Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Is space really expanding? A counterexample   In all Friedman models, the cosmological redshift is widely interpreted as a
consequence of the general-relativistic phenomenon of EXPANSION OF SPACE. Other
commonly believed consequences of this phenomenon are superluminal recession
velocities of distant galaxies and the distance to the particle horizon greater
than c*t (where t is the age of the Universe), in apparent conflict with
special relativity. Here, we study a particular Friedman model: empty universe.
This model exhibits both cosmological redshift, superluminal velocities and
infinite distance to the horizon. However, we show that the cosmological
redshift is there simply a relativistic Doppler shift. Moreover, apparently
superluminal velocities and `acausal' distance to the horizon are in fact a
direct consequence of special-relativistic phenomenon of time dilation, as well
as of the adopted definition of distance in cosmology. There is no conflict
with special relativity, whatsoever. In particular, INERTIAL recession
velocities are subluminal. Since in the real Universe, sufficiently distant
galaxies recede with relativistic velocities, these special-relativistic
effects must be at least partly responsible for the cosmological redshift and
the aforementioned `superluminalities', commonly attributed to the expansion of
space. Let us finish with a question resembling a Buddhism-Zen `koan': in an
empty universe, what is expanding?

11
The categories of this paper are: hep-lat
28
The categories of this paper are: alg-geom
29
The categories of this paper are: funct-an
24
The categories of this paper are: dg-ga
20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
10
The categories of this paper are: q-bio
9
The categories of this paper are: nlin
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math