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My universe was created without stars, of any kind (which includes the sun!), and has continued that way for about 2000 years. To clarify, all life (intelligent life) was created by the deities around the same time as the universe.

I'm aware this poses several problems, but my main concern is whether I can heat up space enough that life can survive (intelligent life, that is, not bacteria), without breaking physics too much.

Can I just make the universe warmer? Or is there something else in the universe that provides heat other than stars?

Thanks in advance!

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    $\begingroup$ I'm not sure to understand, do you want to warm up one or more planets or the space between them? Or perhaps the whole universe's average temperature? Know that vacuum can't really be heated, because thermal energy is kinda defined by particles moving around :). $\endgroup$ Dec 25, 2022 at 22:42
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    $\begingroup$ The idea is to heat up the entire universe to the point where one wouldn't instantly freeze to death. I suppose the planets themselves would be slightly warmer than outer space (volcanoes, etc), but not really enough to keep human- and animalesque creatures alive $\endgroup$ Dec 25, 2022 at 22:49
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    $\begingroup$ Welcome to Worldbuilding! I see this is your first question. Good question! $\endgroup$
    – SRM
    Dec 25, 2022 at 22:56
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    $\begingroup$ There is no such thing as 'heat'. What we refer to as heat is actually the vibration of atoms. The more they vibrate, the 'hotter' we say they are. If there are no atoms, there is no vibration, and thus there is no 'heat'. You can not 'heat up' completely empty space - that is, space with no atoms. $\endgroup$ Dec 26, 2022 at 4:16
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    $\begingroup$ @JustinThymetheSecond actually heat does have a technical definition in thermodynamics. Heat can be transferred by the motion of atoms but also by radiation. There is thermal radiation in space, and you absolutely can heat it up. $\endgroup$
    – N. Virgo
    Dec 26, 2022 at 12:28

7 Answers 7

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There are two ways to transfer heat: convection and radiation. Convection is what you get when you touch something hot, radiation is when high energy photons touch you.

Since there's not a lot of things to touch in space, convection is basically right out. Sure you get some high energy plasma fields and so on, but generally there's not a lot of matter hanging out in the vaccuum... by definition, really. In space what really matters is radiative heating/cooling.

A body with a lot of thermal energy is constantly emitting photons in the infrared band, and each one of those photons carries away a small amount of the thermal energy. Over time the body cools to the lowest possible level due to radiative cooling. At the same time you're receiving photons from everything you can see. Those photons that are absorbed add their meagre energy to your body, exciting your electrons and warming you up. Those excited electrons eventually settle back down and emit photons again.

The equilibrium point, where you are receiving as much as you are emitting is what we often refer to as the temperature of space. In deep space, far from any pesky radiation sources, you're still awash in remnant photons from the microwave background. Leftover energy packets from the pre-expansion universe are still kicking around, and an ideal radiator will still receive enough of them to keep its temperature above absolute zero. Not by much though.

But that's the "real" universe, to the best of my knowledge at least. In your divinely created young universe the rules are clearly different. It's small enough that any reasonable collection of remnant photons would be absorbed fairly quickly, so it won't have a lot of longevity. The ones that aren't quickly absorbed will likely wander off on paths that take them out of the local space and be effectively lost forever.

Unless they have a reason to turn back.

The simplest solution would be a perfect black-body envelope with only one side. If the entire universe is enclosed in a shell of perfectly absorbtive material that radiates energy back into the universe at the same rate then you can maintain an enclosed system at any temperature you like. Objects in space would then be heated fairly evenly from all directions, and any heat they lose would eventually return to them.

Of course now you have the problem that the entire universe will settle into thermal equilibrium pretty quickly. With everything at the same temperature it's hard to get any work done from it. You'd need some way to produce variations in the temperature to get this to work.

One option would be to have significant anisotrophy in the envelope. Instead of releasing energy uniformly the shell could have specific areas that absorb the heat and other areas that emit heat back into the system. This would give you temperature gradients across the enclosed volume, allowing work to be done between areas of differing temperature. Objects in space would have a hot side and a cold side, etc. You could even have the shell emit from different areas in a cycle, vary the emission temperatures and so on. All in IR photons, of course. No lights in the sky, just patches of warm and cold.

