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## Main Question or Discussion Point

What epoch of the universe's evolution would be most conducive to life?

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What epoch of the universe's evolution would be most conducive to life?

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Wallace

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An interesting queistion. http://www.phys.unsw.edu.au/~chas/resources/my/0703429.pdf" [Broken] paper claims that about now is pretty good and argue that the co-incidence problem of the matter and vacuum ( or dark) energy densities can be explained by the fact that cosmologist would be most likely to exist at the epoch where the densities are similar.

I'm not endorsing their argument as such since I haven't read the paper in enough detail to be sure of the argument, but it is an interesting take on the Anthropic Principle.

I'm not endorsing their argument as such since I haven't read the paper in enough detail to be sure of the argument, but it is an interesting take on the Anthropic Principle.

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marcus

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http://arxiv.org/astro-ph/0703429 [Broken]

**The Cosmic Coincidence as a Temporal Selection Effect Produced by the Age Distribution of Terrestrial Planets in the Universe**

Charles H. Lineweaver, Chas A. Egan

Submitted to ApJ

The energy densities of matter and the vacuum are currently observed to be of the same order of magnitude: [tex](\Omega_{m 0} \approx 0.3) \sim (\Omega_{\Lambda 0} \approx 0.7)[/tex]. 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 planets in the Universe. We find a large ([tex]\sim 0.68[/tex]) probability that observations made from terrestrial planets will result in finding [tex]\Omega_m[/tex] at least as close to [tex]\Omega_{\Lambda}[/tex] as we observe today. Hence, we, and any observers in the Universe who have evolved on terrestrial planets, should not be surprised to find [tex]\Omega_m \sim \Omega_{\Lambda}[/tex]. This result is relatively robust if the time it takes an observer to evolve on a terrestrial planet is less than [tex]\sim 10[/tex] Gyr

Lineweaver is one of the best expositors I know. His "inflation and the CMB" helped me and must helped a lot of people.

this is an interesting suggestion for solving the "coincidence problem"

whether or not it is right, I expect it is very clearly and understandably presented. first rate guy

Charles H. Lineweaver, Chas A. Egan

Submitted to ApJ

The energy densities of matter and the vacuum are currently observed to be of the same order of magnitude: [tex](\Omega_{m 0} \approx 0.3) \sim (\Omega_{\Lambda 0} \approx 0.7)[/tex]. 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 planets in the Universe. We find a large ([tex]\sim 0.68[/tex]) probability that observations made from terrestrial planets will result in finding [tex]\Omega_m[/tex] at least as close to [tex]\Omega_{\Lambda}[/tex] as we observe today. Hence, we, and any observers in the Universe who have evolved on terrestrial planets, should not be surprised to find [tex]\Omega_m \sim \Omega_{\Lambda}[/tex]. This result is relatively robust if the time it takes an observer to evolve on a terrestrial planet is less than [tex]\sim 10[/tex] Gyr

Lineweaver is one of the best expositors I know. His "inflation and the CMB" helped me and must helped a lot of people.

this is an interesting suggestion for solving the "coincidence problem"

whether or not it is right, I expect it is very clearly and understandably presented. first rate guy

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Garth

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Would a similar anthropic argument explain the connected coincidental equality of the derived age of the universe (**A**) and Hubble time (**H**_{T}) using the present best estimates of [itex]\Omega_{\Lambda}[/itex], [itex]\Omega_{DM}[/itex], [itex]\Omega_{m}[/itex]?

Note: with an arbitrary proportion of DE and DM, which varies over cosmological time in the standard model, and with a flat universe, the derived age of the universe could be anything from

**A > 2/3 H**_{T} to **A => Infinity**, whereas the present best values actually give a value of

**A**: Age of Universe = **13.81 Gyrs** and

**H**_{T}: Hubble Time = **13.89** Gyrs

Which is some coincidence!

The proportion of DE is constantly growing, because the density of matter (including DM) is [itex]\propto R^{-3}(t)[/itex] whereas the density of DE is constant. The evolving relative abundance of DE and matter determines the age of the universe.

Therefore if this coincidence is significant, which calls for an explanation, one might suggest that__either__ we exist only when the **H**_{T} and **A** happen to be equal (an Anthropic explanation), __or__ the relationship between DE and matter are such that they are always equal.

The latter would give a handle on a possible evolution of DE and therefore its nature.

Garth

Note: with an arbitrary proportion of DE and DM, which varies over cosmological time in the standard model, and with a flat universe, the derived age of the universe could be anything from

Which is some coincidence!

The proportion of DE is constantly growing, because the density of matter (including DM) is [itex]\propto R^{-3}(t)[/itex] whereas the density of DE is constant. The evolving relative abundance of DE and matter determines the age of the universe.

Therefore if this coincidence is significant, which calls for an explanation, one might suggest that

The latter would give a handle on a possible evolution of DE and therefore its nature.

Garth

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hellfire

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IIRC Steven Weinberg *predicted* the value of [itex]\Lambda[/itex] (with about less than 20% deviation) before the SNIa observations making use of the anthropic argument only.

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