No one knows what caused it or why it slowed at the time it did.
It is a naturalistic dogma that should not be contested.
However, recently physicists Anna Ijjas, Paul J. Steinhardt and Abraham Loeb published an article in Scientific American in which they examined “the latest measurements from the European Space Agency relating to cosmic microwave background (CMB)” and suggested that the data did not support the inflation theory.
An article in Newsweek points out some fatal flaws in the theory:
– No one has found primordial gravitational waves—ripples in spacetime created by the Big Bang. – Inflation requires the existence of “inflationary energy,” for which there is no direct evidence.
Ijjas and colleagues think that the idea of a “big bounce”, i.e. periods of expansion and contraction, fits the data better.
However, Stephen Hawking and 32 other astronomers replied with an angry letter, stating that they believe that the Big Bang and its concomitant cosmic inflation is the more credible explanation for the origin of the cosmos.