stefan ballmer fermilab may 14, 2013 experimental challenges in gravitational-wave astronomy
TRANSCRIPT
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!