gravity from entropy quantum gravity

Gravity from Entropy: A Bold New Theory Bridging Quantum Mechanics and General Relativity

Introduction

Professor Ginestra Bianconi, an applied mathematics expert at Queen Mary University of London, presents a pioneering framework in Physical Review D that could redefine the link between gravity and quantum mechanics.

The Study Overview: Gravity from Quantum Relative Entropy

The study, Gravity from Entropy, presents and innovative framework that derives gravity from quantum relative entropy, offering a potential bridge between quantum mechanics and Einstein's general relativity—two historically conflicting theories.

Challenges in Unifying Quantum Gravity

The Struggle to Integrate Quantum Mechanics and General Relativity

Physicists have long grappled with the challenge of unifying quantum mechanics and general relativity—two foundational but seemingly incompatible theories. While quantum mechanics dictates particle behavior at microscopic scales, general relativity governs gravitational forces on a cosmic level. Bridging this theoretical divide remains one of the greatest challenges in modern physics.

Professor Bianconi's Groundbreaking Framework

Reinterpreting the Spacetime Metric as a Quantum Operator

Professor Bianconi's research introduces a novel framework in which the spacetime metric—central to general relativity—is reinterpreted as a quantum operator. By leveraging quantum relative entropy, a principle from quantum information theory, this approach provides new insights into the relationship between spacetime geometry and matter.

The Role of Entropy and the G-field

Entropic Action and Deviations in Spacetime Metrics

The research presents an entropic action framework that measures the deviation between spacetime metrics and those influenced by matter fields.

Modifying Einstein's Equations and Predicting a Cosmological Constant

This framework modifies Einstein's equations, which, under low coupling conditions—characterized by low energy and minimal curvature—converge to classical general relativity. Crucially, it also predicts a small, positive cosmological constant, aligning more accurately with observed cosmic acceleration than alternative theories.

The G-field and its Role in Gravity and Dark Matter

This theory introduces the G-field, an auxiliary field serving as a Lagrangian multiplier. It not only refines the structure of modified gravitational equations but also provides a new theoretical avenue for understanding dark matter, whose elusive nature remains one of modern physics' greatest challenges.

Wider Implications and Future Directions

Quantum Gravity and the Unification of Theoretical Physics

This research carries significant implications, offering a novel connection between gravity and quantum information theory. It paves the way for a potential unification of quantum gravity while also providing fresh perspectives on the elusive nature of dark matter.

The Emergent Cosmological Constant and Expanding Our Understanding of the Universe

"Our research suggests that quantum gravity originates from entropic principles, with the G-field potentially serving as a candidate for dark matter," states Professor Bianconi. "Furthermore, our model's emergent cosmological constant may bridge the gap between theoretical predictions and observed cosmic expansion."

The Future of This Theory and Its Potential Impact

While further investigation is necessary to comprehensively assess this theory's implications, the study constitutes a pivotal stride toward deciphering the fundamental nature of the cosmos.

Disrupting Traditional Paradigms in Physics

Professor Bianconi's research disrupts traditional paradigms, introducing novel avenues for inquiry. By conceptualizing spacetime as a quantum entity and harnessing entropy within spacetime metrics, this work offers profound insights into gravity, quantum mechanics, and the cosmos.

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astronomers unveil cosmic complexity

Astronomers Reveal How the Universe Has Become More Complicated

Understanding the Cosmic Complexity

Throughout cosmic history, immense forces have shaped matter, creating a complex web of structures. Research by Joshua Kim, Mathew Madhavacheril (University of Pennsylvania), and Lawrence Berkeley National Laboratory indicates that over 13.8 billion years, the universe's matter distribution has become unexpectedly less "clumpy" than theoretical models predict, suggesting increasing cosmic complexity.

Combining Datasets for a Comprehensive View of the Universe

"Our study combined two distinct datasets from complementary surveys," explains Madhavacheril, "Overall, our findings align closely with predictions from Einstein's gravity, although we observed a slight deviation in the expected clumpiness of matter around four billion years ago, warranting further investigation."

The Datasets Behind the Discovery: ACT and DESI

The data originates from the Atacama Cosmology Telescope's final data release (DR6) and the Dark Energy Spectroscopic Instrument's Year 1 findings, published in the Journal of Cosmology and Astroparticle Physics and on arXiv.

