## Comments about the article in Nature: Is dark energy really a mystery ?

• The text in italics is copied from the article
• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Figure 1

In the centre of the page there is Figure 1 with the following text:
Cosmic expansion. As the Universe expands, space stretches much like the surface of an inflating balloon, and galaxies get farther apart. Data indicate that the rate of this expansion is accelerating.
```       0 |
|  G  G  G
3,5 bly M..1..2..3
|    x
|   x G     G     G
8,5 bly M .x. 1 . . 2 . . 3
| x
|x             G              G             G
13,5 bly M    .    .    1    .    .    2    .    .   3
time
```
1. The above figure shows the 3 balloons at 10 billion lightyears (bly) ago, at 5 bly ago and at the present.
2. At each epoch there are 4 Galaxies shown: M (Milky way), G1, G2 and G3
3. Galaxy G2 at the 3,5 bly line is a special one. The distance to "M" is 10 bly and we can currently see this young Galaxy. The light path from that Galaxy towards "M" are the letters "x".
4. Galaxy G3 we could see at a younger age/stage as G2 and Galaxy G1 we will see at an older age/stage as G1
5. The problem with the picture is how can we see G2 at a distance of 10 bly, while the age of the Universe is only 3,5 bly ?
6. The above picture shows cosmic expansion. This expansion is homogeneous.
7. The above picture also shows that the expansion is accelerating. A major question is how do we know that the Universe is accelerating. Based on which data.

### No it isn't - Eugenio Bianchi and Carlo Rovelli

See doc 1
Is cosmic acceleration such a great problem?
Yes it is. The problem is it should be based solely on observations. Next we read:
The LabdaCDM model is "almost universilly accepted by cosmologists as the best description of present data".
This sentence is also in doc 1 . Reference there is made to doc 2 .
In document 2 at page 11 we read: "Two major studies, the ‘Supernova Cosmology Project’ and the ‘High-z Supernova Search Team’, found evidence for an accelerating Universe [21], interpreted as due to a cosmological constant, or to a more general dark energy component."
The "Super Cosmological Project" is dicussed in doc 6 . Next we read:
Cosmic acceleration is predicted and simply described by the theory of GR together with a non-vanishing cosmological constant Labda.
That is mathematical correct (except maybe the word predicted). The problem is you need observations in order to quantify this acceleration. You can currently measure one red shift value (z value) or even 50000 (See Document 4) but that is not enough to establish acceleration.
The first objection is known as 'Einstein's blunder'.
The second objection is termed the 'coincidence problem'
The third objection concerns 'vacuum energy'
IMO none of those arguments are trully important to understand the speeding up of the expansion rate.
Labda is a sort of 'zero-point curvature'; it is a repulsive force caused by the intrinsic dynamics of space-time.
I do not understand this sentence. The sentence is not clear. Why such tricky termonology ? Unfortunate nothing is mentioned what the difference is between the "explosive" forces involved in the Big Bang and the cosmological constant labda. I expect physical there is no difference.

### Yes it is - Rocky Kolb

The LabdaCDM is the most complete predictive and succesful cosmological model ever devised, capable of accounting for an enormous number of astronomical observations.
I wished they named a few of those observations. Next we read:
But the success of LabdaCDM comes at a price. In the model only 5% of the total energy of the Universe is observed and understood and 95% of the Universe is dark.
That 95% is for me the mystery, because it can not be observed only inferred.
Cosmologists usually refer to dark matter and dark energy as cosmic mysteries.
The question is who are those cosmologists and what is the meaning of the word mystery. If they mean: "something we do not understand" than I agree. A concept like dark energy is the most difficult to understand i.e. to quantify. Next we read:
IMO a cosmological constant qualifies as a mystery in the mon-theological sense of the word:"Something not understood or beyond understanding"
I do not understand the whole discussion about mystery. IMO the word mystery should not be used by scientist. Scientist should use concepts like: I agree, I do not agree (which both imply understanding) and I do not understand.
Einstein's cosmological constant labda is the simplest explanation for dark energy: it adequatily fits the data and there is no reason to exclude it. But the magtitude of Labda necessary to explain the observations places it far "beyond [our] understanding"
If something fits the data why is it's magtitude than an issue?
If the cosmological constant is the explanation for dark energy Labda must be about (10^28cm)^-2. The length 10^28 cm is absurdly large etc.
If dark energy and the cosmological constant have something to do with the Big Bang and the Big Bang can be viewed as an instantaneous supernova of a billion galaxies than what is the meaning of number that expresses the total energy involved ?
Cosmological models must be grounded in laws of nature that we understand.
Finally they use the word understand. Bravo. IMO Cosmological models should describe the behaviour of physical objects. Newton's Law describes the behaviour of stellar objects. When you include concepts like dark matter and when you claim that of the total universe consists of 25% dark matter (i.e. invisible matter) you places yourself on slippery ice. (Glad ijs in dutch). Claims that the universe includes 70% of dark energy, which does not represent matter, things become even more difficult to understand and accept.

### Documentation

1. Why all these prejudices against a constant? Authors: Eugenio Bianchi, Carlo Rovelli

2. The Cosmological Parameters 2010 Authors: Ofer Lahav, Andrew R Liddle

3. FIVE-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE* OBSERVATIONS: GALACTIC FOREGROUND EMISSION Authors: G. Hinshaw e.a.

4. The Cosmological Constant by Sean M. Carroll
5. The 6dF Galaxy Survey: Final Redshift Release (DR3) and Southern Large-Scale Structures Authors: D. Heath Jones, Mike A. Read e.a.

6. Supernovae, Dark Energy, and the Accelerating Universe Author Saul Perlmutter. April 2003 Physics Today
In that article at page 57 we read:
The best fit to the 1998 supernova data (see figures 3 and 4) implies that, in the present epoch, the vacuum energy density rL is larger than the energy density attributable to mass (rmc2). Therefore, the cosmic expansion is now accelerating
7. Dark Energy and the Accelerating UniverseAuthors: Joshua Frieman (Chicago/Fermilab), Michael Turner (Chicago), Dragan Huterer (Michigan)
8. Lectures on Dark Energy and Cosmic Acceleration Author Joshua A. Frieman
Fermilab Center for Particle Astrophysics, Batavia, IL 60510
9. SNAP

### Reflection

1. This discussion is about the word mystery. IMO it does not belong to the nomenclature of a scientist.
2. When you look at figure 1 you see the same galaxies at three different epochs. I reality you can never make such an observation.
3. The most difficult part of figure 1 is not space expansion, but space acceleration. The whole issue is what are the observations based on which this is based ? IMO concepts like dark energy and the cosmological constant have nothing to do with this. IMO what this means is that the Hubble constant is not a constant, but that fact is never mentioned.

### Feedback

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Created: 24 july 2010