Maulik Parikh, Frank Wilczek, George Zahariade

We develop a formalism to calculate the response of a model gravitational wave detector to a quantized gravitational field. Coupling a detector to a quantum field induces stochastic fluctuations ("noise") in the length of the detector arm. The statistical properties of this noise depend on the choice of quantum state of the gravitational field. We characterize the noise for vacuum, coherent, thermal, and squeezed states. For coherent states, corresponding to classical gravitational configurations, we find that the effect of gravitational field quantization is small. However, the standard deviation in the arm length can be enhanced -- possibly significantly -- when the gravitational field is in a non-coherent state. The detection of this fundamental noise could provide direct evidence for the quantization of gravity and for the existence of gravitons.

Zvi Bern, Julio Parra-Martinez, Radu Roiban, Eric Sawyer, Chia-Hsien Shen

We present the two-body Hamiltonian and associated eikonal phase, to leading post-Minkowskian order, for infinitely many tidal deformations described by operators with arbitrary powers of the curvature tensor. Scattering amplitudes in momentum and position space provide systematic complementary approaches. For the tidal operators quadratic in curvature, which describe the linear response to an external gravitational field, we work out the leading post-Minkowskian contributions using a basis of operators with arbitrary numbers of derivatives which are in one-to-one correspondence with the worldline multipole operators. Explicit examples are used to show that the same techniques apply to both bodies interacting tidally with a spinning particle, for which we find the leading contributions from quadratic in curvature tidal operators with an arbitrary number of derivatives, and to effective field theory extensions of general relativity. We also note that the leading post-Minkowskian order contributions from higher-dimension operators manifest double-copy relations. Finally, we comment on the structure of higher-order corrections.

Maulik Parikh, Frank Wilczek, George Zahariade

For the purpose of analyzing observed phenomena, it has been convenient, and thus far sufficient, to regard gravity as subject to the deterministic principles of classical physics, with the gravitational field obeying Newton's law or Einstein's equations. Here we treat the gravitational field as a quantum field and determine the implications of such treatment for experimental observables. We find that falling bodies in gravity are subject to random fluctuations ("noise") whose characteristics depend on the quantum state of the gravitational field. We derive a stochastic equation for the separation of two falling particles. Detection of this fundamental noise, which may be measurable at gravitational wave detectors, would vindicate the quantization of gravity, and reveal important properties of its sources.

Lorenzo Speri, Nicola Tamanini, Robert R. Caldwell, Jonathan R. Gair, Benjamin Wang

Quasars have recently been used as an absolute distance indicator, extending the Hubble diagram to high redshift to reveal a deviation from the expansion history predicted for the standard, $\Lambda$CDM cosmology. Here we show that the Laser Interferometer Space Antenna (LISA) will efficiently test this claim with standard sirens at high redshift, defined by the coincident gravitational wave (GW) and electromagnetic (EM) observations of the merger of massive black hole binaries (MBHBs). Assuming a fiducial $\Lambda$CDM cosmology for generating mock standard siren datasets, the evidence for the $\Lambda$CDM model with respect to an alternative model inferred from quasar data [Nat. Astron. 3, 272 (2019)] is investigated. By simulating many realizations of possible future LISA observations, we find that for $50\%$ of these realizations (median result) 4 MBHB standard siren measurements will suffice to strongly differentiate between the two models, while 14 standard sirens will yield a similar result in $95\%$ of the realizations. In addition, we investigate the measurement precision of cosmological parameters as a function of the number of observed LISA MBHB standard sirens, finding that 15 events will on average achieve a relative precision of $5\%$ for $H_0$, reducing to $3\%$ and $2\%$ with 25 and 40 events, respectively. Our investigation clearly highlights the potential of LISA as a cosmological probe able to accurately map the expansion of the universe at $z\gtrsim 2$, and as a tool to cross-check and cross-validate cosmological EM measurements with complementary GW observations.

Paul Stankus, Andrei Nomerotski, Anze Složar, Stephen Vintskevich

Improved quantum sensing of photon wave-functions could provide high resolution observations in the optical benefiting numerous fields, including general relativity, dark matter studies, and cosmology. It has been recently proposed that stations in optical interferometers would not require a phase-stable optical link if instead sources of quantum-mechanically entangled pairs could be provided to them, potentially enabling hitherto prohibitively long baselines. A new refinement of this idea is developed, in which two photons from different sources are interfered at two separate and decoupled stations, requiring only a slow classical information link between them. We rigorously calculate the observables and contrast this new interferometric technique with the Hanbury Brown & Twiss intensity interferometry. We argue this technique could allow robust high-precision measurements of the relative astrometry of the two sources. A basic calculation suggests that angular precision on the order of $10\mu$as could be achieved in a single night's observation of two bright stars.

