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Comment by crazygringo | Hacker News Reader

Do I understand correctly that this is actually saying that accelerating expansion of the universe may be in error? That there is, instead, no accelerating at all, but merely constant expansion?

Or can there still be acceleration without dark energy? I'm unclear if they are essentially the same hypothesis or if there are more complicated subtleties I'm missing.

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There are very few ways to modify Einstein's Equations and still have them make sense. An easy one is to add a constant term. Such a term is known as the Cosmological Constant, and a positive value for it would result in a constant repulsive force between masses regardless of distance. This results in an accelerating expansion of the entire universe, but, with a supremely small value it only becomes noticeable at large scales, where it starts to dominate attractive gravity. This appears to model what we're observed happening with galaxy clusters.

Dark Energy is a hypothesis for explaining why such a very small cosmological constant might exist, suggesting that the universe is permeated with a constant negative energy field. However, the article refers to observational evidence measuring the expansion of the universe, not dark energy. There is no evidence for dark energy as such, and if you listen to colloquia on cosmology, you will hear about the cosmological constant, not dark energy. It is not necessary to hypothesize the existence of negative energy particles to model what we've observed, and you'll only really hear the term used in the popular press because it sounds sexy.

> Dark Energy is a hypothesis for explaining why such a very small cosmological constant might exist, suggesting that the universe is permeated with a constant negative energy field.

No, that's not what dark energy is. "Dark energy" is just a general term for "anything that acts like a cosmological constant in the Einstein Field Equation". It could be an actual physical constant, just an inherent property of spacetime, or it could be some kind of energy field. We don't know. (Btw, the energy associated with dark energy is positive, not negative; the pressure is what's negative.)

Thank you for the correction

Well, actually there are infinite ways of modifying the equations and still have them compatible with current observations. Check out for instance https://en.wikipedia.org/wiki/F(R)_gravity but there are many more examples with additional fields.

There is a standard cosmological model called LCDM model, Lambda Cold Dark Matter model, which is basically the minimal thing you can use to fit cosmological observations, it contains a cosmological constant and cold dark matter, that is dark matter that is heavy enough that it is cold in the present universe. From the cosmic microwave background and Baryon Acoustic Oscillations (BAO), that is basically the distribution of galaxies, you can measure values for your favorite cosmological model. There are additionally for the dark matter density galactic rotation curves and for the cosmological constant super nova data.

Now dark energy is basically another name for the cosmological constant. (In general relativity the cosmological constant gives you the curvature of empty space, and dark energy is the same term moved to the right hand side.) However, the philosophical difference is, that sufficiently funny particles, like for example the Higgs, can easily mimic the effect of an cosmological constant. So there is an industry of people who calculate the effects of different extensions of the standard model on LCDM cosmology.

Now, above I said that CMB, BAO and SN data all agree in cosmological observations on the value of the cosmological constant. So for it to be wrong, it needs to either evolve, or several different measurements need to conspire to mask the effect of an constant. So my assumption is, either their non standard evolution is compatible with LCDM cosmology, or there is something wrong with the analysis.

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