A mysterious trio of bright spots has appeared in new images from the James Webb Space Telescope. These three flashes, previously unknown, are intriguing researchers and providing crucial clues about the expansion of the Universe.
These points of light are actually images of the same supernova, named H0pe. Their multiplication is due to the gravitational lensing effect caused by a foreground galaxy cluster.
This cosmic lens, located 3.6 billion light-years away, distorts the light from the distant supernova, creating three distinct images. Each image corresponds to a different moment in the explosion, a true cosmic clock for astronomers.
The supernova H0pe, of type Ia, is a valuable tool for measuring the Hubble constant. This essential indicator describes the rate at which the Universe is expanding, and its precise value is still a topic of debate among scientists.
Thanks to this gravitationally lensed supernova, researchers were able to derive a new estimate of the Hubble constant: 75.4 kilometers per second per megaparsec (46.9 miles/sec/Mpc). This innovative method, combined with the power of the Webb telescope, could finally reduce uncertainties surrounding this fundamental value.
Scientists also used data collected by ground-based telescopes to confirm the nature of the supernova H0pe. These observations corroborate that this star exploded around 3.5 billion years after the Big Bang, long before the appearance of modern galaxies.
A peculiar aspect of this phenomenon is that each light path travels a different distance. This creates "time delays," as if we are viewing the explosion at three distinct moments in its progression.
The Hubble constant: a deepening mystery
The Hubble constant, which measures the expansion rate of the Universe, remains a subject of contention among scientists. Local measurements, meaning those taken from our nearby cosmic environment, are consistent with values around 73 to 75 km/s/Mpc (45.4 to 46.6 miles/sec/Mpc).
New observations of the supernova H0pe, made with the James Webb telescope, confirm this range of values. This reinforces the validity of previous measurements, particularly those taken with the Hubble telescope.
However, these results are in tension with measurements of the Universe's relic radiation, also known as the cosmic microwave background. These latter measurements, based on observations of the young Universe, suggest a lower value for the Hubble constant, around 67 km/s/Mpc (41.6 miles/sec/Mpc).
This discrepancy between the two estimates, known as the "Hubble tension crisis," remains one of the major enigmas in modern cosmology.
Researchers hope that future observations from the PEARLS (Prime Extragalactic Areas for Reionization and Lensing Science) program, leveraging the unique capabilities of the Webb telescope, will help refine the Hubble constant's value. These additional data could either resolve the divergence between the two sets of measurements, or unveil new subtleties about the evolution of the Universe, and perhaps even about the very nature of dark matter or dark energy, which influence cosmic expansion.
What is a gravitational lens?
Gravitational lensing is a cosmic phenomenon predicted by Einstein's theory of general relativity. It occurs when a massive object, such as a galaxy cluster, distorts the space-time around it. This effect bends the light's path from a distant object situated behind the cluster.
When light is bent in this way, it can be magnified and multiplied, creating doubled or even multiple images of the distant object. This phenomenon allows astronomers to observe very distant objects in the Universe, which are normally too faint to detect. Gravitational lenses therefore act as "natural telescopes," making visible objects that would otherwise remain hidden.