LOGO Gravitational Waves I.S.Jang

1. Introduction Contents ii. Waves in general relativity iii. Gravitational wave detectors iv. LIGO v. Future detectors

1. Waves in general relativity

1. Introduction

1.1 Gravitational wave Fluctuation in the curvature of spacetime which propagates as a wave, traveling outward from the source From Wikipidia 1. Introduction

Predicted to exist by Albert Einstein in 1916 where, 1. Introduction

1. Introduction

ii. Waves in general relativity

1.2 Gravitational amplitude - Usually denoted h -Differences between major and minor axis -Shown here is h = 0.5(50%) -Most of sources are weaker than h~ Quantity h can be expressed as below ii. Waves in general relativity

1.2 Gravitational amplitude Examples) Then, we can calculate using this eqn (17Mpc) By using a detector with a baseline of 10km, ( Electron radius=3 x ) “This is rather optimistic estimate” ii. Waves in general relativity

1.3 Gravitational wave frequencies - G.W source can’t much smaller than - Therefore rotation period T is - Finally frequency f From this, we can estimate the maximum mass of G.W source! ( Schwarzshild radius) ii. Waves in general relativity

1.3 Gravitational wave frequencies ii. Waves in general relativity

1.4 Experimental evidence for G.W 1993 Nobel prize! ii. Waves in general relativity

iii. Gravitational wave detectors

 There are 2 kind of detectors Bar(Cylindrical) detector Laser interferometer detector iii. Gravitational wave detectors

Bar(cylindrical) detector Built in 1966 by J.Weber Measure the extremely small resistance difference Bar(Resistance) must cool it down to under the 20k Frequency range ~ 1kHz Sensitivity h~ Only sensitive to extremely powerful GWs! iii. Gravitational wave detectors

First-generation efforts -Weber 1960 ‘bar’ detectors -Weber (’69): Announced that two bar detectors (DC & Chicago) were being excited simultaneously -15 groups (~’77,78,79): No convincing evidence -Sensitivity in early 70s for kiloHz bursts: Weber 1960 iii. Gravitational wave detectors

Bar(cylindrical) detector iii. Gravitational wave detectors

Laser interferometer detector  Measure space-time distortions from light travel time difference  Compare time in two ortho directions transverse to GW  Measure interference phase difference  More sensitive, wider frequency range iii. Gravitational wave detectors

Laser interferometer detector Ground-based (VIRGO-Italy) Space-based iii. Gravitational wave detectors

Ground-based detector iii. Gravitational wave detectors

Space-based detector -Using 2 or 3 satellite -Possible to use very long base line -Sensitive to lower frequency range -LISA (Laser Interferometer Space Antenna) -Omega (OMnidirectional Experiments with Gravitational Antennas) iii. Gravitational wave detectors

iv. LIGO

 Laser Interferometer Gravitational wave Observatory  Two ground observatories, separated by 3000 km – use triangulation to locate source  L-shaped ultra-high vacuum, 4km, 2km on each side iv. LIGO

 Interferometer There are many possible configurations! a)Simple michelson interferometer b)Michelson with delay lines c)Michelson with Fabry-Perot arm cavities d)Power recycled Michelson with Fabry- Perot arm cavities Current LIGO use ‘(d)’ configuration! iv. LIGO

 Interferometer Antenna response function iv. LIGO Ex) freq = 1kHz → length = 10000km!! → impossible! Problems (1) mirror accuracy (2) diffraction limit (3) storage time

 Interferometer Antenna response function iv. LIGO

iv. LIGO

 Suspended Mirrors Mirrors are hung in a pendulum -> ‘freely falling masses’ Provide 100x suppression above 1hz Provide ultraprecise control of mirror displacement (< 1pm) iv. LIGO

Arm (vacuum tube) iv. LIGO

Arm (vacuum tube) Diameter : 1.2m, 3mm thickness Material : special low-hydrogen steel Volume = 20,000 m 3 (most largest!) Pressure : ~ torr iv. LIGO Altitude of 400km! → I S S

Noise iv. LIGO

Noise Fundamental limit -Seismic at low freq -Thermal at mid freq -Shot noise at high freq Facility limit -Gravity gradient -Stray light -Residual gas iv. LIGO

iv. LIGO

Advanced LIGO iv. LIGO Initial LIGOAdvanced LIGO

iv. LIGO

Advanced LIGO iv. LIGO Reduced noise - Higher power laser - Signal recycling - Low loss optics - Active seismic isolation - Multiple suspensions Sensitivity → An order of magnitude improvements!

v. Future detectors

Space-based detectors  LISA (Laser Interferometer Space Antenna)  E.W (Einstein Gravitational Wave Telescope) v. Future detectors

v. Future detectors LISA  Launch due 2018  Arm length : 5 million km Advantages  Away from the earth  Laser interferometry over astronomical distances  Sensitive to lower frequencies  Two independent interferometers