During a one-year period the LIGO and Virgo Collaborations will register science data continuously, and the three detectors will operate as a global observatory Since August 2017, when the second observation period ended (called O2), both collaborations have continued working on their interferometers to improve their performance: their sensivity and accuracy. 

"For this third observation period, we have achieved significantly greater improvements in the sensitivity of the detectors of the last period," says Peter Fritschel, chief scientist of the LIGO detector at MIT. "And with LIGO and Virgo observing together over the next year, we will definitely detect many more gravitational waves and more types of sources than we have ever seen. We are eager to see new events as well, such as the collision of a black hole and a neutron star."

In 2015, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected. The ripples traveled to Earth from the collision of two black holes 1.3 billion light-years away - a discovery that led to the award of the Nobel Prize in Physics in 2017.
Since then, the LIGO-Virgo network of detectors has discovered nine additional black hole fusions and an explosive shock of two neutron stars. That event, labeled GW170817, generated not only gravitational waves but also light, which was observed by dozens of ground and space telescopes.

"With our three detectors now operational with significantly improved sensitivity, the global network of LIGO-Virgo detectors expects to make several new detections. It will also allow precise triangulation of gravitational wave sources. This will be an important step towards our search for multi-messenger astronomy (astronomical phenomena observable through different channels, such as light and gravitational waves)," says Jo van den Brand of Nikhef (Dutch National Institute for Subatomic Physics) and VU University Amsterdam, spokesman for the Virgo collaboration.

"Going from the pioneering era that led to the historical discovery to the present age of observations, where the interferometer and infrastructure will have to operate flawlessly 24 hours a day, 7 days a week, for a full year, was and continues to be a considerable challenge," says Stavros Katsanevas, Director of EGO.

"I am confident, however, that we will face this challenge with the same success with which we faced the previous one.

The sensitivity of the detector is usually given in terms of the distance at which the fusion of a binary system of neutron stars can be observed. "During O2 Advanced Virgo could observe events associated with neutron stars up to a distance of 88 million light years," says Alessio Rocchi, INFN researcher and Virgo launch coordinator. “Both LIGO and Virgo collaborations have been working to improve the sensitivity of the detectors, also taking advantage of updates installed in interferometers. It has not been a direct path at all but certainly very rewarding."

"The quality of the data recorded by the instruments is a determining factor in detecting signals of gravitational waves buried in the noise and measuring their properties," says Nicolas Arnaud, CNRS researcher currently promoted to coordinator of characterization of the EGO and Virgo detector, "Much progress has been made in that direction since O2, thanks to the combined effort of all the collaboration, from instrumentalists to data analysts.”

O3's scientific result is expected to be revolutionary, potentially revealing exciting new signals from new sources such as the fusion of binary systems composed of a black hole and a neutron star. O3 will also target long duration gravitational waves, produced for example by neutron stars spinning non-symmetrically with respect to their axis of rotation. However, the detection of such signals, as well as those from supernova explosions produced after the collapse of stellar cores and other sources, is still an enormous challenge and the LIGO-Virgo collaboration is working to achieve this goal. In addition, thanks to updates from Virgo and LIGO, signals from the fusion of black holes, such as GW150914, the first detection of gravitational waves, are expected to be very common, up to one per week. Scientists also hope to observe perhaps as many as dozens of neutron star fusions, such as GW170817, which opened the era of multi-messenger astronomy as well as provided insights into the evolution of binary systems, nuclear physics, cosmology and fundamental physics.

Scientists have also improved a posteriori and real-time data analysis, and have further developed procedures for communicating Open Public Warnings: they will notify the physics and astronomy communities in a matter of minutes of the observation of a potential gravitational wave candidate. "The new software we have built is capable of sending Open Public Alerts in 5 minutes," says Sarah Antier, postdoctoral researcher at the Université Paris Diderot and responsible for the low-latency program of the Virgo collaboration, "This will allow tracking of the gravitational wave signal with electromagnetic and neutrino searches, thus leading to discoveries in multi-messenger astronomy. The observations of many signals, expected to take place during O3, will provide a census of the population of stellar mass remnants and a better understanding of the violent universe".

Since August 2017 both collaborations, LIGO and Virgo, have updated their observatories and tested them. In particular Virgo has fully replaced the steel wires which were used in O2 to suspend the four main mirrors of the 3 km long interferometer: the mirrors are now suspended with thin fused-silica (‘glass’) fibres, a procedure which has allowed to increase the sensitivity in the low-medium frequency region, and has a dramatic impact in the capabilities to detect mergers of compact binary systems. A second major upgrade was the installation of a more powerful laser source, which improves the sensitivity at high frequencies. Last but not least squeezed vacuum states are now injected into Advanced Virgo, thanks to a collaboration with the Albert Einstein Institute in Hannover, Germany. This technique takes advantage of the quantum nature of light and improves the sensitivity at high frequencies.

The injection technique of compressed vacuum states is a significant update also implemented in the two LIGO interferometers in the United States for the next observation period. In addition, the power of the laser has been doubled in order to more accurately measure the effect of the passage of gravitational waves. Other upgrades have been made to the LIGO mirrors at both observatories, with a total of 5 of the 8 mirrors replaced by better performing versions. 

"We had to break the fibers that held the mirrors, carefully remove them and replace them," says Calum Torrie, head of engineering for LIGO's optical-mechanical system at Caltech. "It was a huge engineering task."

During O3, the LIGO-Virgo collaboration will continue to communicate the new findings to the scientific community and society. In addition, scientists will continue to extract as many physical results as possible from the data.

