Two incredibly rare supernovae that exploded billions of years ago offer a unique opportunity to explain one of the biggest mysteries in cosmology: how fast the universe is expanding.
But there’s a twist. Astronomers have already observed these exploding stars, but we have to wait up to 60 years for their light to reach us again.
A phenomenon called gravitational lensing splits the light from these extinct stars into multiple images, each taking a different path through space and time to reach us. As a result, researchers may one day be able to measure the delay between these ghostly images and provide unprecedented constraints on the rate of expansion of the universe. This problem has puzzled scientists for years because the universe appears to be expanding at different rates depending on where you look.
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Space magnifying glass reveals the invisible
These supernova observations are among the first results of the Vast Exploration for Nascent, Unexplored Sources (VENUS) financial program. The VENUS survey will use the James Webb Space Telescope (JWST) to observe 60 dense galaxy clusters. These galaxy clusters act as cosmic lenses that split and focus light from very distant, invisible sources such as supernovae.
This cosmic phenomenon, called gravitational lensing, is a direct result of gravity’s influence on the fabric of space-time and was first proposed by Albert Einstein in his theory of relativity. This occurs when a massive object, such as a galaxy cluster, bends light from more distant sources behind it, thus magnifying the object.
“Powerful gravitational lensing turns galaxy clusters into nature’s most powerful telescopes,” Seiji Fujimoto, principal investigator of the VENUS program and an astrophysicist at the University of Toronto, said in a statement. “VENUS is designed to maximize the discovery of the most unusual events in the distant universe, and these lenses will [supernovas] This is precisely the kind of phenomenon that only this approach can reveal. ”
SN Ares is the first lenticular supernova discovered by the VENUS program. This explosion occurred about 10 billion years ago, when the universe was about one third as old as it is now. Space-time distortions caused by the foreground galaxy cluster MJ0308 split the light from SN Ares into three images.
One image has already arrived at our telescope. However, the light from the other two images passes much closer to MJ0308’s massive center, causing a significant slowdown due to gravitational time dilation. Therefore, the other two images of SN Ares will arrive about 60 years later. This is an unprecedented delay.
“Such a long-anticipated delay between images of strongly lensed supernovae has never been seen before, and could represent an opportunity for predictive experiments that could impose incredibly precise constraints on cosmological evolution,” Larison said in a statement.
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Meanwhile, delayed images of SN Athena, which erupted as a supernova when the universe was about half its current age, are expected to arrive within the next year or two. Although not as cosmologically accurate as her mythical half-brother Ares, Athena will reveal just how accurate our predictive abilities have become.
A much-needed natural experiment
These supernovae are predicted to reappear, and when compared to their actual arrival times in the future, they will provide precise constraints on the rate of expansion of the universe, known as the Hubble constant.
Curiously, when cosmologists measure the Hubble constant, they get different values based on how they measure it. This is known as the Hubble tension. Calculations based on the cosmic microwave background radiation (the oldest light in the universe, emitted when the universe was just 380,000 years old) show that the universe is expanding at a rate of 67 kilometers per second per megaparsec.
However, calculations based on Hubble Space Telescope observations of pulsating Cepheid stars, which are used as “standard candles” for certain luminosity patterns, yield a value of 73 kilometers per second per megaparsec.
Within the observable reaches of the universe, delayed images from SN Ares and SN Athena could help reconcile Hubble’s strains.
“If we can measure the difference in when these images arrive, we can recover measurements of the physical scale of the lens system across the universe between the supernova and us on Earth,” Larison told Live Science in an email. “Being able to measure distances in the universe in this way tells us how the universe has evolved over cosmic time, because these distances directly depend on this evolution.”
Just as importantly, lensed supernovae allow astronomers to make this measurement in “a single, self-consistent step,” Larison added.
The time delays caused by these supernovae also enable independent measurements unrelated to standard candles such as the cosmic microwave background and Cepheid stars, at a time when such measurements are “desperately needed” to test “unknown potential systems” governing cosmological expansion.
From the big bang to the big mysteries
Coincidentally, 60 years have passed since the first formal proposal to use lensing supernovae as a tool to probe the expansion of the universe. However, fewer than 10 such supernovae had been discovered before the VENUS program’s observations.
“Since VENUS began last July, we have discovered eight new lensing supernovae in 43 observations, almost double the number of known samples in an incredibly fast time frame,” Larison told Live Science. “While lensed supernovae are certainly rare, the real limit seems to be observational capabilities. JWST is really the only one that can achieve the depth and wavelength coverage needed to discover them all at once, and that’s exactly what VENUS was designed for.”
As a result, lensing supernovae may be the most exciting prospect in long-baseline cosmology, the study of how the universe has changed over the 13.8 billion years of its existence.
The answer is up in the air. There is no guarantee that the expansion of the universe will continue to accelerate, especially since dark energy may be weakening. If this is the case, the current expansion of the universe could someday lead to contraction, with profound implications for the universe’s ultimate fate.
Ultimately, SN Ares and SN Athena may hint at the potential death of the universe and whether it will end with a roar or a cry. Will the universe collapse in the Big Crunch, or will it expand into infinity into the thin, cold darkness of the Big Freeze?
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