To close out our series on the Big Bang and inflationary cosmology, today we discuss Alan Guth's inflation model (a similar model developed independently by Alexei Starobinsky should also be acknowledged).
As Guth writes in this MIT Physics magazine-style article, "Despite the striking successes of the big bang theory, there is good reason to believe that the theory in its traditional form is incomplete." Adds Guth:
Could the big bang have been caused by a colossal stick of TNT, or perhaps a thermonuclear explosion? Or maybe a gigantic ball of matter collided with a gigantic ball of antimatter, releasing an untold amount of energy in a powerful cosmic blast. In fact, none of these scenarios can plausibly account for the big bang that started our universe... (p. 30).
Further, as Dan Hooper writes in his book Dark Cosmos (reviewed here), "...three puzzles -- the monopole, flatness, and horizon problems -- confounded cosmologists and particle physicists" (p. 195). The major contribution of Guth's model of dramatic, early expansion of the universe (i.e., in the tiniest infinitesimal fraction of a second) is thus its ability to solve these problems that existed in the traditional Big Bang model.
To get an initial visual image of what the inflation model proposes, this diagram from NASA is very helpful. Rather than emanating outward from a single point in linear fashion (analogously to a spotlight), the universe can be seen in the NASA diagram to have widened out from the initial point very early on. Until recently, the further widening of the universe over billions of years has been relatively modest, thus yielding a timeline that looks like a wastebasket laid on its side (also see Figures 9.2, 10.3, and 10.6 in Brian Greene's book, The Fabric of the Cosmos). An "inflaton" field may be the responsible mechanism.
The first problem listed above involves what are known as magnetic monopoles. As discussed by Hooper (pp. 190-194), there exist some theories of grand unification and symmetry -- invoking Maxwell's electromagnetism -- that call for there to be charged magnetic objects (i.e., with only one pole), just as there are charged electric particles such as electrons. However, mono-polar magnets do not appear to exist. Even after breaking a magnet, each new piece will have both north and south poles.
This Wikipedia entry on Alan Guth explains his solution to this conundrum:
The reason for the missing monopoles was that the universe was so big that the density of monopoles would be very low. The “enormous number of monopoles could have risen in the inflationary universe, yet we and all other observers would find them to be observationally far rarer than snowballs in the Sahara…Inflation would spread them so thin that the average observer would expect to find only a single monopole in the entire observable universe.”
The flatness problem pertains to how amazingly perfectly the universe appears to be situated for it to exist as it does -- where any deviation could throw things askew. To use the popular "Goldilocks" analogy, the physical parameters for the shape of the universe seem to be "just right."
As John Gribbin writes in his essay Inflation for Beginners, "This is the puzzle that the spacetime of the Universe is very nearly flat, which means that the Universe sits just on the dividing line between eternal expansion and eventual recollapse." Mathematically, as described by Guth's aforementioned MIT article, the flatness problem can be characterized thusly:
Unless, however, we postulate that the mass density at one second just happened to have a value between 0.999999999999999 and 1.000000000000001 times the critical density, the theory will not describe a universe that resembles the one in which we live (p. 32).
The proposed solution is described in Guth's Wikipedia profile:
Guth realized from his theory that the reason why the universe appears to be flat was because it was fantastically big, just the same way the spherical Earth appears flat to those on its surface. The observable universe was actually only a very small part of the actual universe.
Brian Greene's balloon depictions in Figure 10.4 of The Fabric... are also helpful in understanding solutions to the flatness problem.
A third difficulty, known as the horizon problem, pertains to the extreme uniformity of the Cosmic Microwave Background. It is characterized as follows by Guth in his MIT article:
Calculations show that energy and information would have to be transported at about 100 times the speed of light in order to achieve uniformity by 300,000 years after the big bang. Thus, the traditional big bang theory requires us to postulate, without explanation, that the primordial fireball filled space from the beginning. The temperature was the same everywhere by assumption, but not as a consequence of any physical process (p. 31).
However, as noted in the Wikipedia entry:
The paradox was resolved, as Guth soon realized, by the inflation theory. Since inflation started with a far small[er] amount of matter than the Big Bang had presupposed, [there was] an amount so small that all parts would have been in touch with each other. Inflation th[e]n blew up the universe so quickly that there was no time for the essential homogeneity to be broken.
This University of Arizona document conveys the situation somewhat differently:
If the Universe just expanded in a uniform way, it would have developed large uniformities over distances where the light communication time would be too long to even them out. The observed high degree of uniformity (to about 1 part in 100,000 for the 3K radiation!) must have been locked in at an early stage and maintained since then.
I recall this idea also being discussed during an episode of the History Channel's "Universe" series, but I cannot find a transcript online.
For those interested in pursuing another concise description of the flatness, horizon, monopole, and other problems -- and their solutions -- I would recommend this University of Maryland document.
As noted above, Guth has critiqued the original Big Bang model for its reliance on assumptions rather than proposed physical processes that may have generated conditions we observe in the universe. Regarding his own model, he asserts the following in his MIT article:
The crucial property of physical law that makes inflation possible is the existence of states of matter which have a high energy density that cannot be rapidly lowered. Such a state is called a false vacuum... (p. 33).
The false-vacuum concept appears relatively complex, so I would encourage interested readers to study this section of Guth's article -- which includes some drawings -- for themselves!
Where does inflationary theory stand today, in terms of scientific acceptance?
This NASA document (different from the one cited earlier) concludes, “So inflation remains a widely accepted but unconfirmed modification to the Big Bang theory.”