Might be quite a pretty sight once you develop IR cameras. But I digress.

Physics in our own universe wouldn't allow a truly closed system like this to exist, but we're talking about a micro-verse created by gods with access to literal miracles. I'm sure they can whip up some perfect perfect heat transfer materials in no time. Or just... make it work by magic. That seems to be the way most godly stuff happens.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Dec 28, 2022 at 20:03
  • $\begingroup$ Equating infrared radiation with heat is a pop-science mistake. IR photons are actually quite low-energy EM radiation. The reason "heat vision" systems scan the infrared band is because IR is the color that objects naturally glow at temperatures that humans typically consider "normal" or "reasonable". Things that are "really hot" emit higher-energy colors and glow in visible light instead. (The next color up from IR is red, which is why objects become "red hot" as they begin to get hot enough to emit visible light instead of IR.) $\endgroup$ Dec 30, 2022 at 2:50
  • $\begingroup$ “It's small enough that any reasonable collection of remnant photons would be absorbed fairly quickly” — nope, as soon as matter and radiation are at equilibrium, every body in the Universe emits as much rad as it absorbs. The Universe may be filled with a cozy +20°C radiation, and will eternally persist in this state, if it's not expanding or contracting. $\endgroup$ Jan 7, 2023 at 1:47
  • $\begingroup$ @kkm The photons that hit the one planet (and whatever moons) will be a one-time warming influence. The ones that miss, and any photons emitted by the planet, would have to be turned around in some way to maintain a high ambient. Space would have to be closed - you'd be able to see yourself with a telescope - or the energy would escape the planet and never return. Planet cools to absolute zero and stays there. $\endgroup$
    – Corey
    Jan 8, 2023 at 22:35
  • $\begingroup$ True, space would have to be either closed or infinite and filled with matter and radiation about equally everywhere. It's kinda hard to imagine a finite lump of matter, of whatever size, in an infinite flat and otherwise empty universe avoiding a gravitational collapse. If you don't like a closed topology, then it has to be filled with matter, but primordial radiation should be limited, and expand from a single source, or a finite number of them. Then yeah, essentially you spread a finite radiation budget into an infinite numbers of planets in the end. $\endgroup$ Jan 9, 2023 at 5:05
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Radioactivity and gravity.

In the real universe, radioactive elements are created in supernovas. In your universe, we can just assume radioactive elements are created in the planet formation by the deities. Really, this, alone, could account for all the heat you need.

But you can also factor in heat from tidal forces. The moons of Jupiter get this, for example. The moon is pulled on by the gravity of Jupiter, creating internal friction that adds heat to the planet.

Also note that it's relatively difficult to shed heat in space. Typically we lose heat via conduction: the atoms of cold air or cold water absorb the heat from your atoms, basically sucking the energy out of you. In space, there is practically no conduction, so you must lose energy by radiating it away, which is far slower. Space is cold, but you'd asphyxiate long before you froze. Heat is a big deal for satellites, because they need to radiate the heat they generate, which they typically do via deployable radiators.

So I think you're safe. Your planets tend to orbit gas giants and come packed with plenty of radioactive materials to keep the core cooking.

If anything, your deities will have to be careful how they form the planets. They must be made all at once (as in poof, here's a planet) because planetary formation -- condensing dust and gas into a planetary body -- is itself a very hot process and 2000 years is actually not nearly enough time to recover from that! They'd be lavaworlds. But we'll assume deities can also set start conditions.

