Searching for cosmic strings With the LIGO-Virgo gravitational-wave - - PowerPoint PPT Presentation

Searching for cosmic strings

With the LIGO-Virgo gravitational-wave experiments

Florent Robinet On behalf of the LIGO Scientific Collaboration and Virgo Collaboration

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Topological strings Topological strings

Cosmic strings were first introduced by Kibble in 1976. Their formation relies on the Higgs mechanism and on the fact that the universe can transition between different ground states when expanding and cooling. Ground states with different phases When a symmetry is broken in multiple and uncorrelated space-time locations, the field phase can take different values. Stable topological defects may form as a result of the universe expansion. Broken U(1) symmetries → one-dimensional topological defect:

Cosmic strings

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Superstrings Superstrings

In the early 2000s, it was realized that cosmic strings can also form in the context of string theory (or M-theory) They are called superstrings CMB measurements show that cosmic strings contribute very little to the primordial density perturbation → Loss of interest for cosmic string Cosmic string rebirth with string theory: – Fundamental strings can grow to macroscopic scales – D-branes partly wrapped in compact dimensions could appear as strings – Brane collisions could result in cosmic strings

Cosmic strings could provide observational tests for string theory

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Cosmic string parameters: Cosmic string parameters: G Gμ μ, , α α, , p p

2 strings 1 string Cosmic string loops Cosmic string loops are produced when 2 string segments meet or when a single string loops back → reconnection Kinks are left behind. We note p, the reconnection probability – it is equal to 1 for topological strings – it can be much smaller than 1 for superstrings The loop size is a fraction of the horizon ( ) Note that is not a "true" parameter. It only reflects our ignorance about the loop size Cosmic strings are characterized by the dimensionless mass per unit length or string tension From now on, CMB data →

G μ<10

−6

G μ/c

2

l∼αt α c=1

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Cosmic string signatures Cosmic string signatures

Kinks are straightened as the strings stretch → GW radiation Loop oscillation/decay Formation of cusps → GW radiation Primordial density inhomogeneities Gravitational lensing effect Keiser-Stebbins effect Stochastic GW background Other possible effects: Electromagnetic radiation form superconducting strings (GRB engines...) Matter wakes behind moving strings Cosmic rays GW bursts

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Cosmic string signatures Cosmic string signatures

Kinks are straightened as the strings stretch → GW radiation Loop oscillation/decay Formation of cusps → GW radiation Primordial density inhomogeneities Galaxy surveys Fits of the CMB angular power spectrum Pulsar timing Gravitational lensing effect Keiser-Stebbins effect Stochastic GW background Image patterns GW bursts Interferometers (LIGO/Virgo) CMB non-Gaussianities CMB line discontinuities

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Cosmic string gravitational-wave signatures Cosmic string gravitational-wave signatures

Kinks are straightened as the strings stretch → GW radiation Loop oscillation/decay Formation of cusps → GW radiation Pulsar timing Stochastic GW background GW bursts Interferometers (LIGO/Virgo)

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Cosmic string gravitational-wave signatures Cosmic string gravitational-wave signatures

Kinks are straightened as the strings stretch → GW radiation Loop oscillation/decay Formation of cusps → GW radiation Pulsar timing Stochastic GW background GW bursts Interferometers (LIGO/Virgo) CMB angular power spectrum (WMAP / Planck)

Indirect bounds on the stochastic GW background can be obtained from CMB data: A large GW background at the time of the decoupling would alter the observed CMB and matter power spectra

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Cosmic string GW radiation Cosmic string GW radiation

The gravitation radiation represents the main mechanism of energy loss of a cosmic string network:

• GW are emitted when kinks straighten
• GW are emitted when loops decay
• GW are emitted by string cusps produced by oscillating loops (~1 cusp per oscillation)

Observational constraints are often given as bounds on . Here we adopt the model and conventions described in Damour, T. and Vilenkin, A. Phys. Rev. D71 (2005) 063510 p=1,ϵ=1

A second loop size parameterization is used: where Standard scenario: Loop density: Small loop scenario: Loop density: The loop size is given by the gravitational back reaction Large loop scenario: Loop density: Density of GW background of unresolved sources:

Γ≈50 l∼αt=ϵ ΓG μt n(t)=(ΓG μ)

−1t −3

ϵ≪1 n(t)=(ΓG μ p)

−1t −3δ(l−ϵΓG μt )

ϵ≫1 n(t)=(ΓGμ p)

−1t −3

ΩGW ( f )= f ρc d ρGW df ∝Gμ p

−1(ΓϵG μ f t 0) −1/3

G μ

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Cosmic string current limits (GW background only) Cosmic string current limits (GW background only)

Pulsar timing C M B ( i n d i r e c t ) L I G O

• s

t

• c

h a s t i c

The searches for a stochastic GW background provide upper limits on the density of GW. These limits can be converted into bounds on the cosmic string parameters.

