Astrophysicists propose a new way to measure cosmic expansion: lensed gravitational waves
(Nanowerk News) the universe is expanding; we’ve had evidence of that for about a century. But how fast the celestial bodies are moving away from each other is debatable.
It’s no small feat to measure the degree of displacement of objects from each other over great distances. Since the discovery of cosmic expansion, its rate has been measured and requantified with increasing precision, with some recent values ranging from 67.4 to 76.5 kilometers per second per megaparsec, which relates the speed of recession (in kilometers per second) to the distance (in megaparsec).
The gap between the various sizes of cosmic expansion is called the “Hubble tension”. Some call it a crisis in cosmology. But for UC Santa Barbara theoretical astrophysicist Tejaswi Venumadhav Nerella and colleagues at the Tata Institute of Fundamental Research in Bangalore, India, and the Inter-University Center for Astronomy and Astrophysics in Pune, India, it is an exciting time.
Since the first detection of gravitational waves in 2015, the detectors have been significantly improved and are ready to generate multiple signals in the coming years. Nerella and his colleagues have devised a method to use these signals to measure the expansion of the universe, and perhaps help settle the debate once and for all. “The ultimate scientific goal of future detectors is to provide a complete catalog of gravitational wave events, and this will be an extraordinary use of an entirely new dataset,” said Nerella, co-author of the paper published in Physical Review Letter (“Cosmography Using Strong Lensed Gravitational Waves from Binary Black Holes”).
The measurement of the rate of cosmic expansion boils down to speed and distance. Astronomers use two types of methods to measure distances: the first is to start with an object of known length (the “standard ruler”) and see how large it appears in the sky. These “stuff” are features in the cosmic background radiation, or in the distribution of galaxies in the universe.
The second class of methods starts with an object of known luminosity (“standard candle”) and measures its distance from Earth using its apparent brightness. These distances are connected to more distant bright objects and so on, forming a series of measurement schemes that are often called the “cosmic distance ladder”. Incidentally, gravitational waves themselves can also help measure cosmic expansion, as the energy released by the collisions of neutron stars or black holes can be used to estimate the distances to these objects.
The method Nerella and co-authors propose belongs to the second class but uses gravitational lensing. This is a phenomenon that occurs when a massive object warps spacetime, and bends any kind of wave that moves near the object. In rare cases, lensing can produce multiple copies of the same gravitational wave signal that reach Earth at different times — the delay between signals for populations of different imaged events can be used to calculate the expansion rate of the universe, according to the researchers.
“We understand very well how sensitive gravitational wave detectors are, and that there are no sources of astrophysical confusion, so we can pinpoint exactly what goes into our catalog of events,” said Nerella. “The new method has sources of error that complement the existing method, which makes it a good differentiator.”
The source of this signal is a binary black hole: a system of two black holes that orbit each other and eventually merge, releasing huge amounts of energy in the form of gravitational waves. We have yet to detect examples of this signal with powerful lenses, but future generations of ground-based detectors are expected to have the required level of sensitivity.
“We expect the first observations of lensed gravitational waves in the next few years,” said study co-author Parameswaran Ajith. In addition, these future detectors should be able to see further into space and detect weaker signals.
The authors expect these advanced detectors to begin their quest to combine black holes within the next decade. They anticipate recording signals from several million black hole pairs, a fraction (about 10,000) of which will appear multiple times in the same detector due to gravitational lensing. This distribution of delays between repeat occurrences encodes Hubble’s expansion rate.
According to lead author Souvik Jana, unlike other measurement methods, it does not rely on knowing the exact location, or distance, of this binary black hole. The only requirement is to accurately identify the large amount of signal this lens has. The researchers added that lensed observations of gravitational waves could even provide clues about other cosmological questions, such as the nature of the invisible dark matter that makes up most of the universe’s energy content.
