March 15, 2008 | David F. Coppedge

How to Avoid Dark Energy

Who needs dark energy?  Copernicus?  George Ellis (U. of Cape Town) said we could get rid of dark energy by throwing the Copernican Principle overboard.  Writing in Nature,1 he said that dark energy may simply be an artifact of the geometry of space-time.
    Copernicus did not invent the Copernican Principle.  He was just trying to simplify calendar-making and astronomical prediction with a new sun-centered geometry.  The revolution begun by On the Revolutions of the Heavenly Spheres in 1543 (see Scientist of the Month) took natural philosophy captive and dragged it into realms far beyond the wildest dreams of Nicolas Copernicus himself.  Removing earth from the center of the scheme of the universe began a series of corollaries.  The solar system was not the center, neither – nor the galaxy, nor anything.
    The Copernican Principle slowly became a dogma.  For nearly a century, it has been nearly a catechism to recite that there is nothing special about the earth or any other point in space or time.  Now, in an astonishing statement of cosmic scope, Ellis gently proposed questioning the cherished assumption that has been a cornerstone of 20th-century cosmology.  He says it is untested and unnecessary.  The troublesome problem of dark energy might be solved, he said, by jettisoning the Copernican principle.

The fundamental dynamics of the Universe are embodied in Albert Einstein’s general-relativistic field equations, which describe how gravity arises through the distortion of space-time by mass and energy.  The simplest class of solution to those equations, that on which the concordance model is based, assumes that matter is distributed both homogeneously (everything is similar in all regions of space) and isotropically (everything looks the same in all directions).  That assumption is consistent with observations, but it is not a direct consequence of them.  It is the favoured solution both because it is the simplest and because it rests on a cherished cosmological assumption.  This is the ‘copernican principle’: that the characteristics of the Universe in our neighbourhood are not special in any way, but are typical of the whole.
    A cherished assumption this might be, but it is also fundamentally untested.  It is consistent with the supernova observations, but only provided that some form of dark energy is present.  The central plank of the new research is the claim that, by jettisoning the copernican principle and our assumptions about the distribution of matter in the Universe, we can also abandon the troublesome chimaera of dark energy.

Whoa; that’s radical.  What would a universe mean that is not homogeneous or isotropic?  He examined whether inhomogeneities might be local or global.  It’s clear that space-time is not homogeneous and isotropic in our neighborhood.  We live in a galaxy.  There’s a lot of empty space between our galaxy and the next one.  Nothing uniform about that.  Moreover, we live in a cluster of galaxies – another level of clumpiness.  Cosmologists presume, however, that they can average out the local clumpiness by looking at large enough scales.  Is that certain?  What if there are sizeable voids and clumps on higher scales that prevent averaging?  We’ve heard reports of gigantic walls of superclusters of galaxies that are a non-trivial part of the visible universe (10/24/2003).  In addition, there are vast regions with almost no galaxies at all.  Further, we know from gravitational lensing that dense areas can distort the light of quasars behind them (07/23/2006).
    These local inhomogeneities can already skew our supernova measurements (11/01/2006) and, with them, our interpretations of cosmic acceleration, of which dark energy is the troublesome chimera.  And then – what if there are clumps and voids at the Hubble scale?  All bets are off.2
    Ellis can’t be serious, can he?  He recognizes that much testing would have to back up any decision to alter the Copernican Principle.  “Spatial homogeneity is one of the foundations of standard cosmology,” he said, winding up his thoughts, “so any chance to check those foundations observationally should be welcomed with open arms.”  He applauded the work of Clarkson et al last year who “show that a simple observation of the copernican principle that is independent of any theory of gravity or model for dark energy is possible through redshift and area�distance observations of distant galaxies.”  This and other papers Ellis referenced were published on ArXiv, an open-access journal where non-traditional proposals are more likely to get a hearing.  That Nature would publish this is an indication Ellis is not crazy.
    Ellis ended by casting doubt on any dark-energy solution that implies the existence of negative kinetic energy terms, like “some adventurous workers propose” in violation of energy conditions.  Instead, “It may be that such observations are trying to tell us that there is something fundamentally wrong in our assumptions; and that the acceleration conundrum could have a geometric, rather than a dynamic, solution.”

1.  George Ellis, “Cosmology: Patchy solutions,” Nature 452, 158-161 (13 March 2008) | doi:10.1038/452158a.
2.  Ellis is not the first to propose geometric inhomogeneities as a solution to the dark-matter problem.  See, for instance, New Scientist a year ago (03/30/2007), and Simon D. M. White’s paper “Why Dark Energy is Bad for Astronomy” on ArXiv.  Another article on New Scientist (13 July 2007) suggested dark energy lurks in hidden spatial dimensions, and another New Scientist article a month later (31 August 2007) suggested that if the universe resembled swiss cheese on large scales (i.e., numerous large voids and dense regions), it could partially mimic the effects of dark energy.

The dark energy problem has caused a lot of head-scratching among cosmologists.  While the popular science writers have confidently stated the consensus idea that 73% or so of the universe is made of dark energy, as if this is a matter of undisputed fact (e.g., 11/02/2002), this article reveals that many cosmologists are still uncomfortable with it.  It’s been embarrassing to not know anything about a substance that allegedly makes up the bulk of the universe (07/23/2007, 04/13/2007).
    Philosophers and historians of science should take note.  Here is a case where one of the most fundamental, indeed, one of the most cherished assumptions in cosmology is on the auction block.  This is not the first time.  Quantum physicists were just about ready to jettison the Law of Conservation of Energy once when observations didn’t square with theory.  There have been other times when radical overhauls of cherished assumptions were considered in order to “save the phenomena” (i.e., match theory to the observations).  So many things have gone wrong with modern cosmology, the crisis in confidence prevails (05/11/2006).  This suggests several paradigm changes are possible in the next few years.
    The Copernican Principle may survive this debate.  If it goes overboard, though, a lot of metaphysical baggage of the Carl Sagan type may just go overboard with it – maybe even the mantra of atheistic evolutionary naturalism, “We are nothing special.”

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Categories: Astronomy, Cosmology, Physics

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