19 research outputs found
Massive spin-2 theories
We give an introduction to massive spin-2 theories and the problem of their
non-linear completion. We review the Boulware-Deser ghost problem and two ways
to circumvent classic no-go theorems. In turn, massive spin-2 theories are not
uniquely defined. In the case of truncated theories, we show that the
Boulware-Deser ghost may only be avoided if the derivative structure of the
theory is not tuned to be Einsteinian.Comment: 14 pages - Invited review for the Central European Journal of
Physics, topical issue devoted to "Cosmology and Particle Physics beyond
Standard Models". v2: References added, extended discussion on massive
gravit
Axion Dark Matter and Planck favor non-minimal couplings to gravity
Constraints on inflationary scenarios and isocurvature perturbations have
excluded the simplest and most generic models of dark matter based on QCD
axions. Considering non-minimal kinetic couplings of scalar fields to gravity
substantially changes this picture. The axion can account for the observed dark
matter density avoiding the overproduction of isocurvature fluctuations.
Finally, we show that assuming the same non-minimal kinetic coupling to the
axion (dark matter) and to the standard model Higgs boson (inflaton) provides a
minimal picture of early time cosmology.Comment: 7 pages, 1 figure, revtex, v3: explanations and references added,
accepted for publication in Phys.Lett.
Human episodic memory retrieval is accompanied by a neural contiguity effect
Cognitive psychologists have long hypothesized that experiences are encoded in a temporal context that changes gradually over time. When an episodic memory is retrieved, the state of context is recovered—a jump back in time. We recorded from single units in the MTL of epilepsy patients performing an item recognition task. The population vector changed gradually over minutes during presentation of the list. When a probe from the list was remembered with high confidence, the population vector reinstated the temporal context of the original presentation of that probe during study—a neural contiguity effect that provides a possible mechanism for behavioral contiguity effects. This pattern was only observed for well-remembered probes; old probes that were not well-remembered showed an anti-contiguity effect. These results constitute the first direct evidence that recovery of an episodic memory in humans is associated with retrieval of a gradually-changing state of temporal context—a neural “jump-back-in-time” that parallels the act of remembering
Physics of Trans-Planckian Gravity
We study the field theoretical description of a generic theory of gravity
flowing to Einstein General Relativity in IR. We prove that, if ghost-free, in
the weakly coupled regime such a theory can never become weaker than General
Relativity. Using this fact, as a byproduct, we suggest that in a ghost-free
theory of gravity trans-Planckian propagating quantum degrees of freedom cannot
exist. The only physical meaning of a trans-Planckian pole is the one of a
classical state (Black Hole) which is described by the light IR quantum degrees
of freedom and gives exponentially-suppressed contributions to virtual
processes. In this picture Einstein gravity is UV self-complete, although not
Wilsonian, and sub-Planckian distances are unobservable in any healthy theory
of gravity. We then finally show that this UV/IR correspondence puts a severe
constraint on any attempt of conventional Wilsonian UV-completion of
trans-Planckian gravity. Specifically, there is no well-defined energy domain
in which gravity could become asymptotically weak or safe.Comment: 23 pages, 4 figures, v2: Paper reorganized to improve clarity;
additional explanations and references added; version accepted for
publication in Phys. Rev.
Human episodic memory retrieval is accompanied by a neural contiguity effect
Cognitive psychologists have long hypothesized that experiences are encoded in a temporal context that changes gradually over time. When an episodic memory is retrieved, the state of context is recovered—a jump back in time. We recorded from single units in the MTL of epilepsy patients performing an item recognition task. The population vector changed gradually over minutes during presentation of the list. When a probe from the list was remembered with high confidence, the population vector reinstated the temporal context of the original presentation of that probe during study—a neural contiguity effect that provides a possible mechanism for behavioral contiguity effects. This pattern was only observed for well-remembered probes; old probes that were not well-remembered showed an anti-contiguity effect. These results constitute the first direct evidence that recovery of an episodic memory in humans is associated with retrieval of a gradually-changing state of temporal context—a neural “jump-back-in-time” that parallels the act of remembering
Human episodic memory retrieval is accompanied by a neural contiguity effect
Cognitive psychologists have long hypothesized that experiences are encoded in a temporal context that changes gradually over time. When an episodic memory is retrieved, the state of context is recovered—a jump back in time. We recorded from single units in the medial temporal lobe of epilepsy patients performing an item recognition task. The population vector changed gradually over minutes during presentation of the list. When a probe from the list was remembered with high confidence, the population vector reinstated the temporal context of the original presentation of that probe during study, a neural contiguity effect that provides a possible mechanism for behavioral contiguity effects. This pattern was only observed for well remembered probes; old probes that were not well remembered showed an anti-contiguity effect. These results constitute the first direct evidence that recovery of an episodic memory in humans is associated with retrieval of a gradually changing state of temporal context, a neural “jump back in time” that parallels the act of remembering.This work was supported by the National Institutes of Health (Grants R01EB022864 and R01MH112169 to M.W.H. and Grants R01MH110831 and U01NS103792 to U.R.), the National Science Foundation (CAREER Award BCS-1554105 to U.R.), and a Memory and Cognitive Disorders Award from the McKnight Foundation for Neuroscience (U.R.). We thank Nigel Stallard, Inder Singh, Zoran Tiganj, Amy Criss, and Rosie Cowell for helpful discussions. (R01EB022864 - National Institutes of Health; R01MH112169 - National Institutes of Health; R01MH110831 - National Institutes of Health; U01NS103792 - National Institutes of Health; BCS-1554105 - National Science Foundation; Memory and Cognitive Disorders Award from the McKnight Foundation for Neuroscience)Accepted manuscrip
Asymptotic freedom of Yang-Mills theory with gravity
We study the behaviour of Yang-Mills theory under the inclusion of gravity.
In the weak- gravity limit, the running gauge coupling receives no contribution
from the gravitational sector, if all symmetries are preserved. This holds true
with and without cosmological constant. We also show that asymptotic freedom
persists in general field-theory-based gravity scenarios including
gravitational shielding as well as asymptotically safe gravity.Comment: 16 pages, 2 figures; v2: explanations added to match published
versio