High School Can particles interact without a mass?

[PeterDonis]

It follows from the fact that there is a finite amount of mass-energy in the Universe

Only in the observable universe. The universe as a whole is spatially infinite.

conservation of mass-energy

Which is a local concept in GR, not a global one. In a non-stationary spacetime, such as the spacetime that describes our universe as a whole, there is no conserved global energy.

which is very basic and uncontroversial premises of the Big Bang theory of cosmology

I think you are either misunderstanding or misstating the theory.

 

[PeterDonis]

Even if you consider gravitational negative energy as a concept (which I am referencing not as authority but just to clarify my terminology) which is more advanced and controversial, it still gets you only to zero

This concept of "total energy" is meaningless since it is zero for any spacetime whatever in GR.

 

[PeterDonis]

A good background discussion of asymptotic safety which I oversimplify in my comment can be found in this published open source paper (Eichhorn 2019) (particularly Part 5). See also here and here. Also here (discussing distinction between classical GR singularity Big Bang at infinite temperature and limitations on that in a quantum gravity scenario).

All of these are speculative quantum gravity models, not mainstream cosmology. Discussion of them belongs in the Beyond the Standard Model forum, not here.

Also, it is rather inconsistent of you to first emphasize that all physics above energy scales we can currently probe is speculative and should be taken with a grain of salt, and then to inundate us with such speculations.

 

[Prishon]

Why should energy not be conserved globally?

 

[PeterDonis]

Why should energy not be conserved globally?

In GR, it's more that there isn't even a well-defined global concept of "energy" except in a special class of spacetimes (the stationary spacetimes).

 

[Prishon]

I don't follow... Can you give a link? I am very interested.

 

[PeterDonis]

Can you give a link?

This Usenet Physics FAQ article gives a decent overview of the issue:

https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html