DSpace Community: 07. Theoretical Physics
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Driven inelastic Maxwell gases
http://hdl.handle.net/2289/6209
Title: Driven inelastic Maxwell gases<br/><br/>Authors: Prasad, V.V.; Sabhapandit, Sanjib; Dhar, Abhishek<br/><br/>Abstract: We consider the inelastic Maxwell model, which consists of a collection of particles that are characterized by only their velocities and evolving through binary collisions and external driving. At any instant, a particle is equally likely to collide with any of the remaining particles. The system evolves in continuous time with mutual collisions and driving taken to be point processes with rates τ−1c and τ−1w, respectively. The mutual collisions conserve momentum and are inelastic, with a coefficient of restitution r. The velocity change of a particle with velocity v, due to driving, is taken to be Δv=−(1+rw)v+η, where rw∈[−1,1] and η is Gaussian white noise. For rw∈(0,1], this driving mechanism mimics the collision with a randomly moving wall, where rw is the coefficient of restitution. Another special limit of this driving is the so-called Ornstein-Uhlenbeck process given by dvdt=−Γv+η. We show that while the equations for the n-particle velocity distribution functions (n=1,2,...) do not close, the joint evolution equations of the variance and the two-particle velocity correlation functions close. With the exact formula for the variance we find that, for rw≠−1, the system goes to a steady state. Also we obtain the exact tail of the velocity distribution in the steady state. On the other hand, for rw=−1, the system does not have a steady state. Similarly, the system goes to a steady state for the Ornstein-Uhlenbeck driving with Γ≠0, whereas for the purely diffusive driving (Γ=0), the system does not have a steady state.<br/><br/>Description: Open AccessTorsional instanton effects in quantum gravity
http://hdl.handle.net/2289/6208
Title: Torsional instanton effects in quantum gravity<br/><br/>Authors: Kaul, Romesh K.; Sengupta, Sandipan<br/><br/>Abstract: We show that in the first-order gravity theory coupled to axions the instanton number of the Giddings-Strominger wormhole can be interpreted as the Nieh-Yan topological index. The axion charge of the baby universes is quantized in terms of the Nieh-Yan integers. Tunneling between universes of different Nieh-Yan charges implies a nonperturbative vacuum state. The associated topological vacuum angle can be identified with the Barbero-Immirzi parameter.<br/><br/>Description: Open AccessAsymptotic symmetries and subleading soft graviton theorem
http://hdl.handle.net/2289/6194
Title: Asymptotic symmetries and subleading soft graviton theorem<br/><br/>Authors: Campiglia, Miguel; Laddha, Alok<br/><br/>Abstract: Motivated by the equivalence between the soft graviton theorem and Ward identities for the supertranslation symmetries belonging to the Bondi, van der Burg, Metzner and Sachs (BMS) group, we propose a new extension (different from the so-called extended BMS) of the BMS group that is a semidirect product of supertranslations and Diff(S2). We propose a definition for the canonical generators associated with the smooth diffeomorphisms and show that the resulting Ward identities are equivalent to the subleading soft graviton theorem of Cachazo and Strominger.<br/><br/>Description: Open AccessNon-linear multipole interactions and gravitational-wave octupole modes for inspiralling compact binaries to third-and-a-half post-Newtonian order
http://hdl.handle.net/2289/6192
Title: Non-linear multipole interactions and gravitational-wave octupole modes for inspiralling compact binaries to third-and-a-half post-Newtonian order<br/><br/>Authors: Faye, Guillaume; Blanchet, Luc; Iyer, B.R.<br/><br/>Abstract: This paper is motivated by the need to improve the post-Newtonian (PN) amplitude accuracy of waveforms for gravitational waves generated by inspiralling compact binaries, both for use in data analysis and in the comparison between post-Newtonian approximations and numerical relativity computations. It presents (i) the non-linear couplings between multipole moments of general post-Newtonian matter sources up to order 3.5PN, including all contributions from tails, tails-of-tails and the non-linear memory effect; and (ii) the source mass-type octupole moment of (non-spinning) compact binaries up to order 3PN, which permits completion of the expressions of the octupole modes $(3,3)$ and $(3,1)$ of the gravitational waveform to order 3.5PN. On this occasion we reconfirm by means of independent calculations our earlier results concerning the source mass-type quadrupole moment to order 3PN. Related discussions on factorized resummed waveforms and the occurence of logarithmic contributions to high order are also included.<br/><br/>Description: Open Access - IOP select