Cosmic deceptions due to peculiar motions

TL;DR

This paper explores how peculiar motions can cause false perceptions of cosmic acceleration, leading observers to incorrectly interpret a decelerating universe as accelerating due to relative motion effects.

Contribution

It demonstrates that bulk peculiar flows can mimic cosmic acceleration signals, explaining potential observational illusions without actual acceleration.

Findings

Observers in bulk flows may falsely perceive acceleration.

Apparent acceleration varies with sky position and redshift.

Signature of relative motion appears as a Doppler-like anisotropy.

Abstract

Relative motions have long been known to mislead the unsuspecting observers to false interpretations of reality. The deceptions are usually brief and unimportant, though relative motions have also led to illusions that were both long-lasting and important. Indeed, in the history of astronomy there are several examples where relative-motion effects have misled us to gross misinterpretations. Here, we consider the possibility that our peculiar motion relative to the cosmic rest-frame can trigger deceptions on cosmological scales. In so doing, we will demonstrate that unsuspecting observers inside bulk peculiar flows may come to the false conclusion of recent accelerated expansion, when their host universe is actually decelerating. The same observers may then erroneously attribute their apparent acceleration to an also recent dramatic change in the nature of the cosmic medium. In reality, however, nothing has really happened. Despite the appearances, the host universe keeps decelerating and its material content retains its conventional form. Nevertheless, there are ways out of these illusions. Our observers can find out that they have been deceived by their own peculiar flow, by looking for the trademark signature of relative motion in their data. This signature is nothing else but a Doppler-like anisotropy in the sky distribution of the measured deceleration parameter. To the bulk-flow observers, the universe should appear to accelerate faster along a certain point on the celestial sphere and equally slower along the antipodal. Moreover, the magnitude of the apparent dipole should decrease with increasing redshift.