Background Gravitational Wave Research Articles

Detecting the heterodyning of gravitational waves

Abstract Gravitational waves modulate the apparent frequencies of other periodic signals. Low-frequency gravitational waves could therefore be detected by observing frequency modulations in signals from higher-frequency sources, e.g. those from binary white dwarfs detected with the LISA gravitational-wave detector. We propose a concrete method to extract these modulations by coherently adding the cross-spectra of a large number of well-resolved quasi-monochromatic signals. We apply this method to the case of LISA, and find this method would enable the detection of background gravitational wave strain amplitudes of, e.g. A ≃ 10 − 10 at a frequency F ≃ 10 − 8 Hz , given current projections for the number and properties of Galactic binary white dwarfs and the sensitivity of the instrument. We also estimate (to within an order of magnitude) that this method could potentially compete with that of current pulsar timing arrays when using signals from binary neutron stars such as those expected to be observed with proposed detectors like DECIGO. Our results show that gravitational-wave detectors could be sensitive at frequencies outside of their designed bandwidth using the same infrastructure, which has the potential to open up unexplored and otherwise inaccessible parts of the gravitational wave spectrum.

Background gravitational waves as signature of the accelerated cosmic expansion

We propose a toy model of a spherical universe made up of an exotic dark gas with temperature T in thermal equilibrium with a black-body in adiabatic expansion. Each particle of this exotic gas mimics a kind of particle of dark energy. So, each dark particle occupies a very small area of space so-called Planck area [Formula: see text], which represents the minimum area of the whole space-time given by the spherical surface with area [Formula: see text], where [Formula: see text] is the Hubble radius. We realize that such spherical surface is the surface of the black-body for representing the dark universe as if it were the surface of an expanding balloon. Thus, we are able to derive the law of universal gravitation, thus leading us to understand the cosmological anti-gravity. We estimate the tiny order of magnitude of the cosmological constant and the acceleration of expansion of the dark sphere. In this toy model, as the dark universe can be thought of as a large black body, when we obtain its power and frequency of emission of radiation, we find very low values. We conclude that such radiation and frequency of the black body made up of dark energy is a background gravitational wave with very low frequency in the order of [Formula: see text] due to the slight stretching of the fabric of space-time.

Background gravitational waves as signature of the adiabatic expansion of a black body that represents the dark universe

We propose a toy model of a spherical universe made up of an exotic dark gas with temperature T in thermal equilibrium with a black body in adiabatic expansion. Each particle of this exotic gas mimics a kind of particle of dark energy represented by the vacuum energy, being quantized into virtual particles with extremely small masses that form such gas representing the own tissue of the expanding space–time governed by a negative pressure whose origin is the equation of state of vacuum, i.e., p = −ρ, where ρ is the vacuum energy density. So, each vacuum particle occupies a tiny area of space so-called Planck area [Formula: see text], which represents the minimum area of the whole space–time given by the spherical surface with area [Formula: see text], where RH is the Hubble radius. We realize that such spherical surface is the surface of the black body for representing the dark universe as if it were the surface of an expanding balloon. Thus, we are able to derive the law of universal gravitation, thus leading us to understand the cosmological anti-gravity. We estimate the very small order of magnitude of the cosmological constant and the acceleration of expansion of the dark sphere. In this toy model, as the dark universe can be thought of as a large black body, when we obtain its power and frequency of emission of radiation, we find very low values. We conclude that such radiation and frequency of the black body made up of dark energy is a background gravitational wave with very low frequency in the order of 10−17Hz due to the slight stretching of the fabric of space–time.

Parametric amplification of electromagnetic plasma waves in resonance with a dispersive background gravitational wave.

It is shown that a subluminal electromagnetic plasma wave, propagating in phase with a background subluminal gravitational wave in a dispersive medium, can undergo parametric amplification. For these phenomena to occur, the dispersive characteristics of the two waves must properly match. The response frequencies of the two waves (medium dependent) must lie within a definite and restrictive range. The combined dynamics is represented by a Whitaker-Hill equation, the quintessential model for parametric instabilities. The exponential growth of the electromagnetic wave is displayed at the resonance; the plasma wave grows at the expense of the background gravitational wave. Different physical scenarios, where the phenomenon can be possible, are discussed.

Primordial non-Guassianity in inflation with gravitationally enhanced friction

The gravitationally enhanced friction can reduce the speed of the inflaton to realize an ultra-slow-roll inflation, which will amplify the curvature perturbations. The amplified perturbations can generate a sizable amount of primordial black holes (PBHs) and induce simultaneously a significant background gravitational waves (SIGWs). In this paper, we investigate the primordial non-Gaussianity of the curvature perturbations in the inflation with gravitationally enhanced friction. We find that when the gravitationally enhanced friction plays a role in the inflationary dynamics, the non-Gaussianity is noticeably larger than that from the standard slow-roll inflation. During the regime in which the power spectrum of the curvature perturbations is around its peak, the non-Gaussianity parameter changes from negative to positive. When the power spectrum is at its maximum, the non-Gaussianity parameter is near zero ($\sim \mathcal{O}(0.01)$). Furthermore, the primordial non-Gaussianity promotes the formation of PBHs, while its effect on SIGWs is negligible.

