Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Problems with Time-Varying Extra Dimensions or "Cardassian Expansion" as
  Alternatives to Dark Energy   It has recently been proposed that the Universe might be accelerating as a
consequence of extra dimensions with time varying size. We show that although
these scenarios can lead to acceleration, they run into serious difficulty when
taking into account limits on the time variation of the four dimensional
Newton's constant. On the other hand, models of ``Cardassian'' expansion based
on extra dimensions which have been constructed so far violate the weak energy
condition for the bulk stress energy, for parameters that give an accelerating
universe.

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The categories of this paper are: eess
26
The categories of this paper are: bayes-an
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
37
The categories of this paper are: supr-con
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
13
The categories of this paper are: quant-ph
5
The categories of this paper are: gr-qc
9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas