In this paper we briefly review the status and the perspectives of the experimental search of gravitational waves, focusing on the ground based interferometers. The current status of the running detectors and the plans to upgrade them are discussed

Accadia, T.

Acernese, F.

Antonucci, F.

Astone, P.

Ballardin, G.

Barone, F.

Barsuglia, M.

Basti, A.

Bauer, Th. S.

Bebronne, M.

Beker, M. G.

Belletoile, A.

Birindelli, S.

Bitossi, M.

Bizouard, M. A.

Blom, M.

Bondu, F.

Bonelli, L.

Bonnand, R.

Boschi, V.

Bosi, L.

Bouhou, B.

Braccini, S.

Bradaschia, C.

Branchesi, M.

Briant, T.

Brillet, A.

Brisson, V.

Budzyński, R.

Bulik, T.

Bulten, H. J.

Buskulic, D.

Buy, C.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10801-6Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010The search for gravitational wavesT. Accadia(1), F. Acernese(2)(3), F. Antonucci(4), P. Astone(4),G. Ballardin(5), F. Barone(2)(3), M. Barsuglia(6), A. Basti(7)(8),Th. S. Bauer(9), M. Bebronne(1), M. G. Beker(9), A. Belletoile(1),S. Birindelli(10), M. Bitossi(7), M. A. Bizouard(11), M. Blom(9),F. Bondu(12), L. Bonelli(7)(8), R. Bonnand(13), V. Boschi(7), L. Bosi(14),B. Bouhou(6), S. Braccini(7), C. Bradaschia(7), M. Branchesi(15)(16),T. Briant(17), A. Brillet(10), V. Brisson(11), R. Budzyn´ski(18),T. Bulik(19)(20), H. J. Bulten(9)(21), D. Buskulic(1), C. Buy(6), G. Cagnoli(15),E. Calloni(2)(22), E. Campagna(15)(16), B. Canuel(5), F. Carbognani(5),F. Cavalier(11), R. Cavalieri(5), G. Cella(7), E. Cesarini(16), O. Chaibi(10),E. Chassande-Mottin(6), A. Chincarini(23), F. Cleva(10), E. Coccia(24)(25),P.-F. Cohadon(17), C. N. Colacino(7)(8), J. Colas(5), A. Colla(4)(26),M. Colombini(26), A. Corsi(4), J.-P. Coulon(10), E. Cuoco(5),S. D’Antonio(24), V. Dattilo(5), M. Davier(11), R. Day(5), R. De Rosa(2)(22),G. Debreczeni(27), M. del Prete(28), L. Di Fiore(2), A. Di Lieto(7)(8),M. Di Paolo Emilio(24)(29), A. Di Virgilio(7), A. Dietz(1), M. Drago(30)(28),V. Fafone(24)(25), I. Ferrante(7)(8), F. Fidecaro(7)(8), I. Fiori(5),R. Flaminio(13), L. A. Forte(2), J.-D. Fournier(10), J. Franc(13),S. Frasca(4)(26), F. Frasconi(7), A. Freise(31), M. Galimberti(13),L. Gammaitoni(14)(32), F. Garufi(2)(22), M. E. Ga´spa´r(27), G. Gemme(23),E. Genin(5), A. Gennai(7), A. Giazotto(7), R. Gouaty(1), M. Granata(6),C. Greverie(10), G. M. Guidi(15)(16), J.-F. Hayau(12), A. Heidmann(17),H. Heitmann(10), P. Hello(11), S. Hild(33), D. Huet(5), P. Jaranowski(34),I. Kowalska(19), A. Kro´lak(35)(36), N. Leroy(11), N. Letendre(1),T. G. F. Li(9), N. Liguori(30)(28), M. Lorenzini(15), V. Loriette(37),G. Losurdo(15)(∗), E. Majorana(4), I. Maksimovic(37), N. Man(10),M. Mantovani(7)(38), F. Marchesoni(14), F. Marion(1), J. Marque(5),F. Martelli(15)(16), A. Masserot(1), C. Michel(13), L. Milano(2)(22),Y. Minenkov(24), M. Mohan(5), N. Morgado(13), A. Morgia(24)(25),S. Mosca(2)(22), V. Moscatelli(4), B. Mours(1), F. Nocera(5),G. Pagliaroli(24)(29), L. Palladino(24)(29), C. Palomba(4), F. Paoletti(7)(5),M. Parisi(2)(22), A. Pasqualetti(5), R. Passaquieti(7)(8), D. Passuello(7),G. Persichetti(2)(22), M. Pichot(10), F. Piergiovanni(15)(16), M. Pietka(34),L. Pinard(13), R. Poggiani(7)(8), M. Prato(23), G. A. Prodi(30)(28),M. Punturo(14), P. Puppo(4), D. S. Rabeling(9)(21), I. Ra´cz(27),P. Rapagnani(4)(26), V. Re(24)(25), T. Regimbau(10), F. Ricci(4)(26),F. Robinet(11), A. Rocchi(24), L. Rolland(1), R. Romano(2)(3), D. Rosin´ska(39),P. Ruggi(5), B. Sassolas(13), D. Sentenac(5), L. Sperandio(24)(25),(∗) E-mail: losurdo@fi.infn.itc© Societa` Italiana di Fisica 107108R. Sturani(15)(16), B. Swinkels(5), M. Tacca(5), A. Toncelli(7)(8),M. Tonelli(7)(8), O. Torre(7)(38), E. Tournefier(1), F. Travasso(14)(32),G. Vajente(7)(8), J. F. J. van den Brand(9)(21), C. Van Den Broeck(9),S. van der Putten(9), M. Vasuth(27), M. Vavoulidis(11), G. Vedovato(40),D. Verkindt(1), F. Vetrano(15)(16), A. Vicere´(15)(16), J.