Theories about the universe
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Theories About the Universe
Quadratic Theories of Gravity and Universe Evolution
Quadratic theories of gravity offer a unique perspective on the evolution of the universe. Research using a dynamical systems approach has shown that Bianchi type I and II universes in these theories exhibit interesting stability properties. Specifically, an isotropic Friedmann-Robertson-Walker (FRW) universe acts as a past attractor, suggesting an isotropisation mechanism at early times. Additionally, Kasner universes, which are anisotropic solutions, also appear as past attractors under certain conditions. The late-time behavior of these universes depends on the parameters of the theory, with some configurations leading to stable de Sitter solutions and others to phantom-like behavior1.
General Relativity and Singularities
Einstein's General Theory of Relativity has been instrumental in understanding the universe's structure and evolution. It predicts that massive stars can collapse into black holes and that the universe began from a singularity. The theory's formulation involves differential geometry and the causal structure of space-time, leading to theorems that establish the inevitability of singularities under certain conditions. These insights have been foundational in cosmology, influencing our understanding of space-time curvature and the universe's initial conditions2.
Alternative Models: Big Rollout and Vacuum Fluctuation
Contemporary theories propose alternative models to the traditional Big Bang. One such model is the "Big Rollout," which suggests that the universe expanded from the Planck length and time, with spacetime growing rapidly at first and then slowing down. This model challenges the need for dark matter or dark energy by proposing that the rate of time's change could be erratic3. Another intriguing model posits that the universe is a vacuum fluctuation, predicting a homogeneous, isotropic, and closed universe consisting equally of matter and anti-matter. This model aligns well with current observations4.
Large Scale Structure and Cosmological Perturbation Theory
Understanding the large-scale structure of the universe involves non-linear perturbation theory (PT). This approach helps in analyzing gravitational instability from linear to non-linear regimes. PT provides quantitative predictions about cosmic fields' statistics, such as correlation functions and probability distribution functions. These predictions are validated through numerical simulations and are crucial for interpreting galaxy catalogs and other observational data. PT also addresses the bias between galaxy and matter distributions and the effects of redshift distortions5.
Dark Energy, Dark Gravity, and Modified Gravity Theories
The accelerating expansion of the universe has led to theories involving dark energy and modified gravity. Traditional models within general relativity struggle to explain this acceleration without invoking dark energy fields with fine-tuned properties. Modified gravity theories, or dark gravity, offer an alternative by suggesting that gravity behaves differently on large scales. However, these theories also face challenges, such as severe fine-tuning and explaining the vacuum energy's gravitational properties. The quest for a convincing model remains a significant challenge in theoretical cosmology6 8.
Simple Relativistic Theory and Universe Dynamics
A Simple Relativistic Theory (ST) of the universe, inspired by Einstein's Special Relativity, removes the postulate of light's invariant velocity and treats photons like terrestrial matter. This theory accounts for various atomic, subatomic, and cosmological phenomena, predicting the amount of normal matter in the universe and explaining the Big Bang's eruption. It proposes that the Big Bang resulted from anti-gravitational forces due to a collision between our universe and a parallel universe of dark matter and dark energy7.
Shape and Fate of the Universe
The shape and fate of the universe have been subjects of extensive study. General relativity suggests three possible geometries: open, closed, and flat. Observations from NASA's WMAP and ESA's Planck probes indicate that the universe is flat with a marginal error. This flat geometry has significant implications for the universe's fate, suggesting it will continue to expand indefinitely10.
Conclusion
Theories about the universe encompass a wide range of models and approaches, from quadratic gravity and general relativity to alternative models like the Big Rollout and vacuum fluctuation. Each theory offers unique insights into the universe's structure, evolution, and ultimate fate. While significant progress has been made, many questions remain, particularly regarding dark energy, dark matter, and the fundamental nature of gravity. The ongoing exploration of these theories continues to push the boundaries of our understanding of the cosmos.
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Most relevant research papers on this topic
Evolution of universes in quadratic theories of gravity
This study reveals that there is an isotropic FRW universe acting as a past attractor in quadratic gravity theories, and that late-time behavior depends on the parameters of the theory.
Highly CitedIs the Universe a Vacuum Fluctuation?
The big bang model suggests a homogeneous, isotropic, closed universe composed equally of matter and anti-matter, supported by observations.
Highly CitedLarge scale structure of the universe and cosmological perturbation theory
Non-linear perturbation theory helps understand the large-scale structure of the universe and its applications in galaxy surveys.
Highly CitedA Simple Theory of the Universe
The Simple Relativistic Theory of the Universe (ST theory) accurately predicts atomic, subatomic, and cosmological phenomena, and proposes the Big Bang as the result of colossal anti-gravitational forces caused by a collision between our universe and an equivalent parallel universe of dark matter and dark energy.
Unveiling the Dynamics of the Universe
Modified gravity models offer an alternative explanation for the universe's recent expansion history, and observational probes constrain the allowed models, offering insights into the early universe and its evolution.
Extended theories of gravity and their cosmological and astrophysical applications
Dark matter and dark energy could be explained as "shortcomings" of General Relativity, with extended theories of gravity potentially addressing these issues without adding unknown exotic ingredients to the cosmic pie.
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