The Observable Universe and Beyond

Parallel Universes, Multiverse - the Unobservable Universe

Figure 02-11 shows recent astronomical observations, which tend to support the hypothesis. The diagram on the left illustrates the WMAP measurementb of the fluctuations in the CMBR temperature. The strongest fluctuations are just over half a degree across, which indicates that space is very large or infinite. In addition, the diagram on the right illustrates the measurements of matter density from WMAP and 2dF Galaxy Redshift Surveyc. They are consistent with uniform distribution of matter on large scales.

Figure 02-11 Universe, Flat & Uniform [view large image]

These observational data support the inflation theory, which suggests that the universe underwent an exponential expansion at 10-35 sec after the Big Bang. The universe became so large that it looks flat within our event horizon, and in addition, the contents in the universe were mixed uniformly as witnessed by the CMBR. Our universe with a size of about 1026 meters (as limited by the event horizon) is just a speck in comparison to this vast expand. The number of ways to arrange matter in the space outside our universe is enormous and each one would have its own event horizon (size); these are the parallel universes. Statistically, an arrangement similar to ours is bound to happen given enough space. Thus there would be universes identical to ours somewhere. However, we cannot communicate with any of these parallel universes because the speed of light is finite. This conclusion is derived from elementary probability and does not assume speculative modern physics, merely that space is infinite (or at least sufficiently large) in size and almost uniformly filled with matter, as observations indicate. In infinite space, even the most unlikely events must take place somewhere.

Figure 02-12 Parallel Universes [view large image]

Followings is a more elaborated deduction with simple mathematics:


• The radius of the observable universe is taken to be 4x10²⁶ m. It is assumed that the matter in the universe consists of nucleons, which has a radius of about 2x10^-13 m.
• Therefore, the universe has roughly 10¹¹⁸ partitions for the nucleons.
• These partitions can be filled or unfilled according to the configuration of the universe. The total number of different arrangements is 2¹⁰¹¹⁸.
• A box with a radius of 2¹⁰¹¹⁸x10²⁶ m ~ 2¹⁰¹¹⁸ m would exhausts all the possibilities. It contains possibly all kinds of parallel universes.
• Beyond that box, universes - including ours - must repeat. The identical parallel universe would be about 2¹⁰¹¹⁸ m away as shown in Figure 02-12.

This is called the Level 1 Multiverse within which other universes basically look like ours. There are even more exotic scenarios in the category of Level 2 Multiverse with very different properties from ours as outlined in the followings. Figure 02-13a shows the different ideas of universe. Originally we were concerned only about the observable universe as that's what we can comprehend (diagram a).

Figure 02-13a Types of Multiverse

Introduction of inflation forces us to recognize that there are more space outside the horizon (diagram b). The latest theoretical consideration suggests that lot more universes exist further out with entirely different properties from ours (diagram c).

More exotic types and weird properties of multiverses are highlighted in the followings:



In quantum mechanics the superposition of quantum states suddenly "collapsed" into a definite quantum state when we make a measurement. For example, measurement of the spin state for a spinning particle would yield either 1 or 0; it would not be any value in between. Generalization of this concept to the macroscopic world suggests that one classical reality would gradually split into superpositions of many as shown in Figure 02-13b. Observation experiences one of the splitting sequences by a decoherent process, which mimics wave function collapse. The classical states (no weird happening such as in two different places at once) are observed because they are in the most robust states.

Figure 02-13b Level 3 Quantum Worlds

This kind of scenario is referred to as Multiverse level 3. It may be realized in the multiverses of level 1 and 2.

		The level 4 multiverse is even more outlandish. It is suggested that the universe we experience is just a simulation running on an advanced civilization's supercomputer (Figure 02-13c). Since advanced civilizations would develop essentially unlimited computing power (see Moore's Law, Figure 02-13d), some of those can run "ancestor simulations" to reconstruct the history of their forebears or other beings. There could be a huge number of such simulations producing a
Figure 02-13c MV Level 4	Figure 02-13d Moore's Law [view large image]	one-to-one relationship between mathematical models and the kind of universes mentioned in level 1, 2, and 3.

