# Cosmological constant

1. Mar 27, 2013

### 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.

2. Mar 27, 2013

### George Jones

Staff Emeritus
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."

3. Mar 27, 2013

### ftr

But how do you measure a theoretical conjecture!

4. Mar 27, 2013

### George Jones

Staff Emeritus
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.

Last edited: Mar 27, 2013
5. Mar 27, 2013

### ftr

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

6. Mar 27, 2013

### George Jones

Staff Emeritus
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:

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".