The Einstein Telescope: a third-generation gravitational wave observatory

TL;DR: The third-generation ground-based observatory Einstein Telescope (ET) project as discussed by the authors is currently in its design study phase, and it can be seen as the first step in this direction.

Abstract: Advanced gravitational wave interferometers, currently under realization, will soon permit the detection of gravitational waves from astronomical sources. To open the era of precision gravitational wave astronomy, a further substantial improvement in sensitivity is required. The future space-based Laser Interferometer Space Antenna and the third-generation ground-based observatory Einstein Telescope (ET) promise to achieve the required sensitivity improvements in frequency ranges. The vastly improved sensitivity of the third generation of gravitational wave observatories could permit detailed measurements of the sources' physical parameters and could complement, in a multi-messenger approach, the observation of signals emitted by cosmological sources obtained through other kinds of telescopes. This paper describes the progress of the ET project which is currently in its design study phase.

Summary

1. Introduction

• Interferometric gravitational wave (GW) detectors have demonstrated the validity of their working principle by coming close to, or even exceeding, the design sensitivity of the initial instruments: LIGO [1] , Virgo [2] , GEO600 [3] and TAMA [4] .
• Hz-a frequency range that is not accessible from ground.
• These detectors, based on technologies currently available, and partly already tested in reduced-scale prototypes, but still to be implemented in full scale, will show a sensitivity improved roughly by a factor of 10 with respect to the initial interferometers.
• This consideration led the GW community to investigate the possibility of building a new generation of detectors, permitting both to observe, with huge SNR, GW sources at distances similar to those detectable in the advanced detectors and to reveal GW signals at distances comparable with the sight distance of electromagnetic telescopes.
• But, mainly, new research facilities hosting the third-generation GW observatory apparatuses must be realized, to circumvent the limitations imposed by the current facilities.

2. ET science reach

• This is by no means the final design goal but it sets the stage for studying what science questions can Figure 1 .
• It is worth underlining that the evaluation of the possible noise level of a third-generation GW observatory is an ongoing activity, still far from being concluded within the ET design study.
• (i) Observation of compact binary coalescences would allow accurate measurement of the masses of neutron stars and masses and spins of black holes [11, 12] .
• For binaries at a given distance, ET will measure masses and spins an order of magnitude better than advanced detectors.
• The formation, abundance and coalescence rates of such systems are highly uncertain although it is plausible that intermediate mass black holes could be seeds of massive black holes that are now found at galactic nuclei, but they could also form in dense star clusters or by other means.

3. Challenges for data analysis and the need for new search algorithms

• A detector with a sensitivity window and span as ET will pose new data analysis challenges.
• As in the case of LISA, there will be many classes of sources all visible at the same time, requiring a paradigm shift in the way data are currently being analysed.
• Most of the following issues are relevant whatever the data analysis method followed, but more so in the case of a matched filtering search, e.g. for binary inspiral signals with a bank of templates.
• Extrapolating the nominal rate of about one neutron star binary merger event per year within a distance of 100 Mpc [7] to the distance reach of ET of about 20 Gpc, one expects to detect an event about every 6 s. Even so, merger signals from BNS and other compact binaries will be observed quite frequently in ET.

4.1. Seismic and gravity gradient noise reduction

• The seismic noise affects the sensitivity at low frequency of the current GW interferometric detectors.
• In the Virgo detector, the so-called super attenuator (SA) [23] has shown its capability to filter the seismic noise below the expected thermal noise.
• The gravity gradient noise is due to the direct coupling of the suspended test-mass displacement with the mass vibration in the soil layers, perturbed by seismic waves, via the mutual attraction force expressed by Newton's universal law of gravitation [26] [27] [28] .
• Obviously the importance of this disturbance depends on the seismic noise level and on the contribution of the other low-frequency noise sources to the noise budget.
• In the third generation of GW detectors, the more stringent requirements in terms of sensitivity at low frequency enhance the importance of this noise source and enforce the need to realize the ET in an underground and quiet site.

