REVIEW
Probing Newton's Constant on Vast Scales: DGP Gravity, Cosmic Acceleration and Large Scale Structure
Astrophysics · Submitted 23 Jan 2004 · 37 references
Pith's verdict: The force-law derivation is solid and worth citing; the sigma_8 tension is real in direction but softer in magnitude than the abstract's 'two sigma' framing suggests.
T0 · AI review
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Referee report
The paper
The paper claims
The paper argues that if cosmic acceleration comes from gravity leaking into a fifth dimension rather than from dark energy, the growth of large-scale structure is suppressed enough to conflict with observed galaxy clustering at roughly the
The reading
What carries the argument
The load-bearing object is the ratio $(r)$ defined in Eq. (2.18), built from the parameter $ = (1 2r_0 H + 2r_0^2 H^2)/(1 2r_0 H)$ (lower signs for the self-accelerating branch), which interpolates the metric of a spherical overdensity between an Einstein phase at small radii and a weak-brane phase at large radii. Its argument is $8r_0^2 R_g/9^2 r^3$, with $R_g$ the effective Schwarzschild radius of the overdensity, so the crossover radius $r_* = (r_0^2 R_g/^2)^{1/3}$ is where the extra scalar switches on. Because $ $ contains the Hubble rate, the force law inherits the expansion history; this is what makes DGP distinguishable from a dark energy model with the same $H(z)$, and it is also why the suppression of growth is roughly twice what a Birkhoff-obeying modification would give. In linear theory the whole effect collapses to the single replacement $G G(1+1/3)$, which feeds directly into the growth equation and into the velocity growth rate $f d D_+/d a$.
Extended reading notes
Core claim
The central claim is that DGP gravity does not obey Birkhoff's law: the force law around an isolated mass depends on the background cosmological expansion, because the brane's extrinsic curvature controls the strength of the scalar gravitational mode. Generalizing an earlier static de Sitter calculation to an expanding matter-dominated background, the authors find two regimes separated by a new scale $r_* = (r_0^2 R_g/^2)^{1/3}$. Inside $r_*$ the metric reduces to the Einstein phase with a small $1/r^{3/2}$ correction; outside it the weak-brane phase sets in and the two gravitational potentials acquire different effective Newton constants, with the Newtonian one becoming $G_{eff} = G(1+1/3)$. Since $ $ is negative and grows in magnitude with time, gravity becomes effectively weaker than Einstein gravity as the universe ages, and linear growth is suppressed relative to a dark energy model engineered to produce the identical expansion history. The suppression is about twice as large as in modified-gravity models that do obey Birkhoff's law. Combined with the supernova-preferred matter density, this drives the present normalization to $_8 0.8$ at roughly two $\sigma$, against a dark-matter value near 1.0 inferred from SDSS galaxy clustering. The authors also show that $^2(-)$ for linear perturbations is exactly the four-dimensional Einstein expression, so the late-time integrated Sachs–Wolfe effect and weak lensing at subhorizon scales are unchanged from a dark energy model with the same expansion history. They are explicit that the treatment is restricted to subhorizon, quasistatic, spherical perturbations, and that a fully consistent linear and second-order treatment, plus the horizon-scale ISW calculation, remains to be done.
Load-bearing premise
The claimed tension rests on turning the measured L* galaxy clustering amplitude into a dark-matter $_8$ through an empirical luminosity-bias relation, and on holding the primordial fluctuation normalization fixed as $_m$ is varied, a chain to which the paper allows only about 10% systematic uncertainty.
Editorial extensions
If this is right
- If DGP is right, the linear growth rate behaves as $f _m^{2/3}$ rather than the $_m^{5/9}$ of a flat cosmological constant model, a difference of at least 10% for $_m$ between 0.2 and 0.3 that redshift-space distortion surveys can reach.
- Because the extra gravitational scalar does not couple to photons, the late-time integrated Sachs–Wolfe effect and weak lensing at subhorizon scales look exactly like a dark energy model with the same expansion history; only the growth rate and the geometrical distances carry the DGP signature.
