Cosmology: a bird's eye view

TL;DR

This paper compares two different observational perspectives in cosmology, highlighting how they can lead to radically different interpretations of the universe's behavior, including scenarios like eternal expansion or a 'big rip' singularity.

Contribution

It demonstrates how different observer frameworks in cosmology can yield contrasting insights into the universe's evolution, without requiring exotic matter.

Findings

Different observers perceive radically different universe behaviors.

One observer sees an eternally expanding universe, the other can experience a 'big rip'.

Observer perspective significantly influences cosmological interpretation.

Abstract

In this essay we discuss the difference in views of the Universe as seen by two different observers. While one of the observers follows a geodesic congruence defined by the geometry of the cosmological model, the other observer follows the fluid flow lines of a perfect fluid with a linear equation of state. We point out that the information these observers collect regarding the state of the Universe can be radically different; while one observes a non-inflating ever-expanding ever-lasting universe, the other observer can experience a dynamical behaviour reminiscent to that of quintessence or even that of a phantom cosmology leading to a 'big rip' singularity within finite time (but without the need for exotic forms of matter).