In 2011, three astrophysicists – Saul Perlmutter from the University of California, Berkeley; Adam Riess from Johns Hopkins University; Brian Schmidt from the Australian National University – were awarded the Nobel Prize in Physics for their discovery that the Universe was expanding at an accelerating rate. The discovery led to widespread acceptance of the idea that our universe is dominated by a mysterious force called dark energy, which altered the standard model of cosmology. Physicists are now questioning this conclusion, backed up with astronomical observations.
During the 1990s, these scientists were part of competing teams measuring distant Type 1A supernovae – explosions of a white dwarf. While a typical white dwarf is only slightly larger than Earth, it will have approximately the same mass as the Sun. Because each Type 1a supernova explodes with roughly the same brightness, the amount of light the supernova gives off can be used as an indication of its distance from Earth – and the shift in its spectrum can be used to determine how fast the white dwarf was moving.
When Perlmutter, Riess, and Schmidt measured all the data for known Type 1a supernovae, recorded by the Hubble space telescope and a number of large ground-based telescopes, they claimed that expansion of the universe was not slowing down, but was accelerating.
By comparing the brightness of distant supernovae with the brightness of nearby supernovae, the scientists discovered that the distant supernovae were about 25 percent too faint. They appeared farther than expected, and the researchers concluded that the Universe was accelerating. The find was backed up by data collected on clustering galaxies and the cosmic microwave background radiation – supposedly the faint afterglow of the Big Bang. Early in 2016, NASA and ESA scientists found that the universe could be expanding around 8 percent faster than originally thought.
The discovery created a sensation among cosmologists and astrophysicists, and presented a seemingly insoluble problem – how to explain this acceleration. If gravity generated by all matter in the universe by the Big Bang has been slowing everything down, how can it be accelerating? Cosmologists theorized that ‘something’ pervading the universe must be causing negative pressure on the universe, causing acceleration in its expansion. They called it ‘dark energy’, which created another problem. What is dark energy, and how was it originally created?
However, the Oxford team led by Professor Subir Sarkar, a fellow at the Neils Bohr Institute, is challenging the acceleration of the universe’s expansion, using a much larger database of Type 1a supernovae observations to back them up. By applying a different analytical model to the identified 740 Type Ia supernovae, the team claims that the statistical techniques used by the original team were too simplistic, and were based on a model devised in the 1930s, which could not reliability be applied to the observable supernovae events.
Instead of finding evidence to support the accelerated expansion of the universe, maintain that the universe is expanding at a constant rate. A more sophisticated theoretical framework accounting for the observation that the Universe is not exactly homogeneous, and that the distribution of matter may not behave like an ideal gas – two key assumptions of standard cosmology – may account for all observations without the requirement for dark energy.
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.”
Delving more deeply into the theory, correcting Newton’s formula for gravity may make the whole problem go away. According to Xiaochun Mei and Ping Yu at the Institute of Innovative Physics in Fuzhou, Department of Physics, Fuzhou University, Fuzhou, China, by transforming the geodesic equation of the Schwarzschild solution of the Einstein’s equation of gravity field to the flat spacetime, a revised Newtonian formula of gravity is generated. The formula also describes the motion of an object with mass in a gravity field, such as the perihelion precession of Mercury. The spacetime singularity in Einstein’s theory of gravity becomes the original point r = 0 in the Newtonian formula of gravity. The singularity problem of gravity in curved spacetime is also eliminated. When the formula is used to describe the expansive universe, the revised Friedmann equation of cosmology is obtained, which accounts for the high red-shift of Ia supernovae. This eliminates the accelerated expansion of the universe and existence of dark energy. The problem of the universe’s actual age can also be solved. Using the revised formula, the theory of gravity returns to the traditional form of dynamic description and becomes normal. The revised equation can then form the foundation for a more rational theory of cosmology.
Given the growing volume of observable astronomical data, uncertainty in the application of Einstein and Newton’s equations, the hypothesis that the expansion of the universe is accelerating, accounted for by introduction of dark energy, leaves lots of room for energetic debate among astrophysicists and cosmologists.
