Does dark energy really exist? Our work identifies cracks in the foundations of today's cosmological model Lisa Lock Scientific Editor Robert Egan Senior Editor According to current 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. Crucial to this realization 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 favor 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-Lemaître-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 that falsifies 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.