According to our best understanding, the universe is expanding - and is doing so at an accelerating rate. This is believed to be caused by something called the "cosmological constant", which was first proposed by Albert Einstein in his theory of general relativity. In recent decades, it has become better known as dark energy , which is believed to make up about 70% of the universe.
Author
- Subir Sarkar
Emeritus Professor, Department of Physics, University of Oxford
Crucial to this realisation were studies of Type Ia supernovae - exploding white dwarf stars. These are thought to emit a specific amount of light, which allows astronomers to determine their distances very accurately and thereby track the expansion of the universe. This work was awarded the 2011 Nobel prize in physics .
The accelerating expansion of the universe is thought to be due to negative pressure, an unusual property of dark energy that allows it to overcome the attractive force of gravity. Yet the exact nature of dark energy remains a puzzle. It cannot be explained by our best theory for the fundamental building blocks of the universe - known as the standard model of particle physics .
However, it is an integral part of another theory, devised to explain how the large-scale universe came to be. This is the standard model of cosmology, also called the lambda cold dark matter (ΛCDM) model .
Other observational evidence supporting the existence of dark energy includes fluctuations in the temperature of the cosmic microwave background (CMB) - the "afterglow of the Big Bang" - and the imprint of baryon acoustic oscillations (BAO) - sound waves from the early universe - in the distribution of galaxies.
But the ΛCDM model has come under increasing scrutiny as new data has emerged. A conference hosted by the UK's Royal Society debated emerging cracks in the standard cosmological model. It has been noted that the model's past successes may partly be due to "confirmation bias" - the unconscious tendency to favour information that supports our existing beliefs while giving less attention to information that challenges them.
Flawed framework?
Most cosmologists remain unconvinced by such criticism and are instead persuaded by the multiple lines of evidence that the universe's expansion is accelerating.
However, all such analyses, for example of the CMB or BAO, are conducted using a special solution of Einstein's equations called the FLRW (Friedmann-Lemaitre-Robertson-Walker) framework. This is a description of space-time that assumes the "cosmological principle", namely that the universe is, on average, homogeneous and isotropic (looks the same in all directions).
However, our recent study found evidence to falsify the FLRW framework, presenting results showing that the universe is asymmetric, or lopsided . We believe this presents a fundamental challenge to dark energy by undermining the very basis for standard analyses of cosmological data.
The observation that the universe may be lopsided is known as the cosmic dipole anomaly . This anomaly cannot be fixed by tweaking parameters in the ΛCDM model. It requires starting again from scratch to build a model of the universe based on observations, rather than on a philosophical principle.
The Nobel prize winning astrophysicist Jim Peebles has said that the cosmic dipole anomaly is about as well established as another cosmological irregularity known as the Hubble tension . However, he notes, the cosmic dipole anomaly has received only a fraction of the interest heaped on the Hubble tension.
With my collaborators, Mohamed Rameez and his PhD student Animesh Sah at the Tata Institute of Fundamental Research, Mumbai, we have also shown that the acceleration of the expansion of the universe inferred from Type Ia supernovae is not the same in all directions .
The signal from supernovae is mainly aligned with a temperature difference in the afterglow of the Big Bang - the cosmic microwave background. This temperature difference, called the CMB dipole anisotropy, is believed to be due to the local motion of the solar system.
'Tilted observers'
The acceleration inferred from supernovae cannot therefore be due to dark energy. Rather, it is probably an illusion, because we are "tilted observers". This is because of Earth's specific location in the cosmos, where our neighbouring galaxies are participating in a large-scale but localised streaming motion called the bulk flow.
The relationship between the intrinsic brightness of Type Ia supernovae and their distance has been crucial to the idea of dark energy. However, this relationship may not be as robust as was first assumed.
Astronomers in South Korea found evidence that the ages of the white dwarf stars that produce Type Ia supernovae (their progenitors) affect their brightness. However, this has been challenged by another team .
When we corrected for the dependence of the intrinsic brightness on the progenitor age, the indication is that the universe is decelerating , rather than accelerating.
This is just as is expected for a universe without dark energy. But it will come as a surprise to most cosmologists who believe that there is compelling independent evidence for ΛCDM, in particular from studies of the CMB. But CMB temperature fluctuations are not influenced by dark energy. Rather, they are related to the spatial curvature of the universe, the baryon density, and the dark matter density.
Whether dark energy is still a valid inference thus depends on whether the distribution of matter in the universe is indeed isotropic - exactly the same in all directions. Observations suggest otherwise. If these findings are confirmed, they could signal a paradigm shift in cosmology.
Subir Sarkar receives funding from the UK Research & Innovation councils and from the Indian Ministry of Science & Technology.
