The Virgo Collaboration

Mario Martínez

The detection of Gravitational Waves (GWs) from a black hole (BH) binary merger by LIGO in 2015 is a milestone that represents the beginning of a new era in the exploration of the universe. Shortly after the addition of the VIRGO antenna into the network lead to the detection of the neutron star (NS) binary merger that could be followed in electromagnetic signals, and thus represents the beginning of multi-messenger astronomy. These events have radically changed the stage for several areas of physics, from astrophysics to particle physics.


IFAE initiates the production of the new instrumented baffle for Virgo after concluding its design and passing successfully production readiness review in 2019

In Fall 2018, IFAE initiated a long-term involvement in Gravitational Wave Physics joining the Virgo Collaboration. The experimental team is currently formed by four senior scientists (M. Cavalli-Sforza, O. Blanch, M. Martínez, and Ll. Mir), one postdoctoral researcher (M. Kolstein) and three PhD students (C. Karathanasis, A. Menendez, A. Romero), in addition to a number of master students.

IFAE has assumed central responsibilities in the experiment in terms of the control of diffuse light in the interferometer, and actively participates in its commissioning and in the identification and reduction of background noise in the detector. In preparation for the future upgrade of the experiment, IFAE is designing new deflectors (baffles), instrumented with photo-sensors around the main mirrors, for a better interferometer control. On the other hand, IFAE actively participates in the LIGO and Virgo data analysis, with special emphasis on the detailed study of binary systems formed by black holes (BHs) and neutron stars (NSs) and on those aspects related to fundamental physics and cosmology.

In 2019, IFAE got strongly involved in the Virgo commissioning, operations, physics analysis, and the preparation of the future upgrades. In the following the activities carried out by the group are described, separated in the different aspects.

Contribution to the commissioning and operations

Members of the IFAE team made significant contributions to the commissioning of the interferometer prior to O3 observation period that started in April 2019. Two PhD students (A. Menéndez and A. Romero) spent 5 months at EGO (January – May 2019) working on noise hunting and magnetic injections with the aim to reduce persistent background and characterize the performance of the interferometer. In October 2019, LIGO and Virgo took one-month shutdown for re-commissioning. All the three PhD students of the team helped in improving further the performance of the interferometer at EGO. As a result, the Virgo interferometer is running with the expected O3 sensitivity of about 50 Mpcs for detecting binary neutron star systems. A publication summarizing the results is in preparation.

IFAE has taken significant responsibilities in the Virgo experiment, translated in a MoU, related to the control of the stray light inside the experiment, which is considered a limiting factor for the sensitivity of interferometers in general. One of the members of the team (Ll. Mir) is the manager of the Stray Light Control (SLC) subsystem in Virgo. As part of the SLC responsibility, members of the team, in collaboration with LIGO-Caltech and Virgo-EGO scientists, have developed simulations of the light propagation in the interferometer. Part of the work is already documented in the Virgo internal note VIR-1175A-19. In addition, IFAE has contributed in understanding the parking position for the new Signal Recycling Mirror (in order to avoid stray light contributions), which is part of the phase I Virgo upgrade program. Results are documented in the internal note VIR-0231A-20.

Contribution to hardware upgrade

As main hardware contribution to AdV+ by 2024, IFAE proposed the construction of new baffles instrumented with photo sensors around the test masses in the suspended areas. The implementation of active baffles with photo sensors, determining online the distribution of light close to the mirrors would allow for: a much more efficient alignment and fine-tune of the parameters of the interferometer during the commissioning phase after each shutdown period; feeding back the observed light distributions into the simulations and the description of the mirror surface; and the identification of developing high modes in the interferometer, beyond its fundamental mode, leading to recognizable patterns in the light distribution in the baffles. The proposed contribution to Virgo has recently drawn the attention of the LIGO community for the long-term future.

A prototype is being built at IFAE in time for summer 2020, when the suspended end mirror in the Input Mode Cleaner cavity of Virgo will be replaced and the corresponding payload reworked. This opens the opportunity to replace the passive baffle surrounding the mirror with an instrumented one, thus acting as a demonstrator of the technology. Figure 1 presents the current design. In 2019, an ad hoc R&D research line was placed in collaboration with Hamamatsu to develop a Si-based solution for an anti-reflecting IR photo-sensor that could work under Virgo conditions (ultra high vacuum and the absence of cooling). Similarly, special R&D was made to achieve edgeless apertures for the sensors in the coated stainless steel (thus minimizing the induced scattering in edges) and to determine the feasibility of a wireless readout (thus avoiding as much as possible cables running throughout the suspension system). In Fall 2019, the design was completed and the corresponding Conceptual Design Report passed successfully the Production Readiness Review carried out internally at Virgo.

Figure 1: Design of the instrumented baffle in the IMC area.
Figure 2: Design of the instrumented baffle in the IMC area.
Figure 2: Supporting frames, baffle (rear side), and photo-sensors for the production of the very first smart baffle for Virgo. The device will be installed under ultra-high vacuum conditions in 2020 at EGO.

