In 1998, it was discovered that the expansion of the universe is accelerating, a finding that earned three astronomers the Nobel Prize in Physics in 2011, but new research from Oxford University has prompted a rethink of that widely accepted idea. By using a much larger dataset, the team found that the evidence for accelerated expansion doesn't meet the certainty standards required by physicists to declare a discovery.

Type Ia supernovae are often called "standard candles" by astronomers, due to the fact that their peak brightness is consistent and predictable. By measuring how bright that visible light is by the time it reaches Earth, scientists are able to accurately determine how far away the object is. Analyzing these supernovae is what led the researchers in the 1990s to the conclusion that the universe is expanding at an accelerated rate, due to the abundance of "dark" or "vacuum energy."

"The discovery of the accelerating expansion of the universe won the Nobel Prize, the Gruber Cosmology Prize, and the Breakthrough Prize in Fundamental Physics," says Professor Subir Sarkar, lead researcher on the new study. "It led to the widespread acceptance of the idea that the universe is dominated by 'dark energy' that behaves like a cosmological constant – this is now the 'standard model' of cosmology."

In the field of physics, scientists use a scale of "sigma" to determine the certainty of an observation. Before a discovery can be declared, it needs to be shown to reach a level of five sigma, which represents about one chance in 3.5 million that the observation is the result of random fluctuation. Recent discoveries like the Higgs Boson and the detection of gravitational waves both reached a 5-sigma level of certainty.

But when Sarkar's team conducted their own analysis of Type Ia supernovae, they found that the evidence for accelerating universal expansion clocks in at just 3 sigma. Their result incorporates the latest list of 740 objects, which is more than 10 times the amount studied in the original research.

While this finding doesn't necessarily prove that the universe isn't expanding at an accelerated rate, it calls into question deeply-held beliefs that may not stand up to scrutiny. Later studies, including, for example, the cosmic microwave background, which supports the theory of accelerated expansion, may then build on these false assumptions, leading scientists further down the wrong path.

"All of these tests are indirect, carried out in the framework of an assumed model, and the cosmic microwave background is not directly affected by dark energy," explains Sarkar. "It is quite possible that we are being misled and that the apparent manifestation of dark energy is a consequence of analyzing the data in an oversimplified theoretical model – one that was in fact constructed in the 1930s, long before there was any real data."

Revised cosmological models may be able to account for what has been observed without resorting to the mysterious dark energy, which, Sarkar says, "is something of which we have absolutely no understanding in fundamental theory."

The finding may go against the grain of many other studies, including a recent project that suggested the universe is expanding faster than previously thought, but the team hopes the research will help others question assumptions and lead to the development of more airtight models.

"Hopefully this will motivate better analyses of cosmological data, as well as inspiring theorists to investigate more nuanced cosmological models," says Sarkar. "Significant progress will be made when the European Extremely Large Telescope makes observations with an ultrasensitive 'laser comb' to directly measure over a 10 to 15-year period whether the expansion rate is indeed accelerating."

The research was published in the journal Scientific Reports.