The properties of our universe appear to be finely-tuned for the existence of life. Cosmologists would like to explain the numbers and values that describe these properties we observe. Their attempt is to show that these constants and values in nature are completely determined as a product of inflation, which entails multiverse scenarios. Inflationary cosmology seems to not only solve fine-tuning implications but it also solves the horizon problem. That is, the early universe’s expansion rate was exponentially fast—faster than the speed of light and if it expanded at such a rate information (light) could not propagate beyond the cosmic horizon. Due to these problems much theoretical focus and work has been implemented in to the field of cosmology and physics developing an inflationary cosmology and string theory.
The eternally inflating multiverse is often used to provide a consistent framework to understand coincidences and fine-tuning in the universe we inhabit. This theory primarily appears in several forms, which attempt to explain the mechanism that drives the rapid expansion of the universe. Before developing these models there are a few basic premises that must be agreed upon: the size of the universe, the Hubble expansion, homogeny and isotropy, and the flatness problem.
It is unanimously agreed upon that the Hubble volume we inhabit is incredibly large. According to standard Friedmann-Lemaître-Robertson-Walker (FRW) cosmology, without inflation, one simply postulates 1090 elementary particles. This number is derived from simply geometrical quantitative measurements. One of the tasks at hand is explaining how the universe got so big. The exponential expansion of inflation reduces the problem of explaining 1090 particles to the problem of explaining 60 or 70 e-foldings of inflation. Inflationary cosmology therefore suggests that, even though the observed universe is incredibly large, it is only an infinitesimal fraction of the entire multiverse.
The Hubble expansion serves as a factor in the initial conditions of the universe. In the 1920’s Edwin Hubble was studying the Andromeda nebula. At least since the time of Immanuel Kant scientists wondered what these distant enormous objects were (galaxies). With further study, Hubble noticed that these galaxies had a red shift; the galaxies were appearing redder than they should have and Hubble postulated that these galaxies were moving away from one another. What was being observed was the same thing that the Doppler effect has on sound. The trajectory of an object has an effect on the wavelength of the sound, or in this case, light. If this expansion is extrapolated the equations of motion then (and even now) can only go but so far—until the universe comes to a singularity. Inflation actually offers the possibility of explaining how this expansion initially began. The repulsive gravity associated with the false vacuum is what contributes to the explanation. The false vacuum energy density is the exact kind of force needed to propel the universe into a pattern of motion in which any two particles are moving apart with a velocity proportional in their separation.
Homogeneity and isotropy refers to the uniformity of the universe. This can be seen in the below image of the Planck Satellite one-year survey results.
The intensity of the cosmic background radiation is the same in all directions. It is calculated to the incredible precision of one part in 100,000 and possibly and even greater precision with developing Planck Survey results. In standard FRW cosmology, the uniformity could be established so quickly only if information could propagate 100 times the speed of light, a proposition clearly contradicting known physics. However, in inflationary cosmology, the uniformity is easily explained by the creation of uniformity on microscopic scales via normal thermal-equilibrium processes. Inflation then takes over and stretches the regions of uniformity to become large enough to encompass the observed universe.
The flatness problem is related to the precision required for the initial value of Ω, the ratio of the actual mass density to the critical mass density. This occurred when Robert Dicke and P.J.E. Peebles pointed out that at t = 1 second after the big bang, nucleosynthesis were just beginning, Ωtot (total) must have equaled one to an accuracy of one part in 1015. If this ratio were not accurate to this degree the resulting universe would not resemble our own.As depicted in figure 5.2 the evolution of the universe differs between inflation scenarios and FRW scenarios.
The standard FRW cosmology does not have an explanation for the Ω value while inflation does.
Magnetic monopoles are extremely massive particles carrying a net magnetic charge, which is a result of predictions made by all the grand unified theories. By combining the grand unified theories with non-inflation scenarios the expected age of the universe is no longer 13.73 billion years old and it becomes about 30,000 years old. Inflation eliminates these monopoles by arranging the parameters so that inflation takes place after or during monopole production, so the monopole density is diluted to a completely negligible level.
These preliminaries for inflation will help understand what exactly inflation accomplishes and what it predicts. Sometime between 1983-1986 Andrei Linde developed and proposed a model of eternal chaotic inflation, for which the energy density, with the initial randomly chosen value of the fields corresponds to a point hill, contra a Mexican hat with a bowl shape, then sufficient inflation can occur as the fields roll towards the state of minimum energy density. Consider the evolution of the scalar field below:
As depicted in figure 4 the evolution of the scalar field leads to many inflationary domains as revealed in this computer-generated depiction. In most parts of the universe, the scalar field decreases (the depressions and the valleys). In other places quantum fluctuations cause the scalar field to augment. In those places, represented as peaks, the universe undergoes inflation and rapidly expands, leading to the creation of inflationary regions. Our Hubble volume is in one of the valleys, where space is no longer inflating. Each of these peaks consists in large domains, which have different laws of physics (represented by the different colors in figure 5.5 below). Sharp peaks are big bangs; their heights correspond to the energy density of the universe there. At the top of the peaks, the colors rapidly fluctuate, indicating that the laws of physics there are not yet settled. They become fixed only in the valleys, one of which corresponds to the universe we live in now.
