Gravitational waves and multi-messenger astronomy. A new way to ‘see’ the Universe.

The discovery of gravitational waves opened up the possibility for astronomers to view the Universe in a whole new way — uncovering objects that may have previously been hidden from us.

In 2017 the Nobel prize for physics was awarded to the theoretical physicist, Kip Thorne, experimental physicist Barry Barish and MIT professor Rainer Weiss for their contribution to the discovery of gravitational waves in 2015.

The discovery of gravitational waves, whilst monumental in its own right, is striking because it does more than simply confirm the theory of general relativity. The direct measurement of gravitational waves by LIGO (Laser Interferometer Gravitational-wave Observatory) opens up a whole new avenue of discovery, a new form of investigation and fundamentally, an entirely new way to ‘see’ the universe both as it is and as it was in the distant past.

Maybe even stretching back to the very dawn of time itself.

The first ripples of a discovery

Gravitational waves were first proposed by Henri Poincaré in 1905 as disturbances in the fabric of space-time propagating at the speed of light. Ten years later Einstein’s theory of General Relativity formalised these ideas. The concept of perturbations in space-time, forbidden in the Newtonian interpretation of gravity, were fully permissible in a theory which treated the universe itself not as a stage on which cosmic events unfold but a player in those events.

Poincaré and Einstein both saw the possibility of gravitational waves propagating through space-time at the speed of light

Gravitational waves arose from the possibility of finding a ‘wave-like’ solution to the general tensor equations at the heart of general relativity. According to Einstein, gravitational waves should be generated en masse by the interaction of massive bodies such as binary systems of super-dense neutron stars and merging black-holes, in fact, they can be generated by any accelerating objects but Earth-bound accelerating objects cause perturbations that are far too small to detect. Hence why our investigations must turn to areas of space where nature provides us with objects that are far more massive.

a simulation of gravity waves emitted by neutron stars in a binary pulsar

Einstein predicted that those ripples, created by objects of great mass, gravitational waves, would be so minute they would be impossible to detect by any technological means imaginable at that time.

Fortunately, Einstein was wrong.

A minor perturbation

A diagram of the Hulse-Taylor binary system (NASA)

The first observations of gravitational waves were indirect in that their effect was strongly implied — but the waves themselves remained elusive. To understand how an indirect observation of the emission of gravitational waves could be made, one must examine the predictions General relativity makes for a binary-system of neutron stars emitting regular pulses of radiation radiowaves, otherwise known as a pulsar.

Einstein’s theory suggests that a system such as this should be losing energy by the emission of gravitational waves. A consequence of this would be that the system’s orbital period should be decreasing in a very predictable way. The stars draw together as there is less energy in the system to resist their mutual gravitational attraction, and as a result, there orbit increases in speed, and thus the pulses of radio waves are emitted at shorter intervals. You can picture this as the time it takes for the radio wave to be directly facing our line of sight to be reduced.

Imagine an ice-skater drawing their arms in, as they do so they spin with increasing speed.

As the ice-skater draws her arms in her rotational speed increases (Kaiser Science)

This is exactly what was observed in the Hulse-Taylor system (PSR B1913±16), discovered in 1974, which is comprised of two rapidly rotating neutron stars. This observation earned the 1993 Nobel prize in physics for the discoverers of PSR B1913±16, Taylor and Hulse.

The cumulative shift of periastron time for PSR 1913+16. This shows the decrease of the orbital period as the two stars spiral together (Weisberg and Taylor, 2004)

In the three decades since this discovery, the effect of the emission of gravitational waves has been further studied in this system and many others and has been confirmed within an error margin of 0.2% which is an amazing line of evidence for general relativity, but still left a frustrating gap in progress for the discovery of gravitational waves. Whilst finding a nibbled block of cheese may strongly indicate the presence of a mouse, it’s no substitute for catching the blighter!

Clearly, the first step in the actual observation of gravitational waves was to build equipment sensitive enough to do so. This came in two forms, ground-based gravitational wave observatories, the previously mentioned LIGO and GEO (the German-British gravitational wave detector) and the space-based system LISA (Laser Interferometer Space Antenor).

Making waves

Aerial view of the West and North arms of the Virgo interferometer. The lasers are housed in the massive vacuum-sealed tunnels seen leading away from the man building.

