Stefan Ballmer Fermilab May 14, 2013 Experimental Challenges in Gravitational-Wave Astronomy

Outline Introduction: The sensitivity of Advanced LIGO What can we achieve in the next 2 decades? – Science case: GW astrophysics – Technological challenges & required R&D

Gravitational Waves NASA/Dana Berry, Sky Works Digital “Matter” “Curvature of Space-Time”

4 The wave’s field “Ripples in Space-Time” Measureable effect: – Stretches/contracts distance between test masses perpendicular to propagation Image credit: Google Amplitude: dL/L = h + polarizationx polarization

The weakness of Gravity NASA/Dana Berry, Sky Works Digital Gravitational waves produced by orbiting masses: For 2 1.4M Sun Neutron stars, at 1 Mpc (3 million light years):

The beginning of LIGO Electromagnetically coupled broad-band gravitational wave antenna, R.Weiss, MIT RLE QPR 1972 NSF funding and construction in the 1990’s

LIGO Livingston Observatory LIGO Hanford Observatory

Currently installing Advanced LIGO…

Interferometer Sensitivity: Quantum noise Laser end test mass 4 km Fabry-Perot cavity recycling mirror input test mass 50/50 beam splitter Michelson Interferometer + Fabry-Perot Arm Cavities + Power Recycling + Signal Recycling GW signal 125 W 6kW 800kW 1064 nm ~2x Hz -1/ m (Numbers for aLIGO design)

Advanced LIGO Noise Budget

NS-NS standard candle (sky-averaged distances) Initial LIGO:20 Mpc Advanced LIGO: 200Mpc – Expect ~40 / year From population synthesis ( Class.Quant.Grav.27:173001,2010 ) Or from SHGRB rate, taking into account beaming Typical Short-Hard GRBs

Outline Introduction: The sensitivity of Advanced LIGO What can we achieve in the next 2 decades? – Science case: GW astrophysics – Technological challenges & required R&D

GW Astronomy Science Goals Fundamental Physics – Is GR the correct theory of gravity? – Do black holes really have “no hair” ? – What is the neutron star equation of state? Astrophysics – What is the black hole mass distribution? – How did supermassive BHs grow? – What are the progenitors of GRBs? Cosmology – Can we directly see past the CMB?

What is needed to achieve this? Advanced LIGO will observe NS/NS mergers, but it is a detection machine – SNR = 10  signal fidelity ~10% – Many interesting science goals out of reach We want to see NS/NS mergers to cosmological distances – …that is where we observe GRB’s… We need better sensitivity… – …probably in 2 stages… G. Galilei

NS-NS standard candle (sky-averaged distances) Initial LIGO:20 Mpc Advanced LIGO: 200Mpc – Expect ~40 / year Future aLIGO upgrade – Observe NS-NS mergers up to redshift ~0.2 – Expect O(2)/week – Multi-messenger observations on a regular basis Typical Short-Hard GRBs

The science case for the next generation GW detector aLIGO Upgrade to aLIGO Next generation – Observe NS-NS mergers larger than redshift 1 – Expect O(10)/day

Expected noise in context Initial LIGO Advanced LIGO Facility upgrade Ultimate goal (new facility, Einstein class)

What about IMBHs? Exploring the Early Relativistic Universe with Intermediate Mass Black Holes. – The existence of intermediate-mass black holes (IMBHs) is an open question in astrophysics. M

Outline Introduction: The sensitivity of Advanced LIGO What can we achieve in the next 2 decades? – Science case: GW astrophysics – Technological challenges & required R&D

Advanced LIGO Noise Budget

Key technological hurdles Quantum noise (radiation pressure/shot) – Quantum mechanical measurement limitations Thermal noise (coating) – Thermal motion of the mirror surface Newtonian (Gravity Gradient) noise – Newtonian gravity short-circuits suspensions (not this talk)

Quantum Noise Go heavy… Squeezing – External squeezed light injection – Filter cavities

Squeezed light source Quantum trade-off between phase and amplitude noise Strain sensing is only sensitive to one of them Schematic representation of Electric field, various states E x-quadrature E p-quadrature

Why does squeezing work? Laser Readout quadrature

Filter cavities Concept: – A cavity operated in reflection:  frequency dependent phase shift – No delay above cavity pole – Used on squeezed light: frequency dependent rotation on squeezing ellipse Keep squeezing ellipse in correct orientation Draw-back: very sensitive to optical losses Φ(f)

Thermal Noise - basics Fluctuation-dissipation theorem: It’s the loss! – Equipartition theorem.: Fluctuation-dissipation theorem – The energy loss per cycle (normalized by the driving force squared) is proportional to the velocity power spectrum vs.

