Search

Galactic white dwarf binaries (WDBs) and black hole binaries (BHBs) will be gravitational wave (GW) sources for LISA. Their detection will provide insights into binary evolution and the evolution of our Galaxy through cosmic history. Here, we make predictions of the expected WDB and BHB population within our Galaxy. We combine predictions of the compact remnant binary populations expected by stellar evolution by using the detailed Binary Population and Spectral Synthesis code (BPASS) with a Milky Way analogue galaxy model from the Feedback In Realistic Environment (FIRE) simulations. We use \textsc{PhenomA} and \textsc{LEGWORK} to simulate LISA observations. Both packages make similar predictions that on average four Galactic BHBs and 673 Galactic WDBs above the signal-to-noise ratio (SNR) threshold of 7 after a four-year mission. We compare these predictions to earlier results using the Binary Star Evolution (BSE) code with the same FIRE model galaxy. We find that BPASS predicts a few more LISA observable Galactic BHBs and a twentieth of the Galactic WDBs. The differences are due to the different physical assumptions that have gone into the binary evolution calculations. These results indicate that the expected population of compact binaries that LISA will detect depends very sensitively on the binary population synthesis models used and thus observations of the LISA population will provide tight constraints on our modelling of binary stars. Finally, from our synthetic populations we have created mock LISA signals that can be used to test and refine data processing methods of the eventual LISA observations.

We report correlations in underground seismic measurements with horizontal separations of several hundreds of meters to a few kilometers in the frequency range 0.01Hz to 40Hz. These seismic correlations could threaten science goals of planned interferometric gravitational-wave detectors such as the Einstein Telescope as well as atom interferometers such as MIGA and ELGAR. We use seismic measurements from four different sites, i.e. the former Homestake mine (USA) as well as two candidate sites for the Einstein Telescope, Sos Enattos (IT) and Euregio Maas-Rhein (NL-BE-DE) and the site housing the MIGA detector, LSBB (FR). At all sites, we observe significant coherence for at least 50% of the time in the majority of the frequency region of interest. Based on the observed correlations in the seismic fields, we predict levels of correlated Newtonian noise from body waves. We project the effect of correlated Newtonian noise from body waves on the capabilities of the triangular design of the Einstein Telescope's to observe an isotropic gravitational-wave background (GWB) and find that, even in case of the most quiet site, its sensitivity will be affected up to $\sim$20Hz. The resolvable amplitude of a GWB signal with a negatively sloped power-law behaviour would be reduced by several orders of magnitude. However, the resolvability of a power-law signal with a slope of e.g. $\alpha=0$ ($\alpha=2/3$) would be more moderately affected by a factor $\sim$ 6-9 ($\sim$3-4) in case of a low noise environment. Furthermore, we bolster confidence in our results by showing that transient noise features have a limited impact on the presented results.

The Laser Interferometer Space Antenna (LISA) is the first scientific endeavour to detect and study gravitational waves from space. LISA will survey the sky for Gravitational Waves in the 0.1 mHz to 1 Hz frequency band which will enable the study of a vast number of objects ranging from Galactic binaries and stellar mass black holes in the Milky Way, to distant massive black-hole mergers and the expansion of the Universe. This definition study report, or Red Book, presents a summary of the very large body of work that has been undertaken on the LISA mission over the LISA definition phase., Comment: 155 pages, with executive summary and table of contents

The Einstein Telescope (ET), the European project for a third-generation gravitational-wave detector, has a reference configuration based on a triangular shape consisting of three nested detectors with 10 km arms, where in each arm there is a `xylophone' configuration made of an interferometer tuned toward high frequencies, and an interferometer tuned toward low frequencies and working at cryogenic temperature. Here, we examine the scientific perspectives under possible variations of this reference design. We perform a detailed evaluation of the science case for a single triangular geometry observatory, and we compare it with the results obtained for a network of two L-shaped detectors (either parallel or misaligned) located in Europe, considering different choices of arm-length for both the triangle and the 2L geometries. We also study how the science output changes in the absence of the low-frequency instrument, both for the triangle and the 2L configurations. We examine a broad class of simple `metrics' that quantify the science output, related to compact binary coalescences, multi-messenger astronomy and stochastic backgrounds, and we then examine the impact of different detector designs on a more specific set of scientific objectives., Comment: 197 pages, 73 figures. v2: corrections in the part on the sensitivity to stochastic backgrounds. Accepted in JCAP