4.2. Thermal noise reduction

• Under the 'thermal noise' label are grouped all those processes that modulate the optical path of the light in the interferometer coupling it to the Brownian fluctuation or to the stochastic fluctuation of the temperature field in the optical components.
• Usually, one distinguishes between the suspension thermal noise, affecting the position of the test masses through the fluctuations of the suspension wires or fibres, and the mirror thermal noise, which is the sum of all the overlapping fluctuation and dissipation processes occurring in the test masses and in its high-reflectivity coatings.
• Hence, cryogenics is one of the most appealing technologies to reduce the thermal noise of the optics suspension in the third generation of GW observatories.
• The design of the cryogenic suspension and of its cooling system is progressing in the ET design study and possible material candidates for the test masses and suspension fibres have been identified in sapphire (as already done [29, 30] for LCGT) and silicon [8, 31, 32] .

4.3. Quantum noise reduction

• Quantum noise in interferometric GW detectors can be understood as the coupling of vacuum fluctuations with the optical readout fields inside the interferometer.
• The phase uncertainty directly spoils the phase measurement of the Michelson interferometer; this effect is called shot noise.
• The amplitude uncertainty, or in other words, the changing amplitude of the light, will cause the light pressure on the test masses to change, which correspondingly causes motions of the test masses; this effect is called radiation pressure noise.
• Techniques to improve the sensitivity beyond the SQL are called quantum noise reduction (QNR) or somewhat misleadingly quantum non-demolition (QND) schemes.
• This method would benefit greatly from a 'xylophone' approach (see section 4.4).

4.4. Multiple interferometer detector

• As described in the previous subsections, to realize a third-generation GW detector, the technologies currently operative in the initial detectors and planned for the advanced detectors must be further advanced and new solutions must be adopted.
• The cross-compatibility between the different solutions becomes a crucial issue; for example, the requirements imposed by the reduction of the quantum noise conflicts with those imposed by the thermal noise suppression.
• This technological difficulty in realizing a single wide-band third-generation detector can be avoided.
• The base line currently favoured in the ET design study [34] is a combination of two interferometers, specialized on different frequency bands: the so-called xylophone philosophy [35] .
• The former could be a cryogenic interferometer at an underground site, with long suspensions, but moderate optical power, whereas the highfrequency interferometer could essentially be a long arm advanced detector, implementing squeezed light states, a very high-power laser and large test masses.

Figures

Figure 1. A possible sensitivity (solid curve) of an underground, long suspension, cryogenic, signal and power recycled single third-generation GW observatory (see table 1 in [9]) compared with a typical sensitivity curve of an advanced detector (dashed curve). It is worth underlining that the evaluation of the possible noise level of a third-generation GW observatory is an ongoing activity, still far from being concluded within the ET design study. For this reason the curves are updated regularly and labelled with progressive letters to be distinguished. In the solid curve (so-called ET-B), corresponding to a single wide-band detector, the suspension thermal noise contribution is not yet included.

Figure 3. Sensitivity of a third-generation GW observatory implemented by two frequencyspecialized (LF and HF) detectors (xylophone topology [34], curve so-called ET-C), with respect to a single wide frequency range interferometer ET implementation [9].

Figure 2. Duration of inspiral signals from binaries of total mass (as given by the x-axis) and mass m2 of one of the component stars (as in the legend). Signals from BNS will last for about 9 days in the detector band. Many signals will last for more than a day but we will also have very short duration signals from intermediate mass black hole binaries. Curves are shown for the mass of one of the components varying from 1 M to 1024 M , increasing by a factor of 2 at each step.

