## Friday, November 30, 2007 ... /////

### Teppo Mattsson: Dark energy as a mirage

Is cosmological constant zero, after all? (PDF)
If this guy were right, the required changes would be tiny. Dark energy would go away, 90% of the mass would be dark matter, 10% would be baryonic, and the age of the Universe would jump to 14.8 billion years.

He says that as you observe distant objects, the Universe is getting emptier, due to a hierarchical clumping of matter, and in these emptier regions, the Hubble constant increases, mimicking the accelerating effects of the cosmological constant.

I am confused about elementary things such as signs in his argument but you should think about it yourself. So far I will keep my estimate of 95% probability that the standard cosmological model including the C.C. is right and the C.C. can't be explained away in this way or a similar way.

Fine-tuning

My problem with an explanation based on hierarchical clumping - which is otherwise a cool old idea - is that the observed cosmological constant seems to be determined rather accurately and uniformly in all directions.

On the other hand, there is no good reason to expect that we are exactly in the middle of our local clumped hierarchies of galaxies and their clusters. If we assume that we are not, the deduced acceleration according to the mirage effect would depend on the direction but it doesn't. Of course, we might be close to the center but it seems as an additional unlikely assumption that makes the whole explanatory framework less likely.

If you care about this argument quantitatively, a small observed cosmological dipole of the microwave background implies that we would have to be a few percent from the center of our local clump. Only a few parts per million of stars live in this small central "core". This determines the degree of fine-tuning. You might still believe that it is better than 10^{-123} fine-tuning for a small but non-zero C.C. But at this moment, I think that physical arguments show that there's nothing wrong with a non-zero C.C. and the new kind of "central" fine-tuning is too high a price to pay for eliminating the C.C.

Thanks to You Know Who(m).