Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A Parameterized Post-Friedmann Framework for Modified Gravity   We develop a parameterized post-Friedmann (PPF) framework which describes
three regimes of modified gravity models that accelerate the expansion without
dark energy. On large scales, the evolution of scalar metric and density
perturbations must be compatible with the expansion history defined by distance
measures. On intermediate scales in the linear regime, they form a
scalar-tensor theory with a modified Poisson equation. On small scales in dark
matter halos such as our own galaxy, modifications must be suppressed in order
to satisfy stringent local tests of general relativity. We describe these
regimes with three free functions and two parameters: the relationship between
the two metric fluctuations, the large and intermediate scale relationships to
density fluctuations and the two scales of the transitions between the regimes.
We also clarify the formal equivalence of modified gravity and generalized dark
energy. The PPF description of linear fluctuation in f(R) modified action and
the Dvali-Gabadadze-Porrati braneworld models show excellent agreement with
explicit calculations. Lacking cosmological simulations of these models, our
non-linear halo-model description remains an ansatz but one that enables
well-motivated consistency tests of general relativity. The required
suppression of modifications within dark matter halos suggests that the linear
and weakly non-linear regimes are better suited for making complementary test
of general relativity than the deeply non-linear regime.

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The categories of this paper are: gr-qc
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The categories of this paper are: chem-ph
36
The categories of this paper are: cmp-lg
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The categories of this paper are: patt-sol
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The categories of this paper are: quant-ph
9
The categories of this paper are: nlin
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The categories of this paper are: hep-ex
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The categories of this paper are: q-fin
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The categories of this paper are: cs
22
The categories of this paper are: chao-dyn
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The categories of this paper are: acc-phys
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The categories of this paper are: adap-org
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The categories of this paper are: comp-gas
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The categories of this paper are: nucl-ex
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The categories of this paper are: hep-ph
37
The categories of this paper are: supr-con