10 research outputs found
Sensitivity of Hawking radiation to superluminal dispersion relations
We analyze the Hawking radiation process due to collapsing configurations in
the presence of superluminal modifications of the dispersion relation. With
such superluminal dispersion relations, the horizon effectively becomes a
frequency-dependent concept. In particular, at every moment of the collapse,
there is a critical frequency above which no horizon is experienced. We show
that, as a consequence, the late-time radiation suffers strong modifications,
both quantitative and qualitative, compared to the standard Hawking picture.
Concretely, we show that the radiation spectrum becomes dependent on the
measuring time, on the surface gravities associated with different frequencies,
and on the critical frequency. Even if the critical frequency is well above the
Planck scale, important modifications still show up.Comment: 14 pages, 7 figures. Extensive paragraph added in conclusions to
clarify obtained result
The cosmological constant: A lesson from the effective gravity of topological Weyl media
Topological matter with Weyl points, such as superfluid 3He-A, provide an
explicit example where there is a direct connection between the properly
determined vacuum energy and the cosmological constant of the effective gravity
emerging in condensed matter. This is in contrast to the acoustic gravity
emerging in Bose-Einstein condensates, where the "value of this constant cannot
be easily predicted by just looking at the ground state energy of the
microscopic system from which spacetime and its dynamics should emerge" (S.
Finazzi, S. Liberati and L. Sindoni, The cosmological constant: a lesson from
Bose-Einstein condensates, Phys. Rev. Lett. 108, 071101 (2012)). The advantage
of topological matter is that the relativistic fermions and gauge bosons
emerging near the Weyl point obey the same effective metric and thus the
effective gravity is more closely related to real gravity. We study this
connection in the bi-metric gravity emerging in 3He-A, and its relation to the
graviton masses, by comparison with a fully relativistic bi-metric theory of
gravity. This shows that the parameter \lambda, which in 3He-A is the bi-metric
generalization of the cosmological constant, coincides with the difference in
the proper energy of the vacuum in two states (the nonequilibrium state without
gravity and the equilibrium state in which gravity emerges) and is on the order
of the characteristic Planck energy scale of the system. Although the
cosmological constant \lambda\ is huge, the cosmological term itself is
naturally non-constant and vanishes in the equilibrium vacuum, as dictated by
thermodynamics. This suggests that the equilibrium state of any system
including the final state of the Universe is not gravitating.Comment: 7 pages, no figure
The Basics of Water Waves Theory for Analogue Gravity
This chapter gives an introduction to the connection between the physics of
water waves and analogue gravity. Only a basic knowledge of fluid mechanics is
assumed as a prerequisite.Comment: 36 pages. Lecture Notes for the IX SIGRAV School on "Analogue
Gravity", Como (Italy), May 201
Massive gravity from bimetric gravity
We discuss the subtle relationship between massive gravity and bimetric
gravity, focusing particularly on the manner in which massive gravity may be
viewed as a suitable limit of bimetric gravity. The limiting procedure is more
delicate than currently appreciated. Specifically, this limiting procedure
should not unnecessarily constrain the background metric, which must be
externally specified by the theory of massive gravity itself. The fact that in
bimetric theories one always has two sets of metric equations of motion
continues to have an effect even in the massive gravity limit, leading to
additional constraints besides the one set of equations of motion naively
expected. Thus, since solutions of bimetric gravity in the limit of vanishing
kinetic term are also solutions of massive gravity, but the contrary statement
is not necessarily true, there is not complete continuity in the parameter
space of the theory. In particular, we study the massive cosmological solutions
which are continuous in the parameter space, showing that many interesting
cosmologies belong to this class.Comment: v1: 25 pages; v2: 6 references added, discussion streamlined; v3: 24
pages, 20 references added, section 2 summarized, new comments added to
section 3, conclusions improved but unchanged. This version accepted for
publication in Classical and Quantum Gravit
Analogue Gravity
Analogue models of (and for) gravity have a long and distinguished history dating back to the earliest years of general relativity. In this review article we will discuss the history, aims, results, and future prospects for the various analogue models. We start the discussion
by presenting a particularly simple example of an analogue model, before exploring the rich history and complex tapestry of models discussed in the literature. The last decade in particular
has seen a remarkable and sustained development of analogue gravity ideas, leading to some hundreds of published articles, a workshop, two books, and this review article. Future prospects for the analogue gravity programme also look promising, both on the experimental
front (where technology is rapidly advancing) and on the theoretical front (where variants of analogue models can be used as a springboard for radical attacks on the problem of quantum
gravity)