According to Superstring theory, there can be 100500 different vacua each one corresponds to a different universe. Our universe and contiguous regions of space is a bubble embedded in an even vaster but mostly empty volume. Other bubbles exist out there, disconnected from ours. They nucleate like raindrops in a cloud. During nucleation, each one may acquire different strength of the forces, and may emerge with different spatial and temporal dimensions. It all depends on the outcome of symmetry breaking. The anthropic principle dictates that we see the universe the way it is because if it were different we would not be here to observe it. (see more in Topic 15 - Elementary Particles)

Figure 02-13e Anthropic Principle [view large image]

Figure 02-13e shows the consequence of the variation in the strength of strong and electromagnetic interactions, and the numbers of spatial and temporal dimensions. It is reported in 2006 that habitable universe is possible without weak interaction. Nuclear burning can somehow be maintained inside stars albeit in a much reduced rate; while heavy elements can still be dispersed into interstellar space via the process of type Ia supernova. Another possibility is the cold big bang (low number of photons/baryon ~ 10⁷) universe where the cosmological constant was 10¹⁷ times that of our universe. In such universe, the stars form so quickly that they are in place before the cosmological repulsion takes hold.

Anyway in this observable universe, there are several parameters which seem to be fine tuned into a particular value beyond or below which life is not possible (see Table 02-03 for some of these so called Goldilocks parameters). Such fine tuning is often used to support the argument that someone, e.g., a God is doing the adjustment so as to allow us to exist. The resonant level of C12 discovered by Fred Hoyle is the most favourite citation for such purpose. According to him, carbon 12 (the stuff that makes life) can be produced from three helium nucleus only if there is an excited state at 7.7 Mev. The existence of such state was quickly

confirmed experimentally as shown in Figure 02-14 (a), in which the 7.3367 Mev is the energy released in the nuclear reaction that produces the C12 nuclei and is in here adopted as the ground state. Further study reveals that there is a range of energies that is capable of producing same amount of C12 for life as shown in Figure 02-14 (b). The range is even wider if the "enough amount" (for life to exist) is included into the deliberation, so there is no "fine tuning" for the C12 resonant level after all. As for the other Goldilocks parameters in Table 02-03, it is pointed out that the tuning is done one by one while keeping all the others constant. The real world may not

Figure 02-14 Resonant Level in C12 [view large image]

be that simple, many of these parameters would depend on each other - varying one would change the others so that the conclusions in the table have to be revised in the future with a more advanced theory.




Parameter	Value	More	Less
Ratio of the Electromagnetic and gravitational forces	1036	OK	Stars become smaller and die earlier making evolution of life unlikely
Proportion of Mass Released as Energy when H Fused into He	0.007	All hydrogen would be consumed during the Big Bang, stars would not exit.	Nuclear fusion is impossible
Ratio of Actual to Critical Cosmic Density	~ 0.3	Universe collapsed long ago	Rapid expansion prevents stars to form
Ratio of Break-up and Total Energy for Galactic Supercluster	10-5	Universe would be dominated by black holes - life is impossible	Universe would be structureless - life is impossible
# of Spatial Dimensions (D)	3	Unstable planetary orbits for D = 4	Life would be impossible for D = 2
Cosmological Constant	~ 0.7	Rapid expansion prevents stars to form	OK

Table 02-03 The Goldilocks Parameters

According to Steven Weinberg, there is not much evidence of fine tuning for these parameters except the cosmological constant, which is 10¹²⁰ times smaller than calculation from quantum theory (See "Vacuum Energy Density" for a 2015 update). If it is not in reality so small, the dark energy would have pushed everything apart so fast that no stars or galaxies had enough time to form. However, the cosmological constant is not the only theory to explain the presence of dark energy. There are other ideas such as "quintessence" needed to be explored as well. And if the idea of multiverse is taken seriously, then the apparent coincidence is just the reflection of so many universes, out of which some must be compatible with life.

