Got stuck in this question, the question asked to use friedmann equation to show that the age of an empty universe (average density = 0) is τ=1/H₀ where H₀ is the present day value of Hubble's Constant. Well, i got the friedmann equation by put the value ρ(average)=0 but the trouble is that u have H₀^2 = - kc^2/R^2. which mean k has to be an imaginary number. But i am kind of not accepting it. Did i miss out something?