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    $\begingroup$ Space is not cold, nor it is hot. Referencing to temperature makes no logical sense in the case of vacuum, as it lacks particles to measure it. It's like telling you have 5 or 10 cookies in a jar, but there's no jar. $\endgroup$ Dec 25, 2022 at 23:33
  • $\begingroup$ @JamieB Yep, everything poofed into existence, with all working parts. Technically, the universe is all set to run without any stars and ergo without any light or heat from them, but I feel it's stretching the laws of believability (at least for me) for human-esque life to exist on a very freezing cold planet. Could you extrapolate on your radiation theory a bit? $\endgroup$ Dec 26, 2022 at 4:36
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    $\begingroup$ @Tortliena there is infrared light in space, and that can be hot or cold. Space in our universe is about 3 kelvin - very cold - but it doesn't have to be so, see my answer. $\endgroup$
    – N. Virgo
    Dec 26, 2022 at 12:27
  • $\begingroup$ I think "space is cold" comes from bodies out in deep space radiating all their heat away. Pluto is around 33 Kelvin at its coldest. Dense molecular clouds are said to be around 10 Kelvin. So space is cold basically because radiation takes away your heat and if you aren't close to a star, or generating your own, then you'll end up at the background temperature which is close to 0 K. You'd definitely freeze in space, it's just not as fast as sci-fi tends to show it. $\endgroup$
    – JamieB
    Dec 26, 2022 at 18:09
  • $\begingroup$ @Tortliena Depends on your definition of "cold." If you define it as X number of particles with Y amount of energy, then no it isn't. But if you define cold as having the ability to accept energy from even low-heat sources, then yes it is. $\endgroup$
    – barbecue
    Dec 27, 2022 at 22:18
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In your , your worlds swim through an actual Luminiferous Aether

In the good old days, long before humanity actually understood that space is remarkably empty, we though there was stuff that the worlds swam through. We called it a "aether," and believed it's what allowed light to travel between the sun and the Earth.

Today, the ability to simply declare such an aether exists in the Real Universe (or something close to it) is difficult because the motion of planets through their orbits and gravity would sweep the aether clear very quickly (on a cosmological scale).

But that's not how your universe works!

In your universe, the aether, let's call it "Dark Matter" (if only for the sake of poking fun at what could be a very realistic way of asserting such an aether in the Real World) is something you can't see... but it is something that can be warmed up... and since there's no where particularly for the heat to go (I'm about the throw thermodynamics out the window, but breaking windows is fun), it's available to warm your planets.

A rule like this does have consequences

If you set a rule that says the aether exists but basically doesn't interact with your planets (meaning it's not just dust being sucked down to the planet surface), that means it's being pushed aside (by the magnetosphere, dontchaknow) as the planet orbits. But that means you have a bow wave leading the planet in its orbit and a wake trailing the planet. That would have some fun consequences with how light is seen by people on your planet, especially at dawn and dusk.

It also means there could be the consequences of friction as your planet moves. In the Real World, friction would eventually slow the planet, causing it to spiral into the Sun. But in your , this doesn't happen. The sun's gravity, perhaps, impels the planet, keeping it moving and overcoming the friction. This means you could have some amazing aurora-like effects during dawn. And if your story includes space flight, avoiding the friction effects would be a whomping big deal.

Another benefit is that spacecraft could get rid of excess heat through convection rather than radiation. Ooooh, that would be a beni. Ridding a craft of heat in the Real World is a big deal.

Finally, allowing a solar system full of planets to swirl around in what can only be called a lovely soup means that the aether is also being stirred, resulting in a whirlpool or vortex around the sun. I'd like to imagine that the aether's natural state is to not do this, otherwise it would negate both of my previous two suggestions. But it does mean that celestial navigators must deal with currents, eddies, and other navigational hazards in three dimensions that are usually only seen by seafaring craft in (simplifying) two dimensions.

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  • $\begingroup$ Thanks for your answer, it sounds really cool :D I think I should clarify, there are no stars. At all. There is no sun for the planet to spin around, and certainly no gas giants, and absolutely no light from any stars (or sun!). (I've decided on other things to solve the light problem, it's just the cold that's the sticking point) $\endgroup$ Dec 26, 2022 at 4:28
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    $\begingroup$ @celerysticks So much the better! It gives planets the capacity to meander, especially if you set a rule that gravity between non-aetherical mass exists such that you can get both motion and, in some cases, orbits where smaller masses orbit around larger masses - even though no stars. Were that the case, you'd need a rule that states fusion never starts due to gravity or pressure. But that just makes the universe more awesome. $\endgroup$
    – JBH
    Dec 26, 2022 at 19:07
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Yes, this is possible and is no problem. In fact, since our universe started at an infinite temperature and gradually cooled to its preset overall temperature of about 3 kelvin (-270 °C), there must have been a time when it was a nice warm 25 °C itself. (This is indeed the case, but that happened before there were any stars or any elements besides hydrogen or helium, so there wasn't any life at that time.)

