Imagine a theoretical parallel universe that does not have and never had a gravitational force. Loose hydrogen atoms would still have formed, and in some cases might have formed H2, but would not coalesce into nebulae or stars. Without gravitational force pulling the hydrogen together, could fusion have occurred to form He? How about heavier elements? The universe would, in any case, be a diffuse cloud of gas, but would the electromagnetic force be strong enough to drive fusion and create heavier elements (not looking for Uranium or anything, but like, Carbon), or would it just be a low-density cloud of hydrogen, for all eternity?
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$\begingroup$ VTC:Needs Focus, There's too many questions here. Stack Exchange is not a discussion forum (see help center), but rather uses the model one-specific-question/one-best-answer. $\endgroup$– JBHSep 4, 2020 at 23:36
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1$\begingroup$ Without gravity you couldn't really have a Big Bang, since the Big Bang is based on Einstein's theory of general relativity which is all about explaining gravity in terms of matter and energy curving spacetime. If you instead assume physics just consists of quantum field theory in "flat" spacetime with no notion of mass causing curvature, then either the universe is infinitely old or it has a starting state which doesn't have any physical explanation (like if you just created a simulation of such a universe and put in some arbitrary initial conditions by hand). $\endgroup$– HypnosiflSep 4, 2020 at 23:45
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1$\begingroup$ The Big Bang is not based on general relativity, it is based on astronomical observations, and must account for the effects of general relativity on those observations. There's no reason to believe that the Big Bang would not have happened if there were no gravity. Also, if there were simply no gravity, general relativity would still be true, and spacetime would still be subject to curvature, there would just be nothing actively curving spacetime. $\endgroup$– cowlinatorSep 5, 2020 at 1:16
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1$\begingroup$ @cowlinator - The theoretical model that is used to explain those observations is based on general relativity, no one has any alternate model that would predict them all--see here for some details. And how can general relativity be true if there is still mass/energy but it doesn't curve spacetime? It's built into the equations of GR that all matter or energy (even the energy of massless particles like photons) causes some curvature, only an empty universe (or one with particles of infinitesimal mass/energy) can be curvature-free in GR. $\endgroup$– HypnosiflSep 5, 2020 at 3:03
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1$\begingroup$ @Hypnosifl "And how can general relativity be true if there is still mass/energy but it doesn't curve spacetime?" I guess one way to interpret the question would be to keep GR's formalism unchanged and send the coupling parameter $G$ to zero (or a negligible value). $\endgroup$– preguntonSep 5, 2020 at 13:38
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3 Answers
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Hydrogen and Helium only
This link tells the story of the big bang aftermath:
https://en.wikibooks.org/wiki/General_Astronomy/The_First_Three_Minutes
It includes the formation of helium using a process which doesn't include gravity at all.
However, the link goes on to explain that nothing other than hydrogen or helium is produced for millions of years.
A million years of cosmic expansion, with all atoms all accelerating away from each other, with nothing to start clustering them. Yeah it'll just be a cloud of hydrogen and helium.
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$\begingroup$ Good answer. Of note, is that since we are changing the laws of physics anyway, greatly increasing the range and strength of the electromagnetic force could possibly result in electromagnetically bound "stars". Maybe. $\endgroup$ Sep 5, 2020 at 1:20
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$\begingroup$ @cowlinator, the range of electromagnetic interaction is already infinite, and increasing its strength would also increase repulsion $\endgroup$– L.Dutch ♦Sep 5, 2020 at 4:35
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$\begingroup$ @cowlinator the problem is isn't that electromagnetism is too weak, it's that electromagnetism has two charges. Electrostatic attraction and repulsion work against buildup of large amounts of charge, since any such buildup repels the charges that would strengthen it and attracts the ones that neutralize it. $\endgroup$ Sep 6, 2020 at 19:43
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$\begingroup$ Not even Helium in more than traces. H1 only. Not even Deuterium and Tritium $\endgroup$– TrishSep 6, 2020 at 20:03
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$\begingroup$ @Trish about 25% helium-4 by mass, actually. And very tiny amounts of lithium. Also, essentially all deuterium came from Big Bang nucleosynthesis, and tiny amounts of tritium and lithium-7, which rapidly decayed. $\endgroup$ Sep 7, 2020 at 16:23
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The universe would, in any case, be a diffuse cloud of gas, but would the electromagnetic force be strong enough to drive fusion and create heavier elements
No.
The reason why the heavier elements are formed is because the atoms are forced in close proximity and at a very high temperature; both phenomena are driven (initially at least) by gravity. Left to itself, the electromagnetic force would actually keep atoms apart, and the nuclear forces would have no chance of entering into play.
Without gravity, atoms will just keep on expanding after the Big Bang, getting thinner and colder all the time. Possibly some random helium atom might form in the very first instants, but nothing else.
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"What if pi was not 3.14..." is what I thought when I read your question. The answer being, we don't know.
Removing gravity from our universe changes everything at such a fundamental Level that, to be quite honest, nobody can imagine what would happen.
To stick to the pi example, if pi was rational, that would indicate something with the geometry of our universe being different. Which in of itself has so many implications that your universe in which pi = 4 might as well just be a different one.
Same with your question, the universe would not be the same in any shape or form.
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$\begingroup$ We can make a lot of concrete assumptions about a universe in which Pi = 4. For instance, since the volume of a cone = pi * r^2 & h / 3, we know that a teepee with the same external size would be much more roomy inside. If teepees exist, of course. $\endgroup$ Sep 5, 2020 at 1:33
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$\begingroup$ You cant make any assumptions. Pi, cannot be Rational in or current Geometry. If Pi was 4, the best you could say is "The Geometry of or Universe is not flat". And thats about it. $\endgroup$ Sep 5, 2020 at 1:43
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1$\begingroup$ You can definitely assume that the geometry of our universe is not flat, that's true. However, non-euclidean geometry is a well studied and understood geometry, complete with axioms, formulas, and laws. We have quite a lot to say about a non-euclidean world. In fact it's still not conclusively proven that our own universe is flat. $\endgroup$ Sep 5, 2020 at 1:47