SN Refsdal
Imagine a celestial magic trick where a single exploding star appears in multiple places at once, and even reappears years later. That's the extraordinary story of SN Refsdal, the first ever observed multiply-lensed supernova, whose ghostly reflections offered unprecedented insights into the cosmos and a decades-old prediction. SN Refsdal was the first supernova observed to be gravitationally lensed, appearing as multiple distinct images due to a massive galaxy cluster. Astronomers accurately predicted the supernova's dramatic reappearance years later, a powerful validation of our understanding of cosmic gravity and spacetime. The precise time delays between these multiple appearances provided a unique and highly accurate method to measure the universe's expansion rate, the Hubble constant.
AI Summary
Imagine a celestial magic trick where a single exploding star appears in multiple places at once, and even reappears years later. That's the extraordinary story of SN Refsdal, the first ever observed multiply-lensed supernova, whose ghostly reflections offered unprecedented insights into the cosmos and a decades-old prediction.
- SN Refsdal was the first supernova observed to be gravitationally lensed, appearing as multiple distinct images due to a massive galaxy cluster.
- Astronomers accurately predicted the supernova's dramatic reappearance years later, a powerful validation of our understanding of cosmic gravity and spacetime.
- The precise time delays between these multiple appearances provided a unique and highly accurate method to measure the universe's expansion rate, the Hubble constant.
SN Refsdal: A Cosmic Time Machine
Our story begins with SN Refsdal, a supernova that broke all the rules. It wasn't just another exploding star; it was the first one ever detected that had been multiply-lensed. This celestial phenomenon was named after Norwegian astrophysicist Sjur Refsdal, who, way back in 1964, first theorized that such an event could be used as a cosmic stopwatch.
Refsdal's groundbreaking idea was this: if a supernova's light were bent by a massive object like a galaxy cluster, it could create multiple images of the same explosion. Crucially, because the light would travel along slightly different paths, these images wouldn't arrive at Earth simultaneously. They'd be delayed, one after another—a cosmic time-lapse of a single event.
The Cosmic Magnifying Glass
Gravitational lensing is a captivating effect predicted by Albert Einstein's theory of general relativity. It states that massive objects warp the fabric of spacetime around them, causing light rays that pass nearby to bend, much like a glass lens focuses light. In SN Refsdal's case, the lens was the colossal galaxy cluster MACS J1149.5+2223.
This galaxy cluster, acting as a natural cosmic magnifying glass, sits at a redshift of 0.54 relative to us. Meanwhile, the supernova's own host galaxy is much more distant, at a redshift of 1.49. This means its light has traveled for an astonishing 9.34 billion years to reach us, placing it about 14.4 billion light-years away in comoving distance.
An Einstein Cross Unveiled
In late 2014, the Hubble Space Telescope captured something extraordinary within MACS J1149.5+2223. It saw not one, but four distinct images of the same supernova, arranged in a distinctive cross-shaped pattern around a massive elliptical galaxy within the cluster. This rare celestial alignment is famously known as an 'Einstein Cross'.
A Predicted Return
What truly set SN Refsdal apart was the prediction that followed its discovery. Because the four initial images were just one component of the complex lensing display, astronomers calculated that the supernova's light would reappear, like an echo, at a different location in the cluster. This was the cosmic time delay at play, just as Sjur Refsdal had predicted decades earlier.
The initial four-image pattern was caused by light paths that were highly magnified and split by the gravitational field. However, other, longer light paths existed, where the light traversed around the densest parts of the cluster, taking more time to reach us. This meant the supernova could also have appeared as a single image some 40–50 years earlier elsewhere in the cluster field, unseen at the time.
The Grand Reappearance
Astronomers eagerly watched the predicted spot. And sure enough, between November 14 and December 11, 2015, the supernova reappeared! Hubble Space Telescope observations confirmed its return, precisely where and when sophisticated models had predicted it would be seen again.
This reappearance was an incredible triumph for astrophysics. It wasn't just a lucky guess; it was the blind prediction by several different models coming true, validating our theoretical understanding of gravitational lensing and the distribution of matter in galaxy clusters. It felt like solving a cosmic riddle decades in the making.