What does it mean for space to be warm? Simply that it is filled with infrared radiation at the right range of frequencies. Our universe is filled with thermal radiation at about 3 kelvin, which is known as the cosmic microwave background, but yours could be filled with thermal radiation at some other temperature. That's all you really need to do to have a warm universe.

Our universe started out really hot and cooled down as it expanded. Since you want your universe to start out warm and stay warm, you probably don't want it to expand. But that's ok. It's your universe and the laws of physics are what you want them to be.

One thing you might have to worry about though is photosynthesis. Life is only possible in our universe because space is cold and the sun is hot. This allows plants to capture high-energy photons from the sun and turn it into useable energy in the form of sugar. When that energy is eventually used up it radiates away into space, which is what prevents our planet from overheating. In your universe there won't be any sunlight and space won't be cold, so you'll need to figure out some other way for your organisms to obtain energy and eventually dissipate it. But I'm sure you'll think of something.

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    $\begingroup$ Life before the light is in fact a recurring pattern in mythologies around the world. Nothing wrong with this, as long as the deities take care to seed some energy-rich substances in whatever the origin of the life is. Also, no need to kill the universe expansion - if the universe looks like our own, the required temperature (~25C) will last for millennia. $\endgroup$
    – fraxinus
    Dec 26, 2022 at 20:11
  • $\begingroup$ If OP doesn't need live living or evolving fast, radioactive decay could also be a replacement for photosynthesis as a "base energy source" $\endgroup$
    – Hobbamok
    Dec 28, 2022 at 19:43
  • $\begingroup$ Despite another answer being accepted, this is the actually correct one. Btw. IR photosynthesis wouldn't be out of the question with intelligent deities seeding live into the universe $\endgroup$
    – Hobbamok
    Dec 28, 2022 at 19:51
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    $\begingroup$ The only correct answer, really. We do not consume heat from the Sun, as we dissipate exactly as much radiation as we absorb, or we'd burn or froze. It's all in the entropy: we receive low-entropy radiation mix (that little 1/2⁰ very hot spot on this huge cool sky), but dissipate featureless, high-entropy spent heat as IR. In a universe at equilibrium, even if it were cozy +20°C, no heat engine or life would be possible—simply, energy doesn't flow. Curiously, it also would have no thermodynamic arrow of time (and also no stuff that could behave usefully as a clock, at least classical). $\endgroup$ Jan 7, 2023 at 2:00
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The Warm Embrace of The Gods

The space is cold. Terribly so. Life wouldn't be able to survive in this cold, uncaring void.

But it does exist, as it was brought forth by the Old Ones. All-mighty beings of pure energy, manifesting themselves as tears in the fabric of space from which they exert their influence. Those beings care deeply for what they created - the planets and the life within them.

So, they safeguard them, in a gentle, loving embrace.

On your cosmos, the deities themselves are the source of heat. They manifest as everflowing loops of potential, circling the planets that watch over, as mighty rivers of starflow.

Those rivers of energy feed life and heat into the planets they come near, allowing them to flourish and prosper, and giving them the much-needed light to break the encroaching, all-devouring darkness that surrounds all.


If those mystical, galactic leylines are actually gods, or some natural phenomena of your universe... that's up for the writer to decide. In any case, they provide the heatsource for the universe to work, without the need for actual, proper stars. Normally things like those wouldn't come up naturally in a regular universe, but yours is no regular universe - with deities being able to break the rules and creating what they need to create life in the cosmos, such unexplicable sources of heat wouldn't be the strangest things around.

