The Big Bang Observer (BBO) is a proposed successor to the Laser Interferometer Space Antenna (LISA) by the European Space Agency. The primary scientific goal is the observation of gravitational waves from the time shortly after the Big Bang, but it would also be able to detect younger sources of gravitational radiation, like binary inspirals. BBO would likely be sensitive to all LIGO and LISA sources, and others. Its extreme sensitivity would come from the higher-power lasers, and correlation of signals from several different interferometers that would be placed around the Sun.
The first phase resembles LISA, consisting of three spacecraft flown in a triangular pattern. The second phase adds three more triangles (twelve spacecraft total), spaced 120° apart in solar orbit, with one position having two overlapping triangles in a hexagram formation.
The individual satellites would differ from those in LISA by having far more powerful lasers. In addition each triangle will be much smaller than the triangles in LISA's pattern, about 50,000km instead of 1 to 5 million km. Because of this smaller size, the test masses will experience smaller tidal deviations, and thus can be locked on a particular fringe of the interferometer — much as in LIGO. By contrast, LISA's test masses will fly in an essentially free orbit, with the spacecraft flying around them, and interferometer fringes will simply be counted, in a technique called "time-delay interferometry".
The BBO instruments present massive technological challenges. Funding has not been allocated for development, and even if selected for development, optimistic estimates place the instrument's launch date many decades away.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These observatories use mirrors spaced four kilometers apart to measure changes in length—over an effective span of 1120 km—of less than one ten-thousandth the charge diameter of a proton.
Interferometry is a technique which uses the interference of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy, quantum mechanics, nuclear and particle physics, plasma physics, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, optometry, and making holograms.
The Laser Interferometer Space Antenna (LISA) is a planned space probe to detect and accurately measure gravitational waves—tiny ripples in the fabric of spacetime—from astronomical sources. LISA will be the first dedicated space-based gravitational-wave observatory. It aims to measure gravitational waves directly by using laser interferometry. The LISA concept features three spacecraft arranged in an equilateral triangle with each side 2.5 million kilometers long, flying in an Earth-like heliocentric orbit. The distance between the satellites is precisely monitored to detect a passing gravitational wave.
The Michelson interferometer is a common configuration for optical interferometry and was invented by the 19/20th-century American physicist Albert Abraham Michelson. Using a beam splitter, a light source is split into two arms. Each of those light beams is reflected back toward the beamsplitter which then combines their amplitudes using the superposition principle. The resulting interference pattern that is not directed back toward the source is typically directed to some type of photoelectric detector or camera. For different applications of the interferometer, the two light paths can be with different lengths or incorporate optical elements or even materials under test.
The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferometer or Sagnac interferometer. A beam of light is split and the two beams are made to follow the same path but in opposite directions. On return to the point of entry the two light beams are allowed to exit the ring and undergo interference. The relative phases of the two exiting beams, and thus the position of the interference fringes, are shifted according to the angular velocity of the apparatus. In other words, when the interferometer is at rest with respect to a nonrotating frame, the light takes the same amount of time to traverse the ring in either direction. However, when the interferometer system is spun, one beam of light has a longer path to travel than the other in order to complete one circuit of the mechanical frame, and so takes longer, resulting in a phase difference between the two beams. Georges Sagnac set up this experiment in 1913 in an attempt to prove the existence of the aether that Einstein's theory of special relativity makes superfluous.
The Max Planck Institute for Gravitational Physics is a Max Planck Institute whose research is aimed at investigating Einstein's theory of relativity and beyond: Mathematics, quantum gravity, astrophysical relativity, and gravitational-wave astronomy. The institute was founded in 1995 and is located in the Potsdam Science Park in Golm, Potsdam and in Hannover where it closely collaborates with the Leibniz University Hannover. Both the Potsdam and the Hannover parts of the institute are organized in three research departments and host a number of independent research groups.
The Beyond Einstein program is a NASA project designed to explore the limits of General theory of Relativity of Albert Einstein. The project includes two space observatories, and several observational cosmology probes. The program culminates with the Einstein Vision probes, after completion of the Great Observatories program.
GEO600 is a gravitational wave detector located near Sarstedt, a town 20 kilometres (12 mi) to the south of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Optics and the Leibniz Universität Hannover, along with University of Glasgow, University of Birmingham and Cardiff University in the United Kingdom, and is funded by the Max Planck Society and the Science and Technology Facilities Council (STFC).
