Gravitational waves are defined as ripples in spacetime. A spacetime is a mathematical model that comprises of the three dimensions of space and the one dimension of time into a single four-dimensional manifold. Scientists used to believe that the three dimensional geometry of the universe had no correlation to one dimensional time. Until it was in 1905 when Einstein developed the concept of Spacetime in his Theory of relativity.
What are ripples in Spacetime? Einstein hypothesized gravity as a curve in a surface called Spacetime under the theory of general relativity. This curve is a result of the presence of mass. The more mass that is contained within a given volume of space, the greater the spacetime curve is at the boundary of its volume. The curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations. It has been proven that everything in this Universe has gravitational force and tends to attract and get attracted to every other thing. Since every mass in the Universe has an effect on every other mass regardless of its size, which gets us to the fact that changes in gravity can tell us about what the objects are doing. As Einstein has devised gravity as a curve in a surface which means a mass in space creates a depression like curve through which it gets attracted to the other mass. The mass is directly proportional to the gravitational force, because greater mass gets deeper depression. This particular mass sends out ripples with the speed of light in Spacetime when moves. These ripples are known as gravitational waves.
LIGO LIGO stands for Laser Interferometer Gravitational Wave Observatory is an astronomical tool which is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. In 2008, the Advanced LIGO Project to enhance the original LIGO detectors began and is supported by the NSF, with important contributions from various tech councils. The LIGO Scientific Collaboration (LSC) and the Virgo Collaboration with the international participation of scientists from several universities and research institutions began operation with improved detectors in 2015 & detected gravitational waves in 2016 . LIGO detected vast ripples of in Spacetime produced by colliding black holes a billion light years away. Since then it has detected 50 similar signals from merging black holes & neutron stars across the universe.
How do we hear gravitational waves? Conventional sound requires a medium to travel through, and is created when particles compress and ratify, making anything from a loud “bang” for a single pulse to a consistent tone for repeating patterns. In space, where there are so few particles that any such signals die away, even solar flares, supernova, black hole mergers, and other cosmic catastrophes go silent before they’re ever heard. But there’s another type of compression and rarefaction that doesn’t require anything other than the fabric of space itself to travel through: gravitational waves. Thanks to the first positive detection results from LIGO, we’re hearing the Universe for the very first time.
Neutron stars are stellar corpses left over from supernova explosions: after the stars explode they collapse in on themselves, crushing atoms down into a super-dense ball of neutrons – essentially compressing a star about the size of the Sun to the size of a city, around 20 kilometres across. These odd objects are not only the densest objects in the universe, but they’re also spinning hundreds of times per second. If a neutron star isn’t spot-on spherical, it will wobble as it rotates and produce a tiny ripple in space-time – a faint “hum” of gravitational waves. The gravitational waves from black hole and neutron star collisions we’ve observed so far are like squawking cockatoos – loud and boisterous, they’re pretty easy to spot. A continuous gravitational wave, however, is like the faint, constant buzz of a faraway bee, which is much more difficult to detect.
Pulsar timing array A pulsar timing array (PTA) is a set of pulsars which is used to search for correlated signatures in the pulse arrival times. It’s best known its use of an array of millisecond pulsars to detect and analyse gravitational waves. Such a detection would result from a detailed investigation of the correlation between arrival times of pulses emitted by the millisecond pulsars as a function of the pulsars’ angular separations. Millisecond pulsars are used because they are not prone to the starquakes and accretion events which can affect the period of classical pulsars.
One influence on these propagation properties are low-frequency gravitational waves, with a frequency of 10−9 to 10−6 hertz; the expected astrophysical sources of such gravitational waves are massive black hole binaries in the centres of merging galaxies, where tens of millions of solar masses are in orbit with a period between months and a few years.
The gravitational waves cause the time of arrival of the pulses to vary by a few tens of nanoseconds over their wavelength (so, for a frequency of 3 x 10 −8 Hz, one cycle per year, one would find that pulses arrive 20 ns early in July and 20 ns late in January). This is a delicate experiment, although millisecond pulsars are stable enough clocks that the time of arrival of the pulses can be predicted to the required accuracy; the experiments use collections of 20 to 50 pulsars to account for dispersion effects in the atmosphere and in the space between the observer and the pulsar. It is necessary to monitor each pulsar roughly once a week; a higher cadence of observation would allow the detection of higher-frequency gravitational waves, but it is unclear whether there would be loud enough astrophysical sources at such frequencies.
Continuous gravitational waves. These are produced by a single spinning massive object like a neutron star. Any bumps on or imperfections in the spherical shape of this star will generate gravitational waves as it spins. If the spin-rate stays constant, so do the gravitational waves it emits i.e., the frequency is constant. That’s why these are called “Continuous Gravitational Waves”. Researchers have created simulations of what an arriving continuous gravitational wave would sound like if the signal LIGO detected was converted into a sound. Click on “Continuous Gravitational Wave Signal” below to hear what the gravitational waves from a spinning neutron star would “sound” like to LIGO.
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