First order phase transitions in the early universe could produce a gravitational-wave background that might be detectable by the Laser Interferometer Space Antenna (LISA). Such an observation would provide evidence for physics beyond the Standard Model. We study the ability of LISA to observe a gravitational-wave background from phase transitions in the presence of an extragalactic foreground from binary black hole mergers throughout the universe, a galactic foreground from white dwarf binaries, and LISA noise. Modelling the phase transition gravitational wave background as a double broken power law, we use the deviance information criterion as a detection statistic, and Fisher matrix and Markov Chain Monte Carlo methods to assess the measurement accuracy of the parameters of the power spectrum. While estimating all the parameters associated with the gravitational-wave backgrounds, foregrounds, and LISA noise, we find that LISA could detect a gravitational-wave background from phase transitions with a peak frequency of 1 mHz and normalized energy density amplitude of $\Omega_{\text p} \simeq 3 \times 10^{-11}$. With $\Omega_{\text p} \simeq 10^{-10}$, the signal is detectable if the peak frequency is in the range $4 \times 10^{-4}$ to $9 \times 10^{-3}$ Hz, and the peak amplitude and frequency can be estimated to an accuracy of 10\% to 1\%.

Correlated noise could impact the search for the gravitational wave background at future Earth-based gravitational-wave detectors. Due to the small distance ($\sim$ 400 m) between the different interferometers of the Einstein Telescope, correlated seismic noise could have a significant effect. To this extent, we study the seismic correlations at the Earth's surface, as well as underground, between seismometers and geophones separated by several hundreds of meters, in the frequency range 0.05 Hz - 50 Hz. Based on these correlated seismic fields we predict the levels of correlated Newtonian noise (NN). We construct upper limits on the allowed seismic coupling function such that correlated seismic noise does not affect the search for an isotropic gravitational wave background. Assuming a facility located 300 m below the surface, the impact on the search for a gravitational wave background of correlated NN from Rayleigh waves are found to be problematic up to $\sim$ 5 Hz. The NN from body waves, however, constitutes a serious threat to the search of a gravitational wave background. Correlated NN from body waves could be up to five to seven orders of magnitude above the planned sensitivity at $\sim$ 3 Hz and it could impede any search for a gravitational wave background below 40 Hz. With a factor 10 of NN reduction via NN cancellation in each interferometer, the effects of the NN on the stochastic search could be eliminated above 30 Hz.

Several proposed gravitational wave interferometers have a triangular configuration, such as the Einstein Telescope and the Laser Interferometer Space Antenna. For such a configuration one can construct a unique null channel insensitive to gravitational waves from all directions. We expand on earlier work and describe how to use the null channel formalism to estimate the power spectral density for the Einstein Telescope interferometers with non-identical as well as correlated noise sources. The formalism is illustrated with two examples in the context of the Einstein Telescope, with increasing degrees of complexity and realism. By using known mixtures of noises we show the formalism is mathematically correct and internally consistent. Finally we highlight future research needed to use this formalism as an ingredient for a Bayesian estimation framework., Comment: The results have been updated (10/10/2022). Please refer to this new version

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational wave observations by LISA to probe the universe.

An important goal of the Laser Interferometer Space Antenna (LISA) is to observe a stochastic gravitational-wave background (SGWB). A study of possible correlated noise in LISA is relevant to establish limits for this future measurement. To test noise investigation methods under somewhat realistic conditions, we use the data of LISA Pathfinder (LPF). We calculate the coherence between the LPF differential acceleration of the two test-masses with magnetic fields, temperature, and micronewton cold gas thruster activity in the spacecraft. We apply our observed correlations to LISA, and estimate how the presence of such correlated noise would affect its search for a SGWB. In the context of a figure of merit, we estimate the effect of noise on the LISA SGWB search.

