How to listen to the Universe?

Transcription

1 How to listen to the Universe? Optimising future GW observatories for astrophysical sources Stefan Hild NIKHEF, May 2009

2 Overview Microphones to detect gravitational waves Why haven t we heard GW so far? How does a microphone for GW work? Michelson interferometer: a brief history Optimisation of the Advanced Virgo sensitivity for astrophysical sources How can we optimise our microphones? What is quantum noise? Signal Recycling and optical rigidity Which is the most promising source for the first detection? When will we hear the first tones from the Universe? Einstein Telescope: The future microphone Stefan Hild Amsterdam, May 2009 Slide 2

3 Looking at a dark spot in the sky For ages mankind has been looking towards the stars wondering about the origin of the Earth and the whole Universe. Today we know the Universe is a zoo of exciting phenomena. Stefan Hild Amsterdam, May 2009 Slide 3

4 Gravitational waves: A new way of exploring the Universe Nearly all of our current knowledge of the cosmos is based on observation of electromagnetic radiation (visible light, radio astronomy, infrared,...). Gravitational astronomy can provide completely new insight to the universe: Multimessenger observations: We can learn more about things we already see in the electromagnetic spectrum by also seeing their GW emission (for instance supernovae). Exclusive GW observations: There are objects that can only be seen by their GW emission Stefan Hild Amsterdam, May 2009 Slide 4

5 Gravitational Waves: Ripples in space time GW are consequence of General Relativity. GW are caused by asymmetric accelerated masses. GW change the metric of space time. Quadrupole waves. We know that GW exist: Indirect detection by Taylor and Hulse (1993 Nobel Price). No direct detection so far. On going search with kilometer-long Michelson interfero-meters looking for tiny length changes. Stefan Hild Amsterdam, May 2009 Slide 5

6 Why haven t we heard GW so far? Stress Energy Tensor Metric Tensor Analogon: Hooke s law Stiffness of space time Space time is extremely stiff! Length changes are really tiny (<10-21 )! Stefan Hild Amsterdam, May 2009 Slide 6

7 How can we detect gravitational waves? A Michelson interferometer is the ideal instrument to measure relative length changes. L 1 = L 2 L 1 > L 2 L 1 = L 2 L 1 < L 2 L 1 = L 2 destructive output destructive output destructive Stefan Hild Amsterdam, May 2009 Slide 7

8 Interaction of GW and laser light TT-gauge: Test masses do not move => but GW changes the distance between test masses: Only considering x-arm: Travel time in x-arm Stefan Hild Amsterdam, May 2009 Slide 8

9 Interaction of GW and laser light (2) Phase Laser freq Assumption of GW signal GW amplitude GW freq Phase shift Produced by GW Geometry term Stefan Hild Amsterdam, May 2009 Slide 9

10 Optimal arm length Maximum Signal: =1 Optimal Arm length: GW wavelength Example: GW signal at 100 Hz => optimal arm length of 750 km (!!) For short arms: develop sine term Signal proportional to h 0, w 0, L Signal independent from GW frequency Stefan Hild Amsterdam, May 2009 Slide 10

11 Going back to the starting point The first Michelson interferometer: Experiment performed by Albert Michelson in Potsdam Measurement accuracy 0.02 fringe (expected Ether effect ~0.04 fringes) Outcome: Not conclusive Stefan Hild Amsterdam, May 2009 Slide 11

12 Michelson in Cleveland, Ohio 2nd attempt in 1887, together with Morley. Increased optical pathlength (multiply-folded arms) Improved seismic isolation: Mercury bath (also stopping traffic around the laboratory building). Stefan Hild Amsterdam, May 2009 Slide 12

13 The first science derived from an Michelson interferometer Measurement accuracy 0.01 fringes, expected Ether effect ~0.4 fringes Stefan Hild Amsterdam, May 2009 Slide 13

14 Michelson Interferometer for GW detection 1970s: Weiss/Forward: first idea and realisation of a Michelson-based gravitational-wave detector Sensitivity: 10-8 of a fringe Stefan Hild Amsterdam, May 2009 Slide 14

15 State-of-the-art Michelson Stefan Hild Amsterdam, May 2009 Slide 15

16 Today s network of GW detectors Today: LIGO, GEO600 and Virgo Sensitivity: of a fringe GEO600: measures the 600m long arms to an accuracy of proton 500 Hz S. Hild for the LSC: The Status of GEO600, Class. Quantum Gravity 23 (2006) Amsterdam, May 2009 Slide 16

