The inflationary scenario constitutes an extension of the cosmological standard big bang model. According to the inflationary model the universe undergoes a phase of extremely rapid expansion starting around 10-35 second and ending 10-33 second after the big bang. Within this short time interval, the universe is believed to have expanded by a factor of about 1030-1050. Although not confirmable, the inflationary theory is considered as integral to the basic cosmological theories.
The epoch of inflation was presumably initiated by a phase transition (comparable with the transition from water to ice below the freezing point), causing the strong interaction to separate from the grand unified force. The basic ideas of an inflationary phase were proposed in 1979 by the Russian physicist Alexei Starobinsky and developed to a first consistent theory 2 years later by the American physicist and cosmologist Alan Guth. In 1982, inflation was brought to its modern shape independently by Andrei Linde, Andreas Albrecht, and Paul Steinhardt. The occasion to postulate an inflationary event in the very early universe was a number of unsolvable problems associated with the standard picture.
The Flatness Problem
Numerous observational hints independently suggest that the density parameter of the universe is Q = 1 with tight tolerance. That is to say, the mean density of the universe is close to a value known as critical density, which separates a universe of eternal expansion (Q < 1) from one that is to turn its expansion to a collapse in a remote future (Q > 1) due to the gravitational deceleration effect of its high mass content. In terms of Einstein’s general theory of relativity, the cosmological classification according to the mean density is identical to 1, according to the intrinsic geometry of space: While a density parameter Q > 1 comes along with a “closed” geometry (the three-dimensional equivalent of a spherical surface), the under-critical “open” Q < 1 universe is affected by the intrinsic geometry of a saddle. The limiting special case Q = 1 finally accords to a flat (“Euclidean”) geometry of a plane surface. The cosmological standard model predicts that any small deviation from Q = 1 in the early universe will be amplified massively in the course of time. Thus the universe should have a density parameter that is orders of magnitude larger or smaller than 1. Or, on the other hand, Q has to be extremely close to 1 right after the big bang. This is a classical problem of fine-tuning.
Solution Within Cosmic Inflation
After rapid expansion comes to an end, the size of space by far exceeds the diameter of the region that makes up our observable universe today. However strong the cosmic curvature may be today, the curvature of “our” space region, the “universe” by definition is tiny, just as the earth’s surface appears to be flat due to our limited perception.
The Horizon Problem
In theories of time and space one frequently uses the term past light cone of an event. An event is a point that determines a “here and now” in spacetime. The past light cone of an event E contains all events (points of spacetime) Ei in the past of E from which light or information could have reached the event E. An event E’ is able to influence a (future) event E only if E’ lies within the past light cone of E.
In 1965, physicists discovered a constant extragalactic microwave radiation that is now known to have been generated less than 400,000 years after the big bang. That so-called cosmological microwave background radiation is reaching us with identical physical properties from all directions of space. A close to perfect isotropy like this clearly seems to contradict the finite propagation velocity of light and information: The birth locations of the background radiation photons measured today have distances of 13 billion light-years from the earth. However, the cosmic background radiation, coming from opposed directions, is measured to have identical temperature and spectral distribution, while the past light cones of the according regions could not have overlapped 400,000 years after the big bang. There is no way to understand the isotropy of the cosmic microwave background in a universe without inflation.
Solution Within Cosmic Inflation
Prior to the onset of inflation the entire space of the universe may well have been in thermodynamic equilibrium. This means there was a common, well- defined temperature throughout the universe and all energy was smoothly distributed. In the course of inflation, all regions of space were torn apart at many times the speed of light and thus lost their causal contact—in other words, their past light cones would no longer overlap after the explosive separation.
Nevertheless, all physical properties have either remained constant or changed in identical ways (density, temperature) within any volume element while inflation happens.
The Monopole Problem
James Clerk Maxwell’s (1831-1879) empirical formulation of the electromagnetic laws contains an asymmetry, running counter to the ambition of highest simplicity in modern physical theories: Classical electrodynamics does not allow for magnetic monopoles; that is, isolated magnetic south or north poles. In fact, a magnetic monopole has so far never been observed in nature. Electric monopoles (charges), on the other hand, are well known. In any case, grand unification theories predict that massive magnetic monopoles formed in the very early universe. Why are these undiscoverable today?
Solution Within Cosmic Inflation
According to grand unification theories, magnetic monopoles should have been created with extremely low numbers within much less than a second after the big bang. The monopole frequency should have diminished during the inflationary expansion in such a manner that there is a close to zero chance of discovering one today.
In terms of quantum theory, the universe was brought from a pure quantum mechanical being into a macroscopic classical one by inflation. Thus the inflationary scenario yields a possible explanation for the onset of structure formation in the universe: Due to Heisenberg’s uncertainty principle, there must have existed tiny quantum fluctuations within matter and radiation (the “primordial soup”) right after the big bang. While the rapid expansion of inflation was acting, these quantum fluctuations were being transformed to a macroscopic density contrast. Starting from this, large structures like galaxies and clusters of galaxies were able to form by gravitational instabilities in the course of time.
There is little agreement on the physical details of how inflation occurred. This is due partly to the uncertain state of knowledge of the corresponding high-energy physics. Second, cosmologists are far from being able to perform experimental tests of the inflationary model. In any event, inflation is in principle falsifiable in terms of Karl Popper’s philosophy of science. This means one can think of feasible experiments that would allow (depending on their results) the inflationary scenario to be disproved. Such an experiment was performed by the NASA space probe WMAP launched in 2001. Its very detailed measurements of the cosmological microwave background radiation are compatible with the inflationary picture. Note, however, that this is by no means a proof of inflation.
Because the big bang itself cannot be considered as encompassed by the time evolution of the universe, inflation marks the first significant change in state of the universe and the beginning of its macroscopic being.
See also Big Bang Theory; Black Holes; Popper, Karl R.; Quantum Mechanics; Universe, Contracting or Expanding; Universe, Evolving; Universes, Baby
Guth, A. H. (1998). The inflationary universe. Reading, MA: Perseus.
Linde, A. (1990). Inflation and quantum cosmology. Boston: Academic Press.
Linde, A. (1990). Particle physics and inflationary cosmology. Chur, Switzerland: Harwood Academic.