̃ p=10−3

NB: The (WMAP) CMB indirect bound

• nly applies to backgrounds

generated before the CMB decoupling The pulsar timing bound mostly constrains large loop scenarios Astrophys.J 653 (2006) 1571 The LIGO bound mostly constrains very small loop scenarios Nature 460 (2009) 990

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Cosmic string GW bursts from cusps Cosmic string GW bursts from cusps

Damour and Vilenkin [1] have shown that the stochastic GW background is strongly non Gaussian. Occasional sharp bursts of GW are expected to be produced by cusps Our search follows the cosmic string model described in [2]. We only consider the small loop regime since the large loop regime is already well-constrained In the frequency domain, the gravitational waveform is: With a high frequency cutoff ( = redshift ) The expected rate of GW events is derived in [2]. It makes use of a generic cosmology including effects from a late time acceleration This model includes ignorance factors of O(1). We define tilde parameters to absorb these factors

[1] Damour, T. and Vilenkin, A. Phys. Rev. Lett. 85, 3761 (2000) [2] Siemens et al. Phys. Rev. D.73 105001 (2006)

hcusp( f )=A(z ;G μ ,ϵ)× f

−4/3

(G ̃ μ , ̃ ϵ , ̃ p)

f h

z

We used the 2005-2010 data set (S5/S6 – VSR1/2/3) at detector design sensitivity We require the coincidence of at least 2 detectors This represents a total of 625 days of observation Florent Robinet Florent Robinet 12

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Our search with the LIGO-Virgo network Our search with the LIGO-Virgo network

Virgo Italy(3 km) LIGO-Livingston USA (4 km) LIGO-Hanford USA (4&2 km)

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Our analysis Our analysis

Cosmic string burst events are extracted using match-filtering techniques. We use a template bank with waveforms predicted by Damour and Vilenkin - only one free parameter: (range: 75-4096 Hz) We perform a time coincidence with at least 2 detectors (from 2 to 4) We use a multivariate likelihood method to rank our events from the less likely to the most likely to be a GW event. Our ranking statistic is tuned using:

• Background events obtained from fake coincidences using time-shifted data
• Simulated cusp events injected in the data with random amplitude, frequency and sky position

f h

4-interferometers network (H1-H2-L1-V1) = 27 discriminative parameters

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Did we detect a cosmic string event? Did we detect a cosmic string event?

1-sigma stat. error

PRELIMINARY

All candidates are less than 1 sigma away from the expected background

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What is the sensitivity to cosmic string cusp signals? What is the sensitivity to cosmic string cusp signals?

PRELIMINARY

The search sensitivity is measured using fake signals injected in the data. The detection efficiency counts how many injections are recovered and ranked above the loudest event of the search

Previous LIGO results

• Phys. Rev. D 80 (2009) 062002

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Cosmic string parameter exclusion results Cosmic string parameter exclusion results

The model we use predicts the expected rate of cosmic string events as a function of the cosmic string parameters Knowing the sensitivity of our search we can constrain the model.

PRELIMINARY

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Comparison with existing constraints Comparison with existing constraints

Pulsar timing C M B ( i n d i r e c t ) L I G O

• s

t

• c

h a s t i c

̃ p=10−3

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Comparison with existing constraints Comparison with existing constraints

Pulsar timing C M B ( i n d i r e c t ) L I G O

• s

t

• c

h a s t i c

̃ p=10−3

PRELIMINARY

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Summary Summary

We analyzed the latest LIGO-Virgo data to search for cosmic string signals. We found no evidence of their existence. The sensitivity of our search allows us to significantly constrain the cosmic string models. We improved the indirect bounds from CMB data.

Outlook Outlook

These results will be published very soon The era of Advanced detectors is coming. A sensitivity improvement of a factor 10 is expected. It should be possible to detect cosmic string signals with amplitudes an order

• f magnitude lower.

Future cosmic string searches could also include the GW radiation from kinks. All the theoretical material is available to perform this search (waveform, rates...).

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Planck results Planck results