Due to the nature of inflation each valley produces a universe with different values, which is a prediction of quantum cosmology. Inflation is not a monolithic in form (eternal, chaotic, new, string, etc.); however, each model has the basic premise as described above. Not only does inflation have scientific attraction for conforming observations, theory, and data but yields a philosophical satisfaction in attempting to explain away fine-tuning. In the standard FRW big bang model inflationists see fine-tuning as ‘ugly.’ The claim is that the need for such fine-tuning of the initial state is removed in the inflationary picture, and this is regarded as a more aesthetically pleasing physical picture. Additionally, if inflation is true then there is not one universe but a multiverse, potentially infinite in number.
This version predicts that different regions of space can exhibit different laws of physics (physical constants, dimensionality, particle content, etc.) corresponding to different localities and a landscape of possibilities. Imagine the multiverse as a bathtub filled with tiny bubbles. Each bubble in this larger system (the bathtub) is a single universe. Or, imagine a pot of boiling water. The bubbles arise from the bottom of the pot analogous to the way inflationary cosmology works. These other domains (or bubble universes) are nearly infinitely far away in the sense that we could never get there even if we traveled faster than the speed of light (due to the constant stretching of space and creation of more volume). It may, however, not be the case that there is an infinite set of universes. Andrei Linde and Vitaly Vanchurin have argued that the way slow-roll inflation works it could only produce a finite number of universes. Hence, they propose that there are approximately 10^10^10^7 universes.
 John D. Barrow, The Constants of Nature: The Numbers Encode the Deepest Secrets of the Universe (New York: Random House, 2003), 182.
 Alan Guth and Yasunori Nomura, “What Can the Observation of Nonzero Curvature Tell Us?” arXiv:1203.6876v2, (July 2012): 32.
 Alan Guth, “Eternal Inflation and Its Implications,” in The Nature of Nature, 487.
 E-foldings are time measurements, which serve as intervals between the exponential growth of a quantity or volume by the factor of e.
 Ibid., 488.
 This all-sky image shows the distribution of the Galactic Haze seen by ESA’s Planck mission at microwave frequencies superimposed over the high-energy sky as seen by NASA’s Fermi Gamma-ray Space Telescope. The Planck data (shown here in red and yellow) correspond to the Haze emission at frequencies of 30 and 44 GHz, extending from and around the Galactic Centre. The Fermi data (shown here in blue) correspond to observations performed at energies between 10 and 100 GeV and reveal two bubble-shaped, gamma-ray emitting structures extending from the Galactic Centre. This becomes important in next chapter. It has been posited that these bubbles in the data may in fact be the result of an early universe collision with another universe’s bubble. ESA/Planck and NASA/DOE/Fermi LAT/Dobler et al./Su et al. http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50008 (accessed May 6, 2012). P.A.R. Ade, et al, “Planck Early Results. I. The Planck Mission,” arXiv: 1101.2022v2 (June 2011). N. Aghanim, et al, “Planck Intermediate Results II: Comparison of Sunyaev-Zeldovich Measurements from Planck and from the Arcminute Microkelvin Imager for 11 Galaxy Clusters,” arXiv:1204.1318v3 (May 2012). Guth, “Eternal Inflation,” 488.
 Andrei Linde, “The Self-Reproducing Inflationary Universe: Recent Versions of the Inflation Scenario Describe the Universe as a Self-Generating Fractal That Sprouts Other Inflationary Universe,” Scientific American (Nov. 1994): 54.
 Alan Guth, The Inflationary Universe, 332. R.H. Dicke and P.J.E. Peebles, (1979) in S.H. Hawking and W. Israel, eds. General Relativity: An Einstein Centenary Survey (Cambridge: Cambridge University Press, 1979).
 Guth, “Eternal Inflation,” 490.
 Roger Penrose, The Road to Reality (New York: Random House, 2004), 737.
 Guth, The Inflationary Universe, 327.
 Linde, “The Self-Reproducing Universe,” 50-51.
 This would be the same space where the Coleman and De Luccia instanton functions, which allows for vacua decay via bubble nucleation (of a bubble universe). Michael P. Salem, “Bubble Collisions and Measures of the Multiverse.” arXiv:1108.0040v2 (Dec. 2011), 2.
 Linde, “The Self-Reproducing Universe,” 49.
 Penrose, 755.
 Max Tegmark, “The Multiverse Hierarchy,” arXiv:0905.1283v1 (May 2009): 2.
 Ibid., 7. Additionally, there has been good scientific evidence suggesting observational grounds for inflation. Researchers have taken the 7-year WMAP data and applied certain algorithms to pick up traces of thermal fluctuations in the early universe. What they found were traces of what could be bubble collisions of the edges of our universe with another universe. Stephen Feeney, et al., “First Observational Tests of Eternal Inflation: Analysis Methods and WMAP 7-year Results,” arXiv:1012.3667v2 (July 2011).
 Linde and Vanchurin, 10.