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What can gravitational waves do to probe early cosmology?
Barry C. BarishCaltech
“Kavli-CERCA Conference on the Future of Cosmology”
Case Western Reserve University October 10-12, 2003
WMAP 2003
LIGO-G030556-00-M
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Signals from the Early Universe
Cosmic Microwave
background
WMAP 2003
The smaller the cross-section, the earlier the particle decouples
Photons: T = 0.2 eV t = 300,000 yrs Neutrinos: T = 1 MeV t ~ 1 sec Gravitons: T = 1019 GeV t ~ 10-43 sec
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GW Stochastic Backgrounds
Primordial stochastic backgrounds: relic GWs produced in the early Universe
Cosmology
Unique laboratory for fundamental physics at very high energy
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GW Stochastic Backgrounds
Astrophysically generated stochastic backgrounds (foregrounds): incoherent superposition of GWs from large populations of astrophysical sources
Populations of compact object in our galaxy and at high redshift
3D distribution of sources
Star formation history
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The Gravitational Wave Signal
The spectrum:
The characteristic amplitude
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What we know limits on primordial backgrounds
The primordial abundance of 4He is exponentially sensitive to the freeze-out temperature. This sets limit on the number of relic gravitons.
The fluctuation of the temperature of the cosmic microwave background radiation. A strong background of gravitational waves at very long wavelengths produces a stochastic redshift on the frequencies of the photons of the 2.7 K radiation, and therefore a fluctuation in their temperature
ms pulsars are fantastic clocks! Stability places limit on gravitational waves passing between the pulsar and us. Integrating for one year gives limit at f ~ 4.4 10-9 Hz
White dwarf and neutron star binary system has second clock, the orbital period! Change in orbital period can be computed in GR, simplifying fitting. Gives limit from 10-11 to 4.4 10-9 Hz
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Some theoretical “prediction”
Strings
Super-string
InflationPhase transition
Theoretical Predictions
Maggiore 2000
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Expected Signal Strength
log Omega(f)
-5
-10
-15
-6 -3 0 3 6 log f
Slow-roll inflation
Nucleosynthesis
?
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Detectionof
Gravitational Waves
Detectors in space
LISA
Gravitational Wave
Astrophysical Source
Terrestrial detectorsVirgo, LIGO, TAMA, GEO
AIGO
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Astrophysics Sourcesfrequency range
Gravitational Waves can be studied over ~10 orders of magnitude in frequency terrestrial + space
Audio band
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Interferometer Concept
Laser used to measure relative lengths of two orthogonal arms
As a wave passes, the arm lengths change in different ways….
…causing the interference
pattern to change at the photodiode
Arms in LIGO are 4km Measure difference in
length to one part in 1021 or 10-18 meters
SuspendedMasses
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International Network
Network Required for:» Detection
Confidence» Waveform
Extraction» Direction by
Triangulation
LIGO Hanford, WA
LIGO Livingston, LA
GEO600 Hanover Germany
TAMA300Tokyo
VIRGOPisa, Italy
+ “Bar Detectors” : Italy, Switzerland, Louisiana, Australia
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Stochastic Background Signalauto-correlation
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Overlap Reduction Function
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Simultaneous DetectionLIGO
Hanford Observatory
LivingstonObservatory
Caltech
MIT
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LIGO Livingston Observatory
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LIGO Hanford Observatory
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What Limits LIGO Sensitivity? Seismic noise limits low
frequencies
Thermal Noise limits middle frequencies
Quantum nature of light (Shot Noise) limits high frequencies
Technical issues - alignment, electronics, acoustics, etc limit us before we reach these design goals
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LIGO SensitivityLivingston 4km Interferometer
May 01
Jan 03
First ScienceRun
17 days - Sept 02
Second ScienceRun
59 days - April 03
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Signals from the Early Universe
Strength specified by ratio of energy density in GWs to total energy density needed to close the universe:
Detect by cross-correlating output of two GW detectors:
First LIGO Science Data
Hanford - Livingston
d(lnf)
dρ
ρ
1(f)Ω GW
criticalGW
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Limits: Stochastic Search
Non-negligible LHO 4km-2km (H1-H2) instrumental cross-
correlation; currently being investigated.
Previous best upper limits:
» Garching-Glasgow interferometers :
» EXPLORER-NAUTILUS (cryogenic bars): 60 (907Hz)ΩGW
61.0 hrs
62.3 hrs
Tobs
GW (40Hz - 314 Hz) < 23
GW (40Hz - 314 Hz) < 72.4
90% CL Upper Limit
LHO 2km-LLO 4km
LHO 4km-LLO 4km
Interferometer Pair
5GW 103(f)Ω
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Gravitational Waves from the Early Universe
E7
S1
S2
LIGO
Adv LIGO
results
projected
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Interferometers in Space
The Laser Interferometer Space Antenna
(LISA)
The center of the triangle formation will be in the ecliptic plane 1 AU from the Sun and 20 degrees behind the Earth.
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LISA: Sources and Sensitivity
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LISA: Astrophysical Backgrounds
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Detecting the Anisotropy
(1) Break-up the yrs long data set in short (say a few hrs long) chunks
(2) Construct the new signal
The LISA motion is periodic
(3) Search for peaks in S(t):
is the observableS
m
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Instrument’s sensitivity
LIGO-ILIGO-I
Advanced LIGO
3rd generation
LISA
Correlation of 2 LISA’s
Slow-roll inflation
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SensitivityPrimordial Stochastic Background
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Stochastic Background Astrophysical Foregrounds
•White-dwarf binary systems (galactic and extra-galactic) (Hils et al, 1990; Schneider et al, 2001)
•Neutron star binary systems (galactic and extra-galactic) (Schneider et al, 2001)
•Rotating neutron stars (galactic and extra-galactic) (Giazzotto et al, 1997; Regimbau and de Freitas Pacheco, 2002)
•Solar mass compact objects orbiting a massive black hole (extra-galactic) (Phinney 2002)
•Super-massive black hole binaries (extra-galactic) (Rajagopal and Romani, 1995)
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Astrophysical SignalsLimit Sensitivity for Primordial Sources
LISA - 1yr10-16
Extra-galacticNS-NS
WD-WD
Extra-galacticWD-WD
10-13
10-10
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Ultimate GW Stochastic Probes
-6 -5 -4 -3 -2 -1 0 1 2 3 log f
log Omega(f) -11 -12
-13
-14 -15
-16
LISA sensitivity limit (1yr)
3rd generation sensitivity limit (1yr)
WD-WD
NS-NS
BH-MBH
NS
?CLEAN
CORRUPTED
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Future experiments in the “gap” (?)
Unresolved Binaries
MBH-MBHcoalescences
LISA
LISA II
LIGOSN
NS
10-18
10-20
10-26
10-24
10-22
10-3 10-1 101 10310-7 10-5
Frequency [Hz]
h
Sun
Earth
60°
50 000 km
90°
90°
A. Vecchio
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Conclusions Primordial Gravitational Wave Stochastic
Background is potentially a powerful probe of early cosmology
Present/planned earth/space-based interferometers will begin to probe the sensitivity regime of interest.
They will either set limits constraining early cosmology or detect the stochastic background
They are ultimately limited to ~10-11 and 10-13 in energy density
A future short arm space probe could probe the gap (0.1 – 1 Hz region looks cleanest)