-1/2 S ] [Hz hh0.5

Gravitational Waves:

a new window to observe the a new window to observe the

Universe Universe

Adamo Angela August, 27 2007

Experimental Techniques in Particle Astrophysics Experimental Techniques in Particle Astrophysics

(KTH 5A5461) (KTH 5A5461)

What are gravitational waves?

What are gravitational waves?

•• fluctuations in the curvature of space-fluctuations in the curvature of space-time which propagate as time which propagate as a wave;

a wave;

•• radiated by accelerated massive objects, provided that the radiated by accelerated massive objects, provided that the

motion is not spherically symmetric (expanding or contracting motion is not spherically symmetric (expanding or contracting sphere) or cylindrically symmetric (spinning disk).

sphere) or cylindrically symmetric (spinning disk).

•• Gravitational radiationGravitational radiation is the energy transported by these is the energy transported by these waves (the angular momentum is radiated away by

waves (the angular momentum is radiated away by gravitational waves, GWs).

gravitational waves, GWs).

•• InspiralsInspirals are very important sources of gravitational waves. are very important sources of gravitational waves.

Two compact objects (white dwarfs, neutron stars, or black Two compact objects (white dwarfs, neutron stars, or black holes) close to each other send out intense gravitational waves holes) close to each other send out intense gravitational waves

•• Predicted by the General Relativity, not yet directly detected!Predicted by the General Relativity, not yet directly detected!

where

Q_ij mass quadrupole moment stress-energy tensor

The gravitational waves are described by:

Amplitude, h

πτ

h

= − 16

From the GR

τ

2

ij ij

h G Q

= − r &&

decreases whit the source distances, r:

galactic sources h ~ 10^-17

extragalactic sources h ~ 10^-20

The gravitational waves are described by:

Amplitude, h

two different polarizations are possible:

cross-polarized plus-polarized

gravitational wave, hx gravitational wave, h₊

The gravitational waves are described by:

Frequency, ν

Upper limit for the frequencies of GWs:

The expected frequency is highest for compact objects with solar mass

M Hz M

GM

c

4 ≈ 10

≤ π

ν

Type Range Run Time Sources Instrument HF 10 Hz →

1000 Hz

Compact stars

bars, LIGO, VIRGO MF 0.1 Hz →

10Hz 10 μHz →

10 mHz 1 nHz →

10 μHz 10 nHz →

0 Hz

NS binary

inspiral Advanced LIGO

LF binaries

SMBHs LISA

one per day one per a few days one per year

VLF ULF

once in a lifetime

cosmic

astrophysics PTA snapshots

only

cosmic structure

COBE, MAP Planck, etc.

The gravitational waves are described by:

Frequency, ν

The first indirect detection of GWs The first indirect detection of GWs by Hulse & Taylor (1974

by Hulse & Taylor (1974 - - 75,Nobel 1993) 75,Nobel 1993)

•• From the GR, a binary From the GR, a binary system louses energy system louses energy emitting GWs (orbital emitting GWs (orbital

decay) decay)

•• PSR B1913+16 binary PSR B1913+16 binary pulsar system

pulsar system

•• 30 years of measured rate 30 years of measured rate of change of orbital period of change of orbital period

in agreement with the in agreement with the

theoretical prediction of theoretical prediction of

the GR the GR

Weisberg & Taylor, 2004

Detecting gravitational waves

The Classical Detection Method:

The Classical Detection Method:

Weber Bars Weber Bars

plane gravitational wave

y x z

solid bar mass M

L

) (

2 sin 0

0 0

0

0 0 1

0

0 1

0 0

0 0 0

0 )

( 2 cos 0

0 0

0

0 1 0

0

0 0

1 0

0 0

0 0

z t h

z t h

h Ω −

⎟⎟

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜⎜

⎜

⎝

⎛ +

− Ω

⎟⎟

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜⎜

⎜

⎝

⎛

= ₊ − _×

μν

L x y

z

) (

2 2 cos

1 xh t z

x− Ω −

= ₊

With l

each particle in the bar is

accelerated ^{dm = ρdx}

The Classical Detection Method:

The Classical Detection Method:

Weber Bars Weber Bars

) (

2 2 cos

1 ₂

2 2

z t h

dt x

d l = Ω ₊ Ω − _transducer

Detection of the motion:

Detection of the motion:

How big a signal will the How big a signal will the transducer see?

transducer see?

