For Einstein gravity wasn’t a force, it was the relationship of two moving bodies in a curved spacetime. In order for everything in the universe to have an accelerating reference frame it must be placed in a dynamic expanding background. His instincts told him this wasn’t the right track so he introduced a cosmological constant to act as background pressure in place of expansion in space.
After all he was an empiricist, he believed in what he could see. He spent many years trying to prove the moon was still there when you’re not looking to the quantum science community among other things.
It was only when Hubble noted the linear relationship between redshift and apparent distance in galaxies that Einstein abandoned the cosmological constant, calling it his greatest blunder.
The cosmological constant is still around, but these days it is used to represent the velocity of the expansion of space. How we frame this is important because in the article it’s noted to have been accurately clocked at around 67km per mega parsec depending on how you measure it.
However another way to say that is every time we try to measure the cosmological constant we get a different number. Is it close? Sure, but every time we measure it given the accuracy of our parameters we should come up with the same number, but we don’t.
This is known as tension in the model, and it means we have something wrong. It could be in the methodology, it could be in the theory, but it’s somewhere and it’s important. One thing to note is for every different experiment and new methodology we try as we end up with a new solution it tends to point at the underlying theory itself rather than the various methods, but not necessarily that the theory itself is wrong. This is an important distinction to consider.
Also worth noting is a universe ruled by general relativity is also a universe ruled by Newtonian mechanics with only slight differences that we as an infantile species would not have had the capacity to measure or appreciate at the time our ideas were developed.
For example Einstein’s equations can be reduced perfectly to Newton where the speed of the gravitational force is less than the speed of light. What’s more Milgrom took the force of gravity and applied a Shwarzchild radius beyond which the strength of the force drops from an inverse square of the distance to just the inverse and those equations are much better at predicting stellar velocities than dark matter.
What this means is it is possible to construct perfectly functional mathematical models of our universe with Newtonian mechanics treating gravity as a dynamic force just like you can with general relativity and dark matter. Mainstream science doesn’t often talk about fight club but guess what, it’s fight club.
In fact when we say “a universe governed by the rules of General relativity” we are also saying “a universe that can be reduced to Newton”. Which means gravity as a force, not only inertial motion, isn’t off the table by a long shot.
Where we are hanging up is in the abstracts, what we think it means when the equations tell us something. Once we have an abstract we pursue it with dogged relentlessness, we can even sometimes ignore evidence which arises to the contrary of our abstract notions in the pursuit of it.
It’s all in the framing. How we say what we think a thing means or doesn’t and how that fits into the bigger picture of the story. Part of the real challenge in science communication is making assumptions for the audience about what they do or don’t already know and trying to give them something they need to build that picture the way we see it. It’s never truly agnostic and sometimes what we leave out was as relevant as what we included because people should be free to build their own ideas from a comprehensive picture and that’s the hardest part of all.
Worth noting is magnetic fields, the only energy field we know which works like gravity, experiences the same kind of Shwarzchild radius between the near and far fields as the one proposed by Milgrom for gravity. The one that works so well at describing galaxies without magic dark matter. Yet we have no physical precedent, or so much as a reasonable explanation to how or what or why dark matter is and yet that is where we are stuck right now.
That.
In the abstracts.
Dropping water down mineshafts and hoping for sparks. Avoiding the real conversation.
So no. We don’t know if the universe is expanding or static, arguably we don’t know if gravity is a force or not even if space is bent in the proximity of mass such that light bends around it and shifts red in it’s presence. We don’t know if Einstein’s universe was correct or his greatest blunder and the evidence in both cases is inconsistent if not contradictory.
What we do know with empirical certainty is someone out there, like Ethan, is working really hard to figure it out, and maybe someday they will. Until they do maybe we shouldn’t frame things like they already have.