Executive Summary




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Proposal for UK Involvement in Advanced LIGO

Executive Summary



It is essential to enhance of the sensitivities of the initial long baseline detectors (LIGO, GEO 600, TAMA 300, and VIRGO) to enter the observatory phase of gravitational wave astronomy. A factor of ten in detection range (a thousand in detection volume) over the initial detectors would bring a great variety and number of sources within reach.

The most efficient way to gain this sensitivity a factor of ten in detection range (a thousand in detection volume) over the initial detectors is to transfer the technology developed for GEO 600 - monolithic suspensions for the test masses and enhanced interferometric techniques - to the three LIGO detectors at the sites in Washington State and Louisiana to create “Advanced LIGO”.

This transfer is currently underway and the UK gravitational wave groups at Glasgow, Cardiff and Birmingham have very strong collaborative relationships with the LIGO project.

This proposal is a request for approximately £6M to supply Advanced LIGO with:

  • technologically advanced suspension systems

  • electronic sensing/actuation systems for suspension control

  • high quality sapphire/silica blanks for the main mirrors of one interferometer

in return for management rights in the strategic planning of the observations and full access to the resulting data.

A similar proposal to BMBF by our German colleagues in GEO 600 (Albert Einstein Institute and the Laser Centre in Hannover), will deal with the design and supply of the high power laser systems for Advanced LIGO.


1. Introduction


Now that the first generation of long baseline gravitational wave detectors are commencing operation there is an urgent need to look to the future. While it is the goal of the experimenters that the network of initial detectors - LIGO, GEO 600, VIRGO and TAMA 300 - will detect signals from coalescing black holes, higher sensitivity is required before more general astronomy can be carried out.

Thus the sensitivity goals for the next generation of long baseline interferometers - approximately a factor of 10 to 15 better in strain sensitivity - are such that we should move from the realm of a realistic likelihood of detection to a highly probable detection.


1.1 Objectives


  • To give Advanced LIGO interferometers with a factor of approximately 10 improvement in strain sensitivity by transferring the GEO developed suspension technology to the LIGO project. Transfer of interferometric technology is being dealt with separately under our present research agreements between Glasgow and the LIGO Science Collaboration.

  • To obtain management rights in the strategic planning of the observations with, and full access to the resulting data from, Advanced LIGO


In particular this proposal is requesting approximately £6M to supply Advanced LIGO with:

  • technologically advanced suspension systems based on GEO designs

  • electronic sensing/actuation systems for control of these systems

  • high quality sapphire/silica blanks for the test mass mirrors of one interferometer.


2. Science goals and overview


T
he types of sources likely to be observed are shown in Figure 1 (from Kip Thorne).


Figure 1. The noise h(f) in several LIGO inteferometers plotted as a function of gravity wave frequency, f, and compared with the estimated signal strengths hs(f) from various sources. The signal strength hs(f) is defined in such a way that, wherever a signal point or curve lies above the interferometers noise curve, the signal, coming from a random direction on the sky and with a random orientation, is detectable with a false alarm probability of less than 1%. This is discussed in more detail in Appendix 1.


2.1 Binary coalescences


The most reliably understood source of gravitational waves for ground based detectors is the inspiral of a compact binary system consisting of two neutron stars. The target sensitivity for Advanced LIGO is such that such events should be detectable out to 300 Mpc with an event rate (Kalagerao et al) in the range 1/year to 2/day. Similarly detection rates in the range 1/year to 4/day are predicted for neutron star/black hole binary coalescences and for black hole/black hole coalescences (~10 solar masses) the rate increases to be in the range of 2/month to 10/day. There is an underlying assumption here that optimal templates for detection are being used and such templates become more difficult to calculate as relativistic effects become stronger. However initial detection of a small number of BH/BH events will allow optimisation of templates and increased understanding of strong relativistic effects such as frame dragging by spins.

