What is dark energy?
Dark energy names whatever drives the observed acceleration of cosmic expansion. A cosmological constant is its simplest description, with pressure equal to minus its energy density. OPH's fixed-capacity branch proposes that the constant is set by how many durable records a cosmological horizon can hold. In Planck units, the proposed relation is Lambda = 3 pi divided by that capacity. A very large capacity then corresponds to a very small constant. Turning a record capacity into a physical horizon and selecting its value are substantive parts of the proposal. On the fixed-capacity interpretation, the pressure-to-density ratio is minus one; a demonstrated change would reject that branch. The mechanism must be compared with observations, rather than inferred from agreement or finite memory alone.
See also Why is the cosmological constant so small?, Why are dark energy and matter important at roughly the same cosmic epoch?, Why does the enormous calculated vacuum energy not curve the universe enormously?
Sources: Observer spacetime and the conditional Einstein reconstruction; Observer-screen synchronization as a candidate inflation-free branch
Also in Chapter 33: Why Was Every Open Problem the Same Problem?, Appendix A: Eighteen Questions.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 29: Why 1/137, and Why Is There a Limit to What the Universe Can Remember?watch a universe make its own particles