High frequency background gravitational waves from spontaneous emission of gravitons by hydrogen and helium

A direct consequence of quantization of gravity would be the existence of gravitons. Therefore, spontaneous transition of an atom from an excited state to a lower-lying energy state accompanied with the emission of a graviton is expected. In this paper, we take the gravitons emitted by hydrogen and helium in the Universe after recombination as a possible source of high frequency background gravitational waves, and calculate the energy density spectrum. Explicit calculations show that the most prominent contribution comes from the 3d-1s transition of singly ionized helium mathrm {He}^{+}, which gives a peak in frequency at sim 10^{13} Hz. Although the corresponding energy density is too small to be detected even with state-of-the-art technology today, we believe that the spontaneous emission of mathrm {He}^{+} is a natural source of high frequency gravitational waves, since it is a direct consequence if we accept that the basic quantum principles we are already familiar with apply as well to a quantum theory of gravity.

Impact of a primordial magnetic field on cosmic microwave background B modes with weak lensing

We discuss the manner in which the primordial magnetic field (PMF) suppresses the cosmic microwave background (CMB) $B$ mode due to the weak-lensing (WL) effect. The WL effect depends on the lensing potential (LP) caused by matter perturbations, the distribution of which at cosmological scales is given by the matter power spectrum (MPS). Therefore, the WL effect on the CMB $B$ mode is affected by the MPS. Considering the effect of the ensemble average energy density of the PMF, which we call "the background PMF," on the MPS, the amplitude of MPS is suppressed in the wave number range of $k>0.01~h$ Mpc$^{-1}$.The MPS affects the LP and the WL effect in the CMB $B$ mode; however, the PMF can damp this effect. Previous studies of the CMB $B$ mode with the PMF have only considered the vector and tensor modes. These modes boost the CMB $B$ mode in the multipole range of $\ell > 1000$, whereas the background PMF damps the CMB $B$ mode owing to the WL effect in the entire multipole range. The matter density in the Universe controls the WL effect. Therefore, when we constrain the PMF and the matter density parameters from cosmological observational data sets, including the CMB $B$ mode, we expect degeneracy between these parameters. The CMB $B$ mode also provides important information on the background gravitational waves, inflation theory, matter density fluctuations, and the structure formations at the cosmological scale through the cosmological parameter search. If we study these topics and correctly constrain the cosmological parameters from cosmological observations including the CMB $B$ mode, we need to correctly consider the background PMF.

Compressed baryonic matter: from nuclei to pulsars

Our world is wonderful because of the negligible baryonic part although unknown dark matter and dark energy dominate the Universe. Those nuclei in the daily life are forbidden to fuse by compression due to the Coulomb repulse, nevertheless, it is usually unexpected in extraterrestrial extreme-environments: the gravity in a core of massive evolved star is so strong that all the other forces (including the Coulomb one) could be neglected. Compressed baryonic matter is then produced after supernova, manifesting itself as pulsar-like stars observed. The study of this compressed baryonic matter can not only be meaningful in fundamental physics (e.g., the elementary color interaction at low-energy scale, testing gravity theories, detecting nano-Hertz background gravitational waves), but has also profound implications in engineering applications (including time standard and navigation), and additionally, is focused by Chinese advanced telescopes, either terrestrial or in space. Historically, in 1930s, L. Landau speculated that dense matter at supra-nuclear density in stellar cores could be considered as gigantic nuclei (the prototype of standard model of neutron star), however, we address that the residual compact object of supernova could be of condensed matter of quark clusters. The idea that pulsars are quark-cluster stars was not ruled out during the last decade, and we are expecting to test further by future powerful facilities.

Gravitational wave detection on the Moon and the moons of Mars

The Moon and the moons of Mars should be extremely quiet seismically and could therefore become sensitive gravitational wave detectors, if instrumented properly. Highly sensitive displacement sensors could be deployed on these planetary bodies to monitor the motion induced by gravitational waves. A superconducting displacement sensor with a 10-kg test mass cooled to 2 K will have an intrinsic instrument noise of 10 −16 m Hz −1/2. These sensors could be tuned to the lowest two quadrupole modes of the body or operated as a wideband detector below its fundamental mode. An interesting frequency range is 0.1–1 Hz, which will be missed by both the ground detectors on the Earth and LISA and would be the best window for searching for stochastic background gravitational waves. Phobos and Deimos have their lowest quadrupole modes at 0.2–0.3 Hz and could offer a sensitivity h min ⩽ 10 −22 Hz −1/2 within their resonance peaks, which is within two orders of magnitude from the goal of the Big Bang Observer (BBO). The lunar and Martian moon detectors would detect many interesting foreground sources in a new frequency window and could serve as a valuable precursor for BBO.

The effect of the cosmological constant on background gravitational waves

The effect of a non-zero cosmological constant on the power spectrum of background gravitational waves is investigated. The power spectra of models with and without a non-zero cosmological constant are compared, and measurable differences between the models are identified. In particular, the power spectrum for non-zero cosmological constant is 0.3–0.6 times lower than the models with a zero constant for large-scale gravitational waves, yet identical at the super-horizon scale.

