Puzzled by new confirmation of dark energy

[phinds]

puzzled by "new" confirmation of dark energy

Just saw an article on BBC:

http://www.bbc.co.uk/news/science-environment-13462926

that says they have used a new technique to confirm the existence of dark energy. I don't know enough to understand why what they did is "new", as I am sure I recall reading previously that the techniques they used have been discussed before. On the other hand, since both types of observations have to do with the formation of galaxies and clusters, I may have seen the discussion in regards to dark matter, not dark energy, or maybe this is a refinement of techniques and the BBC is just calling it new.

Would appreciate any informed comments on the news.

Thanks

 

[marcus]

It's a beautiful study. Thanks for calling our attention to it!

I hope others will comment informatively about this. The technical article in the arxiv explains more about the specific method they used.

As usual, the pop media report uses misleading language. What they were doing was checking the standard LCDM model, and in particular confirming the number 0.73. This is the "dark energy density" , one version of the parameter "Lambda", that is plugged into the model.

There are various different ways of narrowing down what that number can be. by FITTING what the LCDM model predicts to various types of data. There are four or five main ways to check that the right value of the number is 0.73.

They did one of those four or five ways and confirmed that it checks out. The LCDM gives an amazingly neat fit to the real world, to millions of data points. It is amazing.
So simple! Just a very few adjustable parameters to plug in, and then such a good fit to the data!
===============================

You can forget about "dark energy". That is just one possible journalistic way of referring to the parameter Lambda, which is a constant plugged into a differential equation model that simulates the universe.

LCDM means "Lambda cold dark matter". The real business here is in a remarkably simple equation model of how U evolves. We don't have any reason to suppose that the Lambda number corresponds to some kind of "dark energy". The grownup way to think of it is it is a number you plug in. That acts, in a certain way, "like" an extra energy, but not exactly like other more familiar energy fields. More like something analogous to energy. But you couldn't plug in your toaster or run you car, and it doesn't obey conservation or anything you expect from conventional energy.

Presumably this would be totally unsatisfactory to lay viewers of the BBC, so we can't tell them the truth. We say the study "confirms the existence of dark energy". Which it does not! We don't know that Lambda arises from an energy field. So we are lying to them. All the study did was check a model, called LCDM, and verify in yet another way that the correct value of Lambda is 0.73. But this is itself very beautiful and wonderful!

This is my own personal attitude and take on it. I don't speak as a specialist with expert knowledge. The scientific article is here:
http://arxiv.org/abs/1104.2948
http://arxiv.org/pdf/1104.2948

 

[marcus]

==quote http://arxiv.org/pdf/1104.2948 ==
Recent cosmological observations have revealed significant gaps in our understanding of the physics of the Universe. A set of measurements including the anisotropies of the Cos- mic Microwave Background radiation, the shape of the clus- tering power spectrum of galaxies, the brightness of distant supernovae and the projected scales of baryon acoustic os- cillations have indicated the presence of a “dark energy” component which is propelling the cosmic expansion into a phase of acceleration (for recent results see Komatsu et al. 2009, Reid et al. 2009, Percival et al. 2010, Guy et al. 2010).
The physical nature of dark energy is not yet under- stood.* Several explanations have been put forward includ- ing the presence of smoothly-distributed energy such as a cosmological constant or a quintessence scalar field, a large- scale modification to Einstein’s theory of General Relativity, or the effects of spatially-varying curvature in an inhomo- geneous Universe. Further observational data is required to distinguish clearly between the subtly-varying predictions of these very different physical models (e.g., Linder 2005, Wang 2008, Wiltshire 2009).
One of the most important observational datasets for addressing this issue is the large-scale structure of the galaxy distribution. The clustering within this distribution arises through a process of gravitational instability which acts to amplify primordial matter fluctuations. The growth rate of this structure with time is a key discriminant between cos- mological models (e.g., Linder & Jenkins 2003, Linder & Cahn 2007, Nesseris & Perivolaropoulos 2008). Two different physical dark energy scenarios with the same background cosmic expansion generally produce different growth rates of perturbations, hence growth measurements are able to discriminate between models that are degenerate under ge- ometric tests (Davis et al. 2007, Rubin et al. 2009).
The growth of cosmic structure is driven by the motion of matter, for which galaxies act as “tracer particles”. These flows imprint a clear observational signature in galaxy sur- veys, known as redshift-space distortions, because the galaxy redshift is generated by not only the background cosmic ex- pansion but also the peculiar velocity tracing the bulk flow of matter*** (Kaiser 1987, Hamilton 1998). As a consequence the 2-point statistics of the galaxy distribution are anisotropic on large scales, where the amplitude of the anisotropy is re- lated to the velocity of the bulk flow and hence to the growth rate of structure.
Many previous galaxy surveys have measured this anisotropy employing either the galaxy correlation function or power spectrum. In the relatively local Universe, exquisite studies at redshift 0.1...
==endquote==

*It might not even be an energy. It might be a curvature constant. It might be due to an inhomogeneity or unevenness as suggested by David Wiltshire. It might simply be a second constant in the law of gravity, besides Newton G, that we are only beginning to appreciate.
We don't KNOW where this constant Lambda comes from or what its physical nature is.

***Beautiful! Superimposed on the average expansion rate there are these subtle variations that depend on the direction you look and the distance how are or deep you look. Anisotropies=unevennesses. These subtle variations are caused by galaxies and other matter FALLING towards regions of overdensity. They are caused by the FORMATION OF STRUCTURE by the self-gravitation of matter, which is also opposed by the Lambda constant which curves spacetime so that stuff tends to spread out instead of fall together. Lambda retards the formation of structure. So one can delicately "feel" Lambda by sensitively measuring the rate of formation of structure as it has varied over time.

Something has been resisting the formation of structure and slowing it down. It behaves like a constant. what this is we do not know but we can measure it. In four or five different ways and they agree!

To measure something four completely different ways and have them all say 0.73 is beautiful.

One way is with supernovas. Another way is with the Ancient Light (the cosmic microwave background temperature variations, the mottled temperature skymap.) Another way is by galaxy counts---the gross mapping of structure. This way, now, is yet another: mapping little anisotropies of recession speed, the fluctuations (with direction and distance) in the rates of falling together----variations in the speeds of gathering (in different parts of the sky and history.)

 

[Cosmo Novice]

==quote http://arxiv.org/pdf/1104.2948 ==
The physical nature of dark energy is not yet under- stood.* Several explanations have been put forward includ- ing the presence of smoothly-distributed energy such as a cosmological constant or a quintessence scalar field, a large- scale modification to Einstein’s theory of General Relativity, or the effects of spatially-varying curvature in an inhomo- geneous Universe. Further observational data is required to distinguish clearly between the subtly-varying predictions of these very different physical models (e.g., Linder 2005, Wang 2008, Wiltshire 2009).

I find the "effects of spatially varying curvature in an inhomogeneous universe" a very interesting idea behind dark energy.