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  • $\begingroup$ I really like this take. Very poetic. Seems to fit the theme. $\endgroup$
    – Schwern
    Dec 28, 2022 at 22:50
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You don't need to. Your life needs to worry about staying cool not keeping warm.

living things generate heat, anything alive is constantly generating heat. things get cold in space because they don't generate any heat. As soon as you are generating heat in space getting rid of heat is a bigger problem. Any life that evolves in space will have no problem staying warm. If you could somehow keep a human alive while naked in space they would be fine in terms of staying warm. In fact cooling off might be a bigger problem since heavy exercise will will generate excess heat they will have a hard time getting rid of. They will likely need some elaborate cooling radiators they can deploy. Cooling off after heavy exercise will be a dominate evolutionary factor for life in your setting.

Also at the point you have life without stars your universe is so far from reality you can just set whatever rules you want and no one can argue. All elements heavier than lithium are made in big stars as they die so if you have life (which has carbon, oxygen, nitrogen, ect) you are already completely unrealistic.

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Akin to JBH's Aether suggestion but closer to our physics:

Your universe is full of regular gas¹ — which has not yet condensed into stars.

Gravity will eventually amplify density variations, compressing the gas on various scales into proto-galaxies & proto-star systems, translating random velocity unbalances into overall rotation of those systems around heavy centers, and eventually ignite fusion.
But that will take much more than 2000 years!

To make that gas a source of significant life-supporting heat (the whole point of this answer), it should be pretty dense, like an atmosphere.

That doesn't explain presence of any planets!

If you want planets, you could try saying they were "seeded by deities".
They should be small enough (asteroids?) so they don't capture too much inter-planetary gas over 2000 years to reach un-livable pressures...
Or maybe make the global gravitational constant be much weaker than ours (this would slow the all galaxy/star/planet formation).

  • You could have solid structures to live "on" that are not gravity-formed rocks. Say humongous "space trees".

  • But, once you have a cosmos-wide atmosphere, do you really need planets?
    Make it breathable gas(es) — either hydrogen-breathing life, or have the deities seed the universe with whatever mix life needs — and have fun with "space whales" / space birds / winged "angels"...

¹ These all make your universe resemble one giant ocean... Hmm, what if you generalize to universe is full of fluid? Liquids are un-compressable (up to a point) — which could limit or slow cosmological processes driven by gravity. E.g. gas giants like Jupiter have no surface, but a smooth transition from "atmosphere" to "sea" (though at very high pressures). This also helps vary where life can travel — "fish" swim inside "planets" but only "birds" can cross "space"! (Again, better with a low gravitational constant, and maybe invent a fluid with desired phase diagram — e.g. use fictional elements).

Travel limitation ~ speed of sound

Not having vacuum makes cosmic-scale travel impractical. You can't accelerate then coast like spaceships do; air resistance (especially supersonic) eats energy all the way :-(.
But perhaps the gas has fast "cosmic currents" one can ride?

Thermodynamics remains a problem — life wants temperature difference

As other answers have pointed out, life needs somewhere to dump excess heat, and really uses the difference between hot & cold to support useful work.
If your whole universe is equally warm, that's what we call "thermal death of the universe".
So unless deities send you ever-bigger ice cubes, you may want to tweak (or violate) the 2nd law of thermodynamics...

You can use other energy sources, like chemical energy, but will it deplete? (Earth's ecosystem locally uses chemical energy everywhere — food + oxygen — but globally it re-creates those from Sun-light, in a way that wouldn't work if Earth didn't cool into space.)

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  • $\begingroup$ " life wants temperature difference" It depends on what you call 'life'. In photosynthesis, and the electron transfer that leads to stored energy, the electron goes UPHILL to get to the other side, without enough input of energy to actually GO uphill. It is all about quantum tunneling. The electron tunnels through the barrier, without going through it. It is on this side, then it is on the side, never in the barrier itself. No 'temperature' (energy) difference, the electron just 'wants to be' on the other side. $\endgroup$ Dec 29, 2022 at 21:43

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