Eur. Phys. J. C (2020) 80:166
https://doi.org/10.1140/epjc/s10052-020-7738-8
Regular Article - Theoretical Physics
Karmarkar scalar condition
J. Ospino
1,a
, L. A. Núñez
2,3,b
1
Departamento de Matemática Aplicada and Instituto Universitario de Física Fundamental y Matemáticas, Universidad de Salamanca, Salamanca,
Spain
2
Escuela de Física, Universidad Industrial de Santander, Bucaramanga, Colombia
3
Departamento de Física, Universidad de Los Andes, Mérida, Venezuela
Received: 28 January 2020 / Accepted: 9 February 2020 / Published online: 21 February 2020
© The Author(s) 2020
Abstract In this work we present the Karmarkar condition
in terms of the structure scalars obtained from the orthogonal
decomposition of the Riemann tensor. This new expression
becomes an algebraic relation among the physical variables,
and not a differential equation between the metric coeffi-
cients. By using the Karmarkar scalar condition we imple-
ment a method to obtain all possible embedding class I static
spherical solutions, provided the energy density profile is
given. We also analyse the dynamic adiabatic case and show
the incompatibility of the Karmarkar condition with several
commonly assumed simplifications to the study of gravita-
tional collapse. Finally, we consider the dissipative dynamic
Karmarkar collapse and find a new solution family.
1 Introduction
General Relativity is living unprecedented times, witnessing
the transformation of exotic objects – such as black holes
– and feeble phenomena – like gravitational waves – from
mathematical curiosities to observable physical entities.
There are many notable attempts to explore the properties
of physically viable solutions (numeric & analytic) describ-
ing either static, stationary, or collapsing relativistic compact
objects. All known exact solutions have been obtained by
imposing some restrictions, such as symmetry conditions on
the metric, the algebraic structure of the Riemann tensor, new
coupled field equations, meaningful equations of state for the
matter variables, or selecting particular initial and boundary
conditions, to mention the most common strategies.
Einstein’s covariant mathematical description of gravi-
tation contrasts with solution obtained which are strongly
dependent on the coordinate basis. It is not always easy
to understand the qualitative features that these coordinate-
a
e-mail: j.ospino@usal.es
b
e-mail: lnunez@uis.edu.co (corresponding author)
prone solutions might possess, and the analysis of their gen-
eral properties could reveal unforeseen features of the theory.
Thus it is useful to study the general properties through a
coordinate independent formalism.
We have recently implemented a tetrad formalism by an
orthogonal splitting of the Riemann tensor. We introduced
a full set of equations equivalent to the Einstein system and
applied it to the spherical case, showing that it is possible to
obtain relevant information from self-gravitating systems [1,
2]. This formalism provides coordinate-free results expressed
in terms of structure scalars closely related to the kinematical
and physical properties of the fluid.
In this short paper, we shall explore the consequences of
imposing the well-known Karmarkar condition [3], which
implies that a curved four-dimensional metric can be embed-
ded into a five-dimensional pseudo-Euclidean space-time.
The Karmarkar condition provides a geometric relation
between the metric functions and their derivatives; thus, one
can choose one of the metric functions and generate the other.
In the case of an isotropic static fluid sphere – Pascalian
matter distribution – the Karmarkar condition leads to either
a Schwarzschild – homogeneous conformally-flat bounded
solution – or a Kohler–Chao solution – a non-conformally
flat unbounded solution [4]. However, for static anisotropic
matter configurations, it provides a geometrical mechanism
for implementing equations of state relating the radial and
the tangential pressures.
As pointed out by Ivanov [5], the Karmarkar initial embed-
ding motivation changes into a geometical method that gener-
ates matter configurations that may represent compact astro-
physical objects. As shown in Fig. 1 Karmarkar’s condition
has experimented a recent boom, with more than 70 publi-
cations in the last 3 years, most of them devoted in describ-
ing anisotropic compact objects. There are many interesting
models of possible compact objects, depending on the vari-
ety of the metric function selected as input: rational func-
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