Cerenkov radiation from moving straight strings (original) (raw)
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Cerenkov radiation from collisions of straight cosmic (super)strings
2006
We consider Cerenkov radiation which must arise when randomly oriented straight cosmic (super)strings move with relativistic velocities without intercommutation. String interactions via dilaton, two-form and gravity (gravity being the dominant force in the ultra-relativistic regime) leads to formation of superluminal sources which generate Cerenkov radiation of dilatons and axions. Though the effect is of the second order in the couplings of strings to these fields, its total efficiency is increased by high dependence of the radiation rate on the Lorentz-factor of the collision.
Gravitating cosmic strings with flat directions
Journal of High Energy Physics, 2013
We study field theoretical models for cosmic strings with flat directions in curved space-time. More precisely, we consider minimal models with semilocal, axionic and tachyonic strings, respectively. In flat space-time, isolated static and straight cosmic strings solutions of these models have a flat direction, i.e., a uniparametric family of configurations with the same energy exists which is associated with a zero mode. We prove that this zero mode survives coupling to gravity, and study the role of the flat direction when coupling the string to gravity. Even though the total energy of the solution is the same, and thus the global properties of the family of solutions remains unchanged, the energy density, and therefore the gravitational properties, are different. The local structure of the solutions depends strongly on the value of the parameter describing the flat direction; for example, for a supermassive string, the value of the free parameter can determine the size of the space-time.
Cosmic strings in a space–time with positive cosmological constant
2008
We study Abelian strings in a fixed de Sitter background. We find that the gauge and Higgs fields extend smoothly across the cosmological horizon and that the string solutions have oscillating scalar fields outside the cosmological horizon for all currently accepted values of the cosmological constant. If the gauge to Higgs boson mass ratio is small enough, the gauge field function has a power-like behaviour, while it is oscillating outside the cosmological horizon if Higgs and gauge boson mass are comparable. Moreover, we observe that Abelian strings exist only up to a maximal value of the cosmological constant and that two branches of solutions exist that meet at this maximal value. We also construct radially excited solutions that only exist for non-vanishing values of the cosmological constant and are thus a novel feature as compared to flat space-time. Considering the effect of the de Sitter string on the space-time, we observe that the deficit angle increases with increasing cosmological constant.
String Theory in Cosmological Spacetimes
String Theory in Curved Space Times, 1998
Progress on string theory in curved spacetimes since 1992 are reviewed. After a short introduction on strings in Minkowski and curved spacetimes, we focus on strings in cosmological spacetimes. The classical behaviour of strings in FRW and inationary spacetimes is now understood in a large extent from various types of explicit string solutions. Three dierent t ypes of behaviour appear in cosmological spacetimes: unstable, dual to unstable and stable. F or the unstable strings, the energy and size grow for large scale factors R 1 , proportional to R. F or the dual to unstable strings, the energy and size blow up for R 0 a s 1 =R. F or stable strings, the energy and proper size are bounded. (In Minkowski spacetime, all string solutions are of the stable type). Recent progress on self-consistent solutions to the Einstein equations for string dominated universes is reviewed. The energy-momentum tensor for a gas of strings is then considered as source of the spacetime geometry and from the above string behaviours the string equation of state is determined. The selfconsistent string solution exhibits the realistic matter dominated behaviour R (X 0 ) 2=3 for large times and the radiation dominated behaviour R (X 0 ) 1=2 for early times. Finally, w e report on the exact integrability of the string equations plus the constraints in de Sitter spacetime that allows to systematically nd exact string solutions by soliton methods and the multistring solutions. Multistring solutions are a new feature in curved spacetimes. That is, a single worldsheet simultaneously describes many dierent and independent strings. This phenomenon has no analogue in at spacetime and appears as a consequence of the coupling of the strings with the spacetime geometry.
