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The Observable Universe and Beyond

Parallel Universes, Multiverse - the Unobservable Universe

The subject of parallel universes used to belong to the realm of science fictions. The idea is familiar on some TV shows such as "Star Trek", which portrays other worlds that are almost like our own, except ... there is a slight difference. Then some cosmologists propose that our Universe might be just one of many in an ever-multiplying network of parallel universes, which they call the multiverse. Recent observational data open up the possibility that it is conceivable scientifically with some creative imagination.

flat space 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]

Parallel Universes 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]

Types of Multiverse 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).
See more detail in "Mak Tegmark's Universes".
Cosmic Joy 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]

Multiverse Brief Description Theoretical Base
Quilted There must be repeated version of each universe in very large sample Infinite size of the cosmos
Inflationary Bubble universes are created in eternal state of inflation Theory of inflation
Brane Universes exist on 3-D brane Theory of superstring
Cyclic Interacting braneworlds produce cycles of cosmic expansion Theory of superstring
Landscape Universes reside in the valleys of the vacuum energy map Theory of superstring
Quantum Multiverse is created by realization of all the quantum probabilities Quantum Theory
Holographic Each universe has a holographic copy in a lower dimensional surface Holographic principle
Simulated Universes can be simulated by very powerful computer Advanced technology
Ultimate All possible mathematical equations have correspondence to some real universes Philosophy

Table 02-04 Nine Scenarios of Multiverse

On January 8, 2012 at Cambridge University, a symposium was held to honor Stephen Hawking's 70th birthday. The formal title is "The State of the Universe: Stephen Hawking 70th Birthday Symposium" with speakers including Lord Rees, the astronomer royal, Saul Perlmutter, who won the 2011 Nobel prize for the discovery of dark energy, and Kip Thorne, a longtime collaborator of Hawking. Unfortunately, Hawking was too ill to attend the celebration. A pre-recorded lecture, entitle "A Brief History of Mine" was delivered to urge the audience to go into space for the future of humanity (he thinks this world would not last longer than another 1000 years with the imminent threats of global warming and nuclear holocaust). He concluded: "Remember to look up at the stars and not down at your feet. Try to make sense of what you see and wonder about what makes the universe exist. Be curious. And however difficult life may seem, there is always something you can do and succeed at. It matters that you don't just give up."
Hawking at 70 Beginning of the Cosmo 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".
bThe first peak (the whole curve) of the power spectrum moves from left to right with increasing radius of space.

cThe 2dF redshift survey uses the two-degree field spectroscopic facility on the Anglo-Australian Telescope to measure the redshifts of 250,000 galaxies.

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