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Sunday, May 31, 2009

Uncommon Descent Contest 4: Can we save physics by dumping the Copernican principle?

In "Does Dark Energy Really Exist? Or does Earth occupy a very unusual place in the universe?" physicist Timothy Clifton and astrophysicist Pedro G. Ferreira argue just that: If we give up the Copernican principle, we do not need dark energy to explain the composition of the universe. (Scientific American, March 23, 2009)

Copernican principle? Dark energy?

Copernican principle: That's the idea that Earth does not occupy any unusual position in the universe. Indeed, the point was driven home in a recent talk I attended at a science writers' convention.

The Copernican principle is widely believed, to be sure, but that tells me nothing one way or the other about whether it is well supported by evidence. And I already know good reasons for doubting it. (Note: It has nothing whatever to do with Copernicus, who wouldn't likely have agreed with it.)

Dark energy? "Dark" means we are in the dark about it. According to the current model, we don't know what 70 percent, approximately, of the cosmos comprises. Whatever that 70% is, it does not respond to light. It also does not answer e-mail, phone mail, or letter mail. Bummer.

Many physicists believe that maybe 25% of this unknown substance is dark matter. The rest is dark energy.

Actually, we don't even know what dark matter is, according to the cautious SNO Plus physicists who are building a huge underground facility in the Creighton Mine in Sudbury, Canada, to trap a particle a year of the stuff. So they hardly wish to give tell-all interviews on dark energy.

Anyway, here are some excerpts from Clifton and Ferreira on whether we need assume that dark energy even exists:
... the existence of dark energy is still so puzzling that some cosmologists are revisiting the fundamental postulates that led them to deduce its existence in the first place. One of these is the product of that earlier revolution: the Copernican principle, that Earth is not in a central or otherwise special position in the universe. If we discard this basic principle, a surprisingly different picture of what could account for the observations emerges.

Most of us are very familiar with the idea that our planet is nothing more than a tiny speck orbiting a typical star, somewhere near the edge of an otherwise unnoteworthy galaxy. In the midst of a universe populated by billions of galaxies that stretch out to our cosmic horizon, we are led to believe that there is nothing special or unique about our location. But what is the evidence for this cosmic humility? And how would we be able to tell if we were in a special place? Astronomers typically gloss over these questions, assuming our own typicality sufficiently obvious to warrant no further discussion. To entertain the notion that we may, in fact, have a special location in the universe is, for many, unthinkable. Nevertheless, that is exactly what some small groups of physicists around the world have recently been considering.

[ ... ]

In the conventional picture, we talk about the expansion of the universe on the whole. It is very much like when we talk about a balloon blowing up: we discuss how big the entire balloon gets, not how much each individual patch of the balloon inflates. But we all have had experience with those annoying party balloons that inflate unevenly. One ring stretches quickly, and the end takes a while to catch up. In an alternative view of the universe, one that jettisons the cosmological principle [a generalization of t he Copernican principle], space, too, expands unevenly. A more complex picture of the cosmos emerges.

[ ... ]

The possibility that we live in the middle of a giant cosmic void is an extreme rejection of the cosmological principle, but there are gentler possibilities. The universe could obey the cosmological principle on large scales, but the smaller voids and filaments that galaxy surveys have discovered might collectively mimic the effects of dark energy. Tirthabir Biswas and Alessio Notari, both at McGill University, as well as Valerio Marra and his collaborators, then at the University of Padua in Italy and the University of Chicago, have studied this idea. In their models, the universe looks like Swiss cheese uniform on the whole but riddled with holes. Consequently, the expansion rate varies slightly from place to place. Rays of light emitted by distant supernovae travel through a multitude of these small voids before reaching us, and the variations in the expansion rate tweak their brightness and redshift. So far, however, the idea does not look very promising. One of us (Clifton), together with Joseph Zuntz of Oxford, recently showed that reproducing the effects of dark energy would take lots of voids of very low density, distributed in a special way.
Does Guillermo Gonzalez have clones? Is this legal?

Well, never mind that for now. For a free copy of the Privileged Planet DVD, here's the question: To what extent is the Copernican or cosmological principle held for emotional reasons, and not because the evidence supports it? In 400 words, would we be better off or worse off without it?

(Note: I recommend that you read the whole SciAm article before commenting.)

Here are the contest rules.

You must go to Uncommon Descent to comment. Your name will not be put on a mailing list, or sold or given away for any purpose. There is no mailing list. However, if you win and do not send me a mailing address of your choice at, I cannot send you your prize.

I will shortly be judging Contest 3.

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