371 research outputs found
A Note on Boltzmann Brains
Understanding the observed arrow of time is equivalent, under general
assumptions, to explaining why Boltzmann brains do not overwhelm ordinary
observers. It is usually thought that this provides a condition on the decay
rate of every cosmologically accessible de Sitter vacuum, and that this
condition is determined by the production rate of Boltzmann brains calculated
using semiclassical theory built on each such vacuum. We argue, based on a
recently developed picture of microscopic quantum gravitational degrees of
freedom, that this thinking needs to be modified. In particular, depending on
the structure of the fundamental theory, the decay rate of a de Sitter vacuum
may not have to satisfy any condition except for the one imposed by the
Poincare recurrence. The framework discussed here also addresses the question
of whether a Minkowski vacuum may produce Boltzmann brains.Comment: 10 pages, 1 figure; discussion in Section 4 modified and expande
Reanalyzing an Evaporating Black Hole
A coherent picture of the quantum mechanics of a collapse-formed, evaporating
black hole is presented. In a distant frame, semiclassical theory in the zone
describes microscopic dynamics of only the "hard modes," the modes that are
hard enough to be discriminated in the timescale of Hawking emission. The
thermal nature of these modes arises from microcanonical typicality of the full
black hole degrees of freedom, mostly composed of the "soft modes," the modes
that cannot be discriminated at the semiclassical level. The hard modes are
purified by a combined system of the soft modes and early Hawking radiation,
but not by either of them separately. This intrinsically tripartite structure
of entanglement is general, regardless of the age of the black hole. The
interior spacetime emerges only at a coarse-grained level. To describe it, an
effective theory can be erected at each time, which applies only to a limited
spacetime region determined by the time at which the theory is erected. The
entire interior of the black hole can be described only using multiple
effective theories erected at different times, realizing the idea of
complementarity. We analyze implications of the entanglement structure
described here for various phenomena, including Hawking evaporation and general
information retrieval. For multiple entangled black holes, it implies that
semiclassical objects dropped into different black holes cannot meet in the
interior, although each object smoothly enters the horizon of the black hole to
which it is falling. We also discuss physics in Rindler space, elucidating how
it is obtained as a smooth limit of the black hole physics.Comment: 46 pages, 4 figures; clarifications and comments adde
Tensor Modes in Pure Natural Inflation
We study tensor modes in pure natural inflation (arXiv:1706.08522), a
recently-proposed inflationary model in which an axionic inflaton couples to
pure Yang-Mills gauge fields. We find that the tensor-to-scalar ratio r is
naturally bounded from below. This bound originates from the finiteness of the
number of metastable branches of vacua in pure Yang-Mills theories. Details of
the model can be probed by future cosmic microwave background experiments and
improved lattice gauge theory calculations of the theta-angle dependence of the
vacuum energy.Comment: 6 pages, 6 figures; v2: journal versio
Why Firewalls Need Not Exist
The firewall paradox for black holes is often viewed as indicating a conflict
between unitarity and the equivalence principle. We elucidate how the paradox
manifests as a limitation of semiclassical theory, rather than presents a
conflict between fundamental principles. Two principal features of the
fundamental and semiclassical theories address two versions of the paradox: the
entanglement and typicality arguments. First, the physical Hilbert space
describing excitations on a fixed black hole background in the semiclassical
theory is exponentially smaller than the number of physical states in the
fundamental theory of quantum gravity. Second, in addition to the Hilbert space
for physical excitations, the semiclassical theory possesses an unphysically
large Fock space built by creation and annihilation operators on the fixed
black hole background. Understanding these features not only eliminates the
necessity of firewalls but also leads to a new picture of Hawking emission
contrasting pair creation at the horizon.Comment: 15 pages, 4 figures; v2: revised discussion on the number of
configurations for semiclassical excitations, conclusion unchanged; v3: minor
edits, to appear in Phys. Lett.
- …