How Do Perturbation Equations Affect FRW Cosmology Metrics?

In summary, the perturbation equations play a crucial role in analyzing the dynamics of Friedmann-Robertson-Walker (FRW) cosmology metrics by allowing for the study of small deviations from the homogeneous and isotropic solutions of general relativity. These equations help in understanding the evolution of density fluctuations and the growth of structure in the universe, influencing both the cosmic microwave background radiation and large-scale structure formation. By examining how perturbations interact with the underlying FRW metrics, researchers can gain insights into the behavior of the universe under various cosmological models, including those involving dark energy and modified gravity theories.
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Homework Statement
##T^{00} = a^{-2} \bar{\rho}(1+\delta)##
##T^{0i} = a^{-2} \bar{\rho}(1+w)v^i##
##T^{ij} = a^{-2} \bar{\rho} [(1+\delta)\delta^{ij} - h^{ij}]##
Relevant Equations
##\nabla_{\mu} T^{\mu \nu} = 0##
The perturbed line element: ##g = a(\tau)^2[-d\tau^2 + (\delta_{ij} + h_{ij})dx^i dx^j]##
Expanding the covariant derivative with ##\nu = 0##, you get a few pieces. Here on keeping only terms linear in the perturbations,

##\partial_{\mu} T^{\mu 0} = a^{-2} \bar{\rho} \left[ \delta' - 2\mathcal{H} (1+\delta) + (1+w) i\mathbf{k} \cdot \mathbf{v} \right]##

here ##\mathcal{H} = a'/a## and ##i \mathbf{k} \cdot \mathbf{v} = \partial_i v^i##. Then

##\Gamma^{\mu}_{\mu \rho} T^{\rho 0} = a^{-2} \bar{\rho} \left[ 4\mathcal{H}(1+\delta) + \frac{1}{2} h' \right]##

##\Gamma^{0}_{\mu \rho} T^{\mu \rho} = a^{-2} \bar{\rho} \left[ \mathcal{H}(1+\delta)(1+3w) + \frac{1}{2} w h' \right]##

Overall,
##0 = a^{-2} \bar{\rho} \left[ \delta' + 3(1+w) \mathcal{H}(1+\delta) + (1+w) i \mathbf{k} \cdot \mathbf{v} + \frac{1}{2}(1+w)h'\right]##

but the term ##3(1+w) \mathcal{H}(1+\delta)## shouldn't be there. I can't see why not? For reference, the connection coefficients

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FAQ: How Do Perturbation Equations Affect FRW Cosmology Metrics?

What are perturbation equations in the context of FRW cosmology?

Perturbation equations in FRW (Friedmann-Robertson-Walker) cosmology describe small deviations from the perfectly homogeneous and isotropic universe model. These equations help in understanding how small inhomogeneities and anisotropies evolve over time, which is crucial for explaining the formation of large-scale structures like galaxies and clusters in the universe.

Why are perturbation equations important for understanding the early universe?

Perturbation equations are important for understanding the early universe because they provide insights into the growth of initial density fluctuations that eventually led to the formation of cosmic structures. By studying these perturbations, scientists can trace back the origins of the cosmic microwave background (CMB) anisotropies and the distribution of matter in the universe.

How do perturbation equations modify the FRW cosmology metrics?

Perturbation equations modify the FRW cosmology metrics by introducing small perturbations to the metric tensor, which describes the geometry of spacetime. These modifications result in a perturbed metric that accounts for the presence of inhomogeneities and anisotropies. The perturbed metric is then used to study the evolution of these deviations over time.

What role do different types of perturbations (scalar, vector, tensor) play in FRW cosmology?

In FRW cosmology, perturbations are classified into three types: scalar, vector, and tensor. Scalar perturbations are associated with density fluctuations and are the most significant for structure formation. Vector perturbations are related to vortical motions and are generally less important in cosmology. Tensor perturbations correspond to gravitational waves, which can provide information about the early universe and inflationary processes.

How are perturbation equations solved in the context of FRW cosmology?

Perturbation equations in FRW cosmology are typically solved using linear perturbation theory, where the equations are linearized around the background FRW solution. This approach simplifies the equations, making them more tractable. Numerical methods and analytical techniques are then employed to solve these linearized equations and study the evolution of perturbations over time.

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