Physics Letters B 749 (2015) 389–392
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
On the stability of fundamental couplings in the Galaxy
S.M. João
a,b
, C.J.A.P. Martins
a,c,∗
, I.S.A.B. Mota
a,b
, P.M.T. Vianez
a,b
a
Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
b
Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4150-007 Porto, Portugal
c
Instituto de Astrofísica e Ciências do Espaço, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
a r t i c l e i n f o a b s t r a c t
Article history:
Received
27 January 2015
Accepted
6 August 2015
Available
online 10 August 2015
Editor:
S. Dodelson
Keywords:
Astrophysical
observation
Fundamental
couplings
Unification
scenarios
Astrophysical tests of the stability of Nature’s fundamental couplings are a key probe of the standard
paradigms in fundamental physics and cosmology. In this report we discuss updated constraints on the
stability of the fine-structure constant α and the proton-to-electron mass ratio μ = m
p
/m
e
within the
Galaxy. We revisit and improve upon the analysis by Truppe et al. [1] by allowing for the possibility of
simultaneous variations of both couplings and also by combining them with the recent measurements by
Levshakov et al. [2]. By considering representative unification scenarios we find no evidence for variations
of α at the 0.4 ppm level, and of μ at the 0.6 ppm level; if one uses the [2] bound on μ as a prior,
the α bound is improved to 0.1 ppm. We also highlight how these measurements can constrain (and
discriminate among) several fundamental physics paradigms.
© 2015 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
Nature’s dimensionless fundamental couplings are among the
deepest mysteries of modern physics: it is clear that they play a
crucial role in physical theories, and yet we have no ‘theory of
constants’ that describes what this role is. Three rather different
views on the subject are discussed in [3], and a broader overview
of the subject can be found in Uzan’s review [4]. At a phenomeno-
logical
level it is well known that fundamental couplings run with
energy, and in many extensions of the standard model they will
also roll in time and ramble in space (i.e., they will depend on the
local environment). The class of theories with additional spacetime
dimensions, such as string theory, is the most obvious example.
An
unambiguous detection of varying dimensionless fundamen-
tal
couplings will be revolutionary: it will establish that the Ein-
stein
equivalence principle is violated and that there is a fifth force
of nature. We refer the interested reader to [4] as well as to the
recent Equivalence Principle overview by Damour [5] for detailed
discussions of these points. Nevertheless, improved null results are
almost as important. Naively, the natural scale for the cosmological
evolution of one of these couplings (if one assumes the simplest
*
Corresponding author at: Centro de Astrofísica, Universidade do Porto, Rua das
Estrelas, 4150-762 Porto, Portugal.
E-mail
addresses: up201206827@fc.up.pt (S.M. João), Carlos.Martins@astro.up.pt
(C.J.A.P. Martins),
up200908588@fc.up.pt (I.S.A.B. Mota), up201201345@fc.up.pt
(P.M.T. Vianez).
paradigm, in which it is driven by a scalar field) would be the
Hubble time, and we would therefore expect a drift rate of the
order of 10
−10
yr
−1
. However, local tests with atomic clocks [6] re-
strict
any such drift to be at least six orders of magnitude weaker,
and thereby rule out may otherwise viable models. This explains
why tests of the stability of nature’s fundamental couplings are
among the key drivers for the next generation of ESO and ESA
facilities. Additionally, these tests have important implications for
the enigma of dark energy, as discussed in [7].
Evidence
for spacetime variations of the fine-structure con-
stant
α, in the redshift range z ∼ 1–4 and at the few parts per
million level has been provided by [8]. An ongoing Large Program
at ESO’s Very Large Telescope is independently testing these re-
sults,
and the first results of this effort have recently been reported
by [9]. Given the limitations of current optical/UV spectrographs, a
definitive answer may have to wait for a forthcoming generation of
high-resolution ultra-stable spectrographs, such as ESPRESSO and
ELT-HIRES [10,11], both of which include improving these mea-
surements
among their key science/design drivers [12]. Radio/mi-
crowave
measurements of these couplings can also be performed.
While they are typically limited to lower redshifts than their
optical/UV counterparts, they sensitivity is competitive. Ameta-
analysis
of the various recent early universe measurements can be
found in [13].
Another
advantage of the radio/microwave band for our pur-
poses
is that they allow measurements within the Galaxy (ef-
fectively
at z = 0) which provide tests of possible environmental
dependencies. Recently [1] provided improved constraints on the
http://dx.doi.org/10.1016/j.physletb.2015.08.018
0370-2693/
© 2015 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
.