Understanding the rate at which the universe expands is one of the biggest problems modern astronomers face. The most direct approach to measuring its expansion rate is by looking at more distant objects and tracking how quickly they appear to recede from us. However, this method consistently gives us answers about 9% higher than the values we get from looking at signals left by the very early universe.
Just recently, a new proof-of-concept has finally arrived. Two supernovae detected from a galaxy can play a vital role in revealing the universe's expansion rate.
Second Lensed Supernova
In 2016, the Hubble Space Telescope captured the galaxy MRG-M0138, although its images were not thoroughly analyzed until three years later. The light of this galaxy is being distorted into five separate images by the lens of MACS J0138.0-2155, a galaxy cluster that is 4 billion light years away from us.
When astronomers studied the Hubble images in 2019, they noted the bright light of a supernova in MRG-M0138. This supernova named Requiem is classified as type Ia from the explosion of a white dwarf, either through colliding with another white dwarf or by stealing enough matter from a close companion star.
A second type Ia supernova called Encore was recently discovered in the same galaxy using the James Webb Space Telescope. MRG-M0138 is the most distant galaxy with two type Ia supernovae. It is rare to find lensed supernovae, with fewer than a dozen identified. This makes the two type Ia supernovae in MRG-M0138 exceptionally valuable.
While most of the images of the two supernovae have appeared, models of the distribution of dark matter in the lensing cluster predict that one of the light paths will be much longer. The final images are not expected to appear until the mid-to-late 2030s.
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Solving the Greatest Puzzle of Astronomy
In measuring the universe's expansion rate, scientists use the Hubble Constant, which describes how fast the universe is expanding at various distances from a particular point in space. In using this unit, astronomers get two incompatible values, which means there could be an undetected error in their measurements or a new law of physics that must be considered.
One way of measuring the Hubble constant is by analyzing the cosmic microwave background (CMB) radiation left behind by the Big Bang. This 'fossil' radiation is mottled by tiny temperature differences, which equate to differences in the density of primordial matter from the early universe.
Type Ia supernovae are also useful in measuring cosmic distances due to their standardizable maximum luminosity from which we can interpret their accurate intrinsic luminosities. Based on how bright or faint they appear to us, astronomers can calculate how far they are from us. From there, experts can compare this distance with the supernova's redshift, or a measure of how a cosmic object is moving compared to us.
The lensed supernovae in MRG-M0138 have an extra advantage since they appear in five different lensed images of the galaxy. When a supernova explodes behind a gravitational lens, its light reaches us through several paths. Such paths are of different lengths so that the supernova can appear in the images separated by days, weeks, or even years. By measuring differences in the times that the supernova images appear, astronomers can measure the history of the universe's expansion rate.
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