Physics Letters B 798 (2019) 134972
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
The first law for rotating NUTs
Alvaro Ballon Bordo
a,b
, Finnian Gray
a,b
, Robie A. Hennigar
c
, David Kubiz
ˇ
nák
a,b,∗
a
Perimeter Institute, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada
b
Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
c
Department of Mathematics and Statistics, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador, A1C 5S7, Canada
a r t i c l e i n f o a b s t r a c t
Article history:
Received
2 June 2019
Received
in revised form 16 September
2019
Accepted
19 September 2019
Available
online 24 September 2019
Editor:
M. Cveti
ˇ
c
We address a long-standing problem of describing the thermodynamics of rotating Taub–NUT solutions.
The obtained first law is of full cohomogeneity and allows for asymmetric distributions of Misner strings
as well as their potential variable strengths—encoded in the gravitational Misner charges. Notably, the
angular momentum is no longer given by the Noether charge over the sphere at infinity and picks up
non-trivial contributions from Misner strings.
© 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
.
The Lorentzian Taub–NUT spacetime is one of the simplest yet
one of the most puzzling vacuum solutions of general relativity.
The spacetime possesses a Schwarzschild-like horizon that, as a
consequence of the NUT charge, is accompanied by rotating string-
like
singularities on the north/south pole axes, known as Misner
strings.
The
Misner strings can be exiled from the spacetime at the cost
of introducing a periodic time coordinate [1]. However, while this
approach is customary it is not strictly necessary: as suggested by
Bonnor [2], see also [3,4], the Misner strings can instead be in-
terpreted
as due to massless ‘singular’ sources of angular momen-
tum,
and the geometry from this perspective is less pathological
than one might expect. For example, the spacetime is geodesically
complete and while closed time-like curves exist near the axes,
there are no causal pathologies for geodesic observers [5–8].
1
This
raises a possibility that the NUT charge may actually be relevant
for astrophysics. In fact, already in 1997 astrophysicists probed the
possibility of detecting the NUT parameter by microlensing [12,13],
while its impact on the black hole shadow has been investigated in
*
Corresponding author.
E-mail
addresses: aballonbordo@perimeterinstitute.ca (A. Ballon Bordo),
fgray@perimeterinstitute.ca (F. Gray), rhennigar@mun.ca (R.A. Hennigar),
dkubiznak@perimeterinstitute.ca (D. Kubiz
ˇ
nák).
1
Strictly speaking, in [5–8]only the non-rotating Taub–NUT solutions were con-
sidered.
We have explicitly checked that the Kerr–Taub–NUT solution is geodesically
complete, however, the absence of causal pathologies for geodesic observers re-
mains
yet to be shown in this case [9]. Let us also mention that the presence of
the singularity on the axis tentatively generates instability of the solution, e.g. [10].
This should not prevent the study of thermodynamics performed in this paper. For
example, black rings are known to be classically unstable [11]but their sensible
thermodynamics can nevertheless be formulated.
[14–16]. Ongoing and forthcoming tests of general relativity in the
strong field regime, e.g. the Event Horizon Telescope [17], may re-
veal
interesting signatures of these spacetimes or at the very least
provide tighter constraints.
Notwithstanding
their potential pathologies, solutions with NUT
charge have been the source of deep physical insights, for example
in the context of black hole thermodynamics. The thermodynamics
of these solutions has been predominantly studied in the Euclidean
case and in the absence of Misner strings, e.g. [18–25]. The most
striking result to come from these studies is that, in equilibrium,
the entropy of Taub–NUT solutions is not simply one quarter the
horizon area, providing a counter-example to the ‘area law’.
A
full understanding of the properties—in particular the first
law—of the Lorentzian Taub–NUT solutions with Misner strings
present is much subtler. In fact, to the best of our knowledge,
until recently no consistent first law for these solutions has been
presented. It was argued in [26,27] that an additional relationship
between the horizon radius and the NUT charge must be obeyed
in order for the first law to hold. In [10]a Smarr relation was de-
rived
for the Lorentzian solution, but no first law consistent with
the Smarr formula was obtained. The situation was remedied for
the non-rotating solution in [28,29], where it was found that these
issues can be ameliorated through a judicious choice of a potential
and conjugate charge for the NUT parameter.
2
The purpose of this note is to address the first law for Taub–
NUT
solutions with rotation. The Euclidean setup was considered
2
See also [30]where it was found that for Euclidean Taub–NUT solutions with
toroidal bases (which do not feature Misner strings) the analogous terms are re-
quired
for consistency of the first law.
https://doi.org/10.1016/j.physletb.2019.134972
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
.