ito
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Ok, so if we can't observe anything outside our universe, how exactly can we know for certain that it is a closed system. You actually have it backwards, we are physically constrained to seeing past a certain point in any direction (speed limit of light) so you assume that it is a closed system.
That's like saying Earth is a closed system because the atmosphere ends at a certain point.
You mean Energy? Matter can be destroyed and created, energy can't. However the amount of energy in a system can decrease or increase, this can in turn effect Gravity, which effects whether the universe expands or contracts. Also the rate at which energy increases and/or decreases must be considered, and the mass lost too.
Conservation of energy is an absolute law. First law of thermodynamics states that the energy of a closed system must remain constant. In a closed system, energy is not lost but transformed from one form to another. The perfect examples are the Einsteins famous equation E = MC^2 (matter is another form of energy) or equivalence of potential energy and kinetic energy which prove that Universe is a closed system.
Contrary to what you say observable universe is open system while the whole universe is a closed system.