REVIEW
Remarks on Higgs Inflation
High Energy Physics - Phenomenology · Submitted 18 Nov 2010 · 27 references
Pith's verdict: The one new result — that the Germani-Kehagias derivative-coupling model has Φ₀ above its own cutoff — is worth printing, but it rests on a single s-channel estimate with only a factor-of-ten margin.
T0 · AI review
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Referee report
The paper
The paper claims
The paper argues that the minimal Standard Model Higgs cannot be the inflaton without new physics, and that a derivative coupling to the Einstein tensor does not fix the unitarity problem.
The reading
What carries the argument
The argument turns on a background-dependent unitarity cutoff. In the original model the dangerous scale is $ar M_P/xi$ when the Higgs is expanded about small field values, while during inflation the effective cutoff grows with the background field. In the derivative model the coupling $(g^{mu u}-w^2 G^{mu u})partial_muPhi partial_ uPhi$ generates, after expanding about a Friedmann-Robertson-Walker background with $wH gg 1$, an interaction of the form $rac{1}{2}H^2ar M_P,partial^2 h^{mu u}partial_muvarphi partial_ uvarphi$, and power counting converts that into $Lambda simeq (2H^2ar M_P)^{1/3}$. Comparing this cutoff with the fitted inflationary background is what turns a claimed fix into a failure.
Extended reading notes
Core claim
The central claim is negative: neither the original Higgs inflation model nor the derivative Einstein-tensor variant is a self-contained Standard Model explanation of inflation. In the original model the cutoff $ar M_P/xi$ is low enough that new physics must exist in today's universe; a background-dependent cutoff can be above the fields during inflation, but it does not remove the need for that new physics, which would then enter the inflationary dynamics. For the derivative model the paper's power counting gives $Lambda simeq (2H^2ar M_P)^{1/3}$; using $R simeq 5.6 imes10^{-8}ar M_P^2$ and $2.1 imes10^{-2}ar M_P < Phi_0 < 2.7 imes10^{-2}ar M_P$ from the model's cosmic-microwave-background fit, it finds $Lambda simeq 2 imes10^{-3}ar M_P$, so $Phi_0 > Lambda$ during inflation. The positive corollary is that asymptotically safe gravity, with the Planck mass rising and the non-minimal coupling falling in the ultraviolet, could restore consistency without new Standard Model degrees of freedom.
Load-bearing premise
The authors acknowledge that this is a power-counting argument, not a full amplitude calculation, and the step from Eq. (4) to Eq. (5) is asserted rather than derived; the load-bearing premise is that a single direct-channel $2 o 2$ scattering estimate gives the true unitarity cutoff of the derivative-coupling model, without checking whether the $s$-, $t$-, and $u$-channel cancellations that raise the cutoff for a minimally coupled singlet scalar also apply here.
Editorial extensions
If this is right
- The derivative Einstein-tensor model does not solve the unitarity problem it was introduced to solve, so its inflationary predictions cannot be trusted as a complete effective theory without new physics.
- Minimal Higgs inflation with only the Standard Model field content requires new physics below the Planck mass, at least at the scale $ar M_P/xi$ in today's universe.
- Any new physics that unitarizes the model today is likely to remain in the inflationary regime or mix with the Higgs, so inflation would no longer be driven by the Higgs alone.
- Singlet scalars non-minimally coupled to gravity face a related lower cutoff $ar M_P/(sqrt{lambda}xi)$ when they have a self-coupling, so their self-couplings must be chosen carefully.
- If gravity is asymptotically safe with the required non-minimal couplings, a Standard-Model-only Higgs inflaton could be consistent without extra degrees of freedom.
Reading between the lines
- The paper's derivative-model conclusion rests on a power-counting estimate rather than a computed amplitude; an explicit calculation including all graviton-exchange channels is the obvious next check and could reverse the conclusion.
- If asymptotically safe gravity makes $xi$ run to zero in the ultraviolet, the inflationary regime may require a matching condition between the infrared $xi sim 10^4$ and the fixed-point value, which would be a concrete constraint on the scenario.
- The same background-dependent cutoff logic suggests that collider or cosmological probes of the cutoff today are not directly transferable to the inflationary epoch; a model that is safe during inflation can still require new physics at low energies.
- Because the paper treats the Higgs as four real scalars in the high-energy limit, a full Standard Model calculation with electroweak symmetry restored would test whether the single-real-scalar power counting captures the leading cutoff.
Original abstract
We discuss models where the Higgs boson of the electroweak standard model plays the role of the inflaton. We focus on the question of the violation of perturbative unitarity due to the coupling of the Higgs boson either to the Ricci scalar or to the Einstein tensor and discuss the background dependence of the unitarity bounds. Our conclusion is that the simplest model which restricts itself to the standard model Higgs boson without introducing further degrees of freedom has a serious problem. However, in the asymptotically safe gravity scenario, the Higgs boson of the standard model could be the inflaton and no physics beyond the standard model is required to explain both inflation and the spontaneous breaking of the electroweak symmetry of the standard model.
