*Summary:*

*According to General Relativity (GR) a universe with a cosmological constant, Ξ, like ours, is trapped inside an event horizon π < β3/Ξ. Now, Gaztanaga and Fodalba showed that the FLRW metric describes the interior of a trapped surface of size πβ = 1/π»Ξ, like deS metric.**They also explained how exactly our universe evolved.**Using Israel junction conditions, they also showed that there could be a different universe outside. Our Universe looks like a Black Hole for an outside observer. But observer living inside our universe cannot able to see what exactly happening outside our univers*e.*Outgoing radial null geodesics can not escape our universe, but incoming photons can enter and leave an imprint on our CMB sky.**Their recent analysis of the Planck temperature anisotropy data, showed that the distribution of best-fit dark-energy density Ξ©Ξ exhibits three distinct regions or horizons across the CMB sky.**The size of each of these regions is correlated with the mean value of dark energy density (Ξ©Ξ) over that portion of the sky, in good agreement with the BH universeΒ prediction.*

According to General Relativity (GR) a universe with a cosmological constant, Ξ, like ours, is trapped inside an event horizon π < β3/Ξ. Now, Gaztanaga and Fosalba showed that the FLRW metric describes the interior of a trapped surface of size πβ = 1/π»Ξ, like deS metric. (πβ β Event horizon (EH) or trapped surface for a given observer)

Yeah, more generally, the FLRW metric is the background to the Schwarzschild (SCHW) metric solution. The Schwarzschild (SCHW) metric is commonly used to describe the outside of black holes (BHs) or stars and it should be understood as a perturbation inside a larger background. Fosalba and Gaztanaga showed that the SCHW metric is a perturbation of a larger Friedmann-Lemaitre-Robertson-Walker (FLRW) metric, i.e, a BH-like metric embedded in a background described by the FLRW metric. This means that outside πβ = 1/π»Ξ we just have another FLRW metric, like in Matryoshka doll. From outside, the inner FLRW metric looks like a BH. There could be many other BH universes inside and outside πβ = 1/π»Ξ, so the structure could be better described by a fractal.

“We showed using Israel junction conditions that there could be a different universe outside. Our Universe looks like a Black Hole for an outside observer.”β told Fosalba, first author of the study

Now, the question is how our universe exactly evolved? So, for this we have to go quantum. At first, a primordial field π settles or fluctuates into a false (or slow rolling) vacuum which will create a black hole false vaccum (BH.fv) with a junction “Ξ£” (in equation given below),

where the causal boundary is fixed in comoving coordinates and corresponds to the particle horizon during inflation ((πΒ§ = π/(πππ»π)) or the Hubble horizon when inflation begins. The size (π = a (π)πΒ§) of this vacuum grows and asymptotically tends to π β = π/π» following Eq. (given below) with π» = π»π .

The inside of this BH will be expanding exponentially π = π^ππ»π while the Hubble horizon is fixed 1/π»*i*. When this inflation ends, vacuum energy excess converts into matter and radiation (reheating). This results in BH universe (BH.u), where the infinitesimal Hubble horizon starts to grow following the standard Big Bang evolution.

According to authors, this observable universe becomes larger than π β when π > 1, as shown in Fig.1 (compare dotted and dashed lines). This shows that, observers like us, living in the interior of the BH universe, are trapped inside π β but can nevertheless observe what happened outside.

Furthermore, they showed that though outgoing radial null geodesics/light-rays can not escape our universe, but incoming photons can enter and leave an imprint on our CMB sky. They found one of the strong evidence for a violation of the cosmological principle of isotropy from the analysis of the Planck 2018 temperature map. They showed that the distribution of best-fit dark-energy density (Ξ©Ξ) exhibits three distinct regions (or patches or horizons) across the CMB sky (marked by the three large grey circles in Fig.2).

These regions have a radius ranging from 40 to 70 degrees. The size of these structures are in agreement with the scale of the casual boundary, πΒ§, for Ξ dominated universes. **As shown by authors in their last paper, the size of each of these regions is correlated with the mean value of dark energy density (Ξ©Ξ) over that portion of the sky, in good agreement with the BH universe prediction.** The same large-scale anisotropic patterns are observed by them for the distribution of other basic ΞCDM cosmological parameters. This represents a very significant breakdown of the main hypothesis of the big bang model: The assumption that the universe is isotropic on large scales.

“Our analysis points to significant deviations from statistical isotropy on cosmological scales, with a probability βΌ 10Β―^{9}of being a Gaussian fluctuation. This is the largest reported evidence for a violation of the Cosmological principle to our knowledge.”β told Gaztanaga, second authorof the study

If the existence of such horizons is confirmed in future analyses (e.g., in high-quality polarization data) this could lend further support to models that predict the existence of those horizons, such as the Gaztanaga model. This in turn would open the door to unveil the nature of dark-energy and cosmic acceleration, and resolve apparent cosmological parameter tensions reported in recent analyses that combined low and high redshift probes, without the need to invoke new physics beyond our standard model.

**Reference***: (1) Enrique Gaztanaga, Pablo Fosalba, “A peek outside our Universe”, Astronomical Journal, pp. 1-3, 2021. **https://arxiv.org/abs/2104.00521** (2) Pablo Fosalba, Enrique Gaztanaga, “Explaining Cosmological Anisotropy: Evidence for Causal Horizons from CMB data”, ArXiv, pp. 1-26, 2021. **https://arxiv.org/abs/2011.00910*

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