The correspondence between mathematics and physics has been a source of debate that goes as far back as Aristotle and Plato. According to the Aristotelian paradigm, physical reality is fundamental and mathematical language is merely a useful tool. The Platonic argument considers the mathematical structure to be the true reality and observers only perceive it imperfectly. Thus a fundamental asymmetry appears to be built into the very heart of reality - why is only one of the many mathematical structures singled out to describe our universe? It is suggested that complete mathematical symmetry holds: that all mathematical structures exist physically as well. Every mathematical structure corresponds to a parallel universe. As a consequence, each universe is

Figure 02-15a Aristotele and Plato [view large image]

governed by its own fundamental laws of physics (see more in Topic 15 - Elementary Particles). In Figure 02-15a Aristotle points down to the reality on earth while Plato points up to multiverse.




Max Tegmark is a physics professor at MIT. He openly admits in his 2014 book titled "Our Mathematical Universe, My Quest for the Ultimate Nature of Reality" that he is a regular physicist at day and a wacky theorist at night. Currently (in 2014) his pet theory is (not surprisingly) mathematical universe. It posits that the external reality is a mathematical structure similar to the graph with 4 elements at the right side of Figure 02-15b. The human

Figure 02-15b Mathematical Universe [view large image]

world creates artificial objects such as names and sports (picture in the left panel) - some baggage not recognized universally. In mathematical universe, physical theories are here and everywhere; we just discover them and written them down in a form recognizable.

• The theory of eternal inflation posits that the entire eternally inflating spacetime (false vacuum) originated as a minuscule closed multiverse. It tunneled, quantum-mechanically, out of nothing and immediately plunged into the never-ending inflation. Thus, the multiverse is eternal, but it did have a beginning. Inflation rapidly blows the universe up to an enormous size. It stopped where we are, producing our own island universe, and many others. But due to the peculiar structure of inflationary spacetime, it continues in other areas of the multiverse at large. The multiverse contains an unlimited number of island universes. We live in one of them, and our observable region is one of the infinite numbers of observable regions (at different locations) that it contains (see Figure 02-16a). We may be able to travel to the other observable regions, but we are forever confined to our own island universe. Constants

  		of nature that shape the character of our world take different values in other island universes. Most of these universes are drastically different from ours, and only a tiny fraction of them are hospitable to life. Figure 02-16b is another view of the same thing. The square represents an inflating volume, which decays into three with one of them turning into a non-inflating Big Bang universe. The result is a never ending process sprouting out entities like ours in number of 1, 2, 4, ...
  Figure 02-16a Multiverse, Eternal Inflation	Figure 02-16b Multiverse, Eternal Inflation 2 [view large image]

By 2006, the confluence of events has pushed many leading physicists toward the notion of a multiverse. These include: measurements that indicate the universe's expansion is accelerating; empirical tests that bolster the inflationary universe scenario; theories of eternal inflation (see above) that suggest an endless number of Big Bang; and developments in string theory that show how to design universes with widely different properties. In his 2011 book "The Hidden Reality - Parallel Universes and the Deep Laws of the Cosmos" Brian Greene summarizes nine different scenarios of multiverse in a table. All of them (except one) have been mentioned in various sections within this website. Table 02-04 is reproduced from the original in a different format below (Figure 02-16c is an artist's version of the multiverse):

Figure 02-16c Cosmic Joy [view large image]

Table 02-04 Nine Scenarios of Multiverse

		He also missed the opening of a new exhibition on January 19, 2012 at the Science Museum entitled Stephen Hawking: A 70th Birthday Celebration. Figure 02-16d is a picture released by the London Science Museum showing him in his office at Cambridge University. Meanwhile in his absence, some scientists argued that some cosmological theories such as the inflationary and cyclic models (see Table 02-04) require a beginning to address the inconsistencies introduced by the limits on the Hubble constant and runaway entropy respectively (Figure 02-16e).
Figure 02-16d Hawking at 70 [view large image]	Figure 02-16e Cosmic Beginning	Such findings always point to the necessity of supernatural creator contrary to the conclusion in Hawking's latest book: "The Grand Design". Finally, see "Insignificance".

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