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
We discuss models where the Higgs boson of the electroweak standard model plays the role of the inflaton. We focus on the question of the violation of perturbative unitarity due to the coupling of the Higgs boson either to the Ricci scalar or to the Einstein tensor and discuss the background dependence of the unitarity bounds. Our conclusion is that the simplest model which restricts itself to the standard model Higgs boson without introducing further degrees of freedom has a serious problem. However, in the asymptotically safe gravity scenario, the Higgs boson of the standard model could be the inflaton and no physics beyond the standard model is required to explain both inflation and the spontaneous breaking of the electroweak symmetry of the standard model.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We discuss models where the Higgs boson of the electroweak standard model plays the role of the inflaton. | contribution 0.74 | 0.71 | 0.03 | asserted 0.81 |
| s1 | We focus on the question of the violation of perturbative unitarity due to the coupling of the Higgs boson either to the Ricci scalar or to the Einstein tensor and discuss the background dependence of the unitarity bounds. | contribution 0.97 | 0.73 | 0.04 | asserted 0.65 |
| s2 | Our conclusion is that the simplest model which restricts itself to the standard model Higgs boson without introducing further degrees of freedom has a serious problem. | result 0.91 | 0.79 | 0.39 | asserted 1.00 |
| s3 | However, in the asymptotically safe gravity scenario, the Higgs boson of the standard model could be the inflaton and no physics beyond the standard model is required to explain both inflation and the spontaneous breaking of the electroweak symmetry of the standard model. | result 0.77 | 0.56 | 0.46 | hedged possibility 1.00 |
What was examined, and what was not
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References
How the literature uses it
32 arXiv papers cite this in 48 sentences
Papers citing it · 34 sentences without a confident label
Contests it 1 paper
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Therefore, $Λ∼/ξ$ also during inflation, and the numerical value changes by less than a factor of 2 between the EW scale and the inflationary large field region, in contrast to previous work [ref] .
Tree-level unitarity in Higgs inflation in the metric and the Palatini formulation · Conclusions · contest 0.81
Compares with it 1 paper
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Different from the idea of [ref] , in our model the inflationary solution can be obtained purely in the frame of AS gravity without the help of the Higgs boson, but the Higgs field plays the role of seeding primordial power spectrum which explains the CMB observation.
Higgs Boson in RG running Inflationary Cosmology · Introduction · 0.97
Cites it as background 11 papers
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Recently, this approach has been under dense discussion in regard to unitarity violation [ref] .
Higgs-Palatini Inflation and Unitarity · Introduction · 0.89
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This resummation was first considered in when studying the perturbative unitarity of the effective action [ref] [ref] .
The horizon of the lightest black hole · Introduction · 0.89
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Recently, there has been a renewed interest in the gravitational scattering of fields and the question of whether perturbative unitarity could be violated below the Planck scale [ref] .
The Lightest of Black Holes · Introduction · 0.89
What it cites · 45 sentences without a confident label
Contests 1 paper
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We can determine $H≃√(R/12)$ from ([ref]) and we find [equation] In [ref] only the condition $R<Λ^2$ was considered and the model was said to be free of unitarity problems.
New Model of Inflation with Non-minimal Derivative Coupling of Standard Model Higgs Boson to Gravity · New Model of Higgs Inflation · contest 0.78
Uses the method or tool of 2 papers
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As in [ref] , we simplify the calculation by only considering s-channel scattering, we can do this by imposing different in and out states, i.e. $ΦΦ→Φ^'Φ^'$.
On the unitarity of linearized General Relativity coupled to matter · New Model of Higgs Inflation · 0.97
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As in [ref] , we simplify the calculation by only considering s-channel scattering, we can do this by imposing different in and out states, i.e. $ΦΦ→Φ^'Φ^'$.
Scale of Quantum Gravity · New Model of Higgs Inflation · 0.97
Uses data or values from 1 paper
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Expanding around a flat background $g_μν=η_μν+√(2)h_μν/M̅_P + O(M̅_P^-2)$ and the standard model Higgs boson vacuum expectation value (which we take to be zero in the high energy limit being considered), the cut off is found to be [equation] In [ref] , by comparison with the WMAP data the value of the dimensionful parameter $w$ is found to lie in the range [equation] Taking the upper bound for $w^-1$ we find that unitarity is violated at [equation] which is smaller than both $√(R)$ and $Φ_0$ during inflation.
Cosmological Perturbations in the New Higgs Inflation · New Model of Higgs Inflation · 0.92
Cites as support 1 paper
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Indeed, in [ref] it is the presence of the large non-minimal coupling to the new sigma field that is of critical importance for inflation, not the presence of the Higgs boson.
Unitarizing Higgs Inflation · Original Model of Higgs Inflation · 0.98
Cites as background 4 papers
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It has recently been proposed that the standard model Higgs boson could be the inflaton [ref] .
The Standard Model Higgs boson as the inflaton · Introduction · 0.94
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It has recently been proposed that the standard model Higgs boson could be the inflaton [ref] .
Running Inflation in the Standard Model · Introduction · 0.94
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A new model of Higgs inflation was proposed in [ref] with the motivation being that it did not suffer from the unitarity problems that plagued the earlier model.
New Model of Inflation with Non-minimal Derivative Coupling of Standard Model Higgs Boson to Gravity · Conclusions · 0.88
Discussion
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