# B Cosmological constant vis-a-vis dynamical vacuum: bold challenging the ΛCDM

1. Dec 9, 2016

### wolram

I have read this paper, but i do not understand the consequences in the broadest sense.

arXiv:1612.02449 [pdf, ps, other]
Cosmological constant vis-a-vis dynamical vacuum: bold challenging the ΛCDM
Joan Sola
Comments: 31 pages, 2 tables, 9 figures. arXiv admin note: text overlap with arXiv:1605.06104
Journal-ref: Int.J.Mod.Phys. A31 (2016) 1630035
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

Next year we will celebrate 100 years of the cosmological term, Λ, in Einstein's gravitational field equations, also 50 years since the cosmological constant problem was first formulated by Zeldovich, and almost about two decades of the observational evidence that a non-vanishing, positive, Λ-term could be the simplest phenomenological explanation for the observed acceleration of the Universe. This mixed state of affairs already shows that we do no currently understand the theoretical nature of Λ. In particular, we are still facing the crucial question whether Λ is truly a fundamental constant or a mildly evolving dynamical variable. At this point the matter should be settled once more empirically and, amazingly enough, the wealth of observational data at our disposal can presently shed true light on it. In this short review I summarize the situation of some of these studies. It turns out that the Λ=const. hypothesis, despite being the simplest, may well not be the most favored one when we put it in hard-fought competition with specific dynamical models of the vacuum energy. Recently it has been shown that the overall fit to the cosmological observables SNIa+BAO+H(z)+LSS+BBN+CMB do favor the class of "running" vacuum models (RVM's) -- in which Λ=Λ(H) is a function of the Hubble rate -- against the "concordance" ΛCDM model. The support is at an unprecedented level of ∼4σ and is backed up with Akaike and Bayesian criteria leading to compelling evidence in favor of the RVM option and other related dynamical vacuum models. I also address the implications of this framework on the possible time evolution of the fundamental constants of Nature.

2. Dec 9, 2016

### Staff: Mentor

That's much too broad for discussion here. Do you have a specific question?

3. Dec 9, 2016

### wolram

I Think i am asking what is Lamda, I thought it was a constant but this paper says differently.

4. Dec 9, 2016

### Staff: Mentor

We don't know exactly what $\Lambda$ is. It being a constant is the simplest model, which is why it was the first model investigated when it became apparent that $\Lambda$ is nonzero, but the simplest model isn't always the right one. This is still an open area of research and probably will be for some time. This paper is a step in that research, but it's not a final answer.

5. Dec 10, 2016

### Chronos

The author is correctly pointing out the simplest model for Lambda, the cosmological constant, is not necessarily the best fit model. However, this is also true of many things in cosmology. We will rely on these simplest models for as long as they consistently yield results that agree with observation. Consensual acceptance of models is a journey fraught with stubborn resistance and success inherits the resistance to change mantle until a worthy successor is as well vetted as its predecessor. This is neither good nor bad, simply how science is done.

6. Dec 12, 2016

### Chalnoth

Without looking into it in too much detail, let me point out two significant problems that any claim like this must overcome:

1. A varying cosmological constant is mathematically guaranteed to provide a better fit to the data than a constant (provided it reduces to a cosmological constant in some part of the parameter space). This means that we can't simply ask if the varying fit is better: is it better enough to justify the added parameters? That is, unfortunately, always a very hard question to answer.

2. Any measurement of the galaxy across a wide range of redshifts that combines many different forms of data has to contend with the possibility that small systematic errors are influencing the result in subtle ways.

In all, the bar for accepting a varying cosmological constant is necessarily quite high. We'd need to have multiple tests to show that it wasn't just a systematic error, and enough improvement of a fit that there is no doubt that it's just due to the addition of the new parameter.

Fortunately, both of these effects are impacted by the discovery of new data. If the model parameters are consistently measured to take on the same values with different choices of data combinations and do not appreciably change with new data, then we can gain confidence that something is going on here. This data alone wouldn't be a slam dunk for the statement that dark energy varies, but it would mean that there is a component of the universe that $\Lambda$CDM does not take into account. Either way, for now I'd rather wait and see what others say in response.