Types of Thermal Noise 2 types of losses: – Mechanical loss  Brownian noise direct coupling to elastic strain – Thermal loss  Thermo-optic noise thermo-elastic (coupling through thermal expansion) thermo-refractive (coupling through dn/dT)...are coherent

Thermal Noise - basics Two main sources: – Suspension Thermal Noise Due to mechanical losses in suspension wires Use long monolithic suspensions – Coating thermal noise Due to mechanical loss in optical coating Hard to fix…

Mitigating Thermal Noise… Arm length ( pricy ) Beam size ( instability ) Increase sampled mirror area: – Laguerre-Gaussian modes Effective beam area larger (Noise averages) But mode degenerate – Or…

Realizing multiple spots Use multiple spots… – Coating thermal noise becomes (mostly) uncorrelated ( Nakagawa et. al. PRD, vol 65, ) An idea with a twist: – … closed as FB cavity after N-bounces (N<10) (APPLIED OPTICS, Vol. 4, No. 8 (1965) ) Example: 4.5-spot standing-wave cavity

Cryogenic operation Thermal Noise? Cool! – Young’s modulus, mech. loss, thermal conductivity and capacity need to be well-behaved at low T. – New substrate material e.g. crystalline Si (aLIGO uses SiO 2 ) Implications: – Need to change laser wavelength to 1.6u (band edge) – Affects coating choice – Technical integration challenge Vibrations, cooling beam pipes, etc.

Crystal coatings Switch from amorphous to crystalline coating – Lower mechanical loss ~10 -5  lower Brownian noise (aLIGO: Ta 2 O 5 has loss angle ~2.3e-4) Options: – AlGaAs: grows on GaAs Requires lift-off & bonding – AlGaP: Lattice-matched Grows on Si

Crystal coatings - AlGaAs Recent result: – Tenfold reduction of Brownian noise in optical interferometry (G. Cole et. al.,arXiv: ) – Loss angle < 4e-5 G.Cole, W. Zhang, M. Martin, J. Ye, M. Aspelmeyer

Crystal coatings: Remaining Issues Dominated by thermo-optic noise in LIGO band – Cancellation mechanisms between thermo-elastic and thermo-refractive can be exploited Lift-off and bonding for LIGO-size optics non-trivial G.Cole, VCQ, M.Martin, C. Benko, J. Ye JILA Phys.Rev.D78:102003,2008 Evans, Ballmer, Fejer, Fritschel, Harry, Ogin G.Cole, W. Zhang, M. Martin, J. Ye, M. Aspelmeyer

So what can we expect? Where can we get to with these ideas?

‘’RGB’’ design study Simple design studies for aLIGO upgrades. 3 teams formed: – Blue (headed by R.Adhikari) – Red (headed by S.Hild) – Green (headed by myself) Constraints: – Use existing LIGO vacuum systems – Keep as many existing aLIGO systems as possible LIGO Hanford Observatory

‘’RGB’’ design study Questions: – How much improvement over aLIGO is possible? – What is the budget? What are the trade-offs? – Can the upgrade happen incrementally? – When do we need to be ready? – What R&D needs to be carried out?

Parameters  aLIGO coatings!  4-spot standing wave resonant delay line  1.5x spot radius increase  Mass: 160kg  Suspension and Newtonian noise: Copy RB  Laser input: 125Watt (arm power lower…)  10dB frequency dependent squeezing

Conclusion Advanced LIGO will observe NS/NS mergers, and whet our appetite for more A factor of 10x of sensitivity improvement (z=1 for NS/NS) seems possible, but requires research A factor of 3x is possible at existing sites The scientific pay-off would be enormous

Thank you!