Gravitational waves

5.14.2 Gravitational waves

As was described in Chapter 1, a consequence of Einstein’s General Theory of Relativity is that changing mass distributions transfer information across the universe by the emission of gravitational waves. As will be discussed in

Chapter 7, one of the strongest sources of gravitational wave emission would be the coalescence of two neutron stars (each typically weighing more than 1 solar

Figure 5.32 The ‘Moon’s shadow’ as observed by the Soudan 2 muon detector. Image: Argonne National Laboratory, and Minnesota, Tufts and Western Washington Universities, USA, and

Introduction to Astronomy and Cosmology

mass) to form a black hole. At the time of writing, though indirect evidence of such emission has been obtained, no gravitational wave has yet been detected by a gravitational wave detector. It is, however, expected that such a direct detection will be made on a timescale of a few years as the current detectors are made more sensitive and so could detect such events across a wide volume of the universe.

The effect of a passing gravitational wave is to produce a tiny distortion of space–time that could give rise to a temporary distortion of a shape, say a sphere, of material by a factor of less than 1 part in 10 20 . It is somewhat amazing that such a small effect could be detected! The simplest, and fi rst to be built, type of gravitational wave detector is called

a Weber bar – a large cylinder of metal with electronic devices strapped around it to detect the vibrations induced by a passing gravitational wave. Modern versions are cryogenically cooled and use superconducting quantum interference devices to detect any motion. Unfortunately, Weber bars are not very sensitive but could, for example, detect a nearby supernova explosion. More sensitive detectors use laser interferometers to detect the slight change in the separation of masses placed many hundreds of metres to several kilometres apart. In the USA, at

Richland, Washington and Livingston, Louisiana, are two ground-based inter- ferometers forming the Laser Interferometer Gravitational Wave Observatory (LIGO) (Figure 5.33).

Observing the Universe

Each comprises two 4 km arms at right angles to each other whose lengths will

be changed by a passing gravitational wave by no more than 10 ⫺17 m – only a frac- tion of the width of a proton! Laser beams pass through vacuum tubes to measure any difference in the separation of test masses at the ends of the arms and enable LIGO’s interferometers to detect changes at the level required. Upgrades to LIGO were funded in 2008 to improve its sensitivity by at least a factor of 10 to form Advanced LIGO . For a given event, such as the coalescence of two neutron stars, this will expand the volume in which it could be detected by a factor of 1000. As a result, such events are expected to be detected several times a day.

The passing of a gravitational wave must be detected in the presence of seismic noise, not just from earthquakes, but passing cars and trains and waves reaching the nearby coastline. The test masses are thus suspended by a series of wire frames to fi lter out seismic noise and the whole assembly is mounted on a support which has accelerometers to measure major disturbances and apply an opposing correc- tion. This works in a very similar way to that employed in modern camera lenses to provide image stabilization.

A space-based interferometer called the Laser Interferometer Space Antenna ( LISA ) is also being developed (Figure 5.34). Here, three test masses will be placed in spacecraft, 5 million km apart, forming a 60° triangle. Lasers will span the distance between them, detecting the minute changes in their separation as a gravitational wave passes through space. Although LISA will not be affected by seismic noise, it will be affected by other means, such as the Sun’s radiation pressure and solar wind,

Introduction to Astronomy and Cosmology

which would change the relative positions of the test mass. To overcome this, the test masses effectively fl oat free in space protected by their surrounding spacecraft. Additional laser systems control micro thrusters to counteract the forces on the spacecraft and keep its position relative to the test mass accurate to 10 nm – of order 1/100th the wavelength of light!

As we will see in Chapter 9, it is not possible to look back in time further than ∼ 380 000 years after the origin of the universe. Prior to that time, the universe

was suffi ciently hot for photons to ionize the atoms so fi lling space with free electrons which scatter light – rather like water droplets in a fog – with the result that the universe was then opaque to electromagnetic radiation. The sensitivity of LISA might actually allow us to probe even further back in time by detecting gravitational waves that were created less than one-trillionth of a second after the Big Bang!