Kishore Ananda

Influence of structure formation on the cosmic expansion

We investigate the effect that the average backreaction of structure formation has on the dynamics of the cosmological expansion, within the concordance model. Our approach in the Poisson gauge is fully consistent up to second order in a perturbative expansion about a flat Friedmann background, including a cosmological constant. We discuss the key length scales which are inherent in any averaging procedure of this kind. We identify an intrinsic homogeneity scale that arises from the averaging procedure, beyond which a residual offset remains in the expansion rate and deceleration parameter. In the case of the deceleration parameter, this can lead to a quite large increase in the value, and may therefore have important ramifications for dark energy measurements, even if the underlying nature of dark energy is a cosmological constant. We give the intrinsic variance that affects the value of the effective Hubble rate and deceleration parameter. These considerations serve to add extra intrinsic errors to our determination of the cosmological parameters, and, in particular, may render attempts to measure the Hubble constant to percent precision overly optimistic.

Coexistence of matter dominated and accelerating solutions in f ( G ) gravity

Working within the theory of modified Gauss-Bonnet gravity, we show that Friedmann-Lema\^{\i}tre-Robertson-Walker--like power-law solutions only exist for a very special class of

Evolution of cosmological gravitational waves in f(R) gravity

f(\mathcal{G})

The cosmological background of vector modes

theories. Furthermore, we point out that any transition from decelerated to accelerated expansion must pass through

Vector modes generated by primordial density fluctuations

\mathcal{G}=0

Structure growth in f(R) theories of gravity with a dust equation of state

, and no function

A characteristic signature of fourth order gravity

f(\mathcal{G})

Different approaches to the second-order Klein-Gordon equation

that is differentiable at this point can admit both a decelerating power-law solution and any accelerating solution. This strongly constrains the cosmological viability of

Accelerating the Universe with Gravitational Waves

f(\mathcal{G})

On the co‐existence of matter dominated and accelerating solutions in f(G)‐gravity and the case of f(R)‐gravity

gravity, since it may not be possible to obtain an expansion history of the Universe which is compatible with observations. We explain why the same issue does not occur in