# Cosmological constant

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

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If we could calculate the cosmological constant, we'd have a much better idea what it's value is. ;) – David H Mar 28 '13 at 0:55
Sorry, @QSA, this happens quite often and I would like to know why it happens. Could you please explain to us why you think that the word "measure" means something else than "measure" in this context? When people say that they measure the cosmological constant, why do you decide that the word "measure" should mean a totally, totally different verb "calculate"? I am just not getting it. Your modification seems not only obviously wrong but malicious to me, especially because you even seem to accuse the normal folks of inaccuracy - while it's clearly you who is bringing all the noise. – Luboš Motl Mar 28 '13 at 6:08
... No one can calculate the right cosmological constant for our Universe today - if he could, we would probably have a complete TOE. – Luboš Motl Mar 28 '13 at 6:10
@QSA GR was not modified to include the CC. It was always there: the question was just is its value zero, or not. Unfortunately, Einstein (at one point) gave it an arbitrary value for bad reasons, but that doesn't mean it shouldn't exist. – DilithiumMatrix Mar 29 '13 at 13:47
@QSA "I have always wondered about how the electron mass is characterized. So since it is still a hypothesis you often read the “the electron mass is measured to be ….”. Shouldn't the statement read “the electron mass is calculated to be ….” . Or Is it that such semantics does not matter." No difference from your question. After all, we didn't really know where the electron mass came from until last July, although they didn't exactly "measure" the Higgs either. It was inferred through a lot of other measurements and calculations. All of science is done through models. – Michael Brown Mar 29 '13 at 14:09

The cosmological constant can be measured just like any quantity. In fact, the 2011 nobel prize in physics was given for just that.

One of the earlier papers seriously analyzing the issue is linked here. A more general overview can be found here, on scholarpedia; and a even more general one, targeted at a wider audience from Sean Carroll. You can also look at this article on measurements with type Ia supernovae, specifically.

Edit: On the nature of scientific measurement.

In astrophysics, measurements are very rarely done with rulers. Instead, in the majority of cases, they are done by making comparisons between observations (done with rulers---in this case CCDs, etc) and the predictions of models. This is the fundamental nature of scientific inference and deduction.

For example, consider the 'measurement' of mass (any mass):
There is no way to directly measure mass, it can only be inferred using one of many physical laws. For example, by measuring the force of gravity, and using $F = G \frac{m_1 m_2}{r}$. Alternatively, one could use a spring of a known strength, measure the frequency of oscillation and use $\omega^2 = \frac{k}{m}$. Both methods use inference.

As the desired parameter becomes more complex---for example, a dynamic property of the universe (you know, no big deal)---the inferences become more complex. The state of the art technique for inferring model-dependent parameters based on observational measurements is called Bayesian Inference - which can be used to take into account uncertainty on the models themselves, and make comparisons between models. In general, scientists use Bayesian Inference implicitly when comparing between different competing models. One must take into account confidence in a model, in addition to the observational data, to figure out not only what the 'measured values' of parameters are --- but also which models are the most compatible with the data. This is how competing models are compared.

In the end, there can never be 100% certainty in either a 'measurement' (of a parameter) or a model in-and-of-itself. Instead a scientist can only become more and more confident in a range of parameters, or a sufficient accuracy of a model.

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Please see my reply to lubos, and your own references are full of statements that support what I am saying. For example the list of problems in your second ref, this is not exhaustive. "Despite its success, the cosmological constant is not without problems (for further details on all the issues below see Weinberg (1989), Carroll (2001), and Padmanabhan (2003))." – QSA Mar 29 '13 at 10:11
I would say that it isn't really "measured", as in the way you measure your weight on a bathroom scale. A FRLW model is assumed for the universe, and an acceleration parameter is measured, and this predicts a cosmological constant. But there are several assumptions that went into this, and if you model the cosmology a different way, you get different results. – Jerry Schirmer Mar 29 '13 at 15:04
@JerrySchirmer, yes in my answer is a rant on how most scientific measurement does differ from using a bathroom scale (excuse me, I used the term 'ruler'). It is none-the-less measurement. – DilithiumMatrix Mar 29 '13 at 15:12
@JerrySchirmer, first---I do appreciate your points, and I don't think we actually disagree on much (if anything), but again, as I said in my ramble, you can never be 100% sure of any measurement or model. – DilithiumMatrix Mar 29 '13 at 15:39
@zhermes: fine. But you can be much more sure of certain things than others. Anyway, we're getting hung up about connotations of words. I"m out. – Jerry Schirmer Mar 29 '13 at 15:42