-Y. Vinet(10),H. Vocca(14), R. L. Ward(6), M. Was(11) and M. Yvert(1)(1) Laboratoire d’Annecy-le-Vieux de Physique des Particules (LAPP), Universite´ de SavoieCNRS/IN2P3 - F-74941 Annecy-Le-Vieux, France(2) INFN, Sezione di Napoli - I-80126 Napoli, Italy(3) Universita` di Salerno - Fisciano, I-84084 Salerno, Italy(4) INFN, Sezione di Roma - I-00185 Rome, Italy(5) European Gravitational Observatory (EGO) - I-56021 Cascina (PI), Italy(6) Laboratoire AstroParticule et Cosmologie (APC), Universite´ Paris Diderot, CNRS: IN2P3CEA: DSM/IRFU, Observatoire de Paris - 10 rue A. Domon et L. Duquet, 75013 ParisFrance(7) INFN, Sezione di Pisa - I-56127 Pisa, Italy(8) Universita` di Pisa - I-56127 Pisa, Italy(9) NIKHEF, Science Park, Amsterdam - 1081 HV Amsterdam, The Netherlands(10) Universite´ Nice-Sophia-Antipolis, CNRS, Observatoire de la Coˆte d’Azur - F-06304 NiceFrance(11) LAL, Universite´ Paris-Sud, IN2P3/CNRS - F-91898 Orsay, France(12) Institut de Physique de Rennes, CNRS, Universite´ de Rennes 1 - 35042 RennesFrance(13) Laboratoire des Mate´riaux Avance´s (LMA), IN2P3/CNRS - F-69622 Villeurbanne, LyonFrance(14) INFN, Sezione di Perugia - I-06123 Perugia, Italy(15) INFN, Sezione di Firenze - I-50019 Sesto Fiorentino, Italy(16) Universita` di Urbino “Carlo Bo” - I-61029 Urbino, Italy(17) Laboratoire Kastler Brossel, ENS, CNRS, UPMC - Universite´ Pierre et Marie Curie4 Place Jussieu, F-75005 Paris, France(18) Warsaw University - 00-681 Warsaw, Poland(19) Astronomical Observatory Warsaw University - 00-478 Warsaw, Poland(20) CAMK-PAN - 00-716 Warsaw, Poland(21) VU University Amsterdam - De Boelelaan 1105, 1081 HV AmsterdamThe Netherlands(22) Universita` di Napoli “Federico II”, Complesso Universitario di Monte S. AngeloI-80126 Napoli, Italy(23) INFN, Sezione di Genova - I-16146 Genova, Italy(24) INFN, Sezione di Roma “Tor Vergata” - I-00133 Rome, Italy(25) Universita` di Roma “Tor Vergata” - I-00133 Rome, Italy(26) Universita` “La Sapienza” - I-00185 Rome, Italy(27) RMKI - Konkoly Thege Miklo´s u´t 29-33, H-1121 Budapest, Hungary(28) Universita` di Trento - I-38050 Povo, Trento, Italy(29) Universita` dell’Aquila - I-67100 L’Aquila, Italy(30) INFN, Gruppo Collegato di Trento - I-38050 Povo, Trento, Italy(31) University of Birmingham - Birmingham, B15 2TT, UK(32) Universita` di Perugia - I-06123 Perugia, Italy(33) University of Glasgow - Glasgow, G12 8QQ, UK(34) Bialystok University - 15-424 Bialystok, Poland(35) IM-PAN - 00-956 Warsaw, Poland(36) IPJ - 05-400 S´wierk-Otwock, Poland(37) ESPCI, CNRS - F-75005 Paris, France(38) Universita` di Siena - I-53100 Siena, ItalyTHE SEARCH FOR GRAVITATIONAL WAVES 109(39) Institute of Astronomy - 65-265 Zielona Go´ra, Poland(40) INFN, Sezione di Padova - I-35131 Padova, Italy(ricevuto l’ 8 Ottobre 2010)Summary. — In this paper we briefly review the status and the perspectives ofthe experimental search of gravitational waves, focusing on the ground based inter-ferometers. The current status of the running detectors and the plans to upgradethem are discussed.PACS 04.80.-y – Experimental studies of gravity.1. – IntroductionInterferometric detectors of gravitational waves (GW) have taken the baton passed bythe resonant bars (a type of detector greatly contributed by E. Amaldi [1]), continuing theeffort towards the GW discovery and the start of a GW astronomy. Today a world-widenetwork of interferometers is operating:– LIGO [2]: the US LIGO project is made of three interferometers. Two of themare co-located in the same facility at Hanford, WA, and are 4 km and 2 km long,respectively. The third one, 4 km arm length, is in Livingston, LA.