Once upon a time scientists assumed Earth was centre of the Universe, and the planets revolved around us. Since anyone observing the sky sees the planets appear to move backwards in their orbits. To explain retrograde motion scientists added epicycles, wheels within wheels. A divergence of theories grew up, some containing 60-100 epicycles.
Anyone observing the cosmic background also sees that large areas of the sky have reached thermal equilibrium, as if the speed of light were once faster. This is the horizon problem. Nearly 30 years ago Alan Guth and others suggested that the early Universe expanded at warp speed, many times faster than light. This inflation would violate both the First Law of Thermodynamics and Relativity's stipulation that nothing travels faster than light. A divergence of inflation theories has grown up, containing many epicycles. Inflation shares with strings the convenient property of being unprovable. Since we can't time-travel to the Big Bang, or reproduce its unimaginable energies, the inflationary hypothesis may never be proven.
The March issue of PHYSICS TODAY spotlights "Alternatives to Cosmic Inflation." They are kind enough to mention another idea:
"The varying-speed-of-light scenario is a proposed alternative to inflationary cosmology that assumes the speed of light in the very early universe was much larger than it is today. Relative to (Standard Big Bang) cosmology, the enhanced speed of light increases the size of a region that can be in causal contact. Thus... the varying-speed-of-light scenario provides a solution to the horizon problem. It also solves the flatness problem of SBB cosmology."
Unlike inflation, which can never be tested, the speed of light is changing right now at a tiny rate. The best evidence comes from redshifts of distant Type Ia supernovae. Since anyone examining the data can see that the speed of light has slowed, scientists have added a repulsive "dark energy" causing the Universe to accelerate in relation to light. A cottage industry of "dark energy" theories has grown up, containing many epicycles. Since we can't travel to intergalactic Space to collect some, the "dark energy" hypothesis may never be proven.
The March 8 issue of NEW SCIENTIST suggests Dark Energy May Just Be a Cosmic Illusion. Here they ilustrate the fascinating work of David Wiltshire at the University of Canterbury, New Zealand. Wiltshire has come up with a model Universe with 90% dark matter, 10% baryons, and no dark energy at all. This model has an age of 14.7 billion years, but even greater in distant parts of the Universe. In these distant regions, time would pass at a different rate. A changing rate of time is mathematically equivalent to a changing speed of light.
At one time alternatives to inflation and "dark energy" could not reach print at all. Today mainstream publications are beginning to question these prevailing hypotheses. There is room for a Theory that could replace them both, which could be simple as GM=tc^3.