In a universe which does not contain any mass what-so-ever i.e. every single particle is a massless one, would spacetime actually exist? Or would existence be simply a zero-dimensional point?

Many people think that your above scenario will be the ultimate fate of our universe in heat death - just low energy emr after the last super-massive blackholes have evaporated away. as I think we will still have background expansion - and redshifting of the radiation - then I think you can still posit existence of spacetime. it definitely won't be small - cos then any radiation would have to be high-energy, and that could cause particle formation

Many people think that your above scenario will be the ultimate fate of our universe in heat death - just low energy emr after the last super-massive blackholes have evaporated away. as I think we will still have background expansion - and redshifting of the radiation - then I think you can still posit existence of spacetime. it definitely won't be small - cos then any radiation would have to be high-energy, and that could cause particle formation

 

I'm not so sure. Particles only "experience" time when their mass warps spacetime. So surely, in a massless universe, no time would pass? Or does dark energy contribute as mass and therefore there would still be the passage of time?

I'm not so sure. Particles only "experience" time when their mass warps spacetime. So surely, in a massless universe, no time would pass? Or does dark energy contribute as mass and therefore there would still be the passage of time?

 

Energy causes spacetime curvature as well as mass

Energy causes spacetime curvature as well as mass

 

Does this also include dark energy?

Does this also include dark energy?

 

Go on ask me a tough one! Frankly I have little idea. But the oft-quoted figures that normal matter is 4 percent of the mass of the universe, dark matter is 22-23 percent and dark energy is 77 would lead me to believe that yes it does - if you can count it as mass (through I guess mass/energy equivalence) then I presume you have to take it into account - the actual mathematics of GR are beyond me. But as it is constant throughout the universe - and so much seems to depend on gradients, fluxes, changes; I wonder if it might fall out of the equations.