Universe time
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Understanding the Concept of Time in the Universe
The Block Universe and Evolving Block Universe
The concept of the Block Universe suggests that spacetime is a fixed entity where the flow of time is an illusion. This view is based on time-reversible microphysical laws and fails to account for the time-irreversible macro-physical behavior and the development of complex systems, such as life. To address these shortcomings, the Evolving Block Universe model has been proposed. This model posits that spacetime itself evolves as time progresses, with the future continually becoming the past. This evolution is not tied to any preferred surfaces in spacetime but is associated with the evolution of proper time along families of world lines 19.
Quantum Observables and Cosmological Time
In quantum cosmology, time is considered a quantum observable that does not commute with other essential quantum operators, such as the cosmological constant. This non-commutativity implies that the universe may be "delocalized" in time, leading to new cosmological scenarios and invalidating certain paradoxes. For instance, a universe with a sharply defined clock time must have an indeterminate cosmological constant. This framework allows for the emergence of localized histories through backward transitions in quantum time, akin to a time machine cycle .
Cosmic Time and Initial State of the Universe
The concept of cosmic time is crucial in understanding the universe's initial state. The Hamiltonian constraint in canonical gravity reveals a synergy between gravitation and the intrinsic cosmic clock of our expanding universe. This paradigm shift from four covariances to spatial diffeomorphism invariance makes causal time-ordering and quantum evolution in cosmic time meaningful. The addition of a Cotton–York term to the physical Hamiltonian significantly alters the initial data problem, supporting the origin of the universe as a Chern–Simons Hartle–Hawking state. This state features Euclidean–Lorentzian instanton tunneling and manifests transverse traceless quantum metric fluctuations .
Relativity of Cosmic Time
Cosmic time's relativity is explored within the framework of Cosmological Relativity, which incorporates five dimensions: space, time, and velocity. This approach defines a general linearized metric element, showing that the universe is flat. The cosmic aging function fits well with light curve data from Type 1a supernovae and simulated quasar-like light curve power spectra. This framework also helps determine the multipole of the first acoustic peak of the Cosmic Microwave Background radiation anisotropy .
Temporal Finiteness and Cosmic Time
Modern cosmology provides insights into whether the universe is temporally finite or eternal. According to the Standard Model of cosmology, the universe is approximately 13.7 billion years old and temporally finite. Events can be ordered according to "cosmic time," measured by fundamental observers since the "big bang." The future temporal finiteness of the universe can be determined by measuring the actual matter and energy density. If it exceeds the critical value, the universe will collapse back into a "big crunch" .
Characteristics of Cosmic Time
The nature of cosmic time can be understood using Hamilton-Jacobi theory for general relativity. This approach allows for constructing a covariant formalism where all time hypersurfaces are treated equally. Explicit solutions for an inflationary epoch with several scalar fields are provided, and theoretical predictions of double inflation are compared with recent galaxy data and large-angle microwave background anisotropies .
Conclusion
The concept of time in the universe is multifaceted, involving both classical and quantum perspectives. From the evolving block universe model to the non-commutative nature of cosmological time, and from the initial state of the universe to the relativity of cosmic time, each framework offers unique insights into the nature of time. Understanding these concepts is crucial for unraveling the mysteries of the universe's past, present, and future.
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