Charged cosmic strings interacting with gravitational and electromagnetic waves
General Relativity and Gravitation, 2010
Under a particular choice of the Ernst potential, we solve analytically the Einstein-Maxwell equations to derive a new exact solution depending on five parameters: the mass, the angular-momentum (per unit mass), α, the electromagnetic-field strength, k, the parameter-p and the Kerr-NUT parameter, l. This (Petrov Type D) solution is cylindrically-symmetric and represents the curved background around a charged, rotating cosmic string, surrounded by gravitational and electromagnetic waves, under the influence of the Kerr-NUT parameter. A C-energy study in the radiation zone suggests that both the incoming and the outgoing radiation is gravitational, strongly focused around the null direction and preserving its profile. In this case, the absence of the k-parameter from the C-energy implies that, away from the linear defect the electromagnetic field is too weak to contribute to the energy-content of the cylindrically-symmetric space-time under consideration. In order to explain this result, we have evaluated the Weyl and the Maxwell scalars near the axis of the linear defect and at the spatial infinity. Accordingly, we have found that the electromagnetic field is concentrated (mainly) in the vicinity of the axis, while falling-off prominently at large radial distances. However, as long as k = 1, the non-zero Kerr-NUT parameter enhances those scalars, both near
New look at supermassive cosmic strings
Physical Review D, 1991
Although it is well known that cosmic strings produced at grand-unified-theory (GUT) scales give rise to a conical spacetime, this picture must be revised for strings produced at much larger energy scales. An expression for the metric due to a supermassive cosmic string has recently been given by Laguna and Garfinkle. Here, we argue that their metric is not unique, and that a second solution exists which has diR'erent asymptotics. The existence of this new solution is verified numerically. Like the Laguna and Garfinkle metric, the solution we give is singular at finite distance from the core of the string. We further demonstrate that supermassive cosmic strings may also arise from symmetry breaking at GUT scales if the coupling between scalar and gauge fields is very weak. We argue that these low-energy supermassive strings are closely related to U(1) global strings, a, result which is not surprising given their singular nature. By analogy with global strings, it is clear that the singularity of a low-energy supermassive string occurs at extremely large distances from the core of the string.
Gravitational Particle-Production by Cosmic Strings
Nuclear Physics B, 1990
We apply a perturbative treatment of quantum field theory in the presence of weak time-dependent gravitational fields to determine the rate of particle production by cosmic strings of several different types. We evaluate the effect that occurs during the formation of both gauge and global strings, while a current grows along a superconducting string, and by the oscillations of loops as well . The gravitational field of strings can produce very energetic particles, even at present times, but the flux would be too small to have observable consequences . a finite width, so we can not expect the Nambu-Goto action to be a valid
Physical effects of massless cosmic strings
Physical review, 2017
We study massless cosmic strings which are one-dimensional objects moving with the speed of light. Perturbations of velocities of test bodies and anisotropy of cosmic microwave background generated by massless cosmic strings are analysed. These phenomena are analogous to the string wake effect and the Kaiser-Stebbins effect for standard (massive) cosmic strings. There are two regimes depending on the energy E of a massless string per unit length. At low energies, EG/c 4 1, massless and massive strings act similarly. At high energies, EG/c 4 ∼ 1, massless string effects are quite different. Our work provides a method to describe different physical phenomena on spacetimes of massless strings which take into account the presence of a parabolic holonomy.
Physical Review D, 2000
The field equations for a time dependent cylindrical cosmic string coupled to gravity are reformulated in terms of geometrical variables defined on a 2+1-dimensional spacetime by using the method of Geroch decomposition. Unlike the 4-dimensional spacetime the reduced case is asymptotically flat. A numerical method for solving the field equations which involves conformally compactifying the space and including null infinity as part of the grid is described. It is shown that the code reproduces the results of a number of vacuum solutions with one or two degrees of freedom. In the final section the interaction between the cosmic string and a pulse of gravitational radiation is briefly described. This is fully analyzed in the sequel.
Post-Linear Formalism for Gravitating Strings: Crossed Straight Strings Collision
Annals of Physics, 1993
Linear and post-linear formalisms are generalized to incorporate gravitating N ambu and superconducting cosmic strings. The collision of two straight non-parallel strings is analised. When strings scatter at a sufficiently small angle the point of their minimal separation can move with a faster-than-light velocity, and a Cherenkov-like radiation can be anticipated. However, it is shown tliat for Nambu strings in the post-linear order the gravitational reaction is precisely zero. It is argued that any "faster-than-light" crossed colliding strings configuration is essentially equivalent to the parallel one which is described by the 1+2 gravity while any "slower-than-light" configuration may he r~duced to some static distribution of matter. Thus in both cases we have no gravitational radiation. But we can obtain a powerful source of a Cherenkov-like electromagnetic radiation if at least one of the strings is a superconducting one. The same approach can be used in many other applications. It is shown, that this formalism provides a way to calculate vacuum polarization and to consider the problem of topological self-action of a classical charged particle in a multiconical space-time.