It was the ground-based observatory LIGO in collaboration with the Virgo project that made the groundbreaking detection of gravitational waves in August 2015. The LIGO detector uses two laser emitters based at the Hanford and Livingstone observatories, separated by thousands of kilometres apart to form an incredibly sensitive interferometer.

Basic schematic of LIGO’s interferometers with an incoming gravitational wave depicted as arriving from directly above the detector. (LIGO)

An interferometer uses the interference pattern of light, as demonstrated in the double-slit experiment, to detect infinitesimal changes in the length of the ‘arms’ from the beam splitters to the mirrors. As gravitational waves are expected to stretch and compress space as they pass, this causes an incremental change in the interference pattern of the two beams of light.

One of the arms, each of which is 4 km long in LIGO, the world’s largest interferometer, stretches and the other compresses as the gravitational wave passes. This causes the beam in the shorter arm to arrive before the beam in the longer arm. Thus the gravitational waves cause the beams to arrive at the detector out of sync and thus the interference pattern is changed.

This is exactly what was observed at LIGO on the morning of September 14th, 2015 when the interferometer produced two distinct images of the rippling of space-time. The gravitational waves where latter discovered the result of the most violent cosmic event imaginable, the merger of two supermassive black-holes.

The shape of first detection ‘waveform’. The blue line shows the stretching and shrinking of the Hanford Observatory’s arms, and the orange line shows how Livingston’s arms responded in the presence of the gravitational wave. The taller the line, the more stretching/shrinking the arms experienced. (LIGO)

This signal was taken and translated to a small, audible chirp which can be heard here. Hard to believe that a noise, much quieter than that text alert of the average mobile phone, could be the product of two of the most massive objects in the universe violently colliding.

Multi-messenger astronomy: A brighter future

So far, so good, but why is the discovery of gravitational waves so significant for the future of understanding the universe? To answer this, we have to consider that thus far astronomers have been restricted to view the universe using electromagnetic radiation and therefore observations have been confined to that particular spectrum.

Using this spectrum alone, astronomers have been able to discover astronomical bodies and even the cosmic microwave background (CMB) radiation, a ‘relic’ of one of the very first events in the early universe, the recombination epoch when electrons joined with protons thus allowing photons to begin travelling rather than endlessly scattering. Therefore, the CMB is a marker of the point the universe began to be transparent to light. But the use of electromagnetic radiation is severely limited. It does not allow us to directly ‘see’ black-holes, from which light cannot escape. Nor does it allow us to see non-baryonic, non-luminous dark matter, the predominant form of matter in galaxies, which astronomers believe account for 85% of the universe’s total mass. As the term ‘non-luminous’ suggests dark matter does not interact with the electromagnetic spectrum, it neither absorbs or emits light and as such observations in this spectrum will not allow us to see the majority of the matter in the universe.

Clearly, this is a problem. But one that can be avoided by using the gravitational wave spectrum as both black holes and dark matter do have considerable gravitational effects.

The gravitational wave spectrum, (NASA Goddard Space Flight Center)

The beauty of gravitational waves is that not only are they the only detectable form of radiation emitted by black-holes (ignoring so-called Hawking radiation which would likely be too weak to be detected) but they can reach areas of the universe that electromagnetic radiation can’t, unlike light, gravitational waves aren’t intercepted by gas or dust clouds for example. In addition to this gravitational waves interact with dark matter and possibly other unseen objects.

Gravitational-wave astronomy has one other major advantage over traditional electromagnetic wave astronomy. This new form of astronomy measures the amplitude of the travelling wave, whilst electromagnetic wave astronomy measures the energy of the wave, which is proportional to the amplitude of the wave squared.

Therefore the brightness of an object in traditional astronomy is given by 1/distance², whilst the gravitational ‘brightness’ is given by 1/distance. Clearly, this means that the ‘visibility’ of stars falls off much more quickly in traditional astronomy.

Gravitational waves even stand poised to teach us more about elements of the universe we have already examined and understood such as the CMB. To consider the importance of this discovery, count the number of Nobel prize winners involved in its discovery mentioned in this relatively brief article.

Truly, gravitational wave astronomy represents the opening of a whole new window to the universe. It is a most exciting time for the exploration of our universe.