Gravitational waves, ripples in spacetime, offer a new way to observe the universe. Scientists use advanced detectors like LIGO to catch these faint signals from cosmic collisions, opening up a whole new field of astronomy.

Compact binary mergers, where black holes or neutron stars crash together, are the main sources of detectable gravitational waves. These events reveal secrets about the most extreme objects in the universe and help us understand fundamental physics.

Gravitational Wave Detectors

Advanced Ground-Based Interferometers

• LIGO (Laser Interferometer Gravitational-Wave Observatory) consists of two detectors located in Hanford, Washington and Livingston, Louisiana
• LIGO detectors use laser interferometry to measure minute changes in spacetime caused by passing gravitational waves
• Interferometry technique involves splitting a laser beam and sending it down two perpendicular arms
• Mirrors at the end of each arm reflect the light back, recombining at the detector
• Gravitational waves cause slight changes in the arm lengths, creating interference patterns in the recombined light
• LIGO can detect changes in arm length as small as 1/10,000th the width of a proton
• Other ground-based detectors include Virgo in Italy and KAGRA in Japan

Space-Based Detectors and Pulsar Timing Arrays

• Space-based detectors like LISA (Laser Interferometer Space Antenna) planned for future launches
• LISA will consist of three spacecraft in a triangular formation, orbiting the Sun
• Space-based detectors can detect lower frequency gravitational waves than ground-based detectors
• Pulsar timing arrays use precise measurements of pulsar signals to detect very low-frequency gravitational waves
• Pulsars act as cosmic clocks, emitting regular pulses of radio waves
• Gravitational waves passing between Earth and pulsars cause slight variations in pulse arrival times
• International Pulsar Timing Array (IPTA) combines data from multiple radio telescopes worldwide

Compact Binary Mergers

Binary System Evolution and Coalescence

• Compact binary coalescence occurs when two dense objects like black holes or neutron stars orbit each other and eventually merge
• Binary systems lose energy through gravitational wave emission, causing their orbits to decay over time
• As the objects get closer, they orbit faster and emit stronger gravitational waves
• Final stages of merger produce a characteristic "chirp" signal with increasing frequency and amplitude
• Chirp signal provides information about the masses and spins of the merging objects

Black Hole and Neutron Star Mergers

• Black hole mergers produce the strongest gravitational wave signals detectable by current instruments
• First detection of gravitational waves (GW150914) came from a binary black hole merger in 2015
• Neutron star mergers also produce gravitational waves and can be accompanied by electromagnetic counterparts
• GW170817 marked the first detection of gravitational waves from a binary neutron star merger
• Neutron star mergers create heavy elements through r-process nucleosynthesis (gold, platinum)
• Some compact binary mergers may involve a black hole and a neutron star (BHNS systems)

Gravitational Wave Measurements

Strain and Signal Characteristics

• Strain measures the fractional change in distance caused by a passing gravitational wave
• Strain is typically very small, on the order of $10^{-21}$ for detectable events
• Strain amplitude depends on the masses of the merging objects and their distance from Earth
• Gravitational wave frequency increases as the binary system spirals inward (chirp)
• Signal-to-noise ratio (SNR) quantifies the strength of the gravitational wave signal relative to detector noise

Data Analysis and Parameter Estimation

• Matched filtering compares detector output with theoretical waveform templates
• Parameter estimation techniques extract physical properties of the source from the gravitational wave signal
• Properties that can be measured include masses, spins, and sky location of the source
• Multiple detectors allow for improved sky localization through triangulation
• Gravitational wave polarization provides information about the orientation of the binary system
• Hubble constant can be measured using gravitational waves as "standard sirens" when combined with electromagnetic observations