Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Relativistic initial conditions for N-body simulations   Initial conditions for (Newtonian) cosmological N-body simulations are
usually set by re-scaling the present-day power spectrum obtained from linear
(relativistic) Boltzmann codes to the desired initial redshift of the
simulation. This back-scaling method can account for the effect of
inhomogeneous residual thermal radiation at early times, which is absent in the
Newtonian simulations. We analyse this procedure from a fully relativistic
perspective, employing the recently-proposed Newtonian motion gauge framework.
We find that N-body simulations for LambdaCDM cosmology starting from
back-scaled initial conditions can be self-consistently embedded in a
relativistic space-time with first-order metric potentials calculated using a
linear Boltzmann code. This space-time coincides with a simple "N-body gauge"
for z<50 for all observable modes. Care must be taken, however, when simulating
non-standard cosmologies. As an example, we analyse the back-scaling method in
a cosmology with decaying dark matter, and show that metric perturbations
become large at early times in the back-scaling approach, indicating a
breakdown of the perturbative description. We suggest a suitable "forwards
approach" for such cases.

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The categories of this paper are: q-fin
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The categories of this paper are: econ
30
The categories of this paper are: q-alg
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The categories of this paper are: bayes-an
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The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-lat
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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: math
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The categories of this paper are: cs
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The categories of this paper are: cmp-lg
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The categories of this paper are: ao-sci
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The categories of this paper are: nlin