for each of them to be justifiably demonstrated.
Though it would be possible to argue that the ‘Finite Quilted’ multiverse theory is in a category of its own, since virtually all scientists concur that space, with characteristics identical to our universe, extends beyond what we observe. Thus, having a limited number of universes in this expanded space establishes the reality of some parallel universes.
Alan Boyle, Scrunched Up Dimensions Untangled