In 1980, to explain the conditions observed in the universe, astrophysicist Alan Guth proposed cosmic inflation. The term inflation refers to the explosively rapid expansion of space-time that occurred a tiny fraction of a second after the Big Bang. In another tiny fraction of a second, inflation slowed to a more leisurely expansion that continues to this day and is accelerating.

The earliest radiation astronomers can detect is called the Cosmic Microwave Background, or CMB. This is radiation that was released about 380,000 years after the Big Bang.

Astronomers notice that, aside from tiny variations shown here as orange and blue spots, any patch on the CMB looks much the same as the patch on the opposite side of the sky.

One problem with the Big Bang theory is that areas on opposite sides of the observable universe ought not to appear so similar, since the two areas have not been causally connected.

What astronomers call the observable universe (or Hubble volume) is the spherical region, about 90 billion light-years in diameter, that is centered on any given observer.  At the horizon, we see the Cosmic Microwave Background radiation.

Astrophysicists say that everything within a Hubble volume is causally connected. An observer on Earth sees Point A, a spot on the CMB horizon. On the opposite side of Earth's sky is Point B. [The History & Structure of the Universe (Infographic)]

The two points are separated by 90 billion light-years, and are therefore outside each other's Hubble volume. How can both points have similar properties, since they are not causally connected?

Another problem with the Big Bang theory is that it does not explain why spacetime is so flat

Astrophysicists refer to a parameter of the universe pertaining to its density, Omega (chart at left). The value of Omega is observed to be 1.

If Omega had any value other than 1, the universe would be either closed and would eventually collapse, or open and would expand forever.

This universe seems to be 1, the critical density required to make the universe flat. Astrophysicists ask, why is Omega exactly 1?

Both flatness and the horizon problem can be explained if the universe underwent a brief but massive inflation.

It’s hard to talk about the beginning of the universe without resorting to many zeroes. The inflationary era began a tiny fraction of a second after the Big Bang. The figure would be given by writing zero, then a decimal point, then 35 zeroes, followed by a 1. This is roughly one trillionth of a trillionth of a trillionth of a second after the Big Bang.

Inflation only lasted another tiny fractional second — that number would have about 32 zeros.

The scale of the universe expanded exponentially in that brief span. A distance of one nanometer would have been enlarged to a quarter of a billion light-years.

Inflation means that the entire observable universe was once a tiny area. The massive inflation of the scale of spacetime would dilute the effects of curvature, and means that areas widely separated today would have been causally connected at that early time.