16 LQ h = h

L = 1m

Q = 1.6×10⁴ h₊ = 10⁻²³

⇒ h = 10⁻²⁰ Current bars can detect 10⁻¹⁶m

Aluminium cylinder

Resonant frequency 1660 Hz

Elements of a Resonant Bar

Elements of a Resonant Bar

Aluminium cylinder

Resonant frequency 1660 Hz

Elements of a Resonant Bar Elements of a Resonant Bar

Modern Weber Bars

•• cryogenically cooled

with superconducting quantum interference devices to detect the motion

with bars…

800 850 900 950 1000

1x10^-22 1x10^-21 1x10^-20 1x10^-19

S hh

0.5 [Hz-1/2 ]

Frequency [Hz]

supernova in the Galaxy

Bar Network

Bar Network

GRAVITATIONAL ASTRONOMY GRAVITATIONAL ASTRONOMY

OBSERVATORIES OBSERVATORIES

• • Ground Ground - - based: based:

LIGO LIGO VIRGO VIRGO

• • Space Space - - based: based:

LISA LISA

The laser interferometer detectors The laser interferometer detectors

•• masses placed to several kilometers, as two ends of a bar masses placed to several kilometers, as two ends of a bar

•• These are at 90These are at 90 degree angles to each other inside large vacuum degree angles to each other inside large vacuum tubes

tubes

•• The interferometer consists of mirrors suspended at each of the The interferometer consists of mirrors suspended at each of the corners of the L (Michelson interferometer with Fabry

corners of the L (Michelson interferometer with Fabry--Perot arms) Perot arms)

•• A pre-A pre-stabilized laser emits a beam that at the vertex of the L is stabilized laser emits a beam that at the vertex of the L is split into two paths, one for each arm of the L.

split into two paths, one for each arm of the L.

•the Fabry-Perot cavities store the beams and increase the effective path length.

•When a gravitational wave passes through the interferometer, this results in an effective change in the length of one or both of the cavities.

•This length change will cause the light currently in the cavity to become out of phase with the incoming light.

•After the two separate beams leave the arms and recombine at the beam splitter and at the photodiode, indicating a signal.

•Light that does not contain a signal is returned to the

interferometer using a power recycling mirror, thus increasing the power of the light in the arms.

The laser interferometer detectors The laser interferometer detectors

(continued..)

(continued..)

Limits of detections

• All detectors are limited at high frequencies by shot noise, which occurs because the laser beams are made up of

photons. If there are not enough photons arriving in a given time interval it will be impossible to tell whether a

measurement is due to real data, or just random fluctuations in the number of photons.

•• All groundAll ground--basedbased detectors are also limited at detectors are also limited at low low frequencies

frequencies by seismic noise, and must be very well by seismic noise, and must be very well

isolated from seismic disturbances. Passing cars and trains, isolated from seismic disturbances. Passing cars and trains, falling trees, earthquakes are significant sources of noise in falling trees, earthquakes are significant sources of noise in real interferometers

real interferometers.

LIGO

Simultaneously detect signal (within msec) detection

confidence

locate the sources decompose the polarization of gravitational waves

GEO Virgo

TAMA

AIGO

The Interferometer Network

•• 2 observatories:2 observatories:

LIGO Livingston Observatory LIGO Livingston Observatory

LIGO Hanford Observatory LIGO Hanford Observatory

•• the difference in arrival the difference in arrival times can determine the times can determine the source of the wave

source of the wave in the sky

in the sky

•• Each observatory supports Each observatory supports an 4 km L

an 4 km L--shaped systemshaped system

•• Sensitivity 10Sensitivity 10^{-21 -21} inside a inside a bandwidth 100Hz

bandwidth 100Hz (November 2005) (November 2005)

•• Can detect inspiral of two Can detect inspiral of two roughly solar

roughly solar--mass neutronmass neutron stars within about 8x10

stars within about 8x10⁶⁶ pcpc

LIGO (Laser Interferometer LIGO (Laser Interferometer Gravitational

Gravitational - - Wave Observatory) Wave Observatory)

LIGO Livingston Observatory

LIGO Hanford Observatory

• French-Italian interferometric detector

• a Fabry-Perot resonant cavity extends the optical length from 3 to about 100 kilometers because of multiple reflections

• If optics and mirrors would be perfectly stable, no light should reach the detector except when a GW crosses the interferometer

• Sensitivity from 10 to 10⁴ Hz.