Measuring Cosmic Expansion
Beyond the sheer spectacle, the time delay between SN Refsdal's original four images and its single reappearance provided a groundbreaking opportunity. It allowed astronomers to apply Refsdal's original 1964 technique for the very first time: using time-delayed supernova images to measure the Hubble constant.
The Hubble constant, denoted $H0$, describes the rate at which the universe is expanding. It relates a galaxy's recession velocity (v) to its distance (D) from us. By meticulously measuring the arrival times of SN Refsdal's different images and precisely modeling the intervening galaxy cluster, scientists could infer this fundamental cosmic value with unprecedented accuracy.
v = H_0 D
Using data from SN Refsdal and the detailed galaxy cluster lens models, astronomers determined the Hubble constant to be approximately $66.6 \text{ km s}^{-1} \text{ Mpc}^{-1}$. This result offered a vital new data point in the ongoing effort to precisely map the expansion history of our universe and resolve discrepancies between different measurement methods.
Other Lensed Supernovae
While SN Refsdal holds the distinction of being the first multiply-lensed supernova ever detected and used for these measurements, it's no longer alone. Since its discovery, other such cosmic mirages have been reported, including iPTF16geu, SN Requiem (AT2016jka), Supernova Zwicky (SN 2022qmx), Chen et al. SN, SN H0pe, and SN 2022riv.
SN H0pe, for instance, has also been utilized to measure the Hubble constant by analyzing the relative delays in image arrival, furthering the technique pioneered by SN Refsdal. Each new lensed supernova offers another chance to refine our cosmic models and unravel the universe's grandest mysteries.
Article
SN Refsdal
SN Refsdal is the first detected multiply-lensed supernova, visible within the field of the galaxy cluster MACS J1149.5+2223. It was named after Norwegian astrophysicist Sjur Refsdal, who, in 1964, first proposed using time-delayed images from a lensed supernova to study the expansion of the universe. The observations were made using the Hubble Space Telescope.
Einstein cross
SN Refsdal
The host galaxy of SN Refsdal is at a redshift of 1.49, corresponding to a comoving distance of 14.4 billion light-years and a lookback time of 9.34 billion years. The multiple images are arranged around the elliptical galaxy at z = 0.54 in a cross-shaped pattern, also known as an "Einstein cross".
Reappearance
SN Refsdal
The image to the left shows a part of the deep field observation of the galaxy cluster MACS J1149.5+2223 from the Frontier Fields programme. The circle indicates the predicted position of the newest appearance of the supernova. To the lower right, the Einstein cross event from late 2014 is visible. The image on the top right shows observations by Hubble from October 2015, taken at the beginning of the observation programme to detect the newest appearance of the supernova. The image on the lower right shows the discovery of the supernova on 11 December 2015, as predicted by several different models.
After the discovery of the supernova, astronomers predicted that they would be able to see it again in about one year, after the four images had faded away. This is because the initially observed four-image pattern was only one component of the lensing display. The supernova may also have appeared as a single image some 40–50 years ago elsewhere in the cluster field.
The supernova reappeared at the predicted position between 14 November and 11 December 2015 (with the exact date being uncertain by approximately one month which is the interval between two consecutive Hubble observations), in excellent agreement with the blind model predictions made before the reappearance was observed. The time delay between the original quadruplet observed in 2014 and the latest appearance of the supernova in 2015 was used to infer the value of the Hubble constant. This is the first time this technique, originally suggested by Refsdal, has been applied to supernovae.
Using measurements from SN Refsdal and galaxy cluster lens models, astronomers found that the Hubble constant has value H0 = 66.6+4.1 −3.3 km s−1 Mpc−1.
Other multiply-lensed supernova
SN Refsdal
Other reported multiply-lensed supernova are iPTF16geu, SN Requiem (AT2016jka), Supernova Zwicky (SN 2022qmx), Chen et al. SN, SN H0pe and SN 2022riv.
Besides SN Refsdal, SN H0pe has also been used to measure the value of the Hubble constant using the relative delay in the arrival between images.