Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Doppelg\"anger dark energy: modified gravity with non-universal
  couplings after GW170817   Gravitational Wave (GW) astronomy severely narrowed down the theoretical
space for scalar-tensor theories. We propose a new class of attractor models
for Horndeski action in which GWs propagate at the speed of light in the nearby
universe but not in the past. To do so we derive new solutions to the
interacting dark sector in which the ratio of dark energy and dark matter
remains constant, which we refer to as doppelg\"anger dark energy (DDE). We
then remove the interaction between dark matter and dark energy by a suitable
change of variables. The accelerated expansion that (we) baryons observe is due
to a conformal coupling to the dark energy scalar field. We show how in this
context it is possible to find a non trivial subset of solutions in which GWs
propagate at the speed of light only at low red-shifts. The model is an
attractor, thus reaching the limit $c_{T}\to1$ relatively fast. However, the
effect of baryons turns out to be non-negligible and severely constrains the
form of the Lagrangian. In passing, we found that in the simplest DDE models
the no-ghost conditions for perturbations require a non-universal coupling to
gravity. In the end, we comment on possible ways to solve the lack of matter
domination stage for DDE models.

32
The categories of this paper are: atom-ph
34
The categories of this paper are: chem-ph
37
The categories of this paper are: supr-con
1
The categories of this paper are: cs
11
The categories of this paper are: hep-lat
25
The categories of this paper are: patt-sol
15
The categories of this paper are: nucl-ex
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
4
The categories of this paper are: cond-mat
9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
23
The categories of this paper are: solv-int
24
The categories of this paper are: dg-ga