Total energy in cosmology
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Total Energy Density in Cosmology: Definitions and Evolution
In cosmology, the total energy of the universe includes contributions from matter, radiation, dark energy (often modeled as a cosmological constant), and gravitational energy. Traditional models often focus on the dominant energy component in each epoch, such as radiation in the early universe or dark energy in the current era. However, considering all components together, especially during transitions between epochs, provides a more complete and nuanced picture of the universe's evolution. This approach reveals a broader range of possible evolutionary histories than models that only consider the dominant energy density at any given time 1.
Conservation of Total Energy in Cosmological Models
The conservation of total energy in cosmology is a debated topic. Some researchers argue that standard cosmological models violate energy conservation, leading to ambiguities and problems in understanding the universe's evolution. By applying a Machian perspective, where the kinetic energy between particles is considered, it is possible to derive the Friedmann equation and show that conservation of total energy leads to specific allowable spacetime models, such as de Sitter and Minkowski universes. In these models, matter can appear as curvature energy, and total energy as vacuum energy, with the open de Sitter model closely matching current observations 3.
Gravitational Energy and the Zero Total Energy Hypothesis
A key insight in cosmology is that the total energy of the universe may be zero when both matter and gravitational contributions are included, especially in models with a zero cosmological constant. This result holds regardless of the universe's spatial curvature and supports theories suggesting the universe could arise from a quantum fluctuation of the vacuum 5. In homogeneous cosmological models, the total energy can vanish for certain classes (Bianchi class A), while for others (class B), the energy does not vanish and can even be negative, depending on the gravity theory used 7.
The Role of the Cosmological Constant and Modified Energy Density Models
Recent observations require the inclusion of components beyond ordinary matter in the total energy density, such as a varying cosmological constant. Some models propose that the cosmological constant should decay over time, and that the total energy density, rather than just the cosmological constant, should follow this behavior. These generalized models can resolve several cosmological problems but may predict values for matter and dark energy densities that differ from current observations 4.
Total Energy–Momentum in General Relativity with a Cosmological Constant
The presence of a positive cosmological constant complicates the definition and calculation of total energy–momentum in general relativity. Various approaches and expressions have been developed to address these challenges, each with its own conceptual foundations and technical details 2.
Cyclical Cosmology and Energy Conservation
Some theories propose that the universe is cyclical, with total energy conservation playing a fundamental role. These models are based on the cosmological principle of isotropy and homogeneity and introduce new testable consequences, such as large-scale redshift temporal effects 6.
Conclusion
The concept of total energy in cosmology is complex and multifaceted, involving contributions from matter, radiation, dark energy, and gravity. While traditional models often focus on dominant components, a more comprehensive approach that includes all energy contributions provides deeper insights into the universe's evolution and structure. The debate over energy conservation, the role of the cosmological constant, and the possibility of a zero total energy universe continue to shape our understanding of cosmology 1234567.
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