Gravitational Wave Astronomy - Listening to the Universe

Gravitational wave astronomy is a relatively new branch in the field of astrophysics that studies the universe using gravitational waves. Gravitational waves are basically ripples in the fabric of spacetime caused by the acceleration of massive objects, such as black holes and neutron stars. It was originally theorized by Einstein in his general theory of relativity back in the year 1915. Even though it was theorized long ago, the first detection of gravitational waves was made in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration. 

Do we normal human beings produce gravitational waves when we accelerate? Yes, we do, but the amount of waves produce is so small it is practically impossible to detect them with the current technology. For now, we can only detect them from incredibly massive and dense objects like black holes or neutron stars. In addition to studying black holes and neutron stars, gravitational wave astronomy can also be used to study the expansion of the universe and the properties of dark matter and dark energy. 

Historically, scientists have relied only on electromagnetic radiation (visible light, X-rays, radio waves, IR, etc.) to study the universe. Gravitational waves are fundamentally different from EM radiation. They are as distinct from light as hearing is from vision. Imagine if humans were a species that only had eyes to observe and understand nature, and suddenly gained the ability to hear them. This is how LIGO opened a new 'window' on the universe. LIGO itself is a gravitational wave antenna, able to detect the vibrations in the 'fabric' of space-time itself, emanating from the farthest reaches of the cosmos.

There are four types of gravitational waves based on what object or system generates them:

1) Continuous Gravitational Waves

These waves are produced by a single spinning massive object like a neutron star (most cases a pulsar). Any bumps or imperfections in the spherical shape of this star will generate gravitational waves as it spins. As the object rotates, it generates a continuous stream of gravitational waves at a fixed frequency and amplitude. It will seem like a singer is holding a pitch forever while singing. Since these waves do not stop after a single event, it makes them particularly interesting to astronomers, as they provide a persistent signal that can be observed over long periods of time.

2) Compact Binary Inspiral Gravitational Waves

The next class of gravitational waves is Compact Binary Inspiral Waves. Many of you reading this might not know what an inspiral is. An inspiral occurs when a pair of dense compact objects revolve around each other and are tidally locked to each others gravity. They slowly get closer and closer as they revolve faster, generating a higher frequency of gravitational waves until they eventually merge into a single object. There are three subclasses of "compact binary" systems in this category

• Binary Neutron Star (BNS) - two neutron stars orbiting each other
• Binary Black Holes (BBH) - two black holes orbiting each other
• Neutron Star - Black Hole Binary (NSBH) - a neutron star and a black hole orbiting each other

Each binary pair creates a unique pattern of gravitational waves that depends on, among other things, the masses of each object, how their orbits are oriented with respect to Earth, and how far they are.

3) Stochastic Gravitational Waves

Stochastic gravitational waves refer to a type of gravitational wave signal that arises from the collective motion of many small sources distributed randomly throughout the universe. Unlike other types of gravitational waves, stochastic waves do not come from a single source and do not have a fixed frequency or amplitude. They are believed to be a combination of cosmological phenomena, such as inflation or cosmic string networks. Detecting these waves are a challenging task, as the signal is typically much weaker than other types of waves and is buried in background noise. 'Stochastic' means having a random pattern that may be analyzed statistically but not precisely. It is possible that at least a part of this stochastic signal may originate from the Big Bang.

4) Burst Gravitational Waves

The search for 'burst gravitational waves' is to search for the unexpected. It is about detecting gravitational waves which we never knew about before. LIGO has yet to detect them and there are so many unknowns that we don't know what to expect. To search for these kinds of gravitational waves, we cannot assume that they will have well-defined properties like those of other systems. Searching for such waves requires being utterly open-minded. For these kinds of gravitational waves, scientists must recognize a pattern of signals even when such a pattern has not been modeled before.