The Cosmic Microwave Background (CMB) and Gravitational Lensing

Madhavacheril explains that combining these datasets enabled the team to overlay cosmic time, similar to layering ancient and modern cosmic images for a more comprehensive view of the universe.

Understanding the CMB and the Universe's Early Stages

Joshua Kim, the lead author and graduate researcher in the Madhavacheril Group, explains that ACT, which spans about 23% of the sky, offers a glimpse into the universe's early stages by capturing faint light that has been traveling since the Big Bang.

Technically known as the Cosmic Microwave Background (CMB), this light is often referred as the universe's "baby picture" because it represents a snapshot from when the universe was approximately 380,000 years old.

Gravitational Lensing: A Key Tool in Understanding Cosmic History

The journey of this ancient light across cosmic history has not been linear, as Kim explains. Gravitational forces from massive, dense structures like galaxy clusters have bent the CMB, much like how an image is distorted when viewed through glasses.

The "Gravitational lensing effect," predicted by Einstein over a century ago, allows cosmologists to draw conclusions about the universe's properties, such as matter distribution and its age.

DESI and the Modern Perspective on Cosmic Structures

On the other hand, DESI offers a more up-to-date perspective on the cosmos. Located at the Kitt Peak National Observatory in Arizona and managed by Lawrence Berkeley National Laboratory, DESI is charting the universe's three-dimensional structure by examining the distribution of millions of galaxies, with a focus on luminous red galaxies (LRGs). These galaxies serve as cosmic markers, enabling scientists to track the spread of matter over billions of years.

Luminous Red Galaxies (LRGs): A Cosmic Yearbook

According to Kim, the luminous red galaxies (LRGs) from DESI offer a more up-to-date snapshot of the universe, revealing the distribution of galaxies at different distances. He compare it to a high school yearbook photo of the cosmos, providing a clear view of how cosmic structures have evolved from the early CMB map to their current distribution.

Unprecedented Overlap Between Ancient and Modern Cosmic History

By integrating ACT's CMB lensing maps with DESI's LRGs, the team achieved an unprecedented overlap between ancient and modern cosmic history, allowing for direct comparisons between early and late-universe measurements.

A Cosmic CT Scan: Tracing the Evolution of Matter

"This method functions similarly to a cosmic CT scan," says Madhavacheril. "It lets us observe various slices of cosmic history and trace the clustering of matter over time. This approach provides a clear view of how the gravitational pull of matter has changed throughout the ages."

Unexpected Findings the Quest for New Physics

In the process, they observed a slight inconsistency: the expected density fluctuations or clumpiness at later epochs didn't align perfectly with the theoretical predictions.

Sigma 8 (σ₈): A Key Metric in Understanding Cosmic Structures

Kim explains that Sigma 8 (σ₈), a crucial metric that gauges the amplitude of matter density fluctuations, plays a pivotal role. Lower σ₈ values suggest less clumping than anticipated, potentially indicating that cosmic structures haven't evolved as early-universe models predicted and hinting at a slower rate of structural growth than current models account for.

Exploring New Physics: What's Next?

He clarifies that the minor discrepancy with expectations "Isn't sufficient to definitively point to new physics—there remains a possibility that this deviation is purely coincidental."

The Role of Dark Energy in Shaping the Cosmos

If the deviation isn't product of chance, it could indicate the involvement of unaccounted-for physics, potentially altering how structures form and evolve across cosmic history. One possibility is that dark energy—the enigmatic force accelerating the universe's expansion —might be playing a more significant role in shaping cosmic structures than previously realized.

Looking Ahead: Future Collaborations and Advancements in Telescopes

In the future, the team plans to collaborate with advanced telescopes, such as the forthcoming Simons Observatory, which will enhance these measurements, offering a more detailed understanding of cosmic structures.

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dark energy debate timescape expansion

Dark Energy Debate: Physicists Question Its Role in Expanding the 'Lumpy' Universe

Introduction to the Dark Energy Debate

Researchers investigating the expansion of the universe propose that dark energy, long considered one of science's greatest mysteries, may not exist.

The results of their analysis are published in the journal Monthly Notices of the Royal Astronomical Society Letters.

The Traditional Assumption: Isotropic Cosmic Expansion

Over the past 100 years, scientists have generally operated under the assumption of isotopic cosmic expansion, with dark energy serving as a provisional explanation for mysterious, poorly understood physics.

