Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the propagation of gravitational waves in a $\Lambda$CDM universe   We study here how the presence of non-zero matter density and a cosmological
constant could affect the observation of gravitational waves in Pulsar Timing
Arrays. Conventionally, the effect of matter and cosmological constant is
included by considering the redshift in frequency due to the expansion.
However, there is an additional effect due to the change of coordinate systems
from the natural ones in the region where waves are produced to the ones used
to measure the pulsar timing residuals. This change is unavoidable as the
strong gravitational field in a black hole merger distorts clocks and rules.
Harmonic waves produced in such a merger become anharmonic when detected by a
cosmological observer. The effect is small but appears to be observable for the
type of gravitational waves to which PTA are sensitive and for the favoured
values of the cosmological parameters.

18
The categories of this paper are: econ
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas
15
The categories of this paper are: nucl-ex
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
20
The categories of this paper are: acc-phys
9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
37
The categories of this paper are: supr-con
3
The categories of this paper are: physics
1
The categories of this paper are: cs