– Virgo [3]: the French-Italian detector Virgo, 3 km arm length, is in Cascina, nearPisa.– GEO600 [4]: a smaller detector (600m arm length) has been realized near Hannoverby Germany and UK.The LIGO Scientific Collaboration (including the LIGO and GEO600 detectors) andVirgo have signed a Memorandum of Understanding for joint data taking and analysis.They operate as a “single machine”, thus enhancing the chances of detection.2. – StatusAfter years of commissioning the most sensitive detectors of the network (LIGO andVirgo) have achieved two remarkable results: they have reached the design sensitivityalmost on the entire frequency range together with a level of robustness that allows a dutycycle close to 90%. Such results have demonstrated the interferometer technology andconfirmed they are the right instruments for chasing the elusive GW. A joint science run,lasting more than 4 months, has been held in 2009. The network of interferometers haspushed the upper limits on gravitational wave amplitude emitted by several astrophysicalsources to the lowest level ever reached [5-9].With respect to the original design Virgo has undergone a set of upgrades aimed tofurther improve its sensitivity and stability (given the importance of such upgrades thedetector is now called Virgo+):110 T. ACCADIA, F. ACERNESE, F. ANTONUCCI, ETC.Fig. 1. – The Virgo+ monolithic payload: the 21 kg test mass is suspended through fused silicafibers.– The laser power has been increased to reduce the shot noise that limits the sensi-tivity in the high-frequency range.– In parallel, to cope with thermal aberrations induced by laser heating of the mirrors,it has been necessary to install a thermal compensation system, based on a CO2laser that heats the borders of the mirror, reducing the thermal lens created by themain laser.– In October 2009 Virgo was shut down to be upgraded with an important pieceof technology: the monolithic payloads (see fig. 1). New test masses have beensuspended by fused silica fibers instead of the usual steel wires [10], allowing toreduce considerably the suspension thermal noise, one of the main limiting noisesource in the low-frequency range. Though silica fibers are already used in GEO600,it is the first time ever that such an heavy mirror (21 kg) is suspended using thattechnology, with the possibility of improving the low-frequency sensitivity beatingthe Virgo suspension thermal noise limit.– The new test masses form Fabry-Pe´rot cavities with higher finesse (150 instead of50), enhancing the sensitivity in the mid-frequency range.3. – The case for better detectors: the second generationDespite the excellent sensitivity achieved, the detection of a gravitational wave event,even if possible, remains unlikely [11]. In order to enhance the detection probability toa level where several tens of events per year are expected, the sensitivity of the inter-ferometers needs to be improved by one order of magnitude. Such an improvement willincrease by a factor of a ∼ 1000 the number of galaxies explored. As a consequence therate of events of coalescence of binary neutron stars detected is expected to be in therange of 40/year. This number is affected by an uncertainty of plus or minus one orderof magnitude since it is based on the relatively small number of binary neutron starsobserved in our Galaxy.THE SEARCH FOR GRAVITATIONAL WAVES 111The coalescence of binary black holes is expected to be a powerful generator of grav-itational waves. In this case the number of events is based on purely theoretical models.According to the models the number of events per galaxy is smaller but, thanks