Astronomy is entering an era in which gravitational waves and neutrinos will be used to complement existing techniques and to uncover the hidden features of our Universe.
When two black holes or neutron stars in a binary system spiral towards each other, their massive size causes ripples in space-time known as gravitational waves. The strength of these waves increases as the black holes revolve faster, spiralling towards each other until they merge and there is a fall off in the signal (ringdown). The Universe seems to be awash with these cataclysmic collisions, which astronomers expect to tell them how many black holes and neutron stars there are.
How to detect gravitational waves
In the Laser Interferometer Gravitational-Wave Observatory (LIGO), which detected gravitational waves for the first time in 2015, a laser beam is split in two, and each sent down a 4-kilometre tunnel. The beams are reflected back and forth by mirrors at the end of each tunnel, before being recombined at a detector1.
Laser beams travel identical paths and cancel each other out when they recombine at the detector.
Effect of gravitational waves
The waves warp the region of space-time that the tunnels sit in so that the beams seem to have travelled different distances when they merge. The difference is very small — about the width of an atomic nucleus for the first detection.
Global network of detectors
There are three operational gravitational wave detectors around the world: two LIGO arrays and Germany’s smaller GEO600 facility. Kamioka Gravitational Wave Detector (KAGRA) and Virgo are due to come online in 2018 and 2016, respectively, and a third LIGO detector in India is planned. Combining data from multiple detectors will allow scientists to locate the origin of the waves much more precisely. The laser arms of proposed space-based observatories, such as Europe’s eLISA and China’s Taiji and TianQin, would be much longer. They could detect gravitational waves at lower frequencies and reveal events from weaker sources than can be felt on Earth2.
Particles known as neutrinos flood the Universe and are so small that they can zip straight through most matter, making them the ideal cosmic messenger. By studying neutrinos, scientists hope to piece together details of the events that made the particles.
IceCube neutrino observatory
Located beneath the Amundsen–Scott South Pole Station, a US research facility, the detector of the IceCube neutrino observatory is arranged over one cubic kilometre of ice. IceCube’s sensitivity is partly due to the fact that ice in the region is one of the purest and most transparent solids on Earth3.
Timeline of high-energy neutrino discovery
Sources 1. LIGO Scientific Collaboration 2. Nature 531, 150 (2016) 3. IceCube/Univ. Wisconsin–Madison
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Zastrow, M. Revealing the unseen Universe. Nature 537, S198–S199 (2016). https://doi.org/10.1038/537S198a