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The Einstein Telescope: a third-generation gravitational wave observatory
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IOP PUBLISHING CLASSICAL AND QUANTUM GRAVITY
Class. Quantum Grav. 27 (2010) 194002 (12pp) doi:10.1088/0264-9381/27/19/194002
The Einstein Telescope: a third-generation
gravitational wave observatory
M Punturo
1,2
, M Abernathy
3
, F Acernese
4,5
, B Allen
6
, N Andersson
7
,
K Arun
8
, F Barone
4,5
, B Barr
3
, M Barsuglia
9
,MBeker
10
, N Beveridge
3
,
S Birindelli
11
,SBose
12
,LBosi
1
, S Braccini
13
, C Bradaschia
13
, T Bulik
14
,
E Calloni
4,15
, G Cella
13
, E Chassande Mottin
9
, S Chelkowski
16
,
A Chincarini
17
, J Clark
18
, E Coccia
19,20
, C Colacino
13
,JColas
2
,
A Cumming
3
, L Cunningham
3
, E Cuoco
2
, S Danilishin
21
, K Danzmann
6
,
G De Luca
22
, R De Salvo
23
, T Dent
18
, R De Rosa
4,15
,LDiFiore
4,15
,
A Di Virgilio
13
, M Doets
10
, V Fafone
19,20
, P Falferi
24
, R Flaminio
25
,
J Franc
25
, F Frasconi
13
,AFreise
16
,PFulda
16
,JGair
26
, G Gemme
17
,
A Gennai
16
, A Giazotto
2,13
, K Glampedakis
27
, M Granata
9
,HGrote
6
,
G Guidi
28,29
, G Hammond
3
, M Hannam
30
, J Harms
31
, D Heinert
32
,
M Hendry
3
, I Heng
3
, E Hennes
10
, S Hild
3
, J Hough
4
,SHusa
33
,
S Huttner
3
, G Jones
18
, F Khalili
21
, K Kokeyama
16
, K Kokkotas
27
,
B Krishnan
33
, M Lorenzini
28
,HL
¨
uck
6
, E Majorana
34
, I Mandel
35,36
,
V Mandic
31
, I Martin
3
, C Michel
25
, Y Minenkov
19,20
, N Morgado
25
,
S Mosca
4,15
, B Mours
37
,HM
¨
uller–Ebhardt
6
, P Murray
3
, R Nawrodt
3
,
JNelson
3
, R Oshaughnessy
38
,CDOtt
39
,CPalomba
34
, A Paoli
2
,
G Parguez
2
, A Pasqualetti
2
, R Passaquieti
13,40
, D Passuello
13
,
L Pinard
25
, R Poggiani
13,40
, P Popolizio
2
, M Prato
17
, P Puppo
34
,
D Rabeling
10
, P Rapagnani
34,41
, J Read
33
, T Regimbau
11
, H Rehbein
6
,
SReid
3
, L Rezzolla
33
, F Ricci
34,41
, F Richard
2
, A Rocchi
19
, S Rowan
3
,
AR
¨
udiger
6
, B Sassolas
25
, B Sathyaprakash
18
, R Schnabel
6
,
C Schwarz
42
, P Seidel
42
, A Sintes
33
, K Somiya
39
, F Speirits
3
, K Strain
3
,
S Strigin
21
, P Sutton
18
, S Tarabrin
21
,ATh
¨
uring
6
, J van den Brand
10
,
C van Leewen
10
, M van Veggel
3
, C van den Broeck
18
, A Vecchio
16
,
JVeitch
16
, F Vetrano
28,29
, A Vicere
28,29
, S Vyatchanin
21
, B Willke
6
,
G Woan
3
, P Wolfango
43
and K Yamamoto
6
1
INFN, Sezione di Perugia, I-6123 Perugia, Italy
2
European Gravitational Observatory (EGO), I-56021 Cascina (Pi), Italy
3
Department of Physics and Astronomy, The University of Glasgow, Glasgow, G12 8QQ, UK
4
INFN, Sezione di Napoli, Italy
5
Universit
`
a di Salerno, Fisciano, I-84084 Salerno, Italy
6
Max-Planck-Institut f
¨
ur Gravitationsphysik, D-30167 Hannover, Germany
7
University of Southampton, Southampton SO17 1BJ, UK
8
LAL, Universit
´
e Paris-Sud, IN2P3/CNRS, F-91898 Orsay, France
9
AstroParticule et Cosmologie (APC), CNRS; Observatoire de Paris-Universit
´
e Denis
Diderot-Paris VII, France
10
VU University Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam, The Netherlands
11
Universit
´
e Nice ‘Sophia-Antipolis’, CNRS, Observatoire de la C
ˆ
ote d’Azur, F-06304 Nice,
France
12
Washington State University, Pullman, WA 99164, USA
13
INFN, Sezione di Pisa, Italy
0264-9381/10/194002+12$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA 1

