Physics Letters B 797 (2019) 134877
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Physics Letters B
www.elsevier.com/locate/physletb
Raychaudhuri equation with zero point length
Sumanta Chakraborty
a,∗
, Dawood Kothawala
b
, Alessandro Pesci
c
a
School of Physical Sciences, Indian Association for the Cultivation of Science, Kolkata-700032, India
b
Department of Physics, Indian Institute of Technology Madras, Chennai-600040, India
c
INFN Bologna, Via Irnerio 46, I-40126 Bologna, Italy
a r t i c l e i n f o a b s t r a c t
Article history:
Received
29 April 2019
Received
in revised form 16 August 2019
Accepted
19 August 2019
Available
online 21 August 2019
Editor:
M. Cveti
ˇ
c
The Raychaudhuri equation for a geodesic congruence in the presence of a zero-point length has been
investigated. This is directly related to the small-scale structure of spacetime and possibly captures some
quantum gravity effects. The existence of such a minimum distance between spacetime events modifies
the associated metric structure and hence the expansion as well as its rate of change deviates from
standard expectations. This holds true for any kind of geodesic congruences, including time-like and
null geodesics. Interestingly, this construction works with generic spacetime geometry without any need
of invoking any particular symmetry. In particular, inclusion of a zero-point length results into a non-
vanishing
cross-sectional area for the geodesic congruences even in the coincidence limit, thus avoiding
formation of caustics. This will have implications for both time-like and null geodesic congruences, which
may lead to avoidance of singularity formation in the quantum spacetime.
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Raychaudhuri equation governs the flow of geodesics in a given
spacetime manifold and it has been the cornerstone in our un-
derstanding
of formation of trapped surfaces and singularities (for
a recent review, see [1]). Unlike the field equations, the Raychaud-
huri
equation has no connection a priori to the gravitational theory
one is interested in, since it is purely of geometrical origin. It es-
sentially
determines the rate of change of area along a geodesic
congruence, which gets connected to shear and rotation of the
geodesic congruence and the component of Ricci tensor projected
along the geodesics. Only when one tries to connect the Ricci
tensor with matter energy momentum tensor, the gravitational
field equations come into play. In Einstein gravity, with reasonable
assumptions on the matter energy momentum tensor, the Ray-
chaudhuri
equation demonstrates that the geodesics will converge
forming caustics (see also [2]). This is broadly due to the attrac-
tive
nature of gravity. In most of the situations these caustics do
not lead to any spacetime singularities, but under certain circum-
stances
they do, leading to formation of black hole or cosmological
singularities. Removal of these curvature singularities has remained
a puzzle for decades. In this work, we will present a novel ap-
*
Corresponding author.
E-mail
addresses: sumantac.physics@gmail.com (S. Chakraborty),
dawood@physics.iitm.ac.in (D. Kothawala), pesci@bo.infn.it (A. Pesci).
proach where formation of caustics can be avoided which possibly
will lead to avoidance of curvature singularities as well [3–5].
It
is generally believed that the quantum theory of gravity, as
and when it comes into existence must take care of these curva-
ture
singularities. Since we do not have any consistent quantum
theory of gravity yet in sight, one can not attack the problem of
singularity removal head on, but can take a cue from various other
attempts. The single most important fact that is common to all the
candidate theories of quantum gravity is the existence of a zero-
point
length [6,7]. We will incorporate this fact in the spacetime
geometry by postulating that as two points on the manifold co-
incide,
the geodesic distance between them does not vanish. As a
consequence the classical metric g
ab
gets modified to an effective
metric q
ab
(which we will call the qmetric). The qmetric provides a
squared geodesic interval between two events P and p which ap-
proximates
to that provided by g
ab
in the limit of large geodesic
distances, while at the same time approaches a finite value differ-
ent
from zero in the coincidence limit, i.e., as p → P [3–5]. Note
that the above approach incorporates some relics of quantum grav-
ity
irrespective of any specific theory of gravitational interaction.
A
distinguishing aspect of this approach corresponds to the
fact that it can incorporate some generic quantum gravity effects,
but is based on the comfort zone of standard differential geom-
etry.
This provides a useful and at the same time general tool
in describing the small-scale quantum effects. Further it can also
be argued that one can incorporate the qmetric to find out how
https://doi.org/10.1016/j.physletb.2019.134877
0370-2693/
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.