Physics Analysis

The physics program of the IFAE group in Virgo is designed according to the expected number of binary events in O3 (2019 – 2020) and O4 (2022-2023) periods. The shape of the GW signal contains information on the masses, angular momentum and the velocities of the objects involved, and tests GR in very different domains. The distribution of BH masses and the mass and angular momentum ratio are fundamental measurements by themselves since they contain information on their possible origin. In particular, the identification of BHs with masses or the order or below one solar mass would point to primordial BHs, opening the possibility of attributing part of the dark matter content of the universe to primordial BHs.

IFAE adopted already a Deep Learning approach in the analysis of the data since it is particularly suitable for this purpose (the identification of a well-defined signal pattern over a much larger continuous background). In particular, Convoluted Neural Networks (CNN) uses 2-D images on processed events in terms of frequency vs time. As illustrated in Figure 2(left), the signal from a binary coalesce shows up in this case as a distinctive pattern in the image. The use of CNNs in Virgo promises to increase the sensitivity in terms of signal-to-noise ratio and the luminosity distances reachable by the experiment. A CNN was trained using O1+O2 data and a full bank of templates for different masses and luminosity distance of the BH-BH system. Promising results were presented in the 22nd International Conference on General Relativity and Gravitation in Valencia (see Figure 2(right)). At the moment, LIGO+Virgo O3 data is being analysed using the CNN implementation. The study is being performed in collaboration with other EGO scientists and within the framework of the COST17137 EU initiative.

Figure 4: Poster contribution to the 22nd International Conference on General Relativity and Gravitation, Valencia, July 2019
Figure 3: Signal-to-Noise ratio, Spectrogram (frequency vs time) and Strain of a binary candidate (taken from B. P. Abbott et al, Phys. Rev. Lett., 119 (14), pp. 141101, 2017)

Searches for Stochastic GW signals

The detection of GWs from the early universe in the form of a stochastic signal is a major milestone, and constitutes one of the main research lines in IFAE. Models for early universe inflation, reheating, or preheating could be distinguishable in the stochastic GW background, in the presence of a blue-shift mechanism. A detection of inflationary GWs would not only provide further clear evident of inflation but also a direct observation confirming the quantum character of Gravity. In addition, several theoretical developments at IFAE point to the possibility using GW signals of identifying phase transitions related to axion solutions in the early universe, compatible also with an axion-like explanation of the dark matter. The prospects for O3 limits on the GW energy density have the potential sensitivity to identify a stochastic signal related to binary systems. This would constitute a formidable breakthrough since this constitutes the foreground of astrophysical origin to the early universe signal that needs eventually to be subtracted. The stochastic signal is so small that presumably cannot be detected in a single interferometer and relies on cross correlating the signal from the different interferometers, assuming the noise is stationary and uncorrelated across different interferometers. This assumption can be broken if long-distance correlations are introduced. This can be potentially the case of Earth magnetic field variations (Schumann fields) affecting altogether the interferometer network. Therefore, the study, via magnetic field injection campaigns in the interferometers, of potential couplings of interferometer elements with the magnetics fields is crucial. This was an active area of research by IFAE in early 2019, leading to a publication in Classical and Quantum Gravity Journal (see Figure 3). The IFAE group is now part of the LIGO-Virgo working group searching for stochastic signals in the data.

Parameter estimation and the determination of Hubble constant

The GW signals provide a new and independent measurement of the Hubble constant for the expansion of the universe. A first measurement, based on a single binary coalescence event, determined the Hubble constant with an uncertainty of 15%. A rather rapid improvement in the measurements is expected as new events are observed. This part of the program involves multi-messenger aspects for which IFAE is naturally placed in a privileged position, given its participation in other gamma-ray and observational cosmology experiments. The precise determination first relies on an accurate determination of the luminosity distance in the parameter estimation processes, based on complex likelihoods and using a Bayesian approach. IFAE is integrated into the LIGO-Virgo Cosmo group and is contributing to improvements in the parameter estimation software. Secondly, the uncertainty on the luminosity distance determination is limited by the accuracy of the theoretical templates. Coordination with theorist and model builders across LIGO and Virgo collaboration is being put in place. Finally, although the binary neutron star events with identified EM corresponding emissions are the ideal sample, it is a fact that no many events are being accumulated today of this kind. Alternatively, a statistical based determination can be carried out using the nearby BH-BH events together with catalogues of known galaxies. This is being explored together with IFAE members in the Magic/CTA and DES collaborations.

Figure 5: Sensitivity of Virgo to magnetic injections compared to nominal sensitivity to GW signals (taken from A. Cirone et al., ‘Investigation of magnetic noise in Advanced Virgo’, Class. Quant. Grav. 36, no.22, 225004 (2019)).