For over two decades, the concept of dark energy has dominated our understanding of the universe’s accelerating expansion. Introduced as a mysterious force responsible for this acceleration, dark energy was first theorized to explain observations of distant supernovae appearing to move away from us at increasing speeds. However, new research published in the journal Monthly Notices of the Royal Astronomical Society Letters suggests that dark energy may not exist after all. Instead, the findings propose a revolutionary alternative explanation for the universe’s expansion, rooted in the intricacies of cosmic structure and time calibration.
For much of the 20th century, cosmologists adhered to the ΛCDM (Lambda Cold Dark Matter) model, which assumes a uniform, isotropic expansion of the universe. Within this framework, dark energy—accounting for about 68% of the universe’s energy density—was posited as a placeholder to explain the observed acceleration. However, the ΛCDM model has faced mounting challenges, such as the “Hubble tension,” a discrepancy in the measured expansion rate of the universe, and other anomalies in cosmic data.
A team of researchers from the University of Canterbury in Christchurch, New Zealand, has now reanalyzed the evidence using a different approach. They revisited the light curves of supernovae, which are critical for measuring cosmic expansion. Their analysis suggests that the universe’s expansion is not uniform but rather “lumpy” and variable across regions of differing cosmic density. This variability undermines the need for dark energy as an explanatory factor.
The alternative explanation proposed by the team is the “timescape” model of cosmic expansion. This model challenges the simplifying assumptions of the ΛCDM paradigm, particularly the use of Friedmann’s equation—a 100-year-old mathematical formula describing cosmic expansion under the assumption of a homogeneous and isotropic universe. In reality, the cosmos is far from a uniform “featureless soup.” Instead, it consists of a complex web of galaxy clusters, filaments, and vast empty voids.
The timescape model introduces the concept that time itself is not experienced uniformly across different cosmic regions. According to this theory, gravity slows time, so a clock in a dense region, such as the Milky Way, ticks more slowly than a clock in a large, empty void. In practice, this means that billions of additional years would pass in cosmic voids compared to denser regions. Consequently, the apparent acceleration of the universe’s expansion is an artifact of how we calibrate time and distance, rather than evidence of an unknown force like dark energy.
To bolster their argument, the Christchurch team collaborated with the Pantheon+ collaboration, which compiled a catalog of 1,535 well-measured supernovae. Their reanalysis revealed strong support for the timescape model, providing a better fit to the data than the ΛCDM model in explaining cosmic expansion without invoking dark energy. This work also hints at potential resolutions for the Hubble tension, as the variable expansion rates predicted by the timescape model align more closely with observational discrepancies.
One of the most striking implications of the timescape model is its reinterpretation of how cosmic voids dominate the universe’s expansion. As these vast empty regions grow over billions of years, they stretch space more significantly than denser regions, creating the illusion of accelerating expansion when observed from within a galaxy like the Milky Way. The timescape model aligns with Einstein’s general relativity but diverges from the simplistic assumptions of the Friedmann equation.
Further support for this model comes from recent high-precision data collected by the Dark Energy Spectroscopic Instrument (DESI). DESI’s findings suggest that dark energy might not be constant over time, as previously thought, but could evolve. However, these observations remain difficult to reconcile within the ΛCDM framework. The timescape model provides a compelling alternative, explaining these anomalies without requiring dark energy.
Looking ahead, the European Space Agency’s Euclid satellite, launched in July 2023, has the capability to test these competing models. Euclid will provide unprecedented high-quality observations of supernovae and other cosmic phenomena, enabling scientists to distinguish between the predictions of the Friedmann equation and the timescape model. Similarly, NASA’s Nancy Grace Roman Space Telescope, set to launch in the near future, is expected to provide complementary data that could further clarify the nature of cosmic expansion.
While the timescape model offers a promising new perspective, it is not without challenges. Previous tests of the model, including one conducted in 2017, showed only marginal improvements over the ΛCDM model. However, the significantly larger dataset analyzed by the Christchurch team has strengthened the case for timescape, marking a potential paradigm shift in cosmology. The researchers emphasize the need for at least 1,000 independent high-quality supernovae observations to validate their findings definitively.
If the timescape model holds up under further scrutiny, it could fundamentally alter our understanding of the universe. By eliminating the need for dark energy, it simplifies the cosmic puzzle and shifts the focus to the intricate interplay of gravity, time, and structure. This would resolve longstanding anomalies, such as the Hubble tension, and provide a more coherent explanation for the universe’s observed behavior.
The implications of this research extend beyond theoretical physics. Understanding the true nature of cosmic expansion impacts a wide range of scientific disciplines, from astrophysics to cosmology. It also underscores the importance of questioning established paradigms and exploring alternative explanations when faced with unresolved mysteries.
Source: Royal Astronomical Society