Inflationary Theory

Key Insight

To address the unresolved issues of the hot big bang model, particularly the uniformity, critical expansion rate, and density fluctuations, Alan Guth proposed the theory of 'inflation.' This model suggests that the very early universe underwent a period of extraordinarily rapid, accelerating expansion. Guth theorized that the universe, initially hot and chaotic, experienced a 'supercooling' phase. At high temperatures, the fundamental forces (strong, weak nuclear, and electromagnetic) are believed to have been unified. As the universe cooled, it dropped below a critical temperature, but instead of immediately undergoing a phase transition that would break this symmetry, it remained in an unstable, supercooled state. This state possessed excess energy that acted as an effective cosmological constant, generating a repulsive gravitational force.

This repulsive force caused the universe to expand exponentially, dramatically increasing its size by a factor of 1 with 30 zeros after it in a tiny fraction of a second. This rapid expansion had several crucial effects: it smoothed out any initial irregularities, explaining the observed large-scale uniformity of the universe, akin to blowing up a wrinkled balloon. It also allowed light and information to travel across regions that were initially causally disconnected, solving the 'horizon problem' by ensuring that distant parts of the early universe could homogenize their temperature. Furthermore, the inflationary mechanism naturally pushed the universe's expansion rate to be extremely close to the critical value required to avoid either immediate recollapse or an ever-accelerating expansion, thus explaining the observed 'flatness' of the universe without requiring improbable fine-tuning. Inflation also explains the vast amount of matter in the universe, estimated as 1 with 80 zeros after it particles in the observable region. In quantum theory, particles can be created from energy, and the total energy of the universe is zero, with positive matter energy canceled by negative gravitational energy. Inflation allows both positive and negative energies to scale up, providing a mechanism for creating matter without violating energy conservation.

Guth's original 'old inflationary model,' with its sudden phase transitions and the formation of separate 'bubbles' of broken symmetry, faced a problem: the universe expanded too quickly for these bubbles to merge, leading to a non-uniform universe inconsistent with observations. Andrei Linde, and independently Paul Steinhardt and Andreas Albrecht, developed the 'new inflationary model,' which proposed a slow breaking of symmetry within a single, vastly large region. While this addressed the bubble problem, it predicted temperature variations in the microwave background radiation that were greater than observed, ultimately leading to its scientific abandonment by many. Linde subsequently proposed the 'chaotic inflationary model' in 1983. This model dispenses with phase transitions and supercooling; instead, a spin-0 field with quantum fluctuations in the early universe possessed high energy in some regions, acting like a cosmological constant to drive inflationary expansion. As the field energy slowly decreased, the expansion transitioned to that of the hot big bang model. This chaotic inflation successfully explains the universe's uniformity, critical expansion rate, and provides density fluctuations consistent with observations, such as those detected by the COBE satellite in 1992.