* Astronomy

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RE: The Universe

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Title: Entanglement time in the primordial universe
Author: Eugenio Bianchi, Lucas Hackl, Nelson Yokomizo

We investigate the behaviour of the entanglement entropy of space in the primordial phase of the universe before the beginning of cosmic inflation. We argue that in this phase the entanglement entropy of a region of space grows from a zero-law to an area-law. This behaviour provides a quantum version of the classical BKL conjecture that spatially separated points decouple in the approach to a cosmological singularity. We show that the relational growth of the entanglement entropy with the scale factor provides a new statistical notion of arrow of time in quantum gravity. The growth of entanglement in the pre-inflationary phase provides a mechanism for the production of the quantum correlations present at the beginning of inflation and imprinted in the CMB sky.

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Cosmic Latte

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RE: The Universe

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Title: The Physics of the Far Future
Authors: Ruxandra Bondarescu (University of Zurich), Andrew P. Lundgren (Max Planck Institute for Gravitational Physics), Mihai Bondarescu (Universitatea de Vest and University of Mississippi)

We observe the past and present of the universe, but can we predict the far future? Observations suggest that in thousands of billions of years from now most matter and radiation will be absorbed by the cosmological horizon. As it absorbs the contents of the universe, the cosmological horizon is pushed further and further away. In time, the universe asymptotes towards an equilibrium state of the gravitational field. Flat Minkowski space is the limit of this process. It is indistinguishable from a space with an extremely small cosmological constant (\Lambda -> 0) and thus has divergent entropy.

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Title: How Flat is Our Universe Really?
Authors: P. M. Okouma, Y. Fantaye, B. A. Bassett

Distance measurement provide no constraints on curvature independent of assumptions about the dark energy, raising the question, how flat is our Universe if we make no such assumptions? Allowing for general evolution of the dark energy equation of state with 20 free parameters that are allowed to cross the phantom divide, w(z) = -1, we show that while it is indeed possible to match the first peak in the Cosmic Microwave Background with non-flat models and arbitrary Hubble constant, H_0, the full WMAP7 and supernova data alone imply -0.12 < \Omega_k < 0.01 (2\sigma). If we add the HST H_0 prior, this tightens significantly to \Omega_k = 0.002 ±0.009. These constitute the most conservative and model-independent constraints on curvature available today, and illustrate that the curvature-dynamics degeneracy is broken by current data, with a key role played by the Integrated Sachs Wolfe effect rather than the distance to the surface of last scattering. If one imposes a quintessence prior on the dark energy (-1 \leq w(z) \leq 1) then just the WMAP7 and supernova data alone force the Universe to near flatness: \Omega_k = 0.013 ±0.012. Finally, allowing for curvature, we find that all datasets are consistent with a Harrison-Zel'dovich spectral index, n_s = 1, at 2\sigma.

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Title: The Optimal Cosmic Epoch for Precision Cosmology
Authors: Abraham Loeb (Harvard)

The statistical uncertainty in measuring the primordial density perturbations on a given comoving scale is dictated by the number of independent regions of that scale that are accessible to an observer. This number varies with cosmic time and diminishes per Hubble volume in the distant past or future of the standard cosmological model. We show that the best constraints on the initial power spectrum of linear density perturbations are accessible (e.g. through 21-cm intensity mapping) at redshifts z~10, and that the ability to constrain the cosmological initial conditions will deteriorate quickly in our cosmic future.

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The Older We Get, The Less We Know (Cosmologically)

The universe is a marvellously complex place, filled with galaxies and larger-scale structures that have evolved over its 13.7-billion-year history. Those began as small perturbations of matter that grew over time, like ripples in a pond, as the universe expanded. By observing the large-scale cosmic wrinkles now, we can learn about the initial conditions of the universe. But is now really the best time to look, or would we get better information billions of years into the future - or the past?
New calculations by Harvard theorist Avi Loeb show that the ideal time to study the cosmos was more than 13 billion years ago, just about 500 million years after the Big Bang. The farther into the future you go from that time, the more information you lose about the early universe.
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As with interpretations of what happened in the very early universe, advances in fundamental physics are required before it will be possible to know the ultimate fate of the universe with any certainty.

Big freeze: 10^14 years and beyond
This scenario is generally considered to be the most likely[citation needed], as it occurs if the universe continues expanding as it has been. Over a time scale on the order of 10^14 years or less, existing stars burn out, stars cease to be created, and the universe goes dark. Over a much longer time scale in the eras following this, the galaxy evaporates as the stellar remnants comprising it escape into space, and black holes evaporate via Hawking radiation.. In some grand unified theories, proton decay after at least 10^34 years will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons. In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium.