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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)