The information carried by the signals from such coalescences includes the masses of the binary components, spins, distances and locations and any observation of quasi-coincident electromagnetic waves from the same objects will allow comparison of the speeds of gravitational waves and electromagnetic waves to an accuracy of 1 part in 1017.

The exact waveform seen during BH/BH merger should, assuming that initiatives in numerical relativity are successful, yield information about the spacetime curvature around the black holes.


2.2 Spinning neutron stars


Neutron stars with potential ellipticities induced by, for example, non-collinear spin and magnetic axes are potential sources of detectable gravitational waves. For pulsars of known location and spin-down rate, ellipticities of

e > 2 x 10-8 × (100/f)2 × (distance/10 kPc)

should be detectable with Advanced LIGO. This is an interesting level as ellipticities of ~ 10-7 (Cutler et al) ref for this maybe Physical-Review-D. vol.63, no.2; 15 Jan. 2001; p.024002/1-9? are thought reasonable.

For neutron stars of unknown spin period the situation is less favourable as the extra degrees of freedom to be fitted results in a decrease in sensitivity of 5 to 15.

Binary neutron star systems also provide good detection opportunities. For example it seems possible that the low mass X-ray binary. SCO X1 may be detectable in 20 days integration by Advanced LIGO.


2.3 The birth of a neutron star


With reasonable models, Advanced LIGO will be able to detect type I supernovae events – the accretion-induced collapse of a white dwarf to a rapidly spinning neutron star. These are sufficiently luminous that a detection range of 100 Mpc is possible, resulting in as many as 500 detections per year. Also if R-modes are excited in the neutron star, the detection of the quasi-cw signals produced may be detected from a distance of 20 Mpc giving 5 such detections per year.


2.4 Stochastic background


Cross correlation between the Advanced LIGO detectors in Washington State and Louisiana should allow a stochastic background of gravitational waves of energy density greater than 8 × 10-9 times the closure density, in a bandwidth of 40 Hz around a frequency of 40 Hz, to be detected. While such a level is not expected from standard inflation models experiments will let us investigate some of the speculative predictions such as those arising from superstring models of the big bang, or from excitation of cosmic strings or from excitation of the Universe as a 3-D membrane.

3. Detector requirements


To enter the observatory phase of gravitational wave detection we require an array of three or more long baseline detectors at different locations with sensitivities of

10-22/√Hz or better over a frequency range of approximately 15 Hz to 1 kHz. This will allow reasonable source position determination from signal timing measurements.

The GEO 600 detector, despite its advanced suspension and interferometer technology, is limited in sensitivity by its short arm length, and the same argument holds for the shorter TAMA detector. Thus the future observatory array is likely to be centred on upgraded versions of the three LIGO instruments, two in Washington State and one in Louisiana, an upgraded VIRGO detector, ? and on a somewhat longer time scale ? (otherwise why not contribute to EURO now instead) a new long baseline Japanese detector, a proposed new European detector, EURO, and a proposed Australian detector, ACIGA.

The central question to be answered is how a significant improvement in sensitivity level can be achieved for the LIGO detectors. The relevance of GEO 600 to this can be best understood from a discussion of the initial LIGO interferometers and the GEO 600 instrument.


3.1 Present LIGO system


The present LIGO system consists of three interferometers. Two of these have 4 km arm lengths, one at Hanford in Washington State and one at Livingston, Louisiana (Figure 2).

The third interferometer, of arm length 2 km, shares the same vacuum system as the main Hanford instrument.





Figure 2. Aerial view of LIGO Livingston site


These LIGO interferometers use Fabry-Perot cavities in each arm to enhance displacement sensitivity, and use power recycling to enhance the power at the beamsplitter by a factor of 30 over the net usable input power (65 W) which is provided by a 10 W Nd:YAG master oscillator/amplifier system. The main mirrors are suspended on wire slings as single pendulums (see Figure 3) and are hung from vibration isolation stacks consisting of alternate layers of heavy metal and compliantstainless steel and damped springs.

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