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New research suggests that dark energy, the mysterious force driving the accelerating expansion of the universe, may not be operating alone. A recent study analyzing astronomical data indicates that unknown physics might be collaborating with dark energy to impede the formation of large cosmic structures, such as galaxy superclusters. This intriguing finding, stemming from data collected by the Baryon Oscillation Spectroscopic Survey (BOSS), hints at a more complex cosmological model than previously understood, challenging existing theories about the universe’s expansion and the nature of cosmic web formation.
Evidence Suggests Dark Energy is Not Alone in Slowing Cosmic Structure Growth
A fresh examination of astronomical information proposes that unidentified physics is at play, supporting dark energy in functioning almost as “antigravity.” This counteracts gravity’s role in drawing matter together to create extensive formations in the cosmos.
The large-scale structure of the universe encompasses the immense, interconnected arrangements of galaxies, galaxy clusters, and superclusters. These are organized into filaments, voids, and walls that constitute what is known as the “cosmic web.”
Gravity has sculpted this vast network over billions of years. However, the research team discovered that this structure formation is occurring at a slower pace today compared to its rate in the distant past of the 13.8 billion-year-old universe.
Data from BOSS Survey Reveals Clues
Scientists unearthed these indications of novel physics by utilizing data from the Baryon Oscillation Spectroscopic Survey (BOSS).
BOSS meticulously charts the spatial distribution of luminous red galaxies (LRGs) and quasars, which are powered by black holes. This mapping helps detect variations in matter distribution in the early universe, specifically “baryon acoustic oscillations.” These oscillations are imprinted as a “cosmic fossil” in the Cosmic Microwave Background (CMB).
“We observed that the formation of structure in the late universe, as investigated by galaxies within the BOSS survey, appears to be diminished compared to theoretical predictions,” stated Shi-Fan (Stephen) Chen, the team leader and a researcher at Princeton University. “Intriguingly, our findings imply that this suppression is largely independent of dark energy.”
The Mystery of Dark Energy’s Partners
Dark energy serves as a provisional designation for the phenomenon responsible for the accelerating expansion of the universe. Its existence was unveiled in 1998 by two separate teams of astronomers. Current understanding posits that dark energy constitutes roughly 70% of the cosmos’s total matter-energy content.
The prevailing explanation for dark energy is the “cosmological constant,” symbolized by the Greek letter lambda (Λ) in the standard model of cosmology. This model is also recognized as the Lambda Cold Dark Matter (ΛCDM) model.
The cosmological constant embodies “vacuum energy,” the energy inherent in empty space. This notion might seem peculiar, as it is linked to the spontaneous appearance and disappearance of matter and antimatter particle pairs. If a matter-antimatter pair is formed with balanced, opposing energy within a confined region, the overall energy of that space remains zero.
This can be likened to the universe’s line of credit; however, instead of lending currency, it provides energy. Similar to a financial institution, the universe requires this energy to be repaid. Virtual particles achieve this repayment by annihilating one another.
Consequently, “empty space” can never be assured to be truly devoid of content.
Despite the counterintuitive nature of matter emerging from what seems like nothingness, this phenomenon has been experimentally corroborated. The Casimir effect, consistently observed in laboratories globally, stands as a prominent illustration of virtual particles and the quantum fluctuations that give rise to them, and hence, vacuum energy.
A crucial aspect is that if dark energy is indeed the cosmological constant, it should genuinely be constant. Therefore, within the ΛCDM framework, while dark energy influences the rate of universe expansion, the cosmological constant itself should remain unchanged.
Nonetheless, recent discrepancies in findings from the Dark Energy Spectroscopic Instrument (DESI) have generated considerable interest among cosmologists. These results suggested that dark energy might be evolving over time. This dynamic, or “evolving dark energy,” contrasts with the static nature expected by ΛCDM.
“Recent outcomes from DESI indicate that dark energy may not be a cosmological constant, potentially evolving over time. We were curious to investigate if this inconsistency with ΛCDM could be related to the suppressed structure formation,” Chen elaborated.
In an unexpected turn, comparing DESI results with BOSS data has presented the team with an even greater enigma.
Exploring the Link Between Dark Energy and the Cosmological Constant
Regardless of the precise nature of dark energy, whether it is the cosmological constant or an alternative entity, the expansion of spacetime operates at exceedingly large scales. Consequently, everyday experiences, such as coffee cups moving away or commutes lengthening, are unaffected by dark energy.
However, we can observe distant galaxies receding from us at progressively accelerating velocities. We can also discern the imprint of dark energy‘s effects in Baryon Acoustic Oscillation (BAO) fluctuations, preserved within the CMB. This ancient light, originating shortly after the Big Bang, is distributed nearly uniformly throughout the universe.
Therefore, it is logical that dark energy, as a force that propels galaxies apart, has a role in impeding the formation of large-scale structures like galaxy clusters and superclusters.
The remarkable aspect of the results obtained by Chen and his collaborators is their demonstration that large-scale structures are even less prevalent today than predicted not only by the ΛCDM model of cosmology but also when dark energy is considered variable. This suggests the involvement of an additional, unidentified factor.
“Numerous theoretical explanations have been proposed to account for why the observed magnitude of cosmic structure in recent epochs appears to be marginally lower than expected,” Chen noted. “Currently, definitive answers remain elusive.”
Temporal Correlation: Dark Energy and Structure Suppression
Nevertheless, a significant clue exists concerning the diminished formation of extensive cosmic structures. This suppression seems to have commenced around the same period in which dark energy rose to dominance within the universe.
The Era of Matter Dominance in the Early Universe
Although dark energy currently governs the universe, this was not always the case. In the immediate aftermath of the Big Bang, radiation predominated, fueling rapid inflation.
Approximately 70,000 years post-Big Bang, the universe had cooled sufficiently for gravity to overcome radiation pressure. This deceleration of the initial Big Bang-driven expansion led to a near standstill. Subsequently, the first aggregations of matter, stars, and galaxies, were able to coalesce.
Around 9 to 10 billion years following the Big Bang, corresponding to roughly 5 to 4 billion years in the past, an unusual shift occurred. The universe initiated a renewed phase of expansion. Furthermore, this expansion began to accelerate and continues to do so today.
This marks the onset of the dark energy-dominated era. The challenge lies in deciphering the mechanism behind the transition from matter to dark energy dominance.
“The BOSS data provides insights into the universe during the initial phase of dark energy‘s influence, and we hypothesize that this suppression could not have originated significantly earlier,” Chen explained.
Thus, while dark energy appears to be linked to this structural suppression, this enigmatic force alone cannot fully elucidate the reduced formation of large cosmic structures in the contemporary universe.
“By integrating analyses of the peculiar velocities of these galaxies, known as redshift-space distortions, and their correlation with the weak lensing of the CMB, we calculate the probability of our findings being attributable to mere chance as 1 in 300,000,” Chen stated. “This strongly implies either the presence of undiscovered physics or the existence of an unidentified systematic flaw within the BOSS data.”
The researcher added that with enhanced data on the horizon, including the initial public release of galaxy clustering data from DESI from the prior week, the team intends to re-apply their methodologies, contrast new outcomes with current findings, and identify any statistically meaningful differences.
“I believe that currently, more questions than answers exist,” Chen concluded. “This research certainly reinforces the notion that discrepancies are emerging between different cosmological datasets when interpreted within the standard ΛCDM model of cosmology.”
The team’s research has been documented in the journal Physical Review Letters.