Authors: The LIGO Scientific Collaboration and the Virgo Collaboration
Status: Submitted to the Astrophysical Journal; Available on Arxiv
When Gravity Lenses Light
The gravitational attraction of clumps of matter like galaxies and clusters has the power to act as a lens, bending and curving light coming from a luminous source behind it. This phenomenon, known as ‘Gravitational Lensing’, can completely distort images of the light source, either by magnifying and extending it into rings (known as Einstein rings), or by generating multiple copies of it distributed around the lens like a cross (Figure 1, bottom). Studies of gravitational lensing have helped inform us about a wide array of astrophysical phenomena, including dark matter, the Hubble constant, galaxy evolution, to name a few.
When Gravity Lenses Gravity
Since 2015, LIGO and Virgo have opened a new window to study the universe through gravitational waves, ripples in spacetime caused by pairs of compact objects such as black holes and neutron stars spiraling into one another. These observatories record a characteristic chirping strain signal, lasting from a few seconds to a few hundred seconds that rises in frequency as the two objects get closer and finally merge. This peculiar waveform pattern holds the key to unlocking several parameters of the binary, such as the masses and spins of the objects. In addition, its amplitude and relative phase in two detectors can tell us the sky location of the merger, as well as its distance away from us. Mergers closer to the Earth appear louder in the gravitational wave detectors. To date, LIGO-Virgo have announced the detection of over 50 binary black holes and 2 binary neutron star mergers in its latest catalog of events, dubbed the ‘O3a’ catalog.
Gravitational Waves behave in the same way as Electromagnetic waves in many ways. The 2017 detection of gravitational waves accompanied by an electromagnetic counterpart of GW170817, a binary neutron star merger verified that gravitational waves also travel at the speed of light. As such, gravitational waves themselves can undergo lensing due to the large, massive structures, such as galaxies and clusters of galaxies that make up a gravitational lens! The curvature of the fabric of spacetime due to a gravitational lens can bend the ripples in spacetime passing through (Figure 1, top).
What effects does a lensed Gravitational wave event show?
Unlike optical light which appears distorted in a lensed image, gravitational waves are recorded as a time-series and show different effects under lensing. An incoming gravitational wave chirp that gets lensed can get affected in the following ways: (i) The signal can get amplified, which leads us to believe that it is coming from a merger that is closer to us and has heavier masses, or (ii) We may detect multiple copies of the same event signal. The former may provide an explanation for some exceptionally heavy binaries observed, such as GW190521. In case of the latter, the duplicate signal can arrive anywhere from a few minutes, to several days or even years apart, depending on the nature of the lens. Gravitational microlensing can even make copies of a signal appear slightly shifted but overlapping with the original signal.
An important advantage of gravitational wave lensing is that it lets us probe deeper into the universe: events that would normally be too faint to detect being at large distances can get amplified and show up in LIGO-Virgo data due to lensing. Figure 2 shows how the presence of lensed events can contribute to our understanding of the rate of compact binary mergers at different redshifts.
Does the new catalog contain lensed events?
With a population of gravitational wave events being recorded over the past few years, LIGO-Virgo researchers have done a thorough analysis of all available data to check whether any detected signals show signs of being amplified due to lensing, or are duplicates of a previously detected event. In order to find duplicates, they ranked all possible pairs of events in terms of their similarity and the likelihood of the time delay between them being due to lensing (Figure 3).
Even the most likely-lensed pairs from the O3a catalog are not statistically significant candidates for being actual lensed duplicates of each other. Tests for other forms of lensing, including signal amplification and microlensing also do not yield adequate evidence of lensing from the O3a catalog events. As illustrated in Figure 2, even the non-detection of lensing helps us fill in important gaps in our knowledge regarding the distribution of compact objects in our universe. Hovering the magnifying glass over even more gravitational wave detections in the future may one day fill in these gaps through a direct observation of gravitational wave lensing.
Astrobite edited by Wynn Jacobson-Galán
Featured image credit: Ricardo Buscicchio (University of Birmingham); Taken from https://www.ligo.org/news.php