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For a type of multiverse, where physics are the nearly same as ours, but space has a constant atmosphere comparable to our Earth's. How and where would life evolve, if it can? what I mean is that the aspects of physics that would allow for this, like general relativity, are the same. All others that would make this not work are altered

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    $\begingroup$ You mean, there's air not just covering planets but everywhere? How would that even work? $\endgroup$
    – Sach
    Dec 10, 2019 at 23:04
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    $\begingroup$ You say: "where physics are the same as ours" and then: "but space has a constant atmosphere comparable to our Earth's". These two statements are mutually exclusive. I'm not sure what you are asking here. $\endgroup$ Dec 10, 2019 at 23:06
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    $\begingroup$ (1) Earth does not have a constant atmosphere for any reasonable meaning of the word "constant". In particular, once upon a time there was no free oxygen in Earth's atmosphere, and now there is aplenty. (2) "Physics [is] the same as ours" and "interstellar space is full of air" are two mutually imcompatible statements. (3) As a consequence, the proposed world is logically unsound. $\endgroup$
    – AlexP
    Dec 10, 2019 at 23:06
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    $\begingroup$ Oxygen could not have been an element at the birth of the universe. Also, environments with Oxygen are extremely unstable, unless there's something continuously generating it, such as life. Which is something scientists use to detect Earth-like exoplanets. It's pretty unlikely that free Oxygen can exist in space. $\endgroup$
    – Sach
    Dec 10, 2019 at 23:08
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    $\begingroup$ Earth's oxygen doesn't just happen, it's continually replenished by living things (plants, algae, what have you). In the absence of life, it would probably be mostly found in carbon dioxide, water, and metal oxides. Mars is a good example. $\endgroup$
    – Cadence
    Dec 10, 2019 at 23:09

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In that environment, life would evolve in the air.

The devil is in the details. You ask about a world with physics "nearly the same as ours," but such a world has such extraordinarily different physics than ours that we can't even remotely begin to apply the word "nearly." Applying "physics" would be difficult enough on its own. I would consider a world where there are a host of tiny daemons akin to Maxwell's daemons whose sole job is to hold the atmospheric molecules in place, fighting against gravity, and most importantly, preventing the gas from coalescing into new stars.

Your universe has a lot of mass in it. At atmospheric pressure, we find there's something like 10 trillion trillion atoms per cubic meter. There's 1/4 of an atom in every cubic meter of interstellar space, so you're talking about a universe with 40 trillion trillion times as much matter in it.

Accordingly, it is almost guaranteed that, when life forms, it will form in a pocket of air far from a planet. This is just a game of statistics. When there's that much air, it's simply where the fluctuations that start life will form.

Such life is almost guaranteed to derive its energy from the air. Our atmosphere is not stable. Its oxygen will eventually reduce other compounds in the air to form stable compounds like CO2. Accordingly, the daemons will be busy at work disassembling these stable molecules and re-balancing the stellar atmosphere to keep it "comparable to Earth's." This is a massive source of energy when one accounts for just how much air there will be. Combine that with the fact that the stars will be far and few between compared to all of this energy rich air, and you can see where creatures will form.

The energy density here isn't all that great. I would expect great feathery creatures with gill like structures plucking the reactive molecules out of the air and finely tuned enzymes to catalyze their reactions. They will move slowly, carefully tuned to sniff out which pockets of air have been most replenished by the daemons.

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    $\begingroup$ Floating objects that reduce carbon dioxide and provide an energy source for higher-order life are basically just space phytoplankton - so it makes sense that they're fed on by space fish and space whales. As much sense as anything, at least. $\endgroup$
    – Cadence
    Dec 11, 2019 at 0:22

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