Dark energy is unidentified energy that offsets gravity, to explain the observed accelerating expansion of the universe. The explanation need not proffer dark energy, however, as gravity will do. Here’s how: We observe special relativistic time dilation about supernovae receding from us at relativistic velocities. In relativity theory time dilation always goes hand in hand with length contraction, so the receding galaxies must be contracted along their axis of motion relative to us. As this axis is directly along our line of sight, the contraction is wholly hidden from our view, like a photo hides depth. If the universe were decelerating in its expansion then receding galaxies would uncontract, which is to say they would expand toward their full (locally-measured) depth along our line of sight. This is special relativistic length uncontraction (hereafter just “spatial expansion”). Not only the galaxies but the space between the galaxies and us as well would expand (evident since the space between any given galaxy and us could hypothetically be replaced with a galaxy that would likewise expand). While we cannot observe the spatial expansion of the galaxies themselves (this expansion occurring directly along our line of sight), we can observe the spatial expansion between the galaxies and us because it affects their recessional velocities. Given the above and because deceleration is an accelerating decrease in velocity, a universe decelerating in its expansion would observably expand at an accelerating rate until recessional velocities diminished to non-relativistic levels to make the remaining spatial expansion negligible, the space between the galaxies and us having spatially expanded—or uncontracted—to nearly full depth. Then the expansion would decelerate as observed. To explain the observed accelerating expansion of the universe, what is needed is a process that starts the universe expanding at a constant rate (such as proposed by the inflationary model), and gravity since then, to decelerate the expansion. Dark energy is not required and, by Occam’s razor, need not exist.
