# The Theory of Inflation

Inflation Cosmology, or simply Inflation, is the leading theory (or group of theories) of how the universe evolved during the early moments of its expansion. It is a mathematical model that explains what happened before the 380,000-year horizon of the CMB.

Inflation theory brings together ideas from quantum mechanics and particle physics to explore the early moments of the universe. The theory proposes that approximately $10^{-35}$ to \$latex $10^{-32}$ seconds after the Big Bang, the universe expanded exponentially, thus enabling it to maintain an even temperature throughout its cosmic progression. After this brief inflationary era, the expansion rate returned to the more leisurely rate of the standard model of cosmology.

Inflation theory was originally proposed by Alan Guth in 1979, and then improved upon by A.D. Linde and others in 1981. It solves both the horizon and flatness problems of the standard model. During the inflation phase, the diameter of the universe increased by a factor of or more while the temperature diminished and stabilized at about degrees. During the course of this astounding growth all matter and energy in the universe were created via quantum fluctuations from virtually nothing.

 One of the intriguing consequences of inflation is that quantum fluctuations in the early universe can be stretched to astronomical proportions, providing the seeds for the large scale structure of the universe. Alan Guth and others calculated the predicted spectrum of these fluctuations in 1982. These fluctuations manifest as ripples in the CMB radiation today; these ripples were detected by the COBE satellite in 1992, and later to much higher precisions by the WMAP and Planck satellites. Detection of CMB is covered here. Figure 4, 5, and 6 show the ripples in CMB. The properties of the radiation, though found to be in very good agreement with the predictions of the simplest models of inflation, are still not their confirmatory tests.

Tiny ripples in the rapidly expanding energy field eventually grew into the large-scale structures of the universe.

This period of inflation ended rapidly (it only lasted until $10^{-32}$ seconds after the Big Bang) but in spite of its brevity, it is the vital mechanism that spectacularly solved many of the problems of Big Bang, but until recently it was all circumstantial evidence. Like all good theories, the Inflation theory needed to make a prediction that could be verified directly by an experiment.

But the ultimate question is: What produced the energy before inflation? Incredibly, as mentioned earlier, the energy may have come out of nothing!

Not long ago the problem of how the universe came into existence lay beyond the scope of science, and questions about its origin fell into the territory of religion and philosophy. Recently scientists have arrived at a plausible account of the coming-into-being of the entire cosmos. This theory is still in its infancy and may yet prove to be imprecise, but the fact that there now exists a rigorous explanation is a testimony to the triumph of scientific endeavor.

Quantum mechanics (specifically Heisenberg’s Uncertainty Principle) provides a natural explanation for how energy may have emerged out of nothing. According to quantum mechanics, particles and antiparticles spontaneously form and annihilate each other without violating the law of energy conservation. These spontaneous creations and annihilations of the so-called virtual particle pairs are known as quantum fluctuations. Laboratory experiments have shown quantum fluctuations occur everywhere and all the time. Actually, quantum fluctuations have to be taken into account to accurately calculate the energy levels of atoms.

It is possible to imagine many quantum fluctuations occurred before the birth of our universe. Most of them quickly disappeared but one lived sufficiently long and had the right conditions for inflation to initiate.

The grand unified theory (GUT) of particle physics and general relativity, on which the theory of Inflation is based on, allows for the creation of a repulsive gravity field at high energies. According to Guth, the extremely hot early universe at around $10^{-37}$ second after the Big Band could be the ideal place for producing material with repulsive gravity. Incredible as it seems, a patch of about $10^{-28}$ cm in size would have been large enough to ultimately produce the universe. Till about $10^{-35}$ second after the Big Bang, this material expanded exponentially doubling in size at least 65 times after which the region became the size of a marble. Thereafter, the standard Big Bang cosmology sets in and the original tiny volume is magnified by an enormous factor giving rise to the macroscopic universe.

As Alan Guth once remarked, the universe is the ultimate free lunch if quantum fluctuations are ultimately responsible for its being! It came from nothing and its total energy is zero, but it nevertheless has incredible structure and complexity. There could even be many other such universes, spatially distinct from ours.

This leads us to the logical question whether inflation really happened. A team of US scientists has recently unearthed unshakable evidence of inflation in a groundbreaking experiment conducted in the South Pole. In order to appreciate how such an experiment was eventually designed, one needs to understand how gravitational waves interact with the CMB, the central theme of this article. But first, it is important to cover the recent discoveries that have revolutionized the fields of cosmology and particle physics: dark energy and dark matter. A broad understanding of these phenomena is necessary to follow the cosmic progression of our universe, and how everything fits into the big picture.