Vol 466|15 July 2010
NEWS & VIEWS COSMOLOGY FORUM
Is dark energy really a mystery? The Universe is expanding. And the expansion seems to be speeding up. To account for that acceleration, a mysterious factor, ‘dark energy’, is often invoked. A contrary opinion — that this factor isn’t at all mysterious — is here given voice, along with counter-arguments against that view. No it isn’t — Eugenio Bianchi and Carlo Rovelli
QFT’s vacuum energy — or that of any other mysterious substance. Λ is a sort of ‘zero-point curvature’; it is a repulsive force caused by the intrinsic dynamics of space-time. Tests on the ΛCDM model must continue and alternative ideas must be explored. But it is our opinion — and that of many relativists — that saying dark energy is a ‘great mystery’, for a force explained by current theory, is misleading. It is especially wrong to talk about a ‘substance’. It is like attributing the force that pushes us out of a turning merry-go-round to a ‘mysterious substance’.
Is cosmic acceleration1 such a great problem? Many commentators have argued that it is, and that the explanation is to be found in invoking a mysterious substance, dark energy, that as yet has no theoretical underpinning2,3. We disagree. An explanation is to hand and has been for many years. Cosmic acceleration is predicted and simply described by the theory of general relativity, together with a non-vanishing cosmological constant, Figure 1 | Cosmic expansion. As the Universe expands, space stretches much like the Λ. Our case, presented here in surface of an inflating balloon, and galaxies get farther and farther apart. Data indicate summary, is made in detail in that the rate of this expansion is accelerating. a paper4 posted on the arXiv preprint server. history, during which the contributions from Eugenio Bianchi and Carlo Rovelli are at the The Λ cold dark matter (ΛCDM) cosmo- matter and Λ to the cosmic dynamics are Centre de Physique Théorique de Luminy, logical model assumes the presence of Λ and is comparable in magnitude. Such an ‘unlikely Case 907, Luminy, Marseille F–13288, France. “almost universally accepted by cosmologists as coincidence’ is presented as an argument e-mails:
[email protected]; the best description of the present data”5. Three against the ΛCDM hypothesis. But if the ratio
[email protected] objections to Λ are commonly presented, and of these contributions as a function of cosmic nourish the ‘mystery’. We argue that there is time is properly considered on a linear rather Yes it is — confusion — historical or conceptual — in than a logarithmic scale, it can be seen that Rocky Kolb each of them. such a ‘short’ phase lasts for half the life of The ΛCDM is the most complete, predicThe first objection is known as ‘Einstein’s the Universe, and there is no ‘unlikely’ coin- tive and successful cosmological model ever blunder’. Allegedly, Λ was rejected by general cidence. In any case, we should not assume devised, capable of accounting for an enorrelativists, and indeed by Einstein himself, who that we live in a fully random place or time mous number of astronomical observations. At first considered it but later called it his “great- in the Universe, as the coincidence-problem present, there are no observations discrepant est blunder”. But Einstein’s ‘blunder’ was not objection presupposes. The density around with the ΛCDM model. Λ. It was failing to see that — with or with- us, for instance, is very far from the average But the success of ΛCDM comes at a price. out Λ — the Universe isn’t static in his theory cosmic density. In the model, only about 5% of the total mass– of general relativity, thereby missing an easy The third objection concerns ‘vacuum energy of the Universe is observed and underprediction of the cosmic expansion (Fig. 1) energy’. Quantum field theory (QFT) seems stood, and 95% of the Universe is dark. The before its discovery. Λ is not an appendage to to predict a vacuum energy that adds to the dark side includes 25% of the total mass–energy Einstein’s theory added to account for observa- cosmological force due to Λ — just as radia- in the form of dark matter binding together tions: it is an integral and natural part of it. Its tive corrections affect the charge of the elec- galaxies and other large-scale structures, and nature and scale are no more or less mysterious tron. But this hypothetical contribution to Λ is 70% in the form of dark energy driving galaxies than any of the several other constants in our much larger than the observed Λ. The discrep- apart in an accelerating cosmic expansion. fundamental theories. ancy is an open puzzle in QFT in the presence Cosmologists usually refer to dark matter The second objection is termed the of gravity6,7. But it is a conceptual mistake to and dark energy as cosmic mysteries. Bianchi ‘coincidence problem’. Data indicate that we confuse Λ with QFT’s vacuum energy. Λ can- and Rovelli4 argue that dark energy can be happen to live in a ‘short’ phase of cosmic not be reduced to the ill-understood effect of explained by invoking a new constant of © 2010 Macmillan Publishers Limited. All rights reserved
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NEWS & VIEWS nature, a cosmological constant. They assert that this is a simple, acceptable, non-mysterious explanation for dark energy. I disagree. In my opinion, a cosmological constant qualifies as a mystery in the non-theological sense of the word8: “Something not understood or beyond understanding.” Einstein’s cosmological constant Λ is the simplest explanation for dark energy: it adequately fits the data, and there is no reason to exclude it. But the magnitude of Λ necessary to explain the observations places it far “beyond [our] understanding”. If the cosmological constant is the explanation for dark energy, Λ must be about (1028 cm)−2. The length 1028 cm is absurdly large, and cannot at present be related to any
