Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Primordial magnetic seed fields from extra dimensions   Dynamical extra dimensions break the conformal invariance of Maxwell's
equations in four dimensions. A higher dimensional background with $n$
contracting extra dimensions and four expanding dimensions is matched to an
effectively four dimensional standard radiation dominated universe. The
resulting spectrum for the magnetic field is calculated taking into account
also the momenta along the extra dimensions. Imposing constraints from
observations an upper limit on the strength of magnetic seed fields is found.
Depending on the number of extra dimensions, cosmologically interesting
magnetic fields can be created.

23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
30
The categories of this paper are: q-alg
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
21
The categories of this paper are: adap-org
31
The categories of this paper are: ao-sci
16
The categories of this paper are: stat