B Question pertaining to dark energy

[PeterDonis]

The Friedmann equations postulate that general relativity is true.

No, they postulate that GR works within its domain of validity. There's a big difference. See below.

If general relativity were to ever be proven incorrect, then would it be logical to say that the Friedmann equations no longer work?

No, because GR would still be the low energy classical limit of whatever theory replaced it--just as Newtonian gravity is the weak field, slow motion limit of GR. GR proved that Newtonian gravity was "incorrect" in the sense of not being exactly right, but Newtonian gravity still works fine within its domain of validity. We don't need to use GR to calculate the trajectories of artillery shells or space probes.

Similarly, the regime covered by the Friedmann equations, at least from the end of inflation (the Big Bang) to now, is well within the domain of validity of GR, i.e., the domain in which any corrections due to a more fundamental theory that incorporates quantum gravity are expected to be negligible. So any more fundamental theory won't invalidate what Chalnoth said.

 

[Chiclayo guy]

I'm sorry this is an old discussion but I was in Peru so I couldn't have internet to access it.

Where were you in Peru? I live here and even the 'nativeos' in the Andes and jungle are playing Pokemon. : )

 

[Chalnoth]

No, because GR would still be the low energy classical limit of whatever theory replaced it--just as Newtonian gravity is the weak field, slow motion limit of GR. GR proved that Newtonian gravity was "incorrect" in the sense of not being exactly right, but Newtonian gravity still works fine within its domain of validity. We don't need to use GR to calculate the trajectories of artillery shells or space probes.

And incidentally, Newtonian gravity produces the exact same equations for expansion as General Relativity (at least as it relates to matter: Newtonian gravity only describes gravity with matter, and doesn't include gravity from other forms of energy). This is a strong indication that the Friedmann equations are a very good approximation to the behavior of our universe.

There are two places where the Friedmann equations are likely to break down:

1. The Friedmann equations assume a perfectly homogenous, isotropic universe. Our actual universe is not perfectly homogeneous or isotropic. This can lead to subtle modifications of the equations, and is a well-studied area.

2. General Relativity itself is very likely to fail at sufficiently high energy densities, which means that the Friedmann equations cannot describe the earliest beginnings of our universe. There are some that claim that this fact casts doubt on cosmic inflation (which is possible), or that quantum gravity produces a "bounce" (look up Loop Quantum Cosmology if you're curious to learn more). But in any event these only apply to the earliest beginnings of our universe, and have no effect in the later universe beyond setting up the initial conditions.

 

[Chronos]

The isotropy and homogenity of the universe is observer dependent. We cannot assume otherwise based on current theory or observational evidence.

 

[Drakkith]

The isotropy and homogenity of the universe is observer dependent.

What do you mean?