Cosmic Structures’ Growth Rate Defies Einstein’s Predictions, Dark Energy’s Dominance Explored

In a groundbreaking revelation, scientists have unveiled a surprising phenomenon: cosmic structures are expanding at a pace slower than envisaged by Einstein’s Theory of General Relativity. Moreover, this study suggests that dark energy, a mysterious force, wields more influence in suppressing cosmic structure growth than previously understood. This revelation has the potential to reshape our comprehension of dark matter, dark energy, and the fundamental theories governing our cosmos.

As the universe undergoes its relentless evolution, scientists had long anticipated that large cosmic structures would grow at a specific rate. In this envisioned scenario, dense regions such as galaxy clusters would progressively densify, while the vast voids of space would correspondingly grow more desolate.

However, researchers from the University of Michigan have unearthed evidence indicating that the expansion of these colossal structures occurs at a slower pace than the venerable predictions of Einstein’s Theory of General Relativity. Furthermore, their investigations underscored that as dark energy continues to fuel the universe’s global expansion, the inhibition of cosmic structure growth is even more conspicuous than what the theory had originally envisaged. This groundbreaking revelation was published in the journal Physical Review Letters on September 11.

The Cosmic Web

The universe is a vast tapestry where galaxies are intricately woven together, resembling a colossal cosmic spider web. Remarkably, this distribution is not a random occurrence. Instead, galaxies have a proclivity to cluster together. The entire cosmic web’s genesis traces back to minuscule clumps of matter in the early universe, which gradually evolved into individual galaxies and, over time, coalesced into galaxy clusters and interconnected filaments.

Minh Nguyen, the lead author of this study and a postdoctoral research fellow in the U-M Department of Physics, expounds on this phenomenon. “Throughout cosmic time, a minuscule mass clump attracts and accumulates matter from its local surroundings through gravitational interactions. As this region steadily grows denser, it eventually succumbs to its own gravitational forces,” he explains. “This is what we mean by growth. It resembles a weaving loom where one-dimensional, two-dimensional, and three-dimensional collapses manifest as a sheet, a filament, and a node. In reality, it’s a fusion of all three scenarios, with galaxies inhabiting the filaments, while galaxy clusters, the most massive entities in our universe governed by gravity, reside at the nodes.”

Dark Energy and Cosmic Expansion

The universe is not solely comprised of matter; it conceals a enigmatic component known as dark energy. Dark energy impels the universe’s expansion on a grand scale. Yet, intriguingly, it exerts a contrary effect on the development of large cosmic structures.

“If gravity functions as an amplifier, enhancing the growth of matter perturbations into vast-scale structures, then dark energy operates as an attenuator, dampening these perturbations and decelerating structural growth,” elucidates Nguyen. “By scrutinizing the clustering and growth of cosmic structures, we endeavor to fathom the intrinsic nature of gravity and dark energy.”

Methodology and Probes

To unveil these revelations, Nguyen, along with U-M physics professor Dragan Huterer and U-M graduate student Yuewei Wen, harnessed various cosmological probes to scrutinize the temporal evolution of vast-scale structures across cosmic epochs.

Initially, they leveraged the cosmic microwave background, composed of photons emitted shortly after the Big Bang. These photons provide a glimpse into the early universe. As these photons traverse space to reach our telescopes, they may be distorted, or gravitationally lensed, by the vast-scale structures they encounter en route. Analyzing these distortions enables researchers to deduce how matter and structures between us and the cosmic microwave background are distributed.

Additionally, the team utilized the phenomenon of weak gravitational lensing of galaxy shapes. Light from background galaxies experiences distortion as it interacts gravitationally with foreground matter and galaxies. Cosmologists decode these distortions to discern the distribution of intervening matter.

“Crucially, given that the cosmic microwave background and background galaxies are situated at differing distances from our vantage point and our telescopes, galaxy weak gravitational lensing typically probes matter distributions at a later point in time compared to the cosmic microwave background weak gravitational lensing,” Nguyen elucidates.

To extend their scrutiny of structural growth to even later epochs, the researchers examined the motion of galaxies within our local universe. As galaxies gravitate toward the gravitational wells created by the underlying cosmic structures, their movements offer direct insights into structural growth.

Nguyen notes, “The disparities in these growth rates we potentially identified become more apparent as we approach the contemporary era. These diverse probes, both individually and collectively, signify a suppression of growth. This raises the possibility that we may be overlooking systematic errors in each of these probes or overlooking hitherto uncharted late-time physics in our conventional model.”

Addressing the S8 Tension

This revelation holds the promise of resolving a long-standing quandary in cosmology known as the S8 tension. S8 is a parameter that characterizes structural growth. The tension emerges when scientists employ two distinct methods to determine the S8 value, and these values do not align. The first method, which relies on data from the cosmic microwave background, points to a higher S8 value than the one inferred from measurements involving weak gravitational lensing of galaxies and galaxy clustering.

It’s imperative to acknowledge that neither of these probes directly measures the contemporary growth of structures. Instead, they probe structures at earlier junctures in cosmic history and then extrapolate these findings to the present day, under the assumption of the standard model. Cosmic microwave background measurements pertain to structures in the early universe, whereas measurements from weak gravitational lensing and clustering pertain to structures in the later universe.

The researchers’ groundbreaking discovery of a late-time suppression of growth holds the potential to reconcile these two disparate S8 values, as Nguyen suggests. “We were taken aback by the high statistical significance of this unusual growth suppression,” adds Huterer. “Frankly, it feels as though the universe is attempting to communicate with us. The responsibility now rests with cosmologists to interpret these findings fully.”

Nguyen concludes, “We aspire to bolster the statistical evidence for this growth suppression and delve deeper into the intricacies of why structures evolve at a slower pace than predicted by the standard model, which incorporates dark matter and dark energy. The origin of this phenomenon may be attributed to novel attributes of dark energy and dark matter or to an extension of General Relativity and the standard model that has eluded us thus far.”

Reference: “Evidence for Suppression of Structure Growth in the Concordance Cosmological Model” by Nhat-Minh Nguyen, Dragan Huterer, and Yuewei Wen, 11 September 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.131.111001

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