Cosmological constant calculation

[ftr]

I have always wondered about how cosmological constant is characterized. You often read the “cosmological constant measured to be ….”.So since it is still a hypothesis, shouldn't the statement read “cosmological constant calculated to be ….” . Or Is it that such semantics does not matter.

 

[George Jones]

We don't know how to calculate a sensible value for the cosmological constant (that would be a major breakthrough), we can only measure the value of the cosmological constant. Maybe a more accurate statement would be something like "A so-and-so cosmological model with cosmological constant having value such-and-such is consistent with observations."

This is usually what is meant by "measure" in science. For example, "A model for an electron with electric charge such-and-such is consistent with observations."

 

[ftr]

We don't know how to calculate a sensible value for the cosmological constant (that would be a major breakthrough), we can only measure the value of the cosmological constant. Maybe a more accurate statement would be something like "A so-and-so cosmological model with cosmological constant having value such-and-such is consistent with observations."

This is usually what is meant by "measure" in science. For example, "A model for an electron with electric charge such-and-such is consistent with observations."

But how do you measure a theoretical conjecture!

 

[George Jones]

As with any theoretical conjecture, we need a model that makes predictions that can be compared with observation. In this case, cosmological models which predict, e.g., relationships between observed absolute magnitudes, apparent magnitudes, and redshifts of cosmological objects. Different values for the cosmological constant give different relationships between the these things.

 

[ftr]

As with any theoretical conjecture, we need a model that makes predictions that can be compared with observation. In this case, cosmological models which predict, e.g., observed absolute magnitudes, apparent magnitudes, and redshifts of cosmological objects. Different values for the cosmological constant give different relationships between the these things.

I guess I meant that we measure the redshift not the CC. attributing CC to redshift is a conjecture.

 

[George Jones]

This is true for all of physics. "Through this-and-that interactions with our apparatus, we expect to see ... " I don't see anything different in the case of the cosmological constant. What about all the stuff in elementary physics? Indirect observations are viewed through the filters of abstract theories that are far removed from everyday experience.

Physics never proves anything, everything is a conjecture. Physics has never been about proving things.

As an example, consider Newton's theory of gravity. Given the masses of any two objects and the distance that separates the objects, Newtonian gravity gives an expression for the gravitational force between the objects. To prove that Newtonian gravity is true, we would have to verify experimentally its force expression for all possible masses and all possible separation distances. It is impossible, even in principle, to verify this infinite set of possibilities. Even if we verify it a zillion times, tomorrow we could make a measurement that we can't square with its force expression. It only takes one (set of) measurement(s) to prove it wrong.

Newton's theory of gravity, even if it hadn't been falsified by experiment, would still only be a conjecture.

I like what Robert Geroch wrote (in his non-technical book "General Relativity from A to B about physics theories and "proofs" of theories:

It seems to me that "theories of physics" have, in the main, gotten a terrible press. The view has somehow come to be rampant that such theories are precise, highly logical, ultimately "proved". In my opinion, at least, this is simply not the case - not the case for general relativity and not the case for any other theory in physics. First, theories, in my view, consist of an enormous number of ideas, arguments, hunches, vague feelings, value judgements, and so on, all arranged in a maze. These various ingredients are connected in a complicated way. It is this entire body of material that is "the theory". One's mental picture of the theory is this nebulous mass taken as a whole. In presenting the theory, however, one can hardly attempt to present a "nebulous mass taken as a whole". One is thus forced to rearrange it so that it is linear, consisting of one point after another, each connected in some more or less direct way with its predecessor. What is supposed to happen is that one who learns the theory, presented in this linear way, then proceeds to form his own "nebulous mass taken as a whole". The points are all rearranged, numerous new connections between these points are introduced, hunches and vague feelings come into play, and so on. In one's own approach to the theory, one normally makes no attempt to isolate a few of these points to be called "postulates". One makes no attempt to derive the rest of the theory from postulates. (What, indeed, could it mean to "derive" something about the physical world?) One makes no attempt to "prove" the theory, or any part of it. (I don't even know what a "proof" could mean in this context. I wouldn't recognize a "proof" of a physical theory if I saw one.)

Geroch was a very deep thinking, very good, professor in the departments of mathematics and physics at the University of Chicago. He also authored the provocatively titled book "Mathematical Physics".