8 research outputs found
Causality, Analyticity and an IR Obstruction to UV Completion
We argue that certain apparently consistent low-energy effective field
theories described by local, Lorentz-invariant Lagrangians, secretly exhibit
macroscopic non-locality and cannot be embedded in any UV theory whose S-matrix
satisfies canonical analyticity constraints. The obstruction involves the signs
of a set of leading irrelevant operators, which must be strictly positive to
ensure UV analyticity. An IR manifestation of this restriction is that the
"wrong" signs lead to superluminal fluctuations around non-trivial backgrounds,
making it impossible to define local, causal evolution, and implying a
surprising IR breakdown of the effective theory. Such effective theories can
not arise in quantum field theories or weakly coupled string theories, whose
S-matrices satisfy the usual analyticity properties. This conclusion applies to
the DGP brane-world model modifying gravity in the IR, giving a simple
explanation for the difficulty of embedding this model into controlled stringy
backgrounds, and to models of electroweak symmetry breaking that predict
negative anomalous quartic couplings for the W and Z. Conversely, any
experimental support for the DGP model, or measured negative signs for
anomalous quartic gauge boson couplings at future accelerators, would
constitute direct evidence for the existence of superluminality and macroscopic
non-locality unlike anything previously seen in physics, and almost
incidentally falsify both local quantum field theory and perturbative string
theory.Comment: 34 pages, 10 figures; v2: analyticity arguments improved, discussion
on non-commutative theories and minor clarifications adde
Null energy condition and superluminal propagation
We study whether a violation of the null energy condition necessarily implies
the presence of instabilities. We prove that this is the case in a large class
of situations, including isotropic solids and fluids relevant for cosmology. On
the other hand we present several counter-examples of consistent effective
field theories possessing a stable background where the null energy condition
is violated. Two necessary features of these counter-examples are the lack of
isotropy of the background and the presence of superluminal modes. We argue
that many of the properties of massive gravity can be understood by associating
it to a solid at the edge of violating the null energy condition. We briefly
analyze the difficulties of mimicking in scalar tensor theories of
gravity.Comment: 46 pages, 6 figure
Near Scale Invariance with Modified Dispersion Relations
We describe a novel mechanism to seed a nearly scale invariant spectrum of
adiabatic perturbations during a non-inflationary stage. It relies on a
modified dispersion relation that contains higher powers of the spatial
momentum of matter perturbations. We implement this idea in the context of a
massless scalar field in an otherwise perfectly homogeneous universe. The
couplings of the field to background scalars and tensors give rise to the
required modification of its dispersion relation, and the couplings of the
scalar to matter result in an adiabatic primordial spectrum. This work is meant
to explicitly illustrate that it is possible to seed nearly scale invariant
primordial spectra without inflation, within a conventional expansion history.Comment: 7 pages and no figures. Uses RevTeX
Modified-Source Gravity and Cosmological Structure Formation
One way to account for the acceleration of the universe is to modify general
relativity, rather than introducing dark energy. Typically, such modifications
introduce new degrees of freedom. It is interesting to consider models with no
new degrees of freedom, but with a modified dependence on the conventional
energy-momentum tensor; the Palatini formulation of theories is one
example. Such theories offer an interesting testing ground for investigations
of cosmological modified gravity. In this paper we study the evolution of
structure in these ``modified-source gravity'' theories. In the linear regime,
density perturbations exhibit scale dependent runaway growth at late times and,
in particular, a mode of a given wavenumber goes nonlinear at a higher redshift
than in the standard CDM model. We discuss the implications of this
behavior and why there are reasons to expect that the growth will be cut off in
the nonlinear regime. Assuming that this holds in a full nonlinear analysis, we
briefly describe how upcoming measurements may probe the differences between
the modified theory and the standard CDM model.Comment: 22 pages, 6 figures, uses iopart styl
Starting the Universe: stable violation of the null energy condition and non-standard cosmologies
Ghosts, Instabilities, and Superluminal Propagation in Modified Gravity Models
We consider Modified Gravity models involving inverse powers of fourth-order curvature invariants. Using these models' equivalence to the theory of a scalar field coupled to a linear combination of the invariants, we investigate the properties of the propagating modes. Even in the case for which the fourth derivative terms in the field equations vanish, we find that the second derivative terms can give rise to ghosts, instabilities, and superluminal propagation speeds. We establish the conditions which the theories must satisfy in order to avoid these problems in Friedmann backgrounds, and show that the late-time attractor solutions are generically afflicted by superluminally propagating tensor or scalar modes