Physics Letters B 797 (2019) 134830
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Axion-dilaton destabilization and the Hubble tension
Stephon Alexander
a,b
, Evan McDonough
a,b,∗
a
Brown Theoretical Physics Center, Brown University, Providence, RI 02912, USA
b
Department of Physics, Brown University, Providence, RI 02912, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
28 May 2019
Received
in revised form 1 July 2019
Accepted
1 August 2019
Available
online 8 August 2019
Editor:
M. Trodden
The discrepancy in measurements of the Hubble constant indicates new physics in dark energy, dark
matter, or both. Drawing inspiration from string theory, where axions interact with the other moduli
fields, including the dilaton, here we demonstrate that the dynamics of an interacting dilaton and axion
naturally realizes the proposal of Early Dark Energy. In this setup, stabilization of the dilaton is in part
due to the axion, and in the early universe the dilaton contributes to dark energy. The combined axion-
dilaton
system is destabilized when the Hubble constant falls below the mass of the axion, triggering
a phase of fast-roll evolution of the dilaton wherein its equation of state is w = 1, and the early dark
energy redshifts away as a
−6
.
© 2019 The Author(s). 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
With the refined measurement of the distance to the Large
Magellanic Cloud [1], the discrepancy between the observed and
inferred values of the Hubble constant has raised in significance
to 4.4σ . The data supporting this discrepancy comes from a wide
range of redshift, providing the greatest challenge yet to the CDM
model. This growing observational evidence motivates the search
for theoretical explanations.
In
contrast with past cosmological glitches, e.g. anomalies in
the Cosmic Microwave Background, there are still relatively few
working theoretical explanations [2–8]. However, some of the nec-
essary
ingredients of a solution are now known. As emphasized
in [9], to rectify the cosmological distance ladder with the inverse
distance ladder and the CMB requires a modification of the sound
horizon r
s
, and thus any solution must include new dynamics in
the very early universe.
1,2
The scenario of ‘Early Dark Energy’ [2,11]implemented a modi-
fication
of r
s
via the decay of dark energy into a fluid that redshifts
as fast or faster than radiation, i.e. with equation of state w ≥ 1/3.
This fluid can arise via a scalar field with potential that is a poly-
nomial
in the cosine of the field, V = μ
4
(cos φ)
p
[2], with param-
eters
such that the decay happens shortly before recombination.
*
Corresponding author.
E-mail
addresses: stephon_alexander@brown.edu (S. Alexander),
evan_mcdonough@brown.edu (E. McDonough).
1
We thank Matias Zaldarriaga and Christina D. Kreisch for emphasizing this to
us.
2
See also e.g. [10].
As an alternative to such a potential, [7]demonstrated that
single-field models with polynomial potentials can realize the early
dark energy dynamics. The best fit model was found to be V =
eV
4
(φ/m
pl
)
4
, substantiated by a complete perturbation analysis, in
which the perturbed field equations were solved directly.
In
this letter we build on these previous works by demonstrat-
ing
that the requisite dynamics can be realized via the interacting
dynamical system of a dilaton field, referred to as such due to an
exponential potential V = V
0
e
−λφ/m
pl
, and an axion field with co-
sine
potential. At early stages the axion is held up its potential by
Hubble friction, and due to an interaction, the dilaton is also held
up its potential, and thus acts as dark energy. Eventually, when the
Hubble constant falls below the mass of the axion, the axion rolls
down its potential and begins to oscillate. At this point the dila-
ton
is destabilized and also rolls down its potential. For λ > 1, this
rolling can be fast-roll with the energy density in φ predominantly
in kinetic energy. The equation of state of φ in this phase is w = 1
and
the energy density redshifts as a(t)
−6
.
This
realizes early dark energy, wherein the onset of the decay
of dark energy is due to the small axion mass, which is itself in
agreement with general intuition from the string theory axiverse,
and the fluid into which the early dark energy decays is simply the
kinetic energy of the early dark energy field.
As
a final note before we proceed, we remark that the motiva-
tion
for this setup comes from a promising approach to cosmology
in string theory. The existence of moduli fields is one of the few
model-independent predictions of string theory (or more generally,
extra-dimensional theories). The prevailing approach is to stabilize
and subsequently ignore all these fields, in order to focus on the
https://doi.org/10.1016/j.physletb.2019.134830
0370-2693/
© 2019 The Author(s). 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
.
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