- Spherical collapse happens later in DGP (redshift 0.35 rather than 0.66 for the same initial perturbation) but the linearly extrapolated threshold changes only from 1.689 to 1.656, and the hierarchical moments such as the skewness shift by less than a percent, so nonlinear growth can still be used to calibrate galaxy bias as in standard cosmology.
- Fitting the supernova acceleration inside DGP drives $_m$ to about 0.18, where the predicted $_8$ falls to 0.7 or below at one sigma, so sharper supernova evidence for an effective equation of state near $-1$ would deepen the tension rather than relieve it.
Reading between the lines
- The cleanest discriminator suggested by this machinery is a joint measurement of the growth rate and the expansion history at the same redshift: a DGP fit that matches supernovae but predicts a lower $f$ at fixed $_m$ would be separable from a dark energy model with matching $w(z)$.
- Because the paper finds $^2(-)$ unchanged, gravitational-slip measurements, a standard probe of modified gravity, would be blind to this particular modification at subhorizon scales; the signal lives in the growth rate and in distances instead.
- The crossover radius scales as $R_g^{1/3}$, so the Einstein-to-weak-brane transition sits at a few megaparsecs for galaxy-scale masses and moves only weakly with mass; if the null lensing result survives for aspherical lenses, a stacked lensing analysis could in principle map that transition directly.
- The two-sigma tension is only as firm as the conversion of an L* galaxy clustering amplitude into a dark-matter amplitude; a bias-independent $_8$ from cluster counts or cosmic microwave background lensing at $_m$ near 0.2 would settle whether the conflict is real.
Original abstract
The nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. The braneworld theory of Dvali, Gabadadze, and Porrati (DGP) provides a context where late-time acceleration is driven not by some energy-momentum component (dark energy), but rather is the manifestation of the excruciatingly slow leakage of gravity off our four-dimensional world into an extra dimension. At the same time, DGP gravity alters the gravitational force law in a specific and dramatic way at cosmologically accessible scales. We derive the DGP gravitational force law in a cosmological setting for spherical perturbations at subhorizon scales and compute the growth of large-scale structure. We find that a residual repulsive force at large distances gives rise to a suppression of the growth of density and velocity perturbations. Explaining the cosmic acceleration in this framework leads to a present day fluctuation power spectrum normalization sigma_8 <= 0.8 at about the two-sigma level, in contrast with observations. We discuss further theoretical work necessary to go beyond our approximations to confirm these results.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
The nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. The braneworld theory of Dvali, Gabadadze, and Porrati (DGP) provides a context where late-time acceleration is driven not by some energy-momentum component (dark energy), but rather is the manifestation of the excruciatingly slow leakage of gravity off our four-dimensional world into an extra dimension. At the same time, DGP gravity alters the gravitational force law in a specific and dramatic way at cosmologically accessible scales. We derive the DGP gravitational force law in a cosmological setting for spherical perturbations at subhorizon scales and compute the growth of large-scale structure. We find that a residual repulsive force at large distances gives rise to a suppression of the growth of density and velocity perturbations. Explaining the cosmic acceleration in this framework leads to a present day fluctuation power spectrum normalization sigma_8 <= 0.8 at about the two-sigma level, in contrast with observations. We discuss further theoretical work necessary to go beyond our approximations to confirm these results.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. | background 0.91 | 0.04 | 0.02 | asserted 1.00 |
| s1 | The braneworld theory of Dvali, Gabadadze, and Porrati (DGP) provides a context where late-time acceleration is driven not by some energy-momentum component (dark energy), but rather is the manifestation of the excruciatingly slow leakage of gravity off our four-dimensional world into an extra dimension. | background 1.00 | 0.05 | 0.06 | asserted 0.97 |
| s2 | At the same time, DGP gravity alters the gravitational force law in a specific and dramatic way at cosmologically accessible scales. | background 0.99 | 0.08 | 0.03 | asserted 1.00 |
| s3 | We derive the DGP gravitational force law in a cosmological setting for spherical perturbations at subhorizon scales and compute the growth of large-scale structure. | contribution 0.54 | 0.97 | 0.04 | asserted 0.95 |
| s4 | We find that a residual repulsive force at large distances gives rise to a suppression of the growth of density and velocity perturbations. | result 0.94 | 0.95 | 0.20 | asserted 1.00 |
| s5 | Explaining the cosmic acceleration in this framework leads to a present day fluctuation power spectrum normalization sigma_8 <= 0.8 at about the two-sigma level, in contrast with observations. | result 0.93 | 0.89 | 0.77 | asserted 0.91 |
| s6 | We discuss further theoretical work necessary to go beyond our approximations to confirm these results. | future work 0.85 | 0.14 | 0.03 | unclear 0.72 |
What was examined, and what was not
- arXiv listing abstract v2
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:astro-ph/0401515 by the authors.