to theirlarger masses, these events are visible to larger distances making the final statistics com-parable. The observation of the gravitational wave emitted by the final oscillating blackholes will provide the first direct observation of a signal generated by a black-hole gravityfield.Besides the search of coalescing binaries, the second generation detectors will targetseveral other gravitational waves sources both galactic and extragalactic: these includeperiodic waves from rotating neutron stars, as well as impulsive events from supernovaeand soft-gamma-ray repeaters. Electromagnetic counterparts are expected for severalsources. For this reason the advanced gravitational wave detectors will be able to producealerts and to share data with other kind of observatories such as gamma ray observatoriesor optical telescopes.The advanced detectors will also search for the gravitational wave background pro-duced at the epoch of the Big Bang. Initial LIGO has already bounded the energydensity of such stochastic background (normalized by the critical energy density of theUniverse), in the frequency band around 100Hz, to be Ω < 6.9 · 10−6 [6]. The advanceddetectors will be able to push this limit down to ∼ 10−9, thus exploring several Big Bangscenarios among which those based on cosmic strings.Second generation detectors are getting real: the upgrades of LIGO and Virgo tosecond generation detectors (Advanced LIGO and Advanced Virgo) have been fundedand the two projects are in construction phase.4. – Advanced VirgoAdvanced Virgo has been approved by INFN and CNRS at the end of 2009. Besidethe Virgo funding agencies, Advanced Virgo will be contributed by the Dutch laboratoryNIKHEF. The order for the first large procurement, consisting of all the mirror substrateswas placed immediately after the approval. The description of the Advanced Virgobaseline design can be found in [12]. The reference sensitivity for Advanced Virgo isshown in fig. 2.The sensitivity below 50Hz is limited by a combination of suspension thermal noiseand radiation pressure noise. The first is minimized by suspending the large mirrors(42 kg mass) with fused silica fibers similarly to what has been done for Virgo+. Around100Hz the main limitation to the sensitivity is due to the thermal noise in the mirrorcoatings. The sensitivity shown in the figure assumes current coating technology. So,any further improvements from the ongoing R&D will have a direct positive impact onthe detector sensitivity. The effect of coating thermal noise is minimized running theFabry-Pe´rot cavities near the co-focal configuration thus maximizing the beam size onthe mirrors.In order to increase the interferometer sensitivity at higher frequencies the laser inputpower will be around 200W (with 125W injected into the interferometer). The baselinedesign foresees the use of a solid state laser composed by a stabilized master laser followedby two stages of amplification. A possible alternative, currently being developed, is a fiberlaser amplifier. The first tests have shown that very good frequency and power stabilitiescan be achieved. To cope with the larger radiation pressure effects due to the larger powerinjected heavier test masses (42 kg) will be used, and as in Virgo+, will be suspendedthrough fused silica fibers. To this extent, the experience done with the new Virgo+112 T. ACCADIA, F. ACERNESE, F. ANTONUCCI, ETC.Fig. 2. – The reference Advanced Virgo sensitivity (solid black line) compared with the Virgodesign sensitivity (dashed black line). The main noise contributions are also shown.payloads will be precious to understand all the features of this technology, improve itfurther and reduce the risk for the