Stefan BallmerFermilab

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

Einstein’s Equations:When matter moves, or changes its configuration, its gravitational

field changes. This change propagates outward asa ripple in the curvature of space-time: a gravitational wave.

NASA/Dana Berry, Sky Works Digital

T

c

GG

4

8

“Matter”“Curvature ofSpace-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

+ polarization x polarization

The weakness of Gravity

NASA/Dana Berry, Sky Works Digital

• Gravitational waves produced by orbiting masses:

• For 2 1.4MSun Neutron stars, at 1 Mpc (3 million light years):

2

4 Idc

Gh

32

21-

Hz100103

L

dLh

f

2mmWatt / 1~FluxGW

The beginning of LIGO

• Electromagnetically coupled broad-band gravitational wave antenna, R.Weiss, MIT RLE QPR 1972

• NSF funding andconstruction inthe 1990’s

LIGO LivingstonObservatory

LIGO HanfordObservatory

Currently installing Advanced LIGO…

Interferometer Sensitivity:Quantum noise

Laser

end test mass

4 km Fabry-Perot cavity

recyclingmirror input test mass

50/50 beam splitter

MichelsonInterferometer+ Fabry-Perot Arm Cavities+ Power Recycling+ Signal Recycling

GW signal

125 W

6kW 800kW

bL

h0

1064 nm

~2x10-12Hz-1/2

200

4000 m

2/124103 Hz

(Numbers for aLIGO design)

2/120 102.1 HzmdL

Advanced LIGONoise 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

TypicalShort-HardGRBs

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

observationson a regular basis

TypicalShort-HardGRBs

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 L

IGO

Advanced LIGOFacility upgradeUltimate 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 LIGONoise 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 representationof Electric field, various states

Ex-quadrature

Ep-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

noise FxkxxmxZ 22

1 2 Tkxm B vs.

2

diss

F 4

fPTkfS Bvv

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

dSdTTdF ),(

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, 082002)

• 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 bewell-behaved at low T.

– New substrate materiale.g. crystalline Si (aLIGO uses SiO2)

• 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: Ta2O5 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:1302.6489)– 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-refractivecan be exploited

• Lift-off and bonding forLIGO-size optics non-trivial

G.Cole, VCQ,M.Martin, C. Benko, J. Ye JILA

Phys.Rev.D78:102003,2008Evans, 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 studiesfor 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?

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!