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

In summary, the paper reviews the current understanding of the cosmological constant and its possible role in the evolution of the Universe. It finds that the simplest model, which is the cosmological constant being a constant, does not currently have the best fit to the observational data, but that this may change as new data is collected.
  • #1
wolram
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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.
 
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  • #2
wolram said:
i do not understand the consequences in the broadest sense.

That's much too broad for discussion here. Do you have a specific question?
 
  • #3
PeterDonis said:
That's much too broad for discussion here. Do you have a specific question?

I Think i am asking what is Lamda, I thought it was a constant but this paper says differently.
 
  • #4
wolram said:
I Think i am asking what is Lamda, I thought it was a constant but this paper says differently.

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
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
wolram said:
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.
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.
 

Related to Cosmological constant vis-a-vis dynamical vacuum: bold challenging the ΛCDM

1. What is the cosmological constant?

The cosmological constant, denoted by Λ (Lambda), is a term in Einstein's field equations of general relativity that represents the energy density of the vacuum of space. It was originally introduced by Einstein to counteract the effects of gravity and maintain a static universe, but is now used to explain the observed acceleration of the expansion of the universe.

2. How does the cosmological constant relate to the dynamical vacuum?

The cosmological constant and the dynamical vacuum are closely related concepts. The dynamical vacuum refers to the quantum fluctuations of the vacuum of space, which can give rise to virtual particles. These fluctuations contribute to the overall energy density of the vacuum and can be represented by the cosmological constant in Einstein's equations.

3. What is ΛCDM and how does it relate to the cosmological constant?

ΛCDM (Lambda Cold Dark Matter) is the most widely accepted model for the evolution of the universe. It is based on the combination of a cosmological constant (Λ) and cold dark matter (CDM) to explain the observed expansion and structure of the universe. The cosmological constant represents the energy density of the vacuum, while cold dark matter refers to a type of matter that does not interact with light and makes up a large portion of the matter in the universe.

4. What challenges does the ΛCDM model face?

Despite its success in explaining many observational data, the ΛCDM model still faces several challenges. These include the observed discrepancy between the predicted and observed value of the cosmological constant (known as the cosmological constant problem), the nature and origin of dark matter, and the apparent fine-tuning of the model's parameters.

5. How does challenging the ΛCDM model relate to the cosmological constant?

Challenging the ΛCDM model involves exploring alternative theories or modifications to the model that can better explain the observed data. This includes theories that question the need for a cosmological constant or propose alternative explanations for the observed acceleration of the expansion of the universe. These challenges often involve reexamining the role and significance of the cosmological constant in our understanding of the universe.

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