Cosmology

Original questions

The lack of a coherent quantum description of gravity has impeded our understanding of the physics that determined how the Universe began. A synthesis of recent ideas may take us a step farther back in time.

Among the deepest, borderline-philosophical questions in modern physics is that of the origin and formation of the Universe. Earlier attempts to formulate an answer that takes into account existing theories and observations have failed because of obstacles posed by gravity. Mulryne et al.1, writing in Physical Review D, provide a ‘loop quantum gravitational’ model that successfully merges current ideas, and which may enable us to overcome such difficulties.

The most important feature to bear in mind when considering the origin of the Universe is the radiation that was released when the Universe became transparent to light, the so-called cosmic microwave background2. Anisotropies in this radiation — slight variations in its temperature according to the direction in which you look at it — carry infor-mation on the distribution of matter at the time of its release. Through backward evolution of theoretical models of the Universe, we can garner an idea of what the initial seeds for any structure we observe in the cosmos might have been. The currently favoured models are inflationary models, and postulate an accelerated expansion of the early Universe at the time when the initial seeds were being sown.

The trouble with these models is that they require a state at which space is not just tiny, but has no size at all, and where the amount of energy stored becomes infinite — a situation impossible to deal with in the classical theory on which they rely. Mathematically, this is a ‘singularity’, where the main equations and concepts of a theoretical framework become inapplicable. Quite often, this state of zero size is speculatively identified as the ‘initial’ state of the Universe. However, it is simply ill-defined in the theory of general relativity, which is our current best description of the nature of space and time.

Near a singularity, we reach the limits of current theory. At extremely small sizes and high energies, quantum effects are expected to be significant, so a quantum theory of gravity is needed. The required combination of general relativity and quantum theory has so far resisted consistent formulation. We can, however, attempt to apply some promising candidate theories to the early Universe. Loop quantum gravity3,4 is one such theory; it can deal with both strong gravity and a potentially vanishing space, and can be applied to cosmological situations in a framework known as loop quantum cosmology5. The theory gives rise to characteristic effects, such as the energy in matter in quantized space behaving differently, on small scales, from how it does in classical formulations6. To some degree, quantum space can be considered as analogous to a crystal, which, through its atomic structure, changes the propagation of light relative to that through a vacuum.

One characteristic consequence of these features of loop quantum cosmology is a repulsive contribution to the classical, attractive force of gravity. It is easy to imagine that this repulsion could prevent the total collapse of the Universe to zero size7, or even, when it is expanding, accelerate that expansion8. Combined with ideas of inflationary cosmology, the proposition has the ingredients of a well-defined and observationally viable model. Yet by itself it still does not explain the origin of the Universe.

One attempt at such an explanation is the emergent-Universe model9,10. In the absence of additional information on the initial state of a system, it is most economical to assume that it was the simplest possible: for a physicist, this means the most highly symmetrical. Such a state would not have any structure in space or time, but would be homogeneous and static — an assumption already considered by Einstein. Classical examples of such states, called Einstein static spaces, do exist, provided that space is curved and closed.

Static solutions do not evolve, and so are clearly ill-suited as a model for the Universe. But by introducing a perturbation to a static solution, one can slightly change it and thereby start a more interesting history. Unfortunately, the classical solution is unstable: any disturbance grows rapidly, leaving little of the initial state behind. The insight of Mulryne and colleagues1 is that quantum effects could supply all the necessary ingredients where classical solutions do not. Within the framework of loop quantum gravity, repulsion also implies static solutions at small size, but these — in contrast to the classical case — are stable.

According to the authors' model, perturbing such a state leads to small cycles of interchanging expansion and contraction. During this process, matter will evolve slowly, and the cycles will gradually change their behaviour. By itself, this perpetual recurrence and incremental change seems to lack the spark necessary for so momentous an event as the birth of the Universe. And indeed, Mulryne and colleagues identify one final theoretical ingredient that lights this spark: mediated through repulsive effects, potential energy is gradually pushed into the matter during its slow evolution. At the point when potential energy starts to dominate kinetic energy, the mundane cycling is broken by a sudden, dramatic inflationary explosion — the emergent Universe (Fig. 1).

Figure 1: Out of the loop.
figure1

M. BOJOWALD

A small Universe initially cycles through different sizes, and eventually escapes to an inflationary era (line off to right). Colours represent how often the Universe reaches a certain size, growing from left to right; the position in the vertical direction is determined by the amount of expansion or contraction.

Mulryne and colleagues thus supply a promising, well-defined picture of how the Universe with its complicated structure could have emerged from a simple initial state. It is unlikely that the existence of any new observable effects will be postulated soon on the basis of this picture, although it does clarify several conceptual problems: the possibility of non-singular behaviour, for example, and the role of closed spaces. The basic effects interpreted as repulsion have been known as mathematical constructs for some time. But it is only when incorporated into cosmological models such as these, and models governing the physics of black holes, that we see how important quantum-gravitational effects can be, and how naturally they can fill in the gaps in our knowledge.

Work such as that of Mulryne et al.1 gives strong support to general ideas of quantum gravity, although various models and effects must still be better justified and tied in more closely to a full theory. The virtue of cosmological investigations lies not only in their being a supplier of basic ideas, but also in their guiding of developments and showing where, in so complex a theory as quantum gravity, one should look for interesting effects.

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Bojowald, M. Original questions. Nature 436, 920–921 (2005). https://doi.org/10.1038/436920a

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