It would be really, really exciting if a single observation could completely overturn astrophysicists’ current understanding of the universe. But that hasn’t happened yet, at least with regards to dark energy.
This week, a press release proclaimed that “new evidence shows that the key assumption made in the discovery of dark energy is in error,” garnering some attention from astronomers and riling up science sceptics. But scientists have already identified some issues with the paper’s claims.
Today, the most popular theory about the behaviour of the universe on the largest scale is called lambda-CDM, and it might seem a little discomforting. Essentially, it says that the regular matter we can observe directly makes up less than 5 per cent of the universe. Meanwhile, dark matter, stuff whose effects we can see only indirectly via the gravity it exerts, makes up slightly less than a quarter. The rest is dark energy, a mysterious force causing the universe’s expansion to speed up.
Several independent lines of evidence support dark energy’s existence. Scientists use a kind of supernova called Type 1a as a “standard candle”—they understand the brightness of these supernovae well, so the distance to the supernova is easy to calculate based on how bright it appears from Earth. Then, vibrational fluctuations through the early universe locking into place as it cooled left an imprint on the distribution of normal matter, an imprint that scientists can use as a kind of ruler in the sky. Understanding distances allows scientists to calculate the expansion of the universe, further providing evidence of dark energy. Finally, the temperature fluctuations in the universe’s most distant detectable radiation, the cosmic microwave background, seem to show that the universe is flat—a property that would require dark energy for the mathematics to work out right.
Three astronomers won the 2011 Nobel Prize for their 1998 discovery in which they used Type Ia supernovae to determine that the universe’s expansion was accelerating, thus suggesting the existence of dark energy. The standard candles gave them reliable distances, while the amount each supernova’s colour had changed as its light travelled to Earth, something known as its redshift, told them its velocity. Together, this yielded a calculation of the acceleration of the universe’s expansion.
Here’s where the controversial new paper comes in. The work, by researchers at Yonsei University and the Korea Astronomy and Space Science Institute in Korea plus the University of in Lyon at France, took observations of Type Ia supernova in nearby galaxies using the Las Campanas Observatory (LCO) 2.5-metre telescope and the MMT 6.5-metre telescope. They concluded that Type Ia supernovae might not be the standard candles that astronomers initially thought. Their work found statistical evidence of a correlation between the age of stars in 32 nearby galaxies and the brightness of the Type Ia supernovae these galaxies contain. As such, they say that the research casts doubt on the very existence of dark energy, because the major evidence pointing to its existence relies on these supernovae all having a predictable brightness. If the brightness differs from expectations, as the new study finds, then the calculations that lead to dark energy must be wrong.
One researcher not involved in the study, Renée Hložek, professor of astronomy and astrophysics at the Dunlap Institute for Astronomy at the University of Toronto, told Gizmodo that the results rely on extrapolations between these nearby galaxies and farther-away galaxies. Plus, the work implies that some of these galaxies must be older than the age of the universe—something that’s obviously not possible.
Dan Scolnic, assistant professor of physics who studies Type Ia supernovae at Duke University, told Gizmodo that the researchers used a small and biased sample of only old and nearby galaxies. But the dark energy measurements are made with a much larger sample of supernovae and often much younger galaxies that aren’t totally described by this sample.
Hložek told Gizmodo that this work is valuable, especially as these kinds of supernovae will be a target of the upcoming Vera Rubin Observatory. But she’d like to see a more robust study on a larger, more relevant sample.
This paper isn’t the first to rile up cosmologists by contending that dark energy isn’t there. Another paper from last year claimed that our observations of supernovae were actually an artefact of our region of space accelerating. Cosmologists found that paper flawed as well, reported Quanta.
This certainly won’t be the last attempt at overthrowing dark energy, as there’s a fame that comes with proving a paradigm wrong. That’s not to say that dark energy should be treated as dogma; the whole point of science is to falsify enough hypotheses such that only the truth remains. Scientists should absolutely be questioning paradigms and finding discrepancies whenever they can.
But in this case, it seems the evidence won’t be enough to convince mainstream cosmologists.