Speaker
Description
Dark energy drives the present accelerated expansion of the universe, yet its
physical nature remains uncertain. Its properties are characterised by the equation of state w = p/ρ, with w = −1 corresponding to a cosmological constant.
Determining whether w deviates from this value is crucial, since w < −1 would
imply a future Big Rip singularity in homogeneous cosmological models.
I present a hybrid method for reconstructing the dark energy equation of
state that does not assume a predefined parametric form such as the CPL
model. After validation on simulated supernova data, the method is applied
to the Pantheon+SH0ES sample. While broadly consistent with w = −1, the
reconstruction allows a region of parameter space in which w falls below −1,
raising the possibility of phantom behaviour.
Motivated by this observational allowance, I then examine whether a Big
Rip is inevitable once inhomogeneities are included. Using the 1+3 covariant
formalism under the silent-universe approximation, I show that local shear can
significantly modify the late-time dynamics. For certain parameter ranges compatible with observations, anisotropic effects stabilise the local expansion rate
and prevent divergence, suggesting that the fate of cosmic structure may depend
sensitively on inhomogeneous dynamics.
This talk is based on two papers that are currently under review in Phys.
Rev. D, ([1, 2]):
[1] G. Simpson, K. Bolejko, and S. Walters. A hybrid method for re-
construction of the equation of state of dark energy and its application to
Pantheon+SH0ES data. 2026. Submitted to Phys. Rev. D.
[2] G. Simpson, K. Bolejko, and S. Walters. Big Rip and inhomogeneities: can
the Galaxy survive the Big Rip? 2026. Submitted to Phys. Rev. D.
| Research Area | Cosmology: late universe |
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