advanced detectors.The optical scheme is modified by the introduction of a signal recycling cavity. Theuse of signal recycling and of the resonant sideband extraction scheme allows widening thedetector bandwidth thus extending the sensitivity into the kHz region. The sensitivityshown in fig. 2 assumes that the signal recycling is detuned by 0.15 rad such to havethe best sensitivity to binary neutron stars coalescences. In this case the coalescences ofbinary neutron stars will be visible up to distances of 144Mpc. Other tunings are possibleand can be used to target other sources. Also the finesse of the 3 km Fabry-Pe´rot cavitieswill be larger (∼ 900 in the baseline). A DC detection scheme will be adopted to reducesome technical noises.The vacuum system and the infrastructures will be also upgraded to improve thesensitivity in the low and mid frequency range. Large cryotraps will be installed at theextremes of the 3 km vacuum pipes to isolate them from the rest of the vacuum system.This, combined with the baking of the tubes, will allow reducing the residual gas pressureto the target of 10−9 mbar. To reduce the risk of acoustic couplings, the air conditioningmachines as well as several other sources of acoustic noise will be moved out of the centralexperimental hall.According to the present planning in mid 2011 Virgo+ will be shut down and theinstallation of Advanced Virgo will start. The commissioning of the interferometer willstart in 2014 with the goal of having the first data taking at low laser power in 2015.5. – Conclusive remarksInterferometric detectors of GW have proved to be able to reach the promised sensi-tivity and to be robust and reliable detectors. The current technology allows to increasetheir sensitivity by ∼ 10, thus increasing the detection rate by ∼ 1000. Advanced de-tectors are now in construction and are expected to be taking data in 2015. Discoveringthe GW could be a fantastic way to celebrate the centennial of the 1916-18 Einsteinpapers [13,14] predicting their existence.THE SEARCH FOR GRAVITATIONAL WAVES 113REFERENCES[1] Amaldi E. et al., Astron. Astrophys., 216 (1989) 325.[2] Abbot B. et al. (the LIGO Scientific Collaboration), Rep. Prog. Phys., 72 (2009)076901.[3] Acernese F. et al. (the Virgo Collaboration), Class. Quantum Grav., 25 (2009)114045.[4] Grote H. (for the LIGO Scientific Collaboration), Class. Quantum Grav., 25(2009) 114043.[5] Abadie J. et al. (The LIGO Collaboration and The Virgo Collaboration), Phys.Rev. D, 81 (2010) 102001.[6] The LIGO Collaboration and The Virgo Collaboration, Nature, 460 (2009) 990.[7] Abbott B. P. et al. (The LIGO Collaboration and The Virgo Collaboration),Astrophys. J., 713 (2010) 671.[8] Abbott B. P. et al. (The LIGO Collaboration and The Virgo Collaboration),Astrophys. J., 715 (2010) 1438.[9] Abadie J. et al. (The LIGO Collaboration and The Virgo Collaboration),Astrophys. J., 715 (2010) 1453.[10] Accadia T. et al. (the Virgo Collaboration), Progresses in the Realization of aMonolithic Suspension System in Virgo, to appear in the Proceedings of 12th MarcelGrossmann Meeting on General Relativity (MG 12), Paris, France, 12-18 Jul 2009.[11] Abadie J. et al. (The LIGO Collaboration and The Virgo Collaboration), Class.Quantum Grav., 27 (2010) 173001.[12] The Virgo Collaboration, Virgo Internal report VIR-027A-09, https://pub3.ego-gw.it/itf/tds/file.php?callFile=VIR-0027A-09.pdf (2009).[13] Einstein A., Sitzungsber. Ko¨n. Preuss. Akad. Wiss. (1916) 688.[14] Einstein A., Sitzungsber. Ko¨n. Preuss. Akad. Wiss. (1918) 154.

The search for gravitational waves

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The search for gravitational waves

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