Calculation of the Energy Spectrum of Background Gravitational Waves with the Simultaneous Hamiltonian Diagonalization Condition

The energy spectrum of background gravitational waves produced during inflation to the matter-dominated epoch is evaluated for the simultaneous Hamiltonian diagonalizing vacuum. It is shown that the energy spectrum almost coincides with that in Allen's work in which the Bunch-Davies vacuum is adopted as the initial state. The application of the simultaneous Hamiltonian diagonalization condition seems to be valid.

Calculation of the Energy Spectrum of Background Gravitational Waves with the Simultaneous Hamiltonian Diagonalization Condition

Calculation of the Energy Spectrum of Background Gravitational Waves with the Simultaneous Hamiltonian Diagonalization Condition

Background Gravitational Wave Produced in Inflationary Universe

We quantize the gravitational wave which arises from quantum fluctuations in the inflationary universe and compute its time evolution using a simple model of the cosmological evolution. As a result, we obtain the power spectrum of the background gravitational wave and calculate the behavior of the squeezed parameters

Background Gravitational Wave Produced in Inflationary Universe

Background Gravitational Wave Produced in Inflationary Universe

Gravitational radiation from primordial solitons and soliton-star binaries.

The possibility that both the formation of nontopological solitons in a primordial second-order phase transition and binary systems of soliton stars could generate a stochastic gravitational-wave background is examined. The present contribution in gravitational radiation to the energy density of the Universe from these processes is estimated for a number of different models. The detectability of such contributions from the timing measurements of the milisecond pulsar and spaceborne laser interferometry is briefly discussed and compared to other cosmological and local sources of background gravitational waves.

Cosmic background gravitational wave radiation and prospects for its detection

Calculated energy fluxes of gravitational waves from various astrophysical sources, when taken together with estimates of their respective event rates and spatial distributions, indicate that an incoherent isotropic background of gravitational waves. This radiation appears to be most intense in the spectral range from 10 −1 to 10 −5 Hz. Gravitational wave detectors designed to operate in space, which cover this waveband, should see this radiation at measured levels of spatial strain near or below 10 −18 δℓ/ℓ. Several possibilities for operating such detectors are discussed in this paper.

An Upper Limit to the Space Density of Short-period Noninteracting Binary White Dwarfs

view Abstract Citations (77) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Upper Limit to the Space Density of Short-period Noninteracting Binary White Dwarfs Robinson, Edward L. ; Shafter, Allen W. Abstract We have searched for short-period, detached binary stars in which both stars are white dwarfs by looking for radial velocity variations in spectroscopically identified DA and DB white dwarfs. The search was sensitive to binaries with orbital periods between 30 s and 3 hr and, within that range, we would have detected roughly 90% of all binaries, depending on the distribution of their orbital periods, mass ratios, and spectral types. We observed 44 stars without finding any binaries. The fraction of white dwarfs that are binaries is less than 1/20 with a 90% probability and less than 1/37 with a 70% probability. Other surveys sensitive to longer periods have also failed to find a large population of binary white dwarfs. The space density of binary white dwarfs is too low to account for the rate of Type I supernovae in the Galaxy and too low to agree comfortably with recent estimates of their numbers from theoretical calculations of the evolution of close binaries. Binary white dwarfs are the dominant source of the background gravitational waves at periods between about 30 s and 1 hr. The background is likely to be much lower than previously thought. Publication: The Astrophysical Journal Pub Date: November 1987 DOI: 10.1086/165725 Bibcode: 1987ApJ...322..296R Keywords: STARS: BINARIES; STARS: STELLAR STATISTICS; STARS: SUPERNOVAE; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (38)

New millisecond pulsar in a binary system

Recent observations1,2 at the Arecibo Observatory have resulted in the discovery of PSR1855+09, a pulsar with period P=5.362 ms, moving in a nearly circular orbit of period 12.3 days. The pulsar is only the third one known with P < 10 ms, and the sixth known radio pulsar in a binary system. (Discovery of a seventh binary pulsar is announced in an accompanying paper3.) Three of the seven binaries are among the fastest five of more than 400 pulsars—a fact that provides strong support for the conclusion that fast pulsars are ‘recycled’ neutron stars, spun up during a phase of mass accretion from an evolving companion star. The pulsar has a small dispersion measure (13.3 cm−3 pc), suggesting a distance of only ∼350 pc. The proximity of this pulsar and its location within 25 pc of the galactic plane argue that millisecond pulsars form a significant fraction (∼10%) of the pulsar population, leaving many detectable ones undiscovered4,5. Its signal is strong enough to permit pulse-arrival-time measurements with single-day uncertainties of <3µs. Timing observations already suggest that PSR1855+09, like the 1.5-ms pulsar PS R1937+21, will prove to be a natural clock of extremely high stability6. The existence of a second pulsar with extremely small timing uncertainties will greatly aid the search for background gravitational waves using millisecond pulsars as detectors.