Editing Big Bang (section)

=== Cosmic acceleration ===
{{Main|Accelerating expansion of the universe}}
Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as [[dark energy]], which appears to homogeneously permeate all of space. Observations suggest that 73% of the total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together, [[gravity]] predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate.<ref name="peebles" />

Dark energy in its simplest formulation is modeled by a [[cosmological constant]] term in [[Einstein field equations]] of general relativity, but its composition and mechanism are unknown. More generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theory.<ref name="peebles" />

All of this cosmic evolution after the [[inflationary epoch]] can be rigorously described and modeled by the lambda-CDM model of cosmology, which uses the independent frameworks of [[quantum mechanics]] and general relativity. There are no easily testable models that would describe the situation prior to approximately 10<sup>−15</sup> seconds.{{sfn|Manly|2011|loc=chpt. 7: "The Ultimate Free Lunch"{{page needed|date=January 2020}}}} Understanding this earliest of eras in the history of the universe is one of the greatest [[List of unsolved problems in physics|unsolved problems in physics]].