References
How the literature uses it
51 arXiv papers cite this in 76 sentences
Papers citing it · 44 sentences without a confident label
Builds on it 1 paper
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Here we propose a phenomenological extension of these results valid for general $α$ theories, which reduce to the equations of [ref] in the DGP case.
N-Body Simulations of DGP and Degravitation Theories · Cosmological Perturbations · 0.99
Uses its method or tool 3 papers
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One is the spherically symmetric solution (vainshtein), which can be used in modeling a spherical collapse of dark matter halos in DGP [ref] .
Self-Consistent Cosmological Simulations of DGP Braneworld Gravity · Brane bending mode and Vainshtein effect · 0.85
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In this limit, we solve the ordinary differential equation [ref] [equation] for the linear matter density perturbation $Δ_m$.
Cosmological Constraints on DGP Braneworld Gravity with Brane Tension · gISW cross correlations · 0.94
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We begin by reviewing the techniques introduced in Ref. [ref] for the brane bending field but pay special attention to the matching of the various solutions as well as derive a closed form expression for the global field profile.
Spherical Collapse and the Halo Model in Braneworld Gravity · Top-Hat Overdensity in DGP · 0.86
Compares with it 4 papers
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* The above results all agree with [ref] for the DGP case $α=1/2$, with one exception: we have taken into account the modified propagator ([ref]) in the right-hand side of ([ref]).
N-Body Simulations of DGP and Degravitation Theories · Cosmological Perturbations · 0.96
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As another quick check, letting $β→β_DGP$ and setting $D=5$, this agrees with explicit cosmological results in DGP [ref] .
Enhanced Peculiar Velocities in Brane-Induced Gravity · Massive/Resonance Gravity · 0.95
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For comparison, the linearized perturbation equation derived for the full DGP model, derived in [ref] for spherical top-hat perturbations, takes on an identical form to ([ref]), with $β$ in this case given by
Galileon Cosmology · Growth of Density Perturbations · 0.99
Cites it as support 2 papers
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Observations based on structure formation provide further evidence of the difference between DGP and LCDM, since the two models suppress the growth of density perturbations in different ways [ref] .
Dark Energy and Modified Gravity · BRANE-WORLD MODELS · 0.98
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Observations based on structure formation provide further evidence of the difference between DGP and LCDM, since the two models suppress the growth of density perturbations in different ways [ref] .
Brane-World Gravity · `Self-accelerating' DGP · 0.96
Cites it as background 21 papers
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A first study of spherical perturbations in the context of DGP was undertaken in [ref] .
Spherical Collapse and the Halo Model in Braneworld Gravity · Introduction · 0.84
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The cosmology and phenomenology of DGP was first derived in [ref] (see also [ref] ).
Massive Gravity · Phenomenology of DGP · 0.95
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The Vainshtein mechanism occurs not only in modern incarnations of Boulware-Deser [ref] ghost-free massive gravity [ref] but also in galileon cosmology [ref] and braneworld models [ref] .
Equivalence Principle Violation in Weakly Vainshtein-Screened Systems · Introduction · 0.94
Discussion
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