NATURE|Vol 466|15 July 2010
other known or expected length scale in nature. Attempts to explain this new length scale fail by many, many orders of magnitude. We must demand more of cosmology than just piling on components or constants to a model to reproduce observations. Otherwise, we would still happily be adding epicycles to the Ptolemaic model of planetary motion. Cosmological models, along with their constants and components, must be grounded in laws of nature that we understand. The magnitude of the cosmological constant cannot currently be explained by any physics we know. Until it is, it is a mystery. Recalling the warning of astrophysicist Tommy Gold (personal communication), “for every complicated physical phenomenon there is a
CONSERVATION SCIENCE
Trade-in to trade-up Peter Kareiva Nature reserves and protected areas enjoy sacred status in conservation — which translates into a ‘do not touch’ attitude. But selling off some of the less worthy of them would pay conservation dividends. (US$17.4 billion), which could then be reinvested to achieve far more conservation elsewhere (Fig. 1). The idea is to sell off those protected areas that yield the lowest conservation value per assessed land value, and then reinvest the funds in lands that generate the highest conservation value per dollar spent. If these transactions were actually completed, the authors conclude, Australia could achieve a tenfold increase in the total area under conservation protection and a threefold increase
R. SMITH/PHOTOLIBRARY.COM
Protected areas set aside from major human activities and managed for biodiversity are the foundation of modern conservation. Until now, no conservationist would have considered trading them in. Yet trading-in and trading-up is exactly what Fuller and colleagues recommend for Australia’s protected areas, as described on page 365 of this issue1. Fuller et al. estimate that, by selling off 70 of Australia’s nearly 7,000 protected areas, the government could raise Aus$20.6 billion
Figure 1 | Acacia harpophylla — in need of protection. This vegetation type is down to less than 15% of its original extent, and is an example of habitat that would benefit from the scheme of Fuller et al.1. 322
© 2010 Macmillan Publishers Limited. All rights reserved
simple, wrong explanation”, it would be a mistake to be satisfied with the cosmological constant just because it is a simple explanation. O Rocky Kolb is in the Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637, USA. e-mail:
[email protected]
1. Carroll, S. M. Living Rev. Rel. 4, 1–56 (2001). 2. Calder, L. & Lahav, O. Phys. World 23 (June), 32–37 (2010). 3. Tyson, J. A. Nature 464, 172–173 (2010). 4. Bianchi, E. & Rovelli, C. Preprint at http://arxiv.org/ abs/1002.3966 (2010). 5. Lahav, O. & Liddle, A. R. Preprint at http://arxiv.org/ abs/1002.3488 (2010). 6. Wald, R. M. Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics (Univ. Chicago Press, 1994). 7. Rovelli, C. Quantum Gravity (Cambridge Univ. Press, 2004). 8. www.merriam-webster.com/dictionary/mystery
in the diversity of vegetation types under protection. Real-world application of a returnon-investment analysis might be the best thing that could happen to conservation in Australia and, by extrapolation, elsewhere in the world. Historically, the establishment of protected areas has been anything but analytical or efficient. Before conservation emerged as a science, protected areas tended to be located to satisfy the tourist industry or the wishes of a wealthy few, or for convenience. They were often also sited so as to become bargaining chips between large corporate landowners and national governments; rarely were conservation goals a factor2. One of the greatest contributions of the new field of conservation science has been to replace ad hoc establishment of protected areas with networks of nature reserves that are sited using computer-based planning tools3. It has now become clear that the use of data, quantifiable objectives and spatial-optimization programmes provides an opportunity to protect much more nature at far less public expense4. However, the idea of applying a return-oninvestment approach to the design of networks of protected areas has yet to gain full traction in the messy world of non-governmental organizations involved in conservation, or that of national governments. When protected areas are established, it is usually because a conservation group has lobbied for the budget allocation to make it happen. There is no need for economic analysis in such a case, because the problem of getting enough money is seen as a lobbying effort, and when money or land becomes available, opportunity is what counts more than any return-on-investment assessment. The possibility of selling existing protected areas totally changes the nature of the discussion because there is no need to lobby for an opportunity for new protected areas — the opportunity exists by virtue of the funds generated from selling low-return nature reserves. Doubtless, the very thought of such a ‘tradein and trade-up’ scheme for enhancing the efficiency of conservation will cause many
