The energy densities of matter and the vacuum are currently observed to be of the same order of magnitude: (Ω_m 0≈ 0.3) ∼ (Ω_Λ 0≈ 0.7). The cosmological window of time during which this occurs is relatively narrow. Thus, we are presented with the cosmological coincidence problem: Why, just now, do these energy densities happen to be of the same order? Here we show that this apparent coincidence can be explained as a temporal selection effect produced by the age distribution of terrestrial

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... s in the Universe. We find a large (∼ 68 observations made from terrestrial planets will result in finding Ω_m at least as close to Ω_Λ as we observe today. Hence, we, and any observers in the Universe who have evolved on terrestrial planets, should not be surprised to find Ω_m ∼Ω_Λ. This result is relatively robust if the time it takes an observer to evolve on a terrestrial planet is less than ∼ 10 Gyr.