Gravitational waves and red shifts: A space experiment for testing relativistic gravity using multiple time-correlated radio signals

We describe an experimental technique for detecting extremely low-frequency pulses of gravitational radiation ( _GW 1–10 mHz) originating from collapsing supermassive objects (M 10⁶–10⁷ m ) occurring anywhere in the universe. Our technique is the natural outgrowth of a previous gravitational space mission. The novelty of our approach is in placing a highly stable hydrogen maser onboard a deep-space probe that controls a transmitter sending signals to earth. The spacecraft also includes a doppler transponder operating in the conventional two-way mode. Doppler tracking using simultaneously acquired one- and two-way information both on the spacecraft and at the earth station provides four time-records of frequency fluctuations. A single gravitational disturbance manifests itself as a uniquely determined pulse sequence in the two or more data sets whose amplitudes and arrival times depend on a single parameter. The repetition of the signal and the noises in the data can be used in a filtering scheme to improve the amplitude sensitivity by a factor of about 6 in amplitude (36 in energy). We believe the most likely of these gravitational pulse events occurring frequently enough to be detected (more than once per year) will come from the formation of black holes in the cores of ordinary spiral galaxies. We propose a technologically feasible and realistic space mission, using the above technique, to measure two aspects of gravitation with the same experimental equipment. The spaceflight begins in a highly eccentric earth orbit to measure the gravitational red shift and the second-order doppler effects to an accuracy of 5 parts in 10⁶; at this level significant new tests of nonmetric theories of gravity are possible. Later, the spacecraft is sent into a heliocentric orbit to distances beyond 6 AU to search for gravitational radiation.     

Origin of the binary pulsar J0737-3039B

Evolutionary scenarios suggest that the progenitor of the new binary pulsar J0737-3039B was a He star with M>(2.1-2.3)M. . We show that this case implies that the binary must have a large (>120 km/s) center of mass velocity. However, the location, approximately 50 pc from the Galactic plane, suggests that the system has, at high likelihood, a significantly smaller center of mass velocity and a progenitor more massive than 2.1M. is ruled out (at 97% C.L.). A progenitor mass around 1.45M. , involving a new previously unseen gravitational collapse, is kinematically favored. The low mass progenitor is consistent with the recent scintillation based velocity measurement of 66+/-15 km/s and rules out the high mass solution at 99% C.L. Conversely, if the unlikely higher mass solution is the true one we should increase the estimated rate of neutron star mergers by a factor of at least 2.     

High-fidelity DNA sensing by protein binding fluctuations

One of the major functions of RecA protein in the cell is to bind single-stranded DNA exposed upon damage, thereby triggering the SOS repair response. We present fluorescence anisotropy measurements at the binding onset, showing enhanced DNA length discrimination induced by adenosine triphosphate consumption. Our model explains the observed DNA length sensing as an outcome of out-of-equilibrium binding fluctuations, reminiscent of microtubule dynamic instability. The cascade architecture of the binding fluctuations is a generalization of the kinetic proofreading mechanism. Enhancement of precision by an irreversible multistage pathway is a possible design principle in the noisy biological environment.     

Rate-distortion scenario for the emergence and evolution of noisy molecular codes

We discuss, in terms of rate-distortion theory, the fitness of molecular codes as the problem of designing an optimal information channel. The fitness is governed by an interplay between the cost and quality of the channel, which induces smoothness in the code. By incorporating this code fitness into population dynamics models, we suggest that the emergence and evolution of molecular codes may be explained by simple channel design considerations.     

Gamma-ray bursts and related phenomena

Gamma-ray bursts (GRBs) have puzzled astronomers since their accidental discovery in the sixties. The BATSE detector on the COMPTON-GRO satellite has been detecting one burst per day for the last six years. Its findings have revolutionized our ideas about the nature of these objects. They have shown that GRBs are at cosmological distances. This idea was accepted with difficulties at first. However, the recent discovery of an X-ray afterglow by the Italian/Dutch satellite BeppoSAX led to a detection of high red-shift absorption lines in the optical afterglow of GRB970508 and to a confirmation of its cosmological origin. The simplest and practically inevitable interpretation of these observations is that GRBs result from the conversion of the kinetic energy of ultra-relativistic particles flux to radiation in an optically thin region. The “inner engine” that accelerates the particles is hidden from direct observations. Recent studies suggest the “internal-external” model: internal shocks that take place within the relativistic flow produce the GRB while the subsequent interaction of the flow with the external medium produce the afterglow. The “inner engine” that produces the flow is, however, hidden from direct observations. We review this model with a specific emphasis on its implications to underground physics.     

High-energy y-ray emission from gamma-ray bursts - before GLAST

Gamma-ray bursts (GRBs) are short and intense emission of soft γ-rays, which have fascinated astronomers and astrophysicists since their unexpected discovery in 1960s. The X-ray/optical/radio afterglow observations confirm the cosmological origin of GRBs, support the fireball model, and imply a long-activity of the central engine. The high-energy γ-ray emission (> 20 MeV) from GRBs is particularly important because they shed some lights on the radiation mechanisms and can help us to constrain the physical processes giving rise to the early afterglows. In this work, we review observational and theoretical studies of the high-energy emission from GRBs. Special attention is given to the expected high-energy emission signatures accompanying the canonical early-time X-ray afterglow that was observed by the Swift X-ray Telescope. We also discuss the detection prospect of the upcoming GLAST satellite and the current ground-based Cerenkov detectors.     

Inflation — an alternative to the singular Big Bang

Domains larger than the horizon in which > (a few) ×M _pl are required for the onset of inflation. Two different, equally plausible, arguments lead us to opposite conclusions about the feasibility of the existence of such regions. It seems that inflation does not free us completely from the need for special initial conditions. However, Linde has pointed out that inflation can be eternal. He stresses the fact that inflation will never cease, but this also means that it did not necessarily have a beginning. We argue that this is the simplest solution to the initial value problem and that inflation might not only solve the problems of the Big Bang model, it might also provide us with an alternative that will replace it altogether.This essay received the third award from the Gravity Research Foundation, 1990 —Ed.     

Conformally connected universes

A well-known difficulty associated with the conformal method for the solution of the general relativistic Hamiltonian constraint is the appearance of an aphysical “bag of gold” singularity at the nodal surfaces of the conformal factor. This happens whenever the background Ricci scalar is too large. Using a simple model we demonstrate that some of these singular solutions do have a physical meaning, and that these can be considered as initial data for Universes containing black holes, which are connected, in a conformally nonsingular way with each other. The relation between the ADM mass and the horizon area in this solution supports the cosmic censorship conjecture.     

Essay: Cosmic Censorship: The Role of Quantum Gravity

The cosmic censorship hypothesis introduced by Penrose thirty years ago is still one of the most important open questions in classical general relativity. In this essay we put forward the idea that cosmic censorhip is intrinsically a quantum gravity phenomena. To that end, we construct a gedanken experiment in which cosmic censorship is violated within the purely classical framework of general relativity. We prove, however, that quantum effects restore the validity of the conjecture. This suggests that classical general relativity is inconsistent and that cosmic censorship might be enforced only by a quantum theory of gravity.