The New Challenge: Irregular, "Lumpier" Universe Expansion

Researchers at the University of Canterbury in Christchurch, New Zealand, are challenging conventional views, employing refined supernovae light curve analysis to suggest the universe expands in a more irregular, "Lumpier" manner.

The Timescape Model of Cosmic Expansion

The findings bolster the "Timescape" model of cosmic expansion, which eliminates the need for dark energy, attributing differences in light stretching to time and distance calibration rather than cosmic acceleration.

Gravitational Time Dilation and Its Role

The theory incorporates gravitational time dilation, where a clock in the emptiness of space ticks more rapidly than one within a galaxy.

The model proposes that a clock within the Milky Way runs approximately 35% slower than one situated in the average position of large cosmic voids. Consequently, billions of additional years would pass in voids, facilitating greater spatial expansion and creating the illusion of accelerated expansion as these voids increasingly dominate the universe.

Dark Energy Misconception and the Need for a New Approach

Professor David Wiltshire, the study's lead author, remarked, "Our research demonstrates that dark energy is not required to explain the apparent acceleration of the universe's expansion."

"Dark energy is mistakenly identified as variations in the kinetic energy of expansion, which is inherently uneven in a universe as heterogeneous as ours."

He added, "The study offers persuasive evidence that could address some of the fundamental mysteries of our expanding universe."

"New data could resolve the universe's greatest mystery within the next decade."

The Lambda Cold Dark Matter (ΛCDM) Model and Its Challenges

Dark energy is widely believed to be a weak repulsive force, acting independently of matter, comprising approximately two-thirds of the universe's mass-energy density.

The Lambda Cold Dark Matter (ΛCDM) model necessitates dark energy to account for the observed acceleration int he universe's expansion rate.

Researchers derived this conclusion from measurements of supernova distances in distant galaxies, which seem farther than expected without accelerated cosmic expansion.

Discrepancies in Cosmic Expansion and Hubble Tension

Nevertheless, recent observations are progressively questioning the current rate of the universe's expansion.

Initial evidence from the Cosmic Microwave Background (CMB), the afterglow of the Big Bang, reveals discrepancies between the early and current rates of cosmic expansion, a phenomenon referred to as 'Hubble tension."

New Data from DESI and Its Impact

Additionally, a new analysis of high-precision data from the Dark Energy S pectroscopic Instrument (DESI) reveals that models incorporating evolving dark energy fit the observations better than the ΛCDM model.

The Complex Structure of the Universe: A Cosmic Web

Both the Hubble tension and the new insights uncovered by DESI present challenges for models that rely on the simplified cosmic expansion law established 100 years ago—Friedmann's equation.

This assumption relies on the notion that the universe expands uniformly on average, as though all cosmic structures could be blended into a homogeneous state, devoid of distinguishing features. In reality, however, the universe is characterized by a complex web of galaxy clusters arranged in sheets and filaments, interspersed with vast, empty voids.

Why Friedmann's Equation May Not Apply to Our Universe

Professor Wiltshire emphasized, "With the vast amount of data available to us today, the 21st century offers the opportunity to answer the fundamental question: how and why does a simple average expansion law emerge from such complexity?"

"A straightforward expansion law, in line with Einstein's general relativity, need not adhere to Friedmann's equation."

Testing the Timescape Model with the Euclid Satellite

The researchers asset that the European Space Agency's Euclid satellite, launched in July 2023, is equipped to test and differentiate between the Friedmann equation and the timescape alternative. However, this will necessitate a minimum of 1,000 independent, high-quality supernova observations.

Collaborating with Pantheon+ for Improved Data

The last test of the proposed timescape model in 2017 indicated that it was only a marginally better fit than the ΛCDM for explaining cosmic expansion. As a result, the Christchurch team collaborated closely with the Pantheon+ collaboration, which had meticulously compiled a catalog of 1,535 distinct supernovae.

Robust Evidence Supporting the Timescape Model

The researchers assert that the new data offers 'robust evidence' supporting the timescape model. Additionally, it could provide a convincing solution to the Hubble tension and other anomalies associated with the universe's expansion.

The Role of Upcoming Observations in Strengthening the Timescape Model

The researchers emphasize that additional observations from Euclid and the Nancy Grace Roman Space Telescope are crucial to strengthening support for the timescape model. The current focus is on leveraging this new data to uncover the true nature of cosmic expansion and dark energy.

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