Gravity is among the strongest forces in the universe, capable of changing the propagation of light waves. Due to their massive weight, light bends around planets, stars, and galaxies.
Even the gravitational pull exerted by planets can distort light, offering a means to detect exoplanets orbiting distant stars. Experts in the field of astrophysics use the unique process of gravitational lensing to study far-away planets and galaxies and their attributes.
Ring by gravitational lensing due to red galaxy Image Credit: ESA/Hubble & NASA Available at: https://www.cfa.harvard.edu/research/topic/gravitational-lensing
The Role of Gravitational Lenses
Gravitational lensing is the most crucial application of Einstein’s theory in modern astrophysics. The presence of planets and stars distorts space-time and makes them the tool for gravitational lensing. The path of light is changed when it passes in the vicinity of these lenses, and a distorted image of the celestial body is developed. The phenomena are observed in the form of a ring or a halo of light around the celestial body.
Gravitational Lensing for the Study of Supernovae
The gravitational bending of light, known as strong gravitational lensing, and the smaller-scale effect called microlensing, when applied to supernovae (SNe), have become valuable tools for exploring cosmology and astrophysics in recent times. Roughly thirty years ago, the concept of utilizing "gravitational telescopes," represented by known lensing galaxy clusters, gained traction. This approach enhances the faint signals emitted by distant supernovae, with the magnification factor denoted as μ.
An article published in Space Science Reviews highlights the first-ever identification of multiple images resulting from a supernova that occurred with SN Refsdal gravitational lensing at a redshift of z = 1.49, detected by the Hubble Space Telescope (HST). In 2021, a strongly lensed supernova, SN Requiem, was observed through archival HST imaging of the galaxy cluster MACSJ0138.0−2155. More recently, another strongly lensed Type Ia supernova, referred to as Supernova Zwicky or SN 2022qmx, was found in the Zwicky Transient Facility.
Gravitational-lensed supernovae offer potent tools for astrophysical investigations into both supernovae and galaxies. Beyond that, they contribute to probing cosmological aspects by providing insights through time delays. The time delays associated with lensed supernovae present a unique opportunity for obtaining early-phase observations of these celestial events.
How Is Gravitational Lensing Applied to the Astrophysical Study of Fast Radio Bursts?
Astrophysicists have recognized the importance of Fast Radio Bursts (FRBs) and are using them to study planets and stars. These are categorized as transient events and are observable in the frequency range of 100 MHz to 8 GHz. These events have proven valuable for exploring various questions in astrophysics and cosmology. While using gravitational lensing to study modified gravity is not a new idea, its examination, especially concerning timing aspects, has received little attention.
A detailed case study has been published in the Journal of Cosmology and Astroparticle Physics, highlighting the use of gravitational lensing in FRB phenomena for astrophysical studies. This study examined a general theory of modified gravity and explored its implications for gravitational lensing involving FRBs. They utilized a series of FRB observations to limit the proportion of dark matter consisting of primordial black holes in such a theoretical framework.
The investigation focused exclusively on scenarios of strong lensing, where multiple images can be formed. The findings revealed that, with increased modified gravity strength, the time difference between the two lensed images reaching the observer diminished. The constraint on the fraction of dark matter composed of primordial black holes (PBHs), denoted as f PBH bound, weakened when accounting for lensing caused by intergalactic plasma.
This implies that the lensing resulting from modified gravity behaves akin to plasma lensing, introducing signal de-coherence. If any astronomical survey indicates a deficiency of plasma in the direction of a FRB, modified gravity could compensate for it.
Gravitational Lensing: A Crucial Tool for Study of Supermassive Objects
The phenomenon of gravitational lensing by different black holes has garnered considerable attention over recent decades due to its significance. Gravitational lensing by black holes serves as a valuable astrophysical tool, enabling the exploration of the strong field characteristics of gravity and providing insights into distant, faint stars. The distinctive features of gravitational lensing, when examined quantitatively, can differentiate horizonless compact objects from conventional black holes.
A recent study in The Astrophysical Journal has discussed the gravitational lensing effects induced by a specific class of spherically symmetric regular electrically charged (REC) black hole spacetimes. The objective is to understand how this lensing differs from that of corresponding spacetimes without horizons.
The researchers explored the viability of comparing black holes in no-horizon spacetimes through intense gravitational lensing and scrutinized the astrophysical implications for various supermassive black holes, taking electric charge into account.
The investigation unveiled the presence of a photon sphere for values within the range 0 < b < b P ≈ 0.247 for REC spacetimes. Conversely, the anti-photon sphere with stable circular orbits emerged for no-horizon spacetime when b E < b < b P. Notably, the radius of the photon sphere (xps) exhibited a decreasing trend with b, while the anti-photon sphere radius (xaps) demonstrated the opposite behavior, with both converging around x ≈ b E.
Gravitational Lensing for the Study of Dark Matter
Approximately eighty-five percent of the matter in the universe exists as dark matter, and its fundamental nature remains enigmatic. Astronomers, when investigating the evolutionary patterns of galaxies in the cosmos, observe that dark matter exerts gravitational influence. Given its substantial abundance, dark matter takes a leading role in shaping the formation of extensive structures on a large scale within the universe, such as clusters of galaxies.
Galaxies typically occupy central positions within extensive aggregations of dark matter known as haloes, encircling clusters of galaxies. The gravitational lensing phenomenon, where more distant galaxies are influenced by the gravitational pull of dark matter haloes, presents a distinctive and potent method for scrutinizing the precise distribution of dark matter.
Strong gravitational lensing, in particular, produces significantly distorted, magnified, and, at times, multiple images of a single source. Conversely, weak lensing induces subtle but consistently altered shapes in background galaxies, offering reliable constraints on the distribution of dark matter within the clusters. Whatever technique may be used, gravitational lensing plays a vital role in the discovery of planets and galaxies.
More from AZoQuantum: The Theoretical Implications of Gravitational Waves
References and Further Reading
Australian Academy of Science. (2024). Gravitational Lensing: Riding the roller-coaster of curved space. [Online] Australian Academy of Science. Available at: https://www.science.org.au/curious/space-time/gravitational-lensing [Accessed 14 February 2024].
Center for Astrophysics: Harvard and Smithsonian. (2023). Gravitational Lensing. [Online] Center for Astrophysics: Harvard and Smithsonian. Available at: https://www.cfa.harvard.edu/research/topic/gravitational-lensing [Accessed 13 February 2024].
ESA Hubble. (2022). Gravitational Lensing. [Online] ESA Hubble. Available at: https://esahubble.org/wordbank/gravitational-lensing/#:~:text=Gravitational%20lensing%20occurs%20when%20a,as%20if%20by%20a%20lens. [Accessed 12 February 2024].
Suyu, S., et al. (2024). Strong Gravitational Lensing and Microlensing of Supernovae. Space Sci Rev. doi.org/10.1007/s11214-024-01044-7.
Kalita, S., et. al. (2023). Gravitational lensing in modified gravity: a case study for Fast Radio Bursts. Journal of Cosmology and Astroparticle Physics. doi.org/10.1088/1475-7516/2023/11/059.
Kumar, J. et al. (2022). Testing Strong Gravitational Lensing Effects of Supermassive Compact Objects with Regular Spacetimes. The Astrophysical Journal. doi.org/10.3847/1538-4357/ac912c.
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