Chronology of the universe
Inflationary epoch and the rapid expansion of space

At this point of the very early universe, the universe is thought to have expanded by a factor of at least 10 in volume. This is equivalent to a linear increase of at least 10 times in every spatial dimension—equivalent to an object 1 nanometre (10 m, about half the width of a molecule of DNA) in length, expanding to one approximately 10.6 light-years (100 trillion kilometres) long in a tiny fraction of a second. This phase of the cosmic expansion history is known as inflation.

The mechanism that drove inflation remains unknown, although many models have been put forward. In several of the more prominent models, it is thought to have been triggered by the separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field. As this field settled into its lowest energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of universe. Inflation explains several observed properties of the current universe that are otherwise difficult to account for, including explaining how today's universe has ended up so exceedingly homogeneous (spatially uniform) on a very large scale, even though it was highly disordered in its earliest stages.

It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10 and 10 seconds after the Big Bang. The rapid expansion of space meant that elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe. However, the huge potential energy of the inflaton field was released at the end of the inflationary epoch, as the inflaton field decayed into other particles, known as "reheating". This heating effect led to the universe being repopulated with a dense, hot mixture of quarks, anti-quarks and gluons. In other models, reheating is often considered to mark the start of the electroweak epoch, and some theories, such as warm inflation, avoid a reheating phase entirely.

In non-traditional versions of Big Bang theory (known as "inflationary" models), inflation ended at a temperature corresponding to roughly 10 seconds after the Big Bang, but this does not imply that the inflationary era lasted less than 10 seconds. To explain the observed homogeneity of the universe, the duration in these models must be longer than 10 seconds. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation.

After inflation ended, the universe continued to expand, but at a decelerating rate. About 4 billion years ago the expansion gradually began to speed up again. This is believed to be due to dark energy becoming dominant in the universe's large-scale behavior. It is still expanding today.

On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-modes power spectrum which was interpreted as clear experimental evidence for the theory of inflation. However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported and finally, on 2 February 2015, a joint analysis of data from BICEP2/Keck and the European Space Agency 's Planck microwave space telescope concluded that the statistical "significance is too low to be interpreted as a detection of primordial B-modes" and can be attributed mainly to polarized dust in the Milky Way.
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