We investigate the ability of the Laser Interferometer Space Antenna (LISA) to detect a stochastic gravitational-wave background (GWB) produced by cosmic strings, and to subsequently estimate the string tension $G\mu$ in the presence of instrument noise, an astrophysical background from compact binaries, and the galactic foreground from white dwarf binaries. Fisher Information and Markov Chain Monte Carlo methods provide estimates of the LISA noise and the parameters for the different signal sources. We demonstrate the ability of LISA to simultaneously estimate the galactic foreground, as well as the astrophysical and cosmic string produced backgrounds. Considering the expected astrophysical background and a galactic foreground, a cosmic string tension in the $G \mu \approx 10^{-16}$ to $G\mu \approx 10^{-15}$ range or bigger could be measured by LISA, with the galactic foreground affecting this limit more than the astrophysical background. The parameter estimation methods presented here can be applied to other cosmological backgrounds in the LISA observation band.

In its observation band, the Laser Interferometer Space Antenna (LISA) will simultaneously observe stochastic gravitational-wave background (SGWB) signals of different origins; orbitally modulated waveforms from galactic white dwarf binaries, a binary black hole produced background, and possibly a cosmologically produced SGWB. We simulate the emission of gravitational waves from galactic white dwarf binaries based on the Lamberts \cite{Lamberts} distributions and determine a complex waveform from the galactic foreground. We generate the modulated galactic signal detected by LISA due to its orbital motion, and present a data analysis strategy to address it. The Fisher Information and Markov Chain Monte Carlo methods give an estimate of the LISA noise and parameters for the different signal sources. We simultaneously estimate the galactic foreground, the astrophysical and cosmological backgrounds, and estimate detection limits for the future LISA observation of the SGWB in the spectral domain with the 3 LISA channels $ A $, $ E $ and $ T $. In the context of the expected astrophysical background and a galactic foreground, a cosmological background energy density of about $ \Omega_{GW,Cosmo} \approx 8 \times 10^{-13} $ could be detected by LISA with our spectral separation strategy., Comment: Contribution to the 2021 Gravitation session of the 55th Rencontres de Moriond

Within its observational band the Laser Interferometer Space Antenna, LISA, will simultaneously observe orbital modulated waveforms from Galactic white dwarf binaries, a binary black hole produced gravitational-wave background, and potentially a cosmologically created stochastic gravitational-wave background (SGWB). The overwhelming majority of stars end their lives as white dwarfs, making them very numerous in the Milky Way. We simulate Galactic white dwarf binary gravitational-wave emission based on distributions from various mock catalogs and determine a complex waveform from the Galactic foreground with $3.5 \times 10^{7}$ binaries. We describe the effects from the Galactic binary distribution population across mass, position within the Galaxy, core type, and orbital frequency distribution. We generate the modulated Galactic white dwarf signal detected by \textit{LISA} due to its orbital motion, and present a data analysis strategy to address it. The Fisher Information and Markov Chain Monte Carlo methods give an estimation of the \textit{LISA} noise and the parameters for the different signal classes. We estimate the detectable limits for the future LISA observation of the SGWB in the spectral domain with the 3 \textit{LISA} channels $A$, $E$, and $T$. We simultaneously estimate the Galactic foreground, the astrophysical and cosmological backgrounds. Assuming the expected astrophysical background and a Galactic foreground, a cosmological background energy density of around $\Omega_{GW,Cosmo} \approx 8 \times 10^{-13}$ could be detected by LISA. LISA will either detect a cosmologically produced SGWB, or set a limit that will have important consequences.

With the goal of attempting to observe a stochastic gravitational wave background (SGWB) with LISA, the spectral separability of the cosmological and astrophysical backgrounds is important to estimate. We attempt to determine the level with which a cosmological background can be observed given the predicted astrophysical background level. We predict detectable limits for the future LISA measurement of the SGWB. Adaptive Markov chain Monte-Carlo methods are used to produce estimates with the simulated data from the LISA Data challenge (LDC). We also calculate the Cramer-Rao lower bound on the variance of the SGWB parameter uncertainties based on the inverse Fisher Information using the Whittle Likelihood. The estimation of the parameters is done with the 3 LISA channels $A$, $E$, and $T$. We simultaneously estimate the noise using a LISA noise model. Assuming the expected astrophysical background, a cosmological background energy density of around $\Omega_{GW,Cosmo} \approx 1 \times 10^{-12}$ to $1 \times 10^{-13}$ can be detected by LISA., Comment: The paper has been corrected