Physics Letters B 797 (2019) 134904
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Physics Letters B
www.elsevier.com/locate/physletb
Thermodynamic interpretation of the de Sitter swampland conjecture
Min-Seok Seo
Department of Physics Education, Korea National University of Education, Cheongju 28173, Republic of Korea
a r t i c l e i n f o a b s t r a c t
Article history:
Received
15 August 2019
Received
in revised form 28 August 2019
Accepted
28 August 2019
Available
online 3 September 2019
Editor:
M. Cveti
ˇ
c
We interpret the de Sitter swampland conjecture in the thermodynamic point of view. When the number
of degrees of freedom is enhanced as the modulus rolls down the potential, the bound on m
Pl
∇V /V
is
equivalent to the condition for the positive temperature phase. The boundary between the positive-
and
the negative temperature phases is preferred by the classical system since the entropy density is
maximized. The distance conjecture imposes that (quasi-)de Sitter spacetime is entirely in the negative
temperature phase and statistically disfavored. By investigating the concave potential, we also justify the
bound on m
2
Pl
∇
2
V /V .
© 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
The instability of (quasi-)de Sitter (dS) spacetime has been a
long standing issue. While both primordial inflationary paradigm
and CDM model explaining the current accelerating Universe are
well consistent with observations [1], constructing model based
on string theory requires the nontrivial setup [2], as suggested
by
KKLT [3]or large volume scenario [4]. Once the string theory
solution exists, apart from its naturalness, the anthropic principle
[5]or the string landscape [6]might account for the real world
described by the solution. On the other hand, it was recently pro-
posed
that in any parametrically controllable regime (meta-)stable
de Sitter spacetime is not allowed by quantum gravity [7]. Among
many conjectured effective field theory (EFT) properties consis-
tent
with quantum gravity (for reviews, see, [8,9]), this de Sit-
ter
swampland conjecture became controversial. Counter-examples
have been studied through the Higgs and the axion potentials,
from which the role of the curvature, not just the slope of the
potential is emphasized [10–17]. Such a situation calls for the re-
finement
of the conjecture and also the physically acceptable argu-
ments
supporting it. In [18,19], the refined dS swampland bound
was formulated as
m
Pl
|∇V |
V
≥c, or
m
2
Pl
min(∇
i
∇
j
V )
V
≤−c
(1)
E-mail address: minseokseo57@gmail.com.
for some positive order one constants c and c
and more impor-
tantly,
Bousso’s covariant entropy bound [20], as well as the dis-
tance
conjecture [21]were considered to support the conjecture in
[19].
1
That is, in the presence of some modulus along which the
number of physical degrees of freedom N (hence the entropy) in-
creases
rapidly, the condition that the entropy cannot exceed the
Gibbons-Hawking bound S
GH
=m
2
Pl
/H
2
(we ignored the numerical
factor of order one) results in the first bound in (1)as the Hubble
parameter H is also controlled by the modulus. Meanwhile, the
second bound states the breakdown of the semi-classical picture
for dS spacetime by the tachyonic zero point quantum fluctua-
tion.
In
this argument, the inequality itself comes from the thermo-
dynamic
property of spacetime, i.e., the entropy bound. On the
other hand, the existence of the modulus with properties given
above as well as order one values of c and c
are results of the dis-
tance
conjecture which claims that as the modulus traverses along
the trans-Planckian geodesic distance towers of light degrees of
freedom rapidly descend from UV. Motivated by these facts, in this
letter we make a more systematic interpretation of the dS swamp-
land
conjecture using the language of thermodynamics. Moreover
we try to specify the role of the distance conjecture which is ir-
relevant
to thermodynamics. For this purpose, we take the ansatz
S
H
= N
p
(H/m
Pl
)
q
for the entropy inside the horizon as considered
in [19]. The nonzero exponent p stands for the effect from the
1
The dS swampland bound can be written as the condition on the Hubble pa-
rameter
H instead of the potential as given by (1)[22]. This enables us to apply the
dS swampland conjecture to exotic inflationary cosmology models, in which quasi-
dS
spacetime is not necessarily a consequence of the almost flat potential (see also
[23–25]).
https://doi.org/10.1016/j.physletb.2019.134904
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
.