No-boundary solutions are robust to quantum gravity corrections

Jonas, Caroline; Lehners, Jean-Luc

doi:10.1103/physrevd.102.123539

Cited by 13 publications

(13 citation statements)

References 39 publications

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“…If there are infinitely many higher-curvature terms, as expected from loop corrections, then all Lorentzian FLRW backgrounds with standard matter contributions are eliminated. As we already mentioned in section 3.4, the no-boundary proposal shows that if one is prepared to depart from Lorentzian metrics by enlarging the 4-manifolds that are summed in the path integral to the complex domain, then a finite amplitude may be obtained, even for an infinite sum of higher order terms containing the Riemann tensor [89]. But a Lorentzian big bang becomes impossible.…”

Section: Examples Involving Physics Beyond General Relativitymentioning

confidence: 98%

“…What is more, no-boundary solutions are stable to the addition of higher curvature corrections stemming from loop corrections to general relativity [89] (see also [90,91]), thus indicating that no-boundary solutions may persist in full quantum gravity. From this vantage point one may be tempted to reformulate the subject of the present paper as asking whether there exist other prescriptions for semi-classical cosmological amplitudes that are equally sound.…”

Section: No-boundary Proposalmentioning

confidence: 99%

“…We will start with the simplest backgrounds, namely FLRW metrics with spatial curvature k = −1, 0, +1 for open, flat or closed spatial slices. On such backgrounds, the action of a f (R µνρσ ) theory simplifies considerably and may be written as [89] S = dt a 3 N…”

Section: Examples Involving Physics Beyond General Relativitymentioning

confidence: 99%

“…The order in the Riemann tensor of the expression f (R µνρσ ) is P = p 1 + p 2 . The constraint equation (a first integral of the equation of motion) for the action (4.17) is given by [89]…”

Section: Examples Involving Physics Beyond General Relativitymentioning

confidence: 99%

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Cosmological consequences of a principle of finite amplitudes

Jonas,

Lehners,

Quintin

2021

Preprint

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Over 30 years ago, Barrow & Tipler proposed the principle according to which the action integrated over the entire 4-manifold describing the universe should be finite. Here we explore the cosmological consequences of a related criterion, namely that semi-classical transition amplitudes from the early universe up to current field values should be well defined. On a classical level, our criterion is weaker than the Barrow-Tipler principle, but it has the advantage of being sensitive to quantum effects. We find significant consequences for early universe models, in particular: eternal inflation and strictly cyclic universes are ruled out. Within general relativity, the first phase of evolution cannot be inflationary, and it can be ekpyrotic only if the scalar field potential is trustworthy over an infinite field range. Quadratic gravity eliminates all non-accelerating backgrounds near a putative big bang (thus imposing favourable initial conditions for inflation), while the expected infinite series of higher-curvature quantum corrections eliminates Lorentzian big bang spacetimes altogether. The scenarios that work best with the principle of finite amplitudes are the no-boundary proposal, which gives finite amplitudes in all dynamical theories that we have studied, and string-inspired loitering phases. We also comment on the relationship of our proposal to the swampland conjectures.

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Section: Examples Involving Physics Beyond General Relativitymentioning

confidence: 98%

Section: No-boundary Proposalmentioning

confidence: 99%

Section: Examples Involving Physics Beyond General Relativitymentioning

confidence: 99%

Section: Examples Involving Physics Beyond General Relativitymentioning

confidence: 99%

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Cosmological consequences of a principle of finite amplitudes

Jonas,

Lehners,

Quintin

2021

Preprint

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“…We are now looking for no-boundary solutions in this potential, i.e. solutions that are regular and compact [46]. Compactness means that we would like to set a(0) = 0.…”

Section: An 8-dimensional Starobinsky Modelmentioning

confidence: 99%

How to Create Universes with Internal Flux

Lehners¹,

Leung²,

Stelle³

2022

Preprint

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String compactifications typically require fluxes, for example in order to stabilise moduli. Such fluxes, when they thread internal dimensions, are topological in nature and take on quantised values. This poses the puzzle as to how they could arise in the early universe, as they cannot be turned on incrementally. Working with string inspired models in 6 and 8 dimensions, we show that there exist no-boundary solutions in which internal fluxes are present from the creation of the universe onwards. The no-boundary proposal can thus explain the origin of fluxes in a Kaluza-Klein context. In fact, it acts as a selection principle since no-boundary solutions are only found to exist when the fluxes have the right magnitude to lead to an effective potential that is positive and flat enough for accelerated expansion. Within the range of selected fluxes, the no-boundary wave function assigns higher probability to smaller values of flux. Our models illustrate how cosmology can act as a filter on a landscape of possible higher-dimensional solutions.

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Lorentzian Robin Universe

Ailiga,

Mallik,

Narain

2024

J. High Energ. Phys.

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In this paper, we delve into the gravitational path integral of Gauss-Bonnet gravity in four spacetime dimensions, in the mini-superspace approximation. Our primary focus lies in investigating the transition amplitude between distinct boundary configurations. Of particular interest is the case of Robin boundary conditions, known to lead to a stable Universe in Einstein-Hilbert gravity, alongside Neumann boundary conditions. To ensure a consistent variational problem, we supplement the bulk action with suitable surface terms. This study leads us to compute the necessary surface terms required for Gauss-Bonnet gravity with the Robin boundary condition, which wasn’t known earlier. Thereafter, we perform an exact computation of the transition amplitude. Through ħ → 0 analysis, we discover that the Gauss-Bonnet gravity inherently favors the initial configuration, aligning with the Hartle-Hawking no-boundary proposal. Remarkably, as the Universe expands, it undergoes a transition from the Euclidean (imaginary time) to the Lorentzian signature (real time). To further reinforce our findings, we employ a saddle point analysis utilizing the Picard-Lefschetz methods. The saddle point analysis allows us to find the initial configurations which lead to Hartle-Hawking no-boundary Universe that agrees with the exact computations. Our study concludes that for positive Gauss-Bonnet coupling, initial configurations corresponding to the Hartle-Hawking no-boundary Universe gives dominant contribution in the gravitational path-integral.

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No-boundary solutions are robust to quantum gravity corrections

Cited by 13 publications

References 39 publications

Cosmological consequences of a principle of finite amplitudes

Cosmological consequences of a principle of finite amplitudes

How to Create Universes with Internal Flux

Lorentzian Robin Universe

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