Gravitational lensing opens a new frontier in multi-messenger astronomy

The universe is a vast, complex tapestry with incredible events like exploding stars, galaxy formation and merging black holes in every direction. For centuries, we've studied this tapestry mainly by looking at electromagnetic waves or light, first with visible light and eventually other forms of light, like x-rays and gamma rays. But in recent years, scientists have gained new ways to observe the cosmos, particularly with the detection of gravitational waves, which are ripples in spacetime caused by massive cosmic collisions and other high-energy events. This was the dawn of multi-messenger astronomy, where we could observe an event in the universe with more sources than just their light.

Now, an international team of researchers, including researchers from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, @IUCAApune is exploring an exciting new frontier known as multi-messenger gravitational lensing. This method combines different ways of seeing the universe with a phenomenon called gravitational lensing. Gravitational lensing happens when a massive object, like a star, galaxy or cluster of galaxies, sits between us and a distant source. Its gravity bends the path of light and other signals from the source, acting like a giant cosmic magnifying glass. This bending can create multiple images of the distant source, make it appear brighter than it is, and even cause the different images to arrive at Earth at slightly different times.

A new study published in the journal Philosophical Transactions of the Royal Society A introduces the rapidly growing field of multi-messenger gravitational lensing. It highlights how combining signals like gravitational waves, light from gamma rays to radio waves, and even neutrinos can help us discover and study distant cosmic events that are being gravitationally lensed. These events are often transient or variable, meaning they change over time, like supernovae (exploding stars), kilonovae (light from neutron star mergers), gamma-ray bursts (GRBs), and the mergers of black holes and neutron stars (called compact binary coalescences or CBCs). By studying these lensed multi-messenger events, scientists hope to tackle some of the biggest unanswered questions in physics, astronomy, and cosmology.

The paper explains that different cosmic messengers provide different kinds of information. Light from optical and infrared telescopes gives us precise locations in the sky. Gravitational waves, gamma rays, and radio waves, on the other hand, are great for timing or measuring exactly when a signal arrives. Gravitational lensing can magnify faint, distant sources, making them detectable, and create multiple copy-sources of the same event that arrive at different times. By combining the precise timing of gravitational waves or gamma rays with the precise location information from optical telescopes, scientists can pinpoint the lensed source and its host galaxy with incredible accuracy, even if the initial detection signal wasn't exact in pinpointing the location. This synergy is key to unlocking new discoveries.

The researchers found that multi-messenger gravitational lensing offers unique opportunities to test fundamental physics, like the nature of gravity itself. Einstein's theory of General Relativity predicts how gravitational waves should travel and be lensed. Scientists can check if gravity behaves as expected over vast cosmic distances by observing gravitational waves, especially alongside lensed light from the same event. They can also place tighter limits on things like the speed of gravitational waves compared to light, building on groundbreaking measurements from events like the neutron star merger GW170817. Lensing provides multiple paths for the signals, giving more data points to test these theories.

Multi-messenger lensing is also a powerful tool for cosmology – the study of the universe's origin, evolution, and structure. By measuring the time delays between the multiple images of a lensed transient, scientists can find a new way to measure the universe's expansion rate, known as the Hubble Constant. This is particularly exciting because current measurements of the Hubble Constant from different methods don't quite agree. The disagreement has often been termed a crisis in cosmology and one of the biggest mysteries of our current understanding of the universe. Multi-messenger lensing could help resolve this tension.

Lensing also helps probe dark matter, the mysterious substance that makes up most of the matter in the universe. By studying how lensing affects signals, especially on small scales (microlensing), scientists can learn about the distribution and properties of dark matter clumps, including tiny ones called subhalos.

Furthermore, multi-messenger lensing opens new windows into the physics of cosmic sources. For example, observing lensed kilonovae from neutron star mergers could allow scientists to see the early, faint light from these events. This is crucial for understanding the extreme physics of dense nuclear matter.

Lensing can also help probe the structure of powerful jets launched by gamma-ray bursts and potentially reveal if Fast Radio Bursts (mysterious, short radio flashes) are linked to compact object mergers. For black hole and neutron star mergers, lensing can help identify events that appear to fall into mass gaps – ranges of masses where these objects aren't expected to form from normal stellar evolution. By accounting for the magnification caused by lensing, scientists can determine the true mass of the source. Lensing also helps identify the host galaxies of these events, even at high redshifts, providing clues about where and how these cosmic cataclysms occur.

Gravitational lensing has been studied using only light from events like lensed quasars and supernovae or searched for lensing signatures in gravitational wave data alone. Multi-messenger gravitational lensing improves these methods by combining multiple messengers. While previous studies might have been limited by the timing accuracy of optical lightcurves or the sky localisation of gravitational wave detectors, multi-messenger lensing leverages the strengths of each signal. For instance, the precise timing of GWs or gamma rays combined with the exact localisation for optical telescopes allows for much more accurate measurements of time delays and source positions.

However, the paper also points out significant challenges. Identifying candidate lensed multi-messenger events requires sifting through vast amounts of data from different telescopes and detectors. Pinpointing the exact location of events detected by GW or gamma-ray instruments, which often have significant sky uncertainties, is challenging but crucial for finding the lensed images with optical telescopes. Understanding the properties of the lensing galaxies and clusters is also complex and affects the interpretation of the lensed signals. There are also theoretical challenges, like accurately modelling the effects of lensing on different types of signals, especially on small scales (microlensing) or when signals overlap in time. Despite these hurdles, rapid advances in detector sensitivity, larger catalogues of known gravitational lenses from new surveys like the Vera C. Rubin Observatory and Euclid, and improved analysis techniques are paving the way for exciting discoveries in the next 5-10 years.

Ultimately, the research pushes the boundaries of our knowledge about the universe by combining all the ways we can see our universe in a powerful new way. Using multi-messenger gravitational lensing, scientists are gaining unprecedented insights into the fundamental forces that govern the cosmos, the nature of dark matter and energy, and the extreme physics of black holes and neutron stars. These discoveries help us understand our place in the universe and how it has evolved over billions of years.

This research article was written with the help of generative AI and edited by an editor at Research Matters.