Class. Quantum Grav. 27 (2010) 194002 M Punturo et al
14
Astro. Obs. Warsaw Univ. 00-478; CAMK-PAM 00-716 Warsaw; Bialystok Univ. 15-424; IPJ
05-400 Swierk–Otwock; Inst. of Astronomy 65-265 Zielona Gora, Poland
15
Universit
`
a di Napoli ‘Federico II’, Complesso Universitario di Monte S. Angelo, I-80126
Napoli, Italy
16
University of Birmingham, Birmingham, B15 2TT, UK
17
INFN, Sezione di Genova, I-16146 Genova, Italy
18
Cardiff University, Cardiff, CF24 3AA, UK
19
INFN, Sezione di Roma Tor Vergata I-00133 Roma, Italy
20
Universit
`
a di Roma Tor Vergata, I-00133, Roma, Italy
21
Moscow State University, Moscow, 119992, Russia
22
INFN, Laboratori Nazionali del Gran Sasso, Assergi l’Aquila, Italy
23
LIGO, California Institute of Technology, Pasadena, CA 91125, USA
24
INFN, Gruppo Collegato di Trento, Sezione di Padova; Istituto di Fotonica e Nanotecnologie,
CNR-Fondazione Bruno Kessler, 38123 Povo, Trento, Italy
25
Laboratoire des Mat
´
eriaux Avanc
´
es (LMA), IN2P3/CNRS, F-69622 Villeurbanne, Lyon,
France
26
University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
27
Theoretical Astrophysics (TAT) Eberhard-Karls-Universit
¨
at T
¨
ubingen, Auf der Morgenstelle
10, D-72076 T
¨
ubingen, Germany
28
INFN, Sezione di Firenze, I-50019 Sesto Fiorentino, Italy
29
Universit
`
a degli Studi di Urbino ‘Carlo Bo’, I-61029 Urbino, Italy
30
Department of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
31
University of Minnesota, Minneapolis, MN 55455, USA
32
Friedrich-Schiller-Universit
¨
at Jena PF 07737 Jena, Germany
33
Max Planck Institute for Gravitational Physics (Albert Einstein Institute) Am M
¨
uhlenberg 1,
D-14476 Potsdam, Germany
34
INFN, Sezione di Roma 1, I-00185 Roma, Italy
35
Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
36
NSF Astronomy and Astrophysics Postdoctoral Fellow
37
LAPP-IN2P3/CNRS, Universit
´
e de Savoie, F-74941 Annecy-le-Vieux, France
38
The Pennsylvania State University, University Park, PA 16802, USA
39
Caltech–CaRT, Pasadena, CA 91125, USA
40
Universit
`
a di Pisa, I-56127 Pisa, Italy
41
Universit
`
a ‘La Sapienza’, I-00185 Roma, Italy
42
INFN, Sezione di Roma Tre and Universit
`
a di Roma Tre—Dipartimento di Fisica, I-00146
Roma, Italy
43
Universit
`
a degli Studi di Firenze, I-50121, Firenze, Italy
E-mail: michele.punturo@pg.infn.it
Received 19 May 2010, in final form 22 June 2010
Published 21 September 2010
Online at stacks.iop.org/CQG/27/194002
Abstract
Advanced gravitational wave interferometers, currently under realization,
will soon permit the detection of gravitational waves from astronomical
sources. To open the era of precision gravitational wave astronomy, a further
substantial improvement in sensitivity is required. The future space-based
Laser Interferometer Space Antenna and the third-generation ground-based
observatory Einstein Telescope (ET) promise to achieve the required sensitivity
improvements in frequency ranges. The vastly improved sensitivity of the
third generation of gravitational wave observatories could permit detailed
measurements of the sources’ physical parameters and could complement, in a
multi-messenger approach, the observation of signals emitted by cosmological
sources obtained through other kinds of telescopes. This paper describes the
progress of the ET project which is currently in its design study phase.
2

