Right after the big bang, ALL matter was superheated to a plasma state at trillions of degrees. Bodies tend to stay in the same state they are, unless they are affected by an external force. If there was no “space” or other form of cold mass to transmit heat to, why did the superheated matter start to cool down?
I’m thinking it may be related to energy released when particles started to merge/combine with others to form atoms, molecules, etc., but I’m sure someone here has a better explanation.
In: Planetary Science
It’s straightforward- it’s because the universe expanded. Things were very hot in the early universe, but there’s still a finite amount of total heat (or, if we’re talking about an infinite universe, a finite heat density). As the universe expanded the same amount of heat became spread out over greater and greater space, which naturally resulted in things cooling down. If the universe were to ever come back together again (as described in the now-considered-unlikely Big Crunch scenario) it would heat up as the galaxies come closer and start colliding with each other.
Heat is radiated away in the form of electromagnetic radiation, or light. This light also cools down (becomes lower in average frequency) as it is redshifted by the cosmological expansion.
>Bodies tend to stay in the same state they are, unless they are affected by an external force
you are mixing up newton’s laws of motion with the laws of thermodynamics. the “state” you refer is just about motion, not energy. an object in motion will remain at motion, an object at rest will remain at rest. but an object superheated to trillions of degrees will not remain at trillions of degrees.
the second law of thermodynamics states that in a closed system, entropy always tends to increase, which you may picture as a tendency towards matter and energy (and specially temperature) to be spatially homogenous. that means that any object warmer than its surroundings will radiate energy away, until it is at the same temperature of its neighbours.
now you are right that in the early universe, right after the big bang, EVERYTHING was superheated to trillions of degrees. so indeed there was nothing or nowhere to dissipate that heat
>If there was no “space”
except… there was! right after the big bang, space itself was expanding extremely extremely fast, many times faster than the speed of light, the universe expanded in a fraction of a second from something smaller than a molecule of DNA, to something larger than the solar system. so you can understand that the energy density of the early universe quickly decreased, by orders of magnitude. so all that superheated matter suddenly had space to dissipate it’s energy, in the form of electromagnetic radiation, essentially emitting high energy photons.
the universe remained extremely hot for quite some time, though. all those photons emmited by this superheated plasma kept bouncing around all the free electrons, until the universe cooled down enough for those electrons to combine with protons and neutrons. when that happened, roughly 300 thousand years after the big bang, the photons were free to travel across space, and we have a picture of that moment in the form of the [cosmic microwave background radiation](https://en.wikipedia.org/wiki/Cosmic_microwave_background).
Right after the big bang, the universe was expanding extremely rapidly. Hot gases cool as they expand. It took about 380,000 years for the universe to expand enough and cool enough that atoms began to form as electrons joined nuclei. Once the gas was not ionized it became transparent and the universe as we know it began to emerge.
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