Measuring neutron-star properties via gravitational waves from neutron-star mergers

We demonstrate by a large set of merger simulations for symmetric binary neutron stars (NSs) that there is a tight correlation between the frequency peak of the postmerger gravitational-wave (GW) emission and the physical properties of the nuclear equation of state (EoS), e.g., expressed by the radius of the maximum-mass Tolman-Oppenheimer-Volkhoff configuration. Therefore, a single measurement of the peak frequency of the postmerger GW signal will constrain the NS EoS significantly. For optimistic merger-rate estimates a corresponding detection with Advanced LIGO is expected to happen within an operation time of roughly a year.     

Gravitational waves from seismic sources

A fast moving mass radiates part of its energy through gravitational waves. It is possible to consider hurricanes and earthquakes as terrestrial sources of quadrupole gravitational waves. These phenomena have the necessary parameters for the generation of gravitational waves: large quadrupole moments m/r² and also energies and masses moving with large velocities. Estimates indicate a possibility of observing gravitons from the source of a strong earthquake. Local changes in the earth's gravistatic field and the generation of a gravidynamic field during earthquakes will cause variations in the trajectories of the moon and artificial satellites. A mathematical theory of trajectories, containing gravitons, will enable us to fix the graviton according to data from trajectory measurements.Published as a matter for discussion.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 41–46, June, 1974.The author is grateful to Prof. A. A. Sokolov of the Moscow State University for discussing the material expounded here and for interesting remarks.     

Gravitational waves from binary black holes

It is almost a century since Einstein predicted the existence of gravitational waves as one of the consequences of his general theory of relativity. A brief historical overview including Chandrasekhar’s contribution to the subject is first presented. The current status of the experimental search for gravitational waves and the attendant theoretical insights into the two-body problem in general relativity arising from computations of gravitational waves from binary black holes are then broadly reviewed.     

Gravitational waves from an axi-symmetric source in the Nonsymmetric Gravitational Theory

We examine gravitational waves in an isolated axi-symmetric reflexion symmetric system in the framework of the Nonsymmetric Gravitational Theory (ngt). The vacuum field equations are analyzed and the exact solution for the field variables in the metric tensor are found in the form of expansions in powers of a radial coordinate. We find that when the system radiates, the mass decreases monotonically and the energy flux associated with waves is positive.     

Gravitational waves from cosmic strings associated with pseudo-Nambu-Goldstone dark matter

We study stochastic gravitational waves from cosmic strings generated in an ultraviolet-complete model for pseudo-Nambu-Goldstone dark matter with a hidden U(1) gauge symmetry.The dark matter candidate in this model can naturally evade direct detection bounds and easily satisfy other phenomenological constraints.The bound on the dark matter lifetime implies an ultraviolet scale higher than 10⁹ GeV.The spontaneous U(1) symmetry breaking at such a high scale would induce cosmic strings ...     

Constraining nuclear equations of state using gravitational waves from hypermassive neutron stars

Latest general relativistic simulations for the merger of binary neutron stars with realistic equations of states (EOSs) show that a hypermassive neutron star of an ellipsoidal figure is formed after the merger if the total mass is smaller than a threshold value which depends on the EOSs. The effective amplitude of quasiperiodic gravitational waves from such hypermassive neutron stars is approximately 6-7 x 10(-21) at a distance of 50 Mpc, which may be large enough for detection by advanced laser interferometric gravitational wave detectors although the frequency is high, approximately 3 kHz. We point out that the detection of such signals may lead to constraining the EOSs for neutron stars.     

Existence and stability of shock waves in relativistic hydrodynamics with general equation of state

Different forms of criteria for the existence and stability of relativistic shock waves are compared. In case of a single-valued shock adiabat expressed as a function of pressure the requirement of the existence of a shock viscous profile is shown to be most restrictive. This criterion overlaps evolutionarity conditions, the entropy criterion and corrugation stability. The relativistic criterion of the spontaneous radiation of sound is proved to be equivalent to the condition of shock quasi-instability with respect to three-wave branching. The results are qualitatively analogous to those of a non-relativistic consideration.     

Gravitational collapse in quantum mechanics with relativistic kinetic energy

It is proved that the quantum mechanical Hamiltonian $\text{H = Σ}{}_{\mathrm{i=1}}{}^{\mathrm{N}}\text{(p}{}^2\text{+ m}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? κ Σ}{}_{\mathrm{i>j}}\text{|x}{}_{\mathrm{i}}\text{? x}{}_{\mathrm{j}}\text{|}{}^{\mathrm{?1}}$ for bosons (resp, fermions) is bounded from below if N ≤ c_bκ^?1 (resp. $\text{N ≤ c}{}_{\mathrm{f}}\text{κ}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$). H is unbounded from below if N ≥ c_b^lκ^?1 (resp. $\text{N ≥ c}{}_{\mathrm{f}}{}^{\mathrm{l}}\text{κ}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$). The constants c_b and c_b^l (resp. c_f and c_f^l) differ by about a factor 2 (resp. 4).     

Criterion for existence of shock waves in relativistic magnetohydrodynamics with a general equation of state

Relativistic stationary shock waves in an ideal conducting fluid are studied for the general equation of state. We use small viscosity arguments to obtain a criterion that selects physically admissible shock transitions without any supposition about convexity of the Poisson adiabats. The relations between the magnetosound speeds and the speed of the shock obtained as a consequence of this criterion reveal specific differences between relativistic considerations versus classical ones.     

Gravitational waves from mesoscopic dynamics of the extra dimensions

Recent models which describe our world as a brane embedded in a higher dimensional space introduce new geometrical degrees of freedom, associated with spatial variations in the position of the brane and the size of the extra dimensions, that can be coherently excited by symmetry breaking in the early universe even on "mesoscopic" scales as large as 1 mm. The characteristic frequency and intensity of resulting gravitational radiation backgrounds are estimated. Extra dimensions with scale between 10(-14) and 1 mm can produce detectable backgrounds at frequencies f approximately 10(3) to 10(-4) Hz.     

Gravitational waves from the axial perturbations of hyperon stars

The eigen-frequencies of the axial w-mode oscillations of hyperon stars are examined.It is shown that as the appearance of hyperons softens the equation of state of the super-density matter,the frequency of gravitational waves from the axial w-mode of hyperon star becomes smaller than that of a traditional neutron star at the same stellar mass.Moreover,the eigenfrequencies of hyperon stars also have scaling universality.It is shown that the EURO thirdgeneration gravitational-wave detector has the potential to detect the gravitational-wave signal emitted from the axial w-mode oscillations of a hyperon star.     

Gravitational waves from neutron stars: promises and challenges

We discuss different ways that neutron stars can generate gravitational waves, describe recent improvements in modelling the relevant scenarios in the context of improving detector sensitivity, and show how observations are beginning to test our understanding of fundamental physics. The main purpose of the discussion is to establish promising science goals for third-generation ground-based detectors, like the Einstein Telescope, and identify the various challenges that need to be met if we want to use gravitational-wave data to probe neutron star physics.