Cristian Barrera-Hinojosa, Baojiu Li, Marco Bruni, Jian-hua He

We investigate the inherently nonlinear transverse modes of the gravitational and velocity fields in $\Lambda$CDM, based on a high-resolution simulation performed using the adaptive-mesh refinement general-relativistic $N$-body code GRAMSES. We study the generation of vorticity in the dark matter velocity field at low redshift, providing power-law fits to the shape and evolution of its power spectrum from large sub-horizon scales to deeply nonlinear scales. By analysing the gravitomagnetic vector potential, a purely relativistic vector mode of the gravitational field that is absent in Newtonian simulations, in dark matter haloes with masses from $\sim10^{12.5}~h^{-1}{M}_{\odot}$ to $\sim10^{15}~h^{-1}{M}_{\odot}$, we find that the magnitude of this vector correlates with the halo mass, peaking in the inner regions and decreasing towards their outskirts. Nevertheless, on average, its ratio against the scalar gravitational potential remains fairly constant inside the haloes, below percent level, decreasing roughly linearly with redshift at $z<3$ and showing a weak dependence on halo mass. Furthermore, we show that the gravitomagnetic acceleration in dark matter haloes peaks towards the core and reaches almost $10^{-10}$ $h$ cm/s$^2$ in the most massive halo of the simulation. However, regardless of the halo mass, the ratio between the magnitudes of the gravitomagnetic force and the standard gravitational force is typically at around $10^{-3}$ in the innermost parts of the haloes and drops by up to one order of magnitude at the outskirts. This ratio shows a very weak dependence on redshift. This result confirms that the gravitomagnetic field has a negligible effect on cosmic structure formation, even for the most massive structures, although its behaviour in low density regions remains to be explored. Its effects on photons and observations remains to be understood in detail in the future.

Thilo M. Siewert, Matthias Schmidt-Rubart, Dominik J. Schwarz

The origin and contributions to the Cosmic Radio Dipole are of great interest in cosmology. Recent studies revealed open questions about the nature of the observed Cosmic Radio Dipole. We use simulated source count maps to test a linear and a quadratic Cosmic Radio Dipole estimator for possible biases in the estimated dipole directions and contributions from the masking procedure. We find a superiority of the quadratic estimator, which is then used to analyse the TGSS-ADR1, WENSS, SUMSS, and NVSS radio source catalogues, spreading over a decade of frequencies. The same masking strategy is applied to all four surveys to produce comparable results. In order to address the differences in the observed dipole amplitudes, we cross-match two surveys, located at both ends of the analysed frequency range. For the linear estimator, we identify a general bias in the estimated dipole directions. The positional offsets of the quadratic estimator to the CMB dipole for skies with $10^7$ simulated sources is found to be below one degree and the accuracy of the estimated dipole amplitudes is below $10^{-3}$. For the four radio source catalogues, we find an increasing dipole amplitude with decreasing frequency, which is consistent with results from the literature and results of the cross-matched catalogue. We conclude that for all analysed surveys, the observed Cosmic Radio Dipole amplitudes exceed the expectation, derived from the CMB dipole.

Esteban Jimenez, Nelson Padilla, Sergio Contreras, Idit Zehavi, Carlton Baugh, Alvaro Orsi

The next generation of spectroscopic surveys will target emission-line galaxies (ELGs) to produce constraints on cosmological parameters. We study the large scale structure traced by ELGs using a combination of a semi-analytical model of galaxy formation, a code that computes the nebular emission from HII regions using the properties of the interstellar medium, and a large-volume, high-resolution N-body simulation. We consider fixed number density samples where galaxies are selected by either their H$\alpha$, [OIII]$\lambda 5007$ or [OII]$\lambda \lambda 3727-3729$ emission line luminosities. We investigate the assembly bias signatures of these samples, and compare them to those of stellar mass and SFR selected samples. Interestingly, we find that the [OIII]- and [OII]-selected samples display scale-dependent bias on large scales and that their assembly bias signatures are also scale-dependent. Both these effects are more pronounced for lower number density samples. The [OIII] and [OII] emitters that contribute most to the scale dependence tend to have a low gas-phase metallicity and are preferentially found in low-density regions. We also measure the baryon acoustic oscillation (BAO) feature and the $\beta$ parameter related to the growth rate of overdensities. We find a slight tendency for the BAO peak to shift toward smaller scales for [OII] emitters and that $\beta$ is scale-dependent at large scales. Our results suggest that ELG samples include environmental effects that should be modelled in order to remove potential systematic errors that could affect the estimation of cosmological parameters.