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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 register to comment here at Uncommon Descent. 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, you must give me  a mailing address of your choice at [email protected], so I can  send you your prize.

I will shortly be judging Contest 3.

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

  1. This following article is lot more than 400 words but may be of help to someone entering the contest since it covers a lot of the Dark Matter/Energy evidence:

    Intelligent Design – The Anthropic Hypothesis

    http://docs.google.com/View?id=dc8z67wz_0hm7ftjfn

  2. Ha, Lawrence Krauss wronte someting similar here:

    http://www.edge.org/3rd_cultur.....index.html

    Here is a snippet…

    The energy of empty space that isn’t sero (Lawrence Krauss)

    “The new results are either telling us that all of science is wrong and we’re the center of the universe, or maybe the data is [s]imply (sic) incorrect, or maybe it’s telling us there’s something weird about the microwave background results and that maybe, maybe there’s something wrong with our theories on the larger scales. (Lawrence Krauss)

  3. If the universe expands unevenly, I’m not sure how that necessarily makes our place in it “special” — it would just make us our portion of it one of many anomalous ones.

    A key part of one of the quotes seems to be

    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.

    Also:

    In their models, the universe looks like Swiss cheese uniform on the whole but riddled with holes.

    Of course, even if Earth just happens to be in one of those holes, it’s still the only hole we know with life on it, so that makes us special in any case.

  4. I think Stephen Hawking should win the contest. From “The Large Scale Structure of Space-Time” (134-135):

    However we are not able to make cosmological models without some admixture of ideology. In the earliest cosmologies, man placed himself in a commanding position at the center of the universe. Since the time of Copernicus we have been steadily demoted to a medium sized planet going round a medium sized star on the outer edge of a fairly average galaxy, which is itself simply one of a local group of galaxies. Indeed we are now so democratic that we would not claim that our position in space is specially distinguished in any way. We shall, following Bondi (1960), call this assumption the Copernican Principle.

    A reasonable interpretation of this somewhat vague principle is to understand it as implying that, when viewed on a suitable scale, the universe is approximately spatially homogenous…

    … it is difficult to test homogeneity directly by observation…This difficulty is eased by the fact that we can, in principle, fairly easily observe isotropies in extragalactic observations, and isotropies are closely connected with homogeneity.

    …in general, there can exist at most two points ina spherically symmetric space from which the space looks spherically symmetric…[spherically symmetric universes] can only be models of the universe consistent with the isotropy of our observations if we are located near a very special position. The exceptional cases are those in which the universe is isotropic about every point in space time; so we shall interpret the Copernican principle as stating that the universe is approximately spherically symmetric about every point (since it is approximately spherically symmetric about us).

  5. JohnnyB, just for clariff, Dr. Hawking would need to enter the contest in order to win it. People are assumed to enter under their own names/handles. – d.

  6. Since I haven’t even mastered quantum mechanics yet, I am afraid I don’t have any opinion whatsoever on the subject.

  7. Copernicus’ modest proposition was that the solar system is heliocentric and not geocentric. Centuries later came the Copernican principle: the idea that Earth does not occupy any special position in the universe. In the last few decades this principle has been expanded to include the idea that there is nothing special about humans or the Earth. This idea is often called the Copernican principle of mediocrity. In recent years some astronomers have taken the idea further still and have popularized the notion that there is nothing special about our universe, as it is just one among an infinite number of other universes: a multiverse. Although no evidence supports the theory, and as it is not testable no evidence is ever likely to, it is considered the natural and ultimate culmination of the Copernican principle.

    The problem with the principle is how do you define special? In the Rare Earth hypothesis, scientists Ward and Brownlee identify no less than a dozen factors that make complex life possible on Earth. In their view these factors make the Earth, if not special, than certainly very rare. Astronomer Guillermo Gonzalez goes further and identifies factors that make the Earth particularly suitable for scientific discovery. In his view the Earth is more than a rare planet; it is a privileged one. Recently some astronomers have questioned the standard model of the universe that holds that at least 70% of the universe is composed of mystery material. They propose this material is unnecessary if we ignore the Copernican principle and assume instead that the Earth lies at or near the centre of a vast cosmic void with far lower density than other regions of space. Unlike the multiverse, the theory is testable and efforts are underway to confirm or dismiss it. Considering what we have learned about what makes the Earth’s particular location in the solar system and in the galaxy especially suitable for life, will we also discover that Earth’s place in the centre of a vast cosmic void is another necessary precondition for life?

    Do we have further need of the Copernican principle? Or is it instead merely a personal philosophical position about humanity’s place? Does it tell us more about the belief system of those who hold it than it does about the universe?

  8. Just as an aside to my essay, it occurs to me that the confirmation of a “Cosmic Void” (still a very big “if”) could present exciting research opportunities for astronomers open to Intelligent Design.

    I.D proponents identified similar opportunities when Darwinist geneticists advanced the concept of “Junk DNA” to describe non-coding regions of the genome. Not immediately understanding the function of those elements, Darwinists dismissed them as mere left over junk from our distant evolutionary past. I.D proponents assumed instead that those elements would serve some critical but not fully understood purpose. More and more that seems to be the case.

    With the (hypothetical) confirmation that the Earth lies at or near the centre of a vast “Cosmic Void”, an I.D supporting astronomer might also assume that the void plays some kind of critical role in making life on Earth possible. Astronomers have long known that the Earth lies within a relatively narrow habitable zone in our solar system. Proponents of the “Rare Earth” hypothesis have introduced the concept of a “Galactic Habitable Zone”. Could our galaxy lie in the centre of a Cosmic Habitable Zone as well: a vast cosmic void with far lower density than other regions of space?

    Is such a region another necessary precondition for life? If so, would the Earth’s situation at or near the centre of the void also play an important role? An astronomer might ask: if the Earth was not at the centre, but closer to the edge, would that affect the Earth’s ability to support life? Does the size of the void matter? If the void was significantly smaller, would the outcome for our Earth change?

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