Researchers investigating the enigma of the Universe’s expansion claim that one of science’s greatest mysteries — dark energy — may not actually exist.
For the last century, physicists have largely believed that the Universe is expanding uniformly in all directions. They introduced the idea of dark energy to fill in gaps in their understanding of the underlying physics, but this controversial idea has always faced its share of challenges.
Recently, a group of physicists and astronomers from the University of Canterbury in Christchurch, New Zealand, has started to overturn this conventional view. By enhancing the analysis of light patterns from supernovae, they demonstrate that the Universe is expanding in a more complex and ‘lumpier’ manner.
Their new findings support the “timescape” model of cosmic expansion, which eliminates the need for dark energy. According to this model, the shifts in light are not due to an accelerating Universe, but rather how we measure time and distance.
This perspective takes into account the effect of gravity on time; thus, a perfect clock in empty space would tick faster than one immersed in a galaxy.
Under this model, a clock located in the Milky Way would tick roughly 35 percent slower than a similar clock positioned in a large cosmic void, implying that billions more years would pass in those voids. Consequently, this would allow for greater space expansion, making it seem as if the Universe is accelerating when such vast empty regions grow to dominate it.
Professor David Wiltshire, the study’s lead researcher, stated, “Our findings indicate that we do not require dark energy to explain why the Universe appears to be expanding at an accelerating rate.”
“The idea of dark energy mistaking variations in the kinetic energy of expansion is inappropriate in a Universe as heterogeneous as our own.”
He added, “This research offers strong evidence that could clarify some of the major issues concerning the peculiarities of our expanding Universe.
“With new data, we might resolve the Universe’s biggest mystery within the decade.”
This fresh analysis has been published in the journal Monthly Notices of the Royal Astronomical Society Letters.
Typically, dark energy is viewed as a weak anti-gravity force acting independently of matter, accounting for about two-thirds of the Universe’s mass-energy density.
The standard model of the Universe, known as Lambda Cold Dark Matter (ΛCDM), relies on dark energy to account for the observed acceleration in cosmic expansion.
This conclusion arises from measurements of distances to supernovae in remote galaxies, which appear more distant than expected if the Universe were not accelerating in its expansion.
However, the current understanding of the Universe’s expansion is increasingly challenged by new observational evidence.
For instance, data from the aftermath of the Big Bang — referred to as the Cosmic Microwave Background (CMB) — suggests that the early Universe’s expansion contradicts current expansion rates, a discrepancy known as “Hubble tension.”
Also, recent high-precision analyses from the Dark Energy Spectroscopic Instrument (DESI) reveal that the ΛCDM model does not accurately fit the observed phenomena as well as alternative models that allow dark energy to fluctuate over time.
Both the Hubble tension and unexpected results from DESI are proving challenging to resolve within frameworks that employ a simplified century-old equation for cosmic expansion — Friedmann’s equation.
This equation assumes a uniform average expansion of the Universe, as if all cosmic structures could be blended into a uniform mixture without features. In reality, our Universe consists of an intricate cosmic web, with clusters of galaxies arranged in sheets and filaments interspersed with enormous empty voids.
Professor Wiltshire noted, “We now have so much data that in the 21st century we can finally tackle the question — how and why does a simplified average expansion law arise from this complexity?
“A straightforward expansion law that aligns with Einstein’s general relativity need not conform to Friedmann’s equation.”
The researchers assert that the European Space Agency’s Euclid satellite, launched in July 2023, possesses the capability to investigate and differentiate between Friedmann’s equation and the timescape alternative. However, achieving this will necessitate at least 1,000 high-quality supernova observations.
During the previous examination of the timescape model in 2017, results indicated that it offered only a marginally better fit compared to the ΛCDM model for cosmic expansion. Thus, the Christchurch team collaborated with the Pantheon+ team, who meticulously compiled a catalog of 1,535 distinct supernovae.
They assert that the latest data now presents “very strong evidence” in favor of the timescape model, which may also provide a promising solution to the Hubble tension and other anomalies connected to cosmic expansion.
Further observations from the Euclid mission and the Nancy Grace Roman Space Telescope are essential to enhance the case for the timescape model, as researchers are now in a race to utilize the new wealth of data to unveil the true nature of cosmic expansion and dark energy.
