Undergrad If the Universe had a different size, would physics change?

[Suekdccia]

TL;DR

If the Universe had a different size, would physics change?

I found an article by James Bjorken (https://arxiv.org/abs/hep-th/0210202) which argues that universes with different size would have different physics (like different Standard Model parameters).

When applying this reasoning to our own universe, Is this pure conjecture? Or is there some truth in these claims?

 

[phinds]

It is possible (likely, even) that the universe is infinite in extent. If that is indeed the case, then it cannot BE a "different size".

 

[PeterDonis]

When applying this reasoning to our own universe, Is this pure conjecture? Or is there some truth in these claims?

This is a highly advanced speculative paper and asking whether there is any truth in its claims is highly premature.

 

[PeterDonis]

It is possible (likely, even) that the universe is infinite in extent. If that is indeed the case, then it cannot BE a "different size".

The term "size" is really a misnomer as the paper is using it. A better term would be "value of the cosmological constant". There is a perfectly valid family of spatially infinite universes with different values of the cosmological constant.

 

[kimbyd]

Summary: If the Universe had a different size, would physics change?

I found an article by James Bjorken (https://arxiv.org/abs/hep-th/0210202) which argues that universes with different size would have different physics (like different Standard Model parameters).

When applying this reasoning to our own universe, Is this pure conjecture? Or is there some truth in these claims?

Not pure conjecture, based upon what they mean by "size" in that paper: the inverse of the asymptotic value of the Hubble parameter.

This "size" is a function of the cosmological constant, which, in turn, is a function of the laws of physics. A different value strongly suggests different laws. This statement has very little dependence upon the actual theory.

 

[PeterDonis]

the cosmological constant, which, in turn, is a function of the laws of physics

Is it? I get that it's a constant in the field equation (or Lagrangian) in GR, but its value is undetermined by the theory, and any value of the constant is a valid solution of the same laws (the field equation). In what sense is it a "function of the laws of physics"?

 

[kimbyd]

Is it? I get that it's a constant in the field equation (or Lagrangian) in GR, but its value is undetermined by the theory, and any value of the constant is a valid solution of the same laws (the field equation). In what sense is it a "function of the laws of physics"?

It's value is in part determined by the vacuum expectation value of the energy in empty space. This value is generally non-zero and determined by which quantum fields exist.

Theorists have, for a very long time, tried to find a symmetry that would set this to be zero, but none have so far been found to my knowledge.

 

[PeterDonis]

It's value is in part determined by the vacuum expectation value of the energy in empty space. This value is generally non-zero and determined by which quantum fields exist.

Ok, so by "a function of the laws of physics", you mean "a function of which quantum fields exist". Yes, I agree that that's what our best current understanding of QFT says.

 

[Vanadium 50]

This sounds completely backwards - the size of the universe may be determined by the laws of physics, but the reverse?

 

[Delta2]

This sounds completely backwards - the size of the universe may be determined by the laws of physics, but the reverse?

It's about the same thing, If the laws of physics determine the size of the universe then if the universe had different size then this was (possibly) determined by different laws.

 

[PeterDonis]

the size of the universe may be determined by the laws of physics, but the reverse?

As I noted in post #4, "the size of the universe" is really a misnomer; what is actually being referred to is the value of the cosmological constant. The speculative hypothesis in the paper referenced in the OP is basically that other physical constants like the Standard Model parameters are linked to the value of the cosmological constant, so if that were different, those other parameters would be different as well. Whether this counts as "different laws of physics" depends on what you think "laws of physics" refers to.

 

[Orodruin]

It's value is in part determined by the vacuum expectation value of the energy in empty space.

Is it though? As long as we do not have an established theory of quantum gravity it is typically just conjecture as far as I have seen. A conjecture that fails miserably in getting the right value for the cosmological constant one might add.

 

[Vanadium 50]

It's about the same thing

I don't think so. One cannot simply switch cause and effect.

what is actually being referred to is the value of the cosmological constant.

Maybe, maybe not. I would say that "size" is a loose term for "curvature". The real question is what 6the OP thinks it means, but he has not been back. Like tossing a stink bomb and running away.

 

[Delta2]

I don't think so. One cannot simply switch cause and effect.

Sorry I don't understand how I switch cause and effect. All i am saying is that if some laws A have some effect B, then if we observe an effect $B'\neq B$ then it might be caused by some laws A'.

 

[timmdeeg]

Which laws of physics do we talk about?

Which laws of physics determine the size of the universe or the cosmological constant?

Supposed we know the ingredients of the universe and our model is correct we can describe the evolution of the universe but I am not aware of any physical law which determines the question regarding its size finite vs. infinite.

What I want to say in short: If we agree that the question how the universe came into existence is not a physical question then one could think that questioning its size and the value of the cosmological constant it isn't a physical question too.

 

[PeterDonis]

I would say that "size" is a loose term for "curvature".

The paper by Bjorken that the OP referenced is perfectly clear on what it means by "size", which is just what I said.

The real question is what 6the OP thinks it means

If the OP thinks it means anything other than what the paper he referenced explicitly says it means, he needs to rethink.