The global LIGO-Virgo network will provide rapid locations of gravitational wave signals and report events with high reliability through the Open Public Alerts system, with the goal of maximizing the science that the entire scientific community can perform with gravitational wave detections, and minimizing the opportunity to lose any electromagnetic or neutrino counterparts.

The Japanese KAGRA detector is expected to join the global LIGO-Virgo network in the latter part of O3, increasing the detection and pointing capabilities of the global network.

Five groups in Spain are contributing to the gravitational wave astronomy of LIGO-Virgo, in areas ranging from theoretical modeling of astrophysical sources to improved detector sensitivity for current and future observation periods.

Two groups, at UIB and IGFAE-USC, are part of the LIGO Scientific Collaboration, while the Universitat de València (UV), ICCUB and IFAE of Barcelona are members of Virgo.

After the wonderful discoveries that brought the first two observation periods, the Spanish LIGO-Virgo groups are anxiously awaiting the imminent O3 observation period. The significant improvements in sensitivity and the great technological advances achieved in the three detectors by the start-up teams since O2 have been absolutely remarkable. Their efforts will soon be rewarded with another expected and important wave of new and exciting discoveries, which will further boost the emerging field of multi-messenger astrophysics.

The gravitational physics group at the UIB will follow an extensive scientific program to study the gravitational waves emitted by black holes and neutron stars. The team will continue to lead the searches for continuous wave signals from unknown neutron stars, as well as transient signals emitted after the fusion of two neutron stars. Gravitational wave signal models from black hole fusion are an essential part of the data analysis process, and the UIB is involved in the development of one of the two key models used so far.

After about three years of developing an improved and more accurate description of black hole fusion, the UIB group is eager to prove the value of the model by making new discoveries. In addition, a PhD student from the group, Pep Covas, will spend the next three months at LIGO Hanford, and will contribute directly to operating the detector during this exciting time.

The IGFAE Gravitational Wave group at the University of Santiago de Compostela is the 'youngest' member of the LIGO Collaboration in Spain. The group has extensive experience in methods of analysis for detecting gravitational wave signals from the fusion of binary systems of black holes and neutron stars, such as the 11 events catalogued so far by the LIGO-Virgo collaboration.

IGFAE-GW is currently working on updating the channels for detecting this type of event using PyCBC software with the aim of maximizing the scope of binary searches in the new data capture called O3. The group is also involved in the deduction of information concerning populations of gravitational wave sources, including clues that dozens of likely new detections of black hole binaries will provide about the formation and evolution of these mysterious binary systems. Members of the IGFAE-GW group are also working at the Pierre Auger cosmic ray observatory, co-authors along with the LIGO and Virgo collaborations of works in which the most restrictive limits to the emission of ultra-high energy neutrinos from the fusion of the GW170817 binary neutron star system have been established. These researchers will continue to work on multi-messenger tracking of O3 events using data collected at the Pierre Auger observatory.

The Virgo group at the University of Valencia is looking forward to O3's promise to increase the number of detections of binary neutron star systems and, perhaps, the first observations of systems not yet detected, such as the mixed fusions of a black hole and a neutron star and the supernova explosions produced after the collapse of stellar nuclei (this last scenario is highly unlikely due to the low amplitude of the gravitational wave and the low event rate). Astrophysical sources of gravitational waves such as neutron stars and supernova progenitors are the main focus of the Virgo group in València, with respect to research concerning the modeling of waveforms through simulations of numerical relativity, estimation of parameters, and data analysis.

In addition, these scenarios are the main candidates for monitoring observations of associated electromagnetic signals, a research programme in which the Universitat de València group will also be involved during O3.

The Virgo group of the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) will help in the processing and analysis of a vast amount of O3 data in a more efficient and reliable way. The group's experience in the manipulation of massive data and state-of-the-art instrumentation and electronics, acquired thanks to ICCUB's successful participation in large projects in high energy physics (LHCb) and huge astronomical censuses (Gaia), is being transferred to Virgo. In this way, ICCUB's multidisciplinary experts will contribute to the detection and analysis of gravitational waves by providing instrumentation and software, as well as data analysis and their great scientific knowledge especially in the field of cosmology.

IFAE has assumed significant responsibilities in the Virgo experiment related to the control of diffuse light within the experiment. The group has already played an important role in the tuning of the interferometer, prior to the start of O3. This energetic involvement in the experiment will continue in aspects related to interferometer operations and updating. To this end, IFAE is working on the construction of new deflectors instrumented with photosensors around the main mirrors in the suspended areas, allowing ultimately a much more efficient alignment and precise adjustment of the interferometer parameters during operations; a better dynamic description of the mirrors using scattered light distributions and simulations; and the suppression of the development of high modes in the interferometer, leading to recognizable patterns in the distribution of light in the deflectors. The IFAE team has developed a comprehensive research program that focuses on aspects related to fundamental physics. This includes testing exotic models of Gravity beyond General Relativity; searching for primordial black holes as dark matter candidates; accurately determining the expansion factor of the universe; and using gravitational waves as tests of inflation and phase transitions in the early universe. In collaboration with the IFAE team on CTA/MAGIC and Observational Cosmology, the Virgo group is in a privileged position to take full advantage of the multi-messenger astronomy approach.

LIGO is funded by NSF and operated by Caltech and MIT, which conceived LIGO and led to the Initial and Advanced LIGO projects.

Financial support for the Advanced LIGO project was led by the NSF along with Germany (Max Planck Society), the United Kingdom (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Nearly 1300 scientists from around the world participate in the joint effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of some 350 scientists, engineers and technicians from about 70 institutions in Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and has been founded by the Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the groups in the Virgo Collaboration can be found at http://public.virgo-gw.eu/the-virgo-collaboration/.

More information is available on the Virgo website: http://www.virgo-gw.eu.