17 Status and future of GW observatories 1st generation successfully completed: Long duration observations (~1yr) in coincidence mode of 5 oberservatories. Spin-down upper limit of the Crab- Pulsar beaten! 2nd generation on the way: End of design phase, construction about to start (or even started) 10 times better sensitivity than 1st generation. => Scanning 1000 times larger volume of the Universe 3rd generation at the horizon: FP7 funded design study 100 times better sensitivity than 1st generation. => Scanning times larger volume of the Universe Stefan Hild Amsterdam, May 2009 Slide 17

18 Overview Microphones to detect gravitational waves Why haven t we heard GW so far? How does a microphone for GW work? Michelson interferometer: a brief history Optimisation of the Advanced Virgo sensitivity for astrophysical sources How can we optimise our microphones? What is quantum noise? Signal Recycling and optical rigidity Which is the most promising source for the first detection? When will we hear the first tones from the Universe? Einstein Telescope: The future microphone Stefan Hild Amsterdam, May 2009 Slide 18

19 Overview of Advanced Virgo The Virgo is currently the second largest gravitational wave detector in the world (3km). Advanced Virgo will be the 2nd generation upgrade. Main new techniques: Signal recycling, high optical power, non-degerate recycling cavities, monolithic suspension. Thermal compensation and DC-readout. Start of Construction in 2009, Design sensitivity in 2015(?) Stefan Hild Amsterdam, May 2009 Slide 19

20 Optical system design for Advanced Virgo Focus of my current work: Optical design of the Advanced Virgo core interferometer. Advanced Virgo core interferometer Some examples of the topics we are working on: Definition of the optical configuration Optimisation of the sensitivity curve System integrity and interfaces to all other subsystems of Advanced Virgo Topic for the next minutes: How to optimise the Advanced Virgo sensitivity? Stefan Hild Amsterdam, May 2009 Slide 20

21 How to listen to the Universe? Binary Neutron Star inspiral Supernova Pulsar Advanced Virgo is a hyper-sensitivity microphone to listen to the Universe. Each astrophysical source has its own sound or tone. This microphone can be tuned similar to a radio receiver. Stefan Hild Amsterdam, May 2009 Slide 21

22 Fundamental noise limits for Advanced Virgo Advanced Virgo will be limited by quantum noise at nearly all frequencies of interest. GOAL: Optimise quantum noise for maximal science output. Stefan Hild Amsterdam, May 2009 Slide 22

23 Limits of the optimization Our optimisation is limited by Coating thermal noise and Gravity Gradient noise. Quantum noise to be optimised! Stefan Hild Amsterdam, May 2009 Slide 23

24 What is quantum noise? Quantum noise is comprised of photon shot noise at high frequencies and photon radiation pressure noise at low frequencies. The photons in a laser beam are not equally distributed, but follow a Poisson statistic. wavelength Mirror mass Arm length optical power photon radiation pressure noise photon shot noise Stefan Hild Amsterdam, May 2009 Slide 24

25 The Standard Quantum Limit (SQL) While shot noise contribution decreases with optical power, radiation pressure level increases: wavelength optical power Mirror mass Arm length The SQL is the minimal sum of shot noise and radiation pressure noise. Using a classical quantum measurement the SQL represents the lowest achievable noise. V.B. Braginsky and F.Y. Khalili: Rev. Mod. Phys. 68 (1996) Stefan Hild Amsterdam, May 2009 Slide 25

26 Advanced Virgo optical layout We have three knobs available for optimisation: knob 3 Input Light power knob 1 knob 2 Signal Recycling resonance frequency microscopic position of SRM1 (nm scale) optical transmittance of SRM1 Signal Recycling bandwidth Stefan Hild Amsterdam, May 2009 Slide 26

27 Optimization Parameter 1: Signal-Recycling (de)tuning Advanced Virgo, Power = 125W, SR-transmittance = 4% knob 1 Photon radiation pressure noise Photon shot noise Opto-mechanical Resonance (Optical spring) Pure optical resonance Frequency of pure optical resonance goes down with SR-tuning. Frequency of opto-mechanical resonance goes up with SR-tuning Stefan Hild Amsterdam, May 2009 Slide 27

28 Optical Springs & Optical Rigidity Detuned cavities can be used to create optical springs. Optical springs couple the mirrors of a cavity with a spring constant equivalent to the stiffness of diamond. In a full Michelson interferometer detuned Signal Recycling causes an optical spring resonance. Stefan Hild Amsterdam, May 2009 Slide 28

29 Optimization Parameter 2: Signal-Recycling mirror transmittance Advanced Virgo, Power = 125W, SR-tuning = 0.07 knob 2 Resonances are less developed for larger SR transmittance. Stefan Hild Amsterdam, May 2009 Slide 29

