484 research outputs found
Unitary null energy condition violation in cosmologies
A non-singular cosmological bounce in the Einstein frame can only take place
if the Null Energy Condition (NEC) is violated. We explore situations where a
single scalar field drives the NEC violation and derive the constraints imposed
by demanding tree level unitarity on a cosmological background. We then focus
on the explicit constraints that arise in P(X) theories and show that
constraints from perturbative unitarity make it impossible for the NEC
violation to occur within the region of validity of the effective field theory
without also involving irrelevant operators that arise at a higher scale that
would enter from integrating out more massive degrees of freedom. Within the
context of P(X) theories we show that including such operators allows for a
bounce that does not manifestly violate tree level unitarity, but at the price
of either imposing a shift symmetry or involving technically unnatural small
operator coefficients within the low-energy effective field theory.Comment: 35 pages, 1 figur
The sensitivity of cells to the lethal action of X-rays
This thesis consists of three papers which
record an experimental investigation into the
nature of the biological action of X-rays. An
introduction has been added in order to correlate
the papers with each other and to draw attention
to the significance of the experiments described
in the papers in relation to the records of work
by other investigators.Part I. Introduction. •
Part II. The Biological Action of Homogeneous and
Heterogeneous X-rays. Proc. Roy. Soc.
1933. B.112, 365. •
Part III. The Action of X-rays on the Eggs of
Calliphora. Proc. Roy. Soc. B. 1934.
In press. •
Part IV. The influence of temperature on the
sensitivity of calliphorine eggs to
X-rays. (This paper is about to be
submitted to the Royal Society for
publication)
Massive Galileon Positivity Bounds
The EFT coefficients in any gapped, scalar, Lorentz invariant field theory
must satisfy positivity requirements if there is to exist a local, analytic
Wilsonian UV completion. We apply these bounds to the tree level scattering
amplitudes for a massive Galileon. The addition of a mass term, which does not
spoil the non-renormalization theorem of the Galileon and preserves the
Galileon symmetry at loop level, is necessary to satisfy the lowest order
positivity bound. We further show that a careful choice of successively higher
derivative corrections are necessary to satisfy the higher order positivity
bounds. There is then no obstruction to a local UV completion from
considerations of tree level 2-to-2 scattering alone. To demonstrate this we
give an explicit example of such a UV completion.Comment: 31 page
Positivity Bounds for Scalar Theories
Assuming the existence of a local, analytic, unitary UV completion in a
Poincar\'{e} invariant scalar field theory with a mass gap, we derive an
infinite number of positivity requirements using the known properties of the
amplitude at and away from the forward scattering limit. These take the form of
bounds on combinations of the pole subtracted scattering amplitude and its
derivatives. In turn, these positivity requirements act as constraints on the
operator coefficients in the low energy effective theory. For certain theories
these constraints can be used to place an upper bound on the mass of the next
lightest state that must lie beyond the low energy effective theory if such a
UV completion is to ever exist.Comment: 5 page
A de Sitter -matrix for the masses
We define an -matrix for massive scalar fields on a fixed de Sitter
spacetime, in the expanding patch co-ordinates relevant for early Universe
cosmology. It enjoys many of the same properties as its Minkowski counterpart,
for instance: it is insensitive to total derivatives and field redefinitions in
the action; it can be extracted as a particular "on-shell" limit of
time-ordered correlation functions; and for low-point scattering, kinematics
strongly constrains its possible structures. We present explicit formulae
relating the usual observables - in-in equal-time correlators and wavefunction
coefficients at the conformal boundary - to -matrix elements. Finally, we
discuss some of the subtleties in extending this -matrix to light fields (in
the complementary series).Comment: 14 pages, 2 figure
Orbital Precession and Hidden Symmetries in Scalar-Tensor Theories
We revisit the connection between relativistic orbital precession, the
Laplace-Runge-Lenz symmetry, and the -channel discontinuity of scattering
amplitudes. Applying this to scalar-tensor theories of gravity, we compute the
conservative potential and orbital precession induced by both
conformal/disformal-type couplings at second Post-Minkowskian order
(), complementing the known third/first order
Post-Newtonian results. There is a particular tuning of the conformal coupling
for which the precession vanishes at leading PN order, and we show that this
coincides with the emergence of a Laplace-Runge-Lenz symmetry and a
corresponding soft behaviour of the amplitude. While a single scalar field
inevitably breaks this symmetry at higher PN orders, certain supersymmetric
extensions have recently been shown to have an exact Laplace-Runge-Lenz
symmetry and therefore classical orbits do not precess at any PN order. This
symmetry can be used to relate scattering amplitudes at different loop orders,
and we show how this may be used to bootstrap the (classically relevant part of
the) three-loop scattering of charged black holes in
supergravity from existing two-loop calculations.Comment: 34 pages + 4 Appendice
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Cosmological Cutting Rules
Abstract: Primordial perturbations in our universe are believed to have a quantum origin, and can be described by the wavefunction of the universe (or equivalently, cosmological correlators). It follows that these observables must carry the imprint of the founding principle of quantum mechanics: unitary time evolution. Indeed, it was recently discovered that unitarity implies an infinite set of relations among tree-level wavefunction coefficients, dubbed the Cosmological Optical Theorem. Here, we show that unitarity leads to a systematic set of “Cosmological Cutting Rules” which constrain wavefunction coefficients for any number of fields and to any loop order. These rules fix the discontinuity of an n-loop diagram in terms of lower-loop diagrams and the discontinuity of tree-level diagrams in terms of tree-level diagrams with fewer external fields. Our results apply with remarkable generality, namely for arbitrary interactions of fields of any mass and any spin with a Bunch-Davies vacuum around a very general class of FLRW spacetimes. As an application, we show how one-loop corrections in the Effective Field Theory of inflation are fixed by tree-level calculations and discuss related perturbative unitarity bounds. These findings greatly extend the potential of using unitarity to bootstrap cosmological observables and to restrict the space of consistent effective field theories on curved spacetimes
Cosmological Cutting Rules
Abstract: Primordial perturbations in our universe are believed to have a quantum origin, and can be described by the wavefunction of the universe (or equivalently, cosmological correlators). It follows that these observables must carry the imprint of the founding principle of quantum mechanics: unitary time evolution. Indeed, it was recently discovered that unitarity implies an infinite set of relations among tree-level wavefunction coefficients, dubbed the Cosmological Optical Theorem. Here, we show that unitarity leads to a systematic set of “Cosmological Cutting Rules” which constrain wavefunction coefficients for any number of fields and to any loop order. These rules fix the discontinuity of an n-loop diagram in terms of lower-loop diagrams and the discontinuity of tree-level diagrams in terms of tree-level diagrams with fewer external fields. Our results apply with remarkable generality, namely for arbitrary interactions of fields of any mass and any spin with a Bunch-Davies vacuum around a very general class of FLRW spacetimes. As an application, we show how one-loop corrections in the Effective Field Theory of inflation are fixed by tree-level calculations and discuss related perturbative unitarity bounds. These findings greatly extend the potential of using unitarity to bootstrap cosmological observables and to restrict the space of consistent effective field theories on curved spacetimes
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