The Virgo interferometer is a large Michelson interferometer designed to detect the gravitational waves predicted by general relativity. It is in Santo Stefano a Macerata, near the city of Pisa, Italy. The instrument has two arms that are three kilometres long and contain its mirrors and instrumentation in an ultra-high vacuum.
Gravitational waves are transient displacements in a gravitational field – generated by the relative motion of gravitating masses – that radiate outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves. In 1916, Albert Einstein demonstrated that gravitational waves result from his general theory of relativity as ripples in spacetime.
A gravitational-wave detector is any device designed to measure tiny distortions of spacetime called gravitational waves. Since the 1960s, various kinds of gravitational-wave detectors have been built and constantly improved. The present-day generation of laser interferometers has reached the necessary sensitivity to detect gravitational waves from astronomical sources, thus forming the primary tool of gravitational-wave astronomy.
Gravitational-wave astronomy is a subfield of astronomy concerned with the detection and study of gravitational waves emitted by astrophysical sources.
Bernard F. Schutz FInstP FLSW is an American and naturalised British physicist. He is well known for his research in Einstein's theory of general relativity, especially for his contributions to the detection of gravitational waves, and for his textbooks. Schutz is a Fellow of the Royal Society and a Member of the US National Academy of Sciences. He is a professor of physics and astronomy at Cardiff University, and was a founding director of the Max Planck Institute for Gravitational Physics in Potsdam, Germany, where he led the Astrophysical Relativity division from 1995 to 2014. Schutz was a founder and principal investigator of the GEO gravitational wave collaboration, which became part of the LIGO Scientific Collaboration (LSC). Schutz was also one of the initiators of the proposal for the space-borne gravitational wave detector LISA, and he coordinated the European planning for its data analysis until the mission was adopted by ESA in 2016. Schutz conceived and in 1998 began publishing from the AEI the online open access (OA) review journal Living Reviews in Relativity, which for many years has been the highest-impact OA journal in the world, as measured by Clarivate.
In cosmology, primordial black holes (PBHs) are hypothetical black holes that formed soon after the Big Bang. In the inflationary era and early radiation-dominated universe, extremely dense pockets of subatomic matter may have been tightly packed to the point of gravitational collapse, creating primordial black holes without the supernova compression typically needed to make black holes today. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of stellar black holes.
The DECi-hertz Interferometer Gravitational wave Observatory is a proposed Japanese, space-based, gravitational wave observatory. The laser interferometric gravitational wave detector is so named because it is designed to be most sensitive in the frequency band between 0.1 and 10 Hz, filling in the gap between the sensitive bands of LIGO and LISA. Its precursor mission, B-DECIGO, is currently planned for launch in the 2030s, with DECIGO launching at some time afterward.
LISA Pathfinder, formerly Small Missions for Advanced Research in Technology-2 (SMART-2), was an ESA spacecraft that was launched on 3 December 2015 on board Vega flight VV06. The mission tested technologies needed for the Laser Interferometer Space Antenna (LISA), an ESA gravitational wave observatory planned to be launched in 2035. The scientific phase started on 8 March 2016 and lasted almost sixteen months. In April 2016 ESA announced that LISA Pathfinder demonstrated that the LISA mission is feasible.
The TianQin Project is a proposed space-borne gravitational-wave observatory consisting of three spacecraft in Earth orbit. The TianQin project is being led by Professor Luo Jun, President of Sun Yat-sen University, and is based in the university's Zhuhai campus. Construction on project-related infrastructure, which will include a research building, ultra-quiet cave laboratory, and observation center, began in March 2016. The project is estimated to cost 15 billion RMB, with a projected completion date in the mid-2030s. In December 2019, China launched Tianqin-1, a technology demonstration.
The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. Previously, gravitational waves had been inferred only indirectly, via their effect on the timing of pulsars in binary star systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of two black holes and the subsequent ringdown of a single, 62 M☉ black hole remnant. The signal was named GW150914. It was also the first observation of a binary black hole merger, demonstrating both the existence of binary stellar-mass black hole systems and the fact that such mergers could occur within the current age of the universe.
The Taiji program is a proposed Chinese satellite-based gravitational-wave observatory. It is scheduled for launch in 2033 to study ripples in spacetime caused by gravitational waves. The program consists of a triangle of three spacecraft orbiting the Sun linked by laser interferometers.
Lisa Barsotti is a research scientist at the Massachusetts Institute of Technology Kavli Institute.
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