30 Optimization Parameter 3: Laser-Input-Power Advanced Virgo, SR-tuning=0.07, SR-transmittance = 4% knob 3 High frequency sensitivity improves with higher power (Shotnoise) Low frequency sensitivity decreases with higher power (Radiation pressure noise) Stefan Hild Amsterdam, May 2009 Slide 30

31 Figure of merit: Inspiral Inspiral ranges for BHBH and NSNS coalesence: Total mass Symmetric mass ratio Frequency of last stable orbit (BNS = 1570 Hz, BBH = 220 Hz) Spectral weighting = f -7/3 [1] Damour, Iyer and Sathyaprakash, Phys. Rev. D 62, (2000). [2] B. S. Sathyaprakash, Two PN Chirps for injection into GEO, GEO Internal Document Detector sensitivity Parameters usually used: NS mass = 1.4 solar masses BH mass = 10 solar masses SNR = 8 Averaged sky location Stefan Hild Amsterdam, May 2009 Slide 31

32 Example: Optimizing 2 Parameters Inspiral ranges for free SR-tuning and free SRMtransmittance, but fixed Input power NSNS-range BHBH-range Stefan Hild Amsterdam, May 2009 Slide 32

33 Example: Optimizing 2 Parameters Parameters for maximum Maximum NSNS-range Maximum BHBH-range Parameters for maximum Different source usually have their maxima at different operation points. It is impossible to get the maximum for BNS AND BBH both at the same time! Stefan Hild Amsterdam, May 2009 Slide 33

34 Example: Optimizing 3 Parameter for Inspiral range Scanning 3 parameter at the same time: SR-tuning SR-trans Input Power Using a video to display 4th dimension. Stefan Hild Amsterdam, May 2009 Slide 34

35 Optimal configurations Curves show the optimal sensitivity for a single source type. Stefan Hild Amsterdam, May 2009 Slide 35

36 Which is the most promising source? Binary neutron star inspirals: Based on observations of existing binary stars Based on models of binary star formation and evolution Expected event rates seen by Advanced Virgo: ~1 to 10 events per year. Binary neutron star inspirals are chosen to be the primary target for Advanced Virgo. Binary black hole inspirals: Binary neutron star inspirals: C.Kim, V.Kalogera and D.Lorimer: Effect of PSRJ on the DNS Merger Rate and Implications for GW Detection, astro-ph: abs/astro-ph/ K.Belczynski, R.E.Taam, V.Kalogera, F.A.Rasio, T.Buli:, On the rarity of double black hole binaries: consequences for gravitational-wave detection, The Astrophysical Journal 662:1 (2007) Stefan Hild Amsterdam, May 2009 Slide 36

37 When will we detect gravitational waves?? When Advanced Virgo and Advanced Ligo come online WE WILL SEE GRAVITATIONAL WAVES! if not, then something is completely wrong with our understanding of General Relativity. Stefan Hild Amsterdam, May 2009 Slide 37

38 Overview Microphones to detect gravitational waves Why haven t we heard GW so far? How does a microphone for GW work? Michelson interferometer: a brief history Optimisation of the Advanced Virgo sensitivity for astrophysical sources How can we optimise our microphones? What is quantum noise? Signal Recycling and optical rigidity Which is the most promising source for the first detection? When will we hear the first tones from the Universe? Einstein Telescope: The future microphone Stefan Hild Amsterdam, May 2009 Slide 38

39 Start around 2020(?) Underground location ~30km integrated tunnel length (?) Myriads of new possibilities and challenges!! Plenty of new Science NIKHEF, 08 Stefan Hild Amsterdam, May 2009 Slide 39

40 Tackling Gravity Gradient noise: going underground Ohasi et al: Class. Quantum Grav. 20 (2003) S Fiori et al: VIR-NOT-PIS Surface (Pisa) Underground (Kamioka) about about Stefan Hild Amsterdam, May 2009 Slide 40

41 How to achieve the ambitious sensitivity? Start Result Stefan Hild Amsterdam, May 2009 Slide 41

42 Xyolophone: More than one detector to cover the full bandwidth Low Frequency IFO: low optical power, cryogenic test masses, sophisticated low frequency suspension, underground, heavy test masses. High Frquency IFO: high optical power, room temperature, surface location, squeezed light Stefan Hild Amsterdam, May 2009 Slide 42

43 If we do a good job over the next few years then will soon listen to the symphony of the Universe!! Stefan Hild Amsterdam, May 2009 Slide 43

44 END