@@ -7,7 +7,7 @@ A detailed structure of the thesis then follows.
\section{Gravitational waves and their detection}
Gravitational waves are an astrophysical event that takes place when massive objects move and deform the fabric of spacetime \footnote{An in-depth source about how gravitational waves have been computed and their features is \cite{mag}.}. They have been theorized by Albert Einstein in 1915 and discovered a hundred years later by a joint collaboration of two detectors \cite{nar}\cite{first}, which was worthy of the Nobel Prize for Physics in 2017 \footnote{See Appendix C for some information about the first detection of gravitational waves.}.\\
Gravitational waves are an astrophysical event that takes place when massive objects move with a quadrupolar momentum and deform the fabric of spacetime \footnote{An in-depth source about how gravitational waves have been computed and their features is \cite{mag}.}. They have been theorized by Albert Einstein in 1915 and discovered a hundred years later by a joint collaboration of two detectors \cite{nar}\cite{first}, which was worthy of the Nobel Prize for Physics in 2017 \footnote{See Appendix C for some information about the first detection of gravitational waves.}.\\
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The effect of gravitational waves when they pass through an object is to produce a deformation of the physical lengths (L). This effect is very small ($\Delta$L/L $\sim$ 10$^{-21}$): masses able to deform the fabric of spacetime and generate gravitational waves are of the order of more than the solar mass $M_{\odot}$, so such massive objects need to be looked for in the Universe.\\
@@ -24,7 +24,7 @@ The best modelled sources are binary systems, orbiting around a common central p
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Gravitational waves from binary systems can provide information about the equation of state of Neutron stars, masses and spin of Black Holes and allow for test of General Relativity \cite{wei}\cite{mag}. The interest of the scientific community for these events and their detectors is therefore linked to the possibility of new astrophysical discoveries.\\
Currently, the ground-based observatories are tuned to detect emission from binary systems: the interferometers are the only instruments that have been able to detect gravitational waves from these kind of sources.\\
Currently, the ground-based observatories are tuned to detect emission mostly from binary systems: the interferometers are the only instruments that have been able to detect gravitational waves from these kind of sources.\\
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The first detection of gravitational waves happened on the 14th September 2015 and confirmed the Theory of General Relativity, opening a new window on the Universe: the signal from a merger of two black holes have been observed thanks to the emission of gravitational waves, confirming the existence of these objects, still mostly unknown \cite{first}.\\
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@@ -32,7 +32,7 @@ The first detection of gravitational waves happened on the 14th September 2015 a
The detector responsible of the new discovery is based in the USA and it is one of the terrestrial interferometers currently in use for gravitational-wave detection \footnote{The working principles of the interferometers and details about the US instrument are explained in Chapter \ref{LIGO}}.
\section{Opening the low frequency window}
As we will see in the next chapter, the ground-based detectors involved in the search of gravitational waves cover a wide range of frequencies, but they are affected by some noises which make them unable to detect waves from sources emitting below 30 Hz. We will later see the nature of these noises. The reason why it is important to open the lower frequency window is that it can give access to the detection of gravitational waves emitted by sources whose physical structure and astrophysical features are still unknown.\\
As we will see in the next chapter, the ground-based detectors involved in the search of gravitational waves cover a wide range of frequencies, but they are affected by some noises which make them unable to detect waves from sources emitting below 30 Hz. We will later see the nature of these noises. The reason why it is important to open the lower frequency window is that it can give access to the detection of gravitational waves emitted by sources whose physical structure and astrophysical features are still unknown and also to explore the features of a wave in the earlier phases of the inspiral.\\
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This is the end towards which a significant part of the scientific collaboration is directed.
Most of the work reported in this thesis has been carried out in laboratories and on LIGO sites. In this chapter, I will briefly introduce interferometers and LIGO, and I will explain in detail only the structures at LIGO that have been the subject of study in this thesis work: this is essential to fully embrace the work reported in Chapter \ref{CPSdiff} in particular, and in general for the devices described in the whole thesis. The information contained in this chapter will often be referred throughout this thesis.
\section{Interferometric detectors}
The interaction of gravitation waves with two objects moving along the x axis produces effects on their distance $d = x_2- x_1$ and hence the effect of the gravitational waves can be measured by looking at the variation of the distance of the masses involved.\\
The interaction of gravitation waves with two objects placed along the x axis produces effects on their distance $d = x_2- x_1$ and hence the effect of the gravitational waves can be measured by looking at the variation of the distance of the masses involved.\\
A method to do it is to measure the time light takes to travel from one mass to the other: this is the basic principle of the \textit{interferometer}.
