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Continue to ChatOn the left-hand side of the field equation we may add the fundamental tensor guv, multiplied by a universal constant, -λ, at present unknown, without destroying the general covariance. - Albert Einstein from https://whatsmyquote.com/quote/on-the-left-hand-side-of-the-field-equation-we-may-add-the-fundamental-tensor-guv-multiplied-by-a-universal/page/10