Out of the White Hole: A Holographic Origin for the Big Bang

Abstract: While most of the singularities of General Relativity are expected to be safely hidden behind event horizons by the cosmic censorship conjecture, we happen to live in the causal future of the classical big bang singularity, whose resolution constitutes the active field of early universe cosmology. Could the big bang be also hidden behind a causal horizon, making us immune to the decadent impacts of a naked singularity? We describe a braneworld description of cosmology with both 4d induced and 5d bulk gravity (otherwise known as Dvali-Gabadadze-Porati, or DGP model), which exhibits this feature: The universe emerges as a spherical 3-brane out of the formation of a 5d Schwarzschild black hole. In particular, we show that a pressure singularity of the holographic fluid, discovered earlier, happens inside the white hole horizon, and thus need not be real or imply any pathology. Furthermore, we outline a novel mechanism through which any thermal atmosphere for the brane, with comoving temperature of 20% of the 5D Planck mass can induce scale-invariant primordial curvature perturbations on the brane, circumventing the need for a separate process (such as cosmic inflation) to explain current cosmological observations. Finally, we note that 5D space-time is asymptotically flat, and thus potentially allows an S-matrix or (after minor modifications) AdS/CFT description of the cosmological big bang.

• The paper proposes a holographic origin for the Big Bang by modeling our universe as a 4D brane in a 5D bulk, circumventing singularity issues.
• It employs the DGP model with modified FRW dynamics to integrate holographic fluid contributions alongside standard matter sources.
• The study demonstrates that thermal fluctuations in the brane-bulk equilibrium yield near-scale-invariant perturbations, offering an alternative to inflation.

Holographic Perspectives on the Big Bang in Braneworld Cosmology

In this paper, the authors explore a novel approach to cosmology through the lens of braneworld scenarios, particularly focusing on the Dvali-Gabadadze-Porrati (DGP) model. Their central thesis posits an unconventional origin for the universe, replacing the traditional big bang singularity with a holographic description emerging from a higher-dimensional space.

Alternative Cosmological Framework

Their framework is based on the premise that our universe is a four-dimensional (4D) brane embedded in a five-dimensional (5D) bulk spacetime, resembling a Schwarzschild black hole. This approach leverages the DGP model, where the brane features its own gravitational dynamics due to induced gravity, competing with the bulk gravity. The cosmological evolution of the 4D universe follows an adaptation of the standard Friedmann-Robertson-Walker (FRW) dynamics, modified by contributions from the bulk, the so-called holographic fluid, and standard matter sources.

Big Bang as a Holographic Emergence

A significant insight from this research is the potential resolution of the big bang singularity issue. The study suggests that the big bang might be hidden behind a white hole horizon in the higher-dimensional bulk, thus circumventing the singularity problem. This description becomes viable within their model when the density conditions of the holographic fluid avoid pathological singularities. The model elucidates conditions under which these singularities, termed "pressure singularities," occur inside a white hole horizon.

Scale-Invariant Cosmological Perturbations

The paper introduces a mechanism for generating cosmological perturbations that align with observed scale-invariance without invoking inflation. By hypothesizing an equilibrium between the brane and a thermal atmosphere in the surrounding 5D bulk, this model can produce near-scale-invariant curvature perturbations. The critical result here is that these thermal fluctuations retain scale invariance thanks to their power spectrum's behavior, a notable deviation from the classical inflationary predictions while still remaining consistent with cosmic microwave background observations.

Implications and Further Considerations

This research suggests several broader implications. Practically, the model offers an alternative narrative to the cosmic inflation paradigm, potentially explaining large-scale cosmic features without the need for a rapid expansion epoch. Theoretically, the framework opens discussions on employing holographic principles—specifically, AdS/CFT-like correspondences—when formulating cosmological theories that transcend traditional boundaries set by General Relativity.

The authors acknowledge fundamental challenges, such as aligning the slight deviations from scale-invariance and adherence to non-gaussianity constraints observed today. Future investigations could explore the potential of this framework to address other standard cosmological puzzles (e.g., flatness, horizon, and monopole problems) and whether it might predict novel gravitational wave signals or modifications in primordial nucleosynthesis rates.

Overall, this paper contributes a noteworthy perspective to the ongoing inquiry into the initial conditions of our universe, potentially bridging high-energy theoretical physics with observable cosmological phenomena. As the authors suggest, further exploration into the implications and refinements of this model could pave the way for more comprehensive theories in fundamental physics.