Vector-tensor theories in metric-affine geometry

We investigate ghost-free vector-tensor theories in metric-affine geometry. In all of our analysis, we start with the Lagrangian containing up to quadratic terms of first-order derivatives of a vector field. To obtain ghost-free vector-tensor theories efficiently, we consider two options; the theories satisfy the torsionless condition or have the projective symmetry. We first explore the vector-tensor theories under the former condition. We then investigate the projective-invariant vector-tensor theories in metric-affine geometry. To systematically construct a projective-invariant Lagrangian, we use two different approaches. First, we construct a Lagrangian by contracting the epsilon tensor. Second, we construct a Lagrangian by use of projective-invariant combinations. We find that to obtain a ghost-free Lagrangian in metric-affine geometry, imposing the projective invariance would be more useful than imposing the torsionless condition. However, we also prove that the projective invariance (or the torsionless condition) alone is insufficient for vector-tensor theories in metric-affine geometry to be ghost-free.Comment: 21 pages, no figur

Consistency of higher derivative couplings to matter fields in scalar-tensor gravity

Recently, a generalization of invertible disformal transformations containing higher-order derivatives of a scalar field has been proposed in the context of scalar-tensor theories of gravity. By applying this generalized disformal transformation to the Horndeski theory, one can obtain the so-called generalized disformal Horndeski theories which are more general healthy scalar-tensor theories than ever. However, it is unclear whether or not the generalized disformal Horndeski theories can be coupled consistently to matter fields because introducing a matter field could break the degeneracy conditions of higher-order scalar-tensor theories and hence yield the unwanted Ostrogradsky ghost. We investigate this issue and explore the conditions under which a minimal coupling to a matter field is consistent in the generalized disformal Horndeski theories without relying on any particular gauge such as the unitary gauge. We find that all the higher derivative terms in the generalized disformal transformation are prohibited to avoid the appearance of the Ostrogradsky ghost, leading to the conclusion that only the theories that are related to the Horndeski theory through a conventional disformal transformation remain ghost-free in the presence of minimally coupled matter fields.Comment: 11pages, no figur

Parametrized Love numbers of non-rotating black holes

A set of tidal Love numbers quantifies tidal deformation of compact objects and is a detectable imprint in gravitational waves from inspiralling binary systems. The measurement of black hole Love numbers allows to test strong-field gravity. In this paper, we present a parametrized formalism to compute the Love numbers of static and spherically symmetric black hole backgrounds, connecting the underlying equations of a given theory with detectable quantities in gravitational-wave observations in a theory-agnostic way. With this formalism, we compute the Love numbers in several systems. We further classify black hole Love numbers according to whether they vanish, are nonzero or are "running" (scale-dependent), in theories or backgrounds which deviate perturbatively from the GR values. The construction relies on static linear perturbations and scattering theory. Our analytic and numerical results are in excellent agreement. As a side result, we show how to use Chandrasekhar's relations to relate basis of even parity to odd parity.Comment: 24 pages, 4 figure

高階微分重力の理論的側面

立教大学RIKKYO UNIVERSITY博士（理学）Doctor of Philosophy in Physicsdoctoral thesi

‘We are the same as everyone else just with a different and unique backstory’: Identity, belonging and ‘othering’ within education for young people who are ‘looked after’

This paper develops understandings of how being publicly identified and consequently labelled as ‘looked after’ can have damaging consequences for young people, particularly in how they are perceived by their peers in the context of schooling. Based on qualitative research in northern England utilising participatory approaches with young people and interviews with support staff, we explore barriers that inhibit young people’s sense of belonging. We highlight how the very processes and practices set up to support the young people can often have unintended consequences by routinely positioning them as Other, before considering the implications for education and schooling in particular

‘It's so much better than contact’: A qualitative study exploring children and young people's experiences of a sibling camp in the United Kingdom

This article explores children and young people's experiences of a sibling camp based in the United Kingdom. Sibling camps are an intervention based on children's activity holidays that aim to promote meaningful contact for siblings separated in public care. This study adopted a qualitative approach using semi-structured interviews with 11 children and young people; this included one sibling group of three and four sibling groups of two. The children's ages ranged from 8 to 17 years old, and they had all attended at least one camp with their sibling. Findings highlighted how the children valued the extended time they could spend with their siblings at camp, and how they felt this enabled them to better understand their siblings and improve their relationships. Findings also showed how the children developed close supportive relationships with the staff at the camps, who ensured they were cared for, and they also supported them with managing their relationships, which some participants acknowledged at times could be challenging. The participants also valued spending time with other sibling groups who also experienced separation. The study found camps provided a space for these children to maintain links with their siblings and to strengthen their sibling bonds

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