Friedmann Equation and the age of empty universe

darkar
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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?
 
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Does spatial curavture have to be positive?

Regards,
George
 
Ummm, rite. Just recheck, K can be negative and it mean the universe is open.

Btw, still i will get some constand infront 1/H₀
 
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