The conservation of energy can be applied to the universe by considering the total energy content, including matter and radiation. Defining the energy of the universe involves complex factors, such as gravitational energy and dark energy, which may vary for different observers due to the effects of relativity. The thermodynamic universe, which includes a system and its environment, differs from the cosmological universe, as thermodynamics is typically formulated for specific observers and conditions. Observers in different frames of reference may measure energy differently, highlighting the relativity of energy definitions. Understanding these distinctions is crucial for comprehending the broader implications of energy conservation in cosmology.
