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How the energy conservation law is obeyed by the universe during itsevolution is an important but not yet unanimously resolved question. Does theuniverse have a constant mass during its evolution or has its mass beenincreasing with its radius? Here, we evaluate the two contending propositionswithin the context of the Friedmann equations and the standard big bang theory.We find that though both propositions appeal to the Friedmann equations forvalidity, an increasing mass with increasing radius is more in harmony with thethermal history of the big bang model. In addition, temperature and flatnessproblems that plague the constant mass proposal are mitigated by the increasingmass with radius proposal. We conclude that the universe has been increasing inmass and radius in obedience to the energy conservation law.

Does the universe obey the energy conservation law by a constant mass or an increasing mass with radius during its evolution?