Dark energy and gravity |
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Authors: | T Padmanabhan |
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Institution: | (1) IUCAA, Post Bag 4, Ganeshkhind, Pune, 411 007, India |
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Abstract: | I review the problem of dark energy focussing on cosmological constant as the candidate and discuss what it tells us regarding
the nature of gravity. Section 1 briefly overviews the currently popular “concordance cosmology” and summarizes the evidence
for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as a
candidate and emphasizes why no other approach really solves the conceptual problems usually attributed to cosmological constant.
Section 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract certain
key ingredients which must be present in any viable solution. In the conventional approach, the equations of motion for matter
fields are invariant under the shift of the matter Lagrangian by a constant while gravity breaks this symmetry. I argue that
until the gravity is made to respect this symmetry, one cannot obtain a satisfactory solution to the cosmological constant
problem. Hence cosmological constant problem essentially has to do with our understanding of the nature of gravity. Section 3 discusses such an alternative perspective on gravity in which the gravitational interaction—described in terms
of a metric on a smooth spacetime—is an emergent, long wavelength phenomenon, and can be described in terms of an effective
theory using an action associated with normalized vectors in the spacetime. This action is explicitly invariant under the
shift of the matter energy momentum tensor T
ab
→ T
ab
+ Λ
gab
and any bulk cosmological constant can be gauged away. Extremizing this action leads to an equation determining the background geometry which gives Einstein’s theory at the lowest order with Lanczos–Lovelock type corrections. In this approach, the observed
value of the cosmological constant has to arise from the energy fluctuations of degrees of freedom located in the boundary
of a spacetime region. |
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Keywords: | |
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