Physicist Alan Guth's theory of cosmic inflation describes an almost immeasurably small event that happened incomprehensibly quickly at the origin of the universe roughly 13.8 billion years ago. A patch of material a fraction of the size of a proton was stretched by a unique form of gravity (repelling, rather than attracting) to the size of a marble. When cosmic inflation stopped, the Big Bang took over.
A patch of a peculiar material, about 100-billionth the size of a single proton, generates a repulsive form of gravity.
Repulsive gravity doubled the patch in size every tenth of a trillionth of a trillionth of a trillionth of a second. A novel feature of repulsive gravity allowed the matter to maintain a constant density even as it kept growing.
Inflation gravity was unique
Normally gravity is an attractive force.
Repulsive gravity, created by the peculiar material present in cosmic inflation, caused matter to repel instead.
Patch grows in size
This doubling repeated about 100 times until the patch was about the size of a marble.
Then inflation ended. The peculiar material that had caused the repulsive gravity was unstable, and it decayed, releasing energy that produced ordinary particles and, eventually, the universe we know today. When cosmic inflation was complete, the region the size of a marble contained the ingredients for what would become our entire observable universe.
Inflation's fossil evidence
A group of scientists led by John Kovac found evidence for inflation by looking through a telescope at light that formed 380,000 years after inflation occurred, what's called the cosmic microwave background. A relic of that first expansion, swirling patterns in the polarization of the cosmic microwave background indicated gravitational radiation dating from that first fraction of a second of the universe's existence.
SOURCES: The Inflationary Universe: The Quest for a New Theory of Cosmic Origins, Alan H. Guth; BICEP2 Collaboration