Class. Quantum Grav. 27 (2010) 194002 M Punturo et al
PACS number: 40.80.Nn
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Interferometric gravitational wave (GW) detectors have demonstrated the validity of their
working principle by coming close to, or even exceeding, the design sensitivity of the initial
instruments: LIGO [1], Virgo [2], GEO600 [3] and TAMA [4]. In the same infrastructures,
currently hosting the initial GW detectors (and their limited upgrades, called ‘enhanced’
interferometers: eLIGO and Virgo+) a second generation of interferometers (so-called
advanced detectors: ‘Advanced LIGO’ [5], ‘Advanced Virgo’ [6] and GEO-HF [3]) will
be implemented over the next few years. The Laser Interferometer Space Antenna (LISA), a
joint ESA–NASA mission expected to fly around 2020, is a space-borne detector to observe
in the frequency range of 0.1 mHz–0.1 Hz—a frequency range that is not accessible from
ground. These detectors, based on technologies currently available, and partly already tested
in reduced-scale prototypes, but still to be implemented in full scale, will show a sensitivity
improved roughly by a factor of 10 with respect to the initial interferometers. Hence, a
detection rate about a factor of 1000 larger than with the initial interferometers is expected,
strongly enhancing the probability of detecting the signals generated by astro-physical sources.
In particular, considering the predicted detection rate of the GW signal generated by a binary
system of coalescing neutron stars [7], the sensitivity of the advanced interferometers is
expected to guarantee the detection within months to a year at most.
Apart from extremely rare events, the signal-to-noise ratio (SNR) of detections in the
‘advanced’ detectors is likely to be still too low for precise astronomical studies of the GW
sources and for complementing optical and x-ray observations in the study of fundamental
systems and processes in the Universe. This consideration led the GW community to
investigate the possibility of building a new (third) generation of detectors, permitting both
to observe, with huge SNR, GW sources at distances similar to those detectable in the
advanced detectors and to reveal GW signals at distances comparable with the sight distance of
electromagnetic telescopes. As LISA will do for super-massive black holes (M  10
6
M
Sun
),
the Einstein Telescope (ET), thanks to this capability to inspect the GW signal in great detail,
could herald a new era of routine GW astronomy for lighter astrophysical bodies.
To realize a third-generation GW observatory, with a significantly enhanced sensitivity
(considering a target of a factor of 10 improvement over advanced detectors in a wide frequency
range), several limitations of the technologies adopted in the advanced interferometers must be
overcome and new solutions must be developed to reduce the fundamental and technical noises
that will limit the next-generation detectors. But, mainly, new research facilities hosting the
third-generation GW observatory apparatuses must be realized, to circumvent the limitations
imposed by the current facilities. Hereafter, we will describe some of the possible scientific
goals and some of the challenges of a third-generation GW observatory, as evaluated within
the framework of the ET design study [8].
2. ET science reach
In figure 1 we plot a possible sensitivity curve of a third-generation GW detector [9]. This is
by no means the final design goal but it sets the stage for studying what science questions can
3

Class. Quantum Grav. 27 (2010) 194002 M Punturo et al
Figure 1. A possible sensitivity (solid curve) of an underground, long suspension, cryogenic, signal
and power recycled single third-generation GW observatory (see table 1 in [9]) compared with a
typical sensitivity curve of an advanced detector (dashed curve). It is worth underlining that the
evaluation of the possible noise level of a third-generation GW observatory is an ongoing activity,
still far from being concluded within the ET design study. For this reason the curves are updated
regularly and labelled with progressive letters to be distinguished. In the solid curve (so-called
ET-B), corresponding to a single wide-band detector, the suspension thermal noise contribution is
not yet included.
be addressed with a third-generation detector. A detector with a sensitivity a factor 10 better
than an advanced detector will open a new avenue for understanding the physical phenomena
of extreme objects in the Universe. The study team has started putting together a vision
document [10] detailing the scope of such a detector. Here we list a few examples of the
science questions we can expect to pose with ET.
(i) Observation of compact binary coalescences would allow accurate measurement of the
masses of neutron stars and masses and spins of black holes [11,12]. For instance, for
binaries at a given distance, ET will measure masses and spins an order of magnitude
better than advanced detectors. More importantly, it should be possible to determine the
component masses of binaries to better than 5% (except when the component objects are
of comparable masses) over a wide range of masses from a few solar masses to hundreds
of solar masses. From a population of such observations, it will be possible to infer the
maximum mass of a neutron star (a long-standing open problem in theoretical physics)
and constrain its equation of state [10]. The way this can be done is as follows: it is widely
believed that short, hard gamma-ray bursts (shGRBs) are triggered by the coalescence of
a compact binary in which one of the stars is a neutron star and the other a neutron star
or a black hole. If this is the case, then one can reliably assume that the lighter of the
components of a binary coalescence observed in coincidence is definitely a neutron star.
A large enough sample should then give the mass function of neutron stars and tell us
where the cutoff in the mass distribution is.
(ii) Advanced detectors should make the first coincident observations of binary mergers and
shGRBs. One might not accumulate a sufficiently large population of such events with
advanced detectors to fully understand the population of GRBs and their precursors.
Advanced detectors could shed light on GRB progenitors (an outstanding problem
4