The birth of the universe is arguably the greatest unsolved mystery in physics. It continues to arouse curiosity among physicists and laymen. Physicists mathematically modelled the history and predicted the past, analysing electromagnetic radiation such as visible light, x-rays, radio waves, microwaves and sub atomic particles, However, the surge of new theories only weaved ambiguity which resulted in various contradictory pictures concerning our very own existence. Albert Einstein first realized that the process of the evolution of the universe from ‘nothing’ to ‘everything’ has almost certainly left footprints, which can be studied and analysed even today to ascertain the truth about the origin of our universe.
Related: A Noob’s Guide To Quantum Gravity
Relativity’s Gravitational Waves
The General theory of Relativity (formulated almost exactly a hundred years from now) was one of the boldest and most pioneering contributions made by Einstein. GTR, much like its predecessor, the Special Theory of Relativity revolutionised our understanding of space and time. Einstein went beyond Newton’s Law of Universal gravitation and conceptualised gravity as not merely a force, but as a geometric property of the space-time model and a consequence of the inherent curvature of space-time.
According to the GTR, space-time isn’t flat at all but curved because of the influence of mass-energy. The underlying mathematics of General Relativity is Riemannian geometry, which is a generalised form of Euclidean geometry that deals with Non Euclidean space. The curvature of space time is directly related to the energy or momentum of the present matter or radiation expressed in Einstein’s set of ten field equations. The curvature is caused by the presence of mass and changes proportionally with mass volume ratio i.e. density. As objects with mass move around in spacetime, the curvature changes to reflect the changed locations of those objects. In some cases, massive accelerating objects disrupt space and time in such way that the “information” passes outwards in the form of waves and with same speed as that of electromagnetic waves. These wave are known as gravitational waves.
Chandra data on J0806 shows that its X-rays vary with a period of 321.5 seconds, or slightly more than five minutes. This implies that the X-ray source is a binary star system where two white dwarf stars are orbiting each other (above, illustration) only 50,000 miles apart, making it one of the smallest known binary orbits in the Galaxy. According to Einstein’s General Theory of Relativity, such a system should produce gravitational waves – ripples in space-time – that carry energy away from the system and cause the stars to move closer together. X-ray and optical observations indicate that the orbital period of this system is decreasing by 1.2 milliseconds every year, which means that the stars are moving closer at a rate of 2 feet per year.
These ripples are of great importance to the understanding of our universe. Large celestial bodies radiate these waves which, astonishingly, are an encryption of their “genes”. The carried information not only reveals their cataclysmic origin but also provides thorough insights to the nature of gravity itself. After the prediction of gravitational waves, an inevitable consequence of GTR, science waited until 1974 to behold the practical existence of gravitational waves. Two astronomers from the Arecibo Radio Observatory in Puerto Rico discovered a binary pulsar (highly magnetized rotating neutron star); two extremely dense and heavy stars orbiting around each other. This was the ideal system to show gravitational waves. After forty years of constant study, the system fitted into the GTR predictions with a very high level of accuracy which indirectly hinted at the presence of gravitational waves.
However, the discoveries were all indirect and were insufficient to prove the existence of gravitational waves, for which direct detection was essential. With this aim, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and ESA’s Laser Interferometer Space Antenna (Elisa) along with multiple other pulsar timing arrays were made to aid in the process of detection.
Decoding Gravitational Waves
Gravitational waves are not electromagnetic waves. Two colliding black holes release almost no electromagnetic energy. However, the system sends highly energized gravitational waves which are distortion free and noise less as they stay aloof when it comes to interaction with other objects.
Gravitational waves are generated even with the slightest acceleration change of humans, cars, moving objects etc. But they are of extremely negligible range which makes them practically undetectable by even the most sophisticated instruments. Additionally, gravitational waves are nearly impossible to generate on earth, unlike the famous Higgs Boson which was detected in the LHC. For these reasons, scientists wait for the minutest signal to be detected which originate from black holes, neutron stars and new galaxy formations etc.
These waves come are of four different types namely Continuous Gravitational waves, Compact Binary Inspiral Gravitational Waves, Stochastic Gravitational Waves and Burst Gravitational waves. Each have their unique characteristics:
Continuous Gravitational waves: Generated from large single spinning bodies as massive as a Neutron star – any deformations or bumps in the spin can produce a huge ripple which has a time independent frequency and amplitude. Hence the name Continuous Gravitational waves. Researchers simulated their predictions on CGW and processed a sound which is the sound equivalent of gravitational waves.
Compact Binary Inspiral Gravitational Waves: These waves are produced by orbiting pairs of massive and dense (hence ‘compact’) objects like white dwarf stars, black holes and neutron stars. There are three kinds of “compact binary” systems in this category of gravitational wave generators:
- Binary Neutron Star (neutron star-neutron star) or BNS
- Binary Black Hole (black hole-black hole) or BBH
- Neutron Star-Black Hole Binary (NSBH)
Each of the mentioned types produce possess their own set of properties.
Stochastic Gravitational Waves: Small signals detected from all the distributed unknown sources are known as a “Stochastic Signal”. Stochastic has a unique meaning which concerns a random pattern, that can be statistically explained but might not have a precise meaning.
Burst Gravitational Waves: These gravitational waves are those which do not have definite properties and are completely unknown and mysterious. Thus, these are the toughest to discover.
Recently, the BICEP 2 radio telescope was thought to have found conclusive evidence of primordial gravitational waves in the instant universe. In 2015, however, the BICEP 2 findings were confirmed to be the result of cosmic dust in the Milky Way.
A little while ago, a fresh rumour on the discovery of the elusive gravitational waves resurfaced following a series of tweets from Laurence Krauss, a popular author and theoretical physicist at the Arizona State University. The news emerged from the $500 million LIGO project, built specifically to detect these gravitational waves.
Physicists working with LIGO scanned for gravitational waves from 2002 to 2010, with the initial incarnation of the observatory, which consists of two humongous L-shaped optical instruments in Hanford, Washington, and Livingston, Louisiana. To detect the stretching of the fabric of space itself, researchers compare the lengths of an interferometer’s two 4-kilometer-long arms to within a billionth the diameter of an atom. From 2010 to 2015, LIGO researchers rebuilt the instruments entirely, aiming to scale up the sensitivity by a factor of 10.
On 18th September, 2015 a largely upgraded quest for the gravitational waves was initiated by the LIGO team. On the 25th of September, 2015 Laurence Krauss’ set of tweets suggesting a possible discovery ignited a new controversy among the scientific community. Krauss has been accused of rumour mongering after insiders from the LIGO failed to validate the claims.
Gravitational waves are one of the most important predictions of Einstein’s General Theory of Relativity and hold the key to uncovering several mysterious aspects of our early universe. Although gravitational radiation hasn’t been detected directly, there are several indirect evidences substantiating its presence. Many of Einstein’s predictions, like black holes, have already been confirmed and it might not be long since we get conclusive evidence of Gravitational Waves. If the discovery is validated, it will be a landmark one in today’s physics and once again prove the undisputed applicability of one of the two pillars of modern physics – Einstein’s General Relativity.