Physics Letters B 736 (2014) 317–320
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Physics Letters B
www.elsevier.com/locate/physletb
Planck-scale dimensional reduction without a preferred frame
Giovanni Amelino-Camelia
a
, Michele Arzano
a,∗
, Giulia Gubitosi
a
, João Magueijo
b
a
Dipartimento di Fisica, Università “La Sapienza” and Sez. Roma1 INFN, P.le A. Moro 2, 00185 Roma, Italy
b
Theoretical Physics, Blackett Laboratory, Imperial College, London, SW7 2BZ, United Kingdom
a r t i c l e i n f o a b s t r a c t
Article history:
Received
18 June 2014
Received
in revised form 16 July 2014
Accepted
17 July 2014
Available
online 23 July 2014
Editor: M.
Cveti
ˇ
c
Several approaches to quantum gravity suggest that the standard description of spacetime as probed at
low-energy, with four dimensions, is replaced in the Planckian regime by a spacetime with a spectral
dimension of two. The implications for relativistic symmetries can be momentous, and indeed the most
tangible picture for “running” of the spectral dimension, found within Horava–Lifshitz gravity, requires
the breakdown of relativity of inertial frames. In this Letter we incorporate running spectral dimensions
in a scenario that does not require the emergence of a preferred frame. We consider the best studied
mechanism for deforming relativistic symmetries whilst preserving the relativity of inertial frames, based
on a momentum space with curvature at the Planck scale. We show explicitly how running of the spectral
dimension can be derived from these models.
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/3.0/). Funded by SCOAP
3
.
1. Introduction
One of the most robust predictions of quantum-gravity is that
spacetime itself should acquire quantum properties, when probed
at the Planck scale ( 10
28
eV). Over the past decade it was grad-
ually
appreciated that two key issues deserve priority in investiga-
tions
of quantum models of spacetime: 1) What is the fate of rel-
ativistic
symmetries in the Planck-scale description of spacetime?
2) Is spacetime still four-dimensional in the Planck-scale regime?
The second question may appear to be ill-defined, since our in-
tuitive
notion of spacetime dimensionality is based on properties
of purely classical geometries. The most intuitive such notion is
the Hausdorff dimension, captured by the scaling exponent of the
volume of a sphere. Unlike the case of a smooth classical space-
time,
in a quantum or discrete geometry the notion of Hausdorff
dimension is technically more challenging to define. This has led
quantum-gravity researchers to employ an alternative definition:
the spectral dimension. This concept is encoded in the spectral
properties of the scalar Laplacian for the theory of interest. For
smooth classical spacetimes the spectral dimension coincides with
the Hausdorff dimension. In a quantum geometry the latter is in
general inapplicable, but the spectral dimension of spacetime is
still well-defined.
Interestingly,
as the spectral dimension criterion became
adopted
in a growing number of approaches, it emerged that
rather generically the spectral dimension in the UV regime is
*
Corresponding author.
smaller than 4 (e.g. [1] and references therein). It is particularly
intriguing that some of the most studied, but ostensibly very dif-
ferent,
quantum gravity theories predict that the value of the
spectral dimension in the UV is 2. This conclusion finds support
in the CDT (Causal-Dynamical-Triangulation) approach [2], Asymp-
totic
Safety [3], Horava–Lifshitz (HL) gravity [4], and Loop Quantum
Gravity (LQG) [5].
Irrespective
of the alleged UV dimensional reduction phe-
nomenon,
the fate of relativistic symmetries in the Planckian
regime has attracted interest from other angles (see e.g. [6–8]).
Relativistic symmetries may be left unscathed by the new struc-
tures
at the Planck scale (e.g. [9]), but there are at least two other
possibilities. Planck-scale effects may break relativistic invariance,
introducing a preferred-frame [10–14]; or they may deform the
relativistic symmetry transformations, preserving the relativity of
inertial frames [15–20]. In this Letter we contribute to the under-
standing
of the interplay between spectral dimensional reduction
and the fate of relativistic symmetries at the Planck scale.
It
is evident that any model of spacetime with dimensional
reduction must bring relativistic transformations under scru-
tiny [21,22].
Yet, in most studies the analysis is confined to the
perspective of a single observer, without mention of how a boosted
observer would describe the same phenomenon. An exception is
found in HL gravity, where an explicit breakdown of the equiva-
lence
of inertial observers is vividly manifest [4,22]. The fate of rel-
ativistic
invariance in CDT, Asymptotic Safety and LQG remains the
subject of a lively debate (e.g. [12,23,24]). We hope to contribute
to this debate by showing that the phenomenon of running spec-
tral
dimension arises naturally within the most studied mechanism
http://dx.doi.org/10.1016/j.physletb.2014.07.030
0370-2693/
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). Funded by
SCOAP
3
.