• Expected detection of GWs by coalescent binary systems (stars, BHs, NSs) and SNs from the Virgo cluster

About VIRGO..

About VIRGO..

• Each optical element is suspended to a seismic isolation system contained in a vacuum “tower”

• The towers are linked by vacuum tubes located inside tunnels

• The seismic isolation is achieved

through a chain of suspended seismic filters made of triangular cantilever blade springs providing the vertical isolation while the pendulum provides horizontal isolations.

• In order to minimize the thermal noise the interferometer is cooled close to T=20° Kelvin (-253 °C) .

About VIRGO

• construction completed in June 2003

• started its first science run in May 2007 and it is

currently running

• Direct GWs detection not obtained yet

The Laser Interferometer The Laser Interferometer

Space Antenna (LISA) Space Antenna (LISA)

•• sponsored by the ESA and sponsored by the ESA and NASANASA

•• planned launch in 2015 and planned launch in 2015 and planned duration of five years.

planned duration of five years.

•• Consist of 3 test masses placed Consist of 3 test masses placed 5x105x10^{6 6} km apart, in 3 identical km apart, in 3 identical

spacecrafts working as spacecrafts working as

interferometers.

interferometers.

•• The arms will be at 60The arms will be at 60 degree degree angles to each other.

angles to each other.

•• Detect GWs by measuring the Detect GWs by measuring the changes in distances between changes in distances between

freely floating test masses freely floating test masses

•• Although LISA will not be Although LISA will not be

affected by seismic noise, it will affected by seismic noise, it will

be affected by other noise be affected by other noise

sources, like cosmic rays and sources, like cosmic rays and

solar wind and shot noise.

solar wind and shot noise.

Orbit

5 ×10⁶ km

Spacecraft (no mechanical

contact) Free falling masses

( 3 10^-15 ms^-2 Hz^-1/2 @ 0.1 mHz)

Laser Tracking Signals ( 40 pm Hz^-1/2 @ 3 mHz)

The LISA Concept

The LISA Concept

Instruments on board Instruments on board

Each of the spacecraft is made up of two

optical assemblies, which contain the main optics, lasers, and a free-falling gravitational

reference sensor.

Instruments on board Instruments on board

• The sensor contains the

"test masses“, two cubes allowed to float freely

within the spacecraft.

• These cubes, are shielded from external and internal disturbances so that they detect only the force of gravity.

Instruments on board Instruments on board

The cubes are highly polished so they act as mirrors in an interferometer.

The relative motion of these cubes on different

spacecraft are what will detect passing

gravitational waves

Sensitivity

LISA and ground detectors are

complementary rather than competitive.

• Current bar detectors will see gravitational waves if there is another supernova in the Galaxy

• Advanced LIGO and VIRGO will probably detect gravitational waves and may begin to do astronomy on compact binaries or pulsars

• LISA will:

9Detect gravitational waves from known sources 9Survey all NS-NS binaries in the Galaxy

9Determine WD-WD statistics to inform common-envelope evolution studies

9See mergers of massive black holes in galactic nuclei and inform models of hierarchal galaxy formation and evolution 9Map out the field of a black hole (“seeing a black hole”)

9Test GR in the strong field regime

Conclusions

Conclusions

References:

•• Daniel Sigg, Daniel Sigg, ““Gravitational Waves”Gravitational Waves”, LIGO-, LIGO-P980007-P980007-0000--DD

•• Weisberg & Taylor, 2004, ASP Conference SeriesWeisberg & Taylor, 2004, ASP Conference Series

•• http://en.wikipedia.org/wiki/Gravitational_waveshttp://en.wikipedia.org/wiki/Gravitational_waves

•• http://lisa.jpl.nasa.gov/http://lisa.jpl.nasa.gov/

•• http://sci.esa.int/sciencehttp://sci.esa.int/science--e/www/area/index.cfm?fareaid=27e/www/area/index.cfm?fareaid=27

•• http://www.egohttp://www.ego--gw.it/virgodescription/pag_4.htmlgw.it/virgodescription/pag_4.html

•• http://www.ligo.caltech.edu/http://www.ligo.caltech.edu/

•• http://www.apc.univhttp://www.apc.univ-paris7.fr/The_Violent_Universe/-paris7.fr/The_Violent_Universe/

(Hellings talk about gravitational waves) (Hellings talk about gravitational waves)