Brane
World: Spacetime as a Membrane in Higher Dimensions |

**In General** > s.a. boundaries
in field theory; cosmological constant;
modified general relativity [signature change]; membranes;
string theory.

* __Idea__: (4+*k*)-D
models of spacetime and matter, in which the extra dimensions are either compactified
but not small as in Kaluza-Klein models, with size *R* \(\gg\) *l*_{P},
or non-compact; The usual spacetime is a 4D brane, or a set of closely stacked
parallel ones, and the bulk is usually assumed to be flat or have constant curvature;
Ordinary matter corresponds to open strings, trapped on the brane because
they must have endpoints there; Only gravitons, which correspond to
closed strings, propagate off the brane (and get a mass in 4D) – might
explain why gravity is so weak at large scales.

* __Motivation__: The 4D
Planck mass could be much smaller than the 4 + *n* one, *m*_{(4)}^{2}
= *R*^{n} *m*_{(4+n)}^{2+n},
and unification could occur at small energies, ~ 10
TeV (2000, experiments already put a lower bound at 1300 GeV); Could solve the hierarchy
problem without supersymmetry [this hope does not seem to be realized], and
the standard cosmological model problems without inflation.

* __History__:
1990, Joseph Polchinski and
others developed the theory of branes and showed that they are essential to string theory.

* __Types of models__: Flat
or warped; Compact or non-compact extra dimensions; Untwisted or twisted (e.g., Randall-Sundrum).

* __Bounds on extra dimensions__: 2002, the strongest bounds come from
astrophysics and cosmology, rather than collider experiments; The hierarchy problem could
be solved only if there are at least 4 extra dimensions.

> __Online resources__: see Wikipedia page.

**Matter Phenomenology** > s.a. brane
cosmology and gravity; higher-dimensional
theories; matter
in quantum gravity; string phenomenology.

* __Idea__: The constants
of nature we see are just shadows of the higher-dimensional ones, and can vary
in time and space, if the size of the extra dimensions varies; Constraints
on deviations from Newtonian gravity put upper bounds on the extra scales;
Constraints on Casimir effect contributions to
the cosmological constant put lower bounds on them.

* __High-energy phenomenology__:
2000, The idea does not contradict observations if the size of the extra dimensions
is up to about 0.1 mm; Better measurements
of gravity at smaller scales may put tighter bounds; Predicts
non-trivial spacetime foamy refractive index for photons and other massless
probes; 2001, Possible formation of black holes in the TeV realm of the LHC
or high energy cosmic ray interactions.

@ __High-energy physics__: Dvali et al MPLA(00),
Youm PRD(00)ht [4D
forces]; Abbott et al (D0) PRL(01)
[*p*-bar *p* → *e*^{+}*e*^{–} or *γγ*];
Shaposhnikov & Tinyakov
PLB(01)ht,
Dvali et al PRD(02)ht/01 [Higgs
alternative]; Kazanas & Nicolaidis
GRG(03)hp/01 [cosmic
ray spectrum]; Cheung ht/03-conf;
Nicolaidis & Sánchez MPLA(05)hp/03;
Cembranos et al IJMPD(04)hp-GRF
[dark matter]; Choudhury et al JHEP(04)
[Higgs production]; Aquino et al PRL(07)
[at LHC]; Jalalzadeh et al PS(07)
[4D forces].

@ __Neutrino oscillations__: Dvali & Smirnov NPB(99);
Davoudiasl et al PRD(02)hp [bounds].

@ __Supersymmetry breaking__: de Boer et al NPB(98)
[dynamical]; Bagger et al JHEP(02)ht/01;
Anisimov et al PRD(02).

@ __Bounds from astrophysics__: Hannestad & Raffelt PRL(01)hp [gamma
rays from supernovas],
PRL(02)hp/01 [neutron
stars]; González et al a1601 [compact stars].

@ __Other astrophysics__: Barger et al PLB(99)
[supernovas]; Sigurdsson IJMPD(01)ap [*R* ≈ 80
micron from dust aggregation]; Gnedin ap/01 [photon propagation
and TeV physics];
Burgess et al JHEP(02)hp [graviton
dispersion]; > s.a. black-hole
phenomenology.

@ __Other phenomenology__: Cardoso et al PRD(06)ht [diffraction
radiation]; Berenji et Fermi-LAT JCAP(12)-a1201 [neutron stars]; Berezhiani &
Nesti EPJC(12), Sarrazin et al PLB(12)-a1201 + news popsci(12)jan, io9(12)jun ["neutron loss" or disappearance].

> __Related topics__: see
black-hole radiation; cosmic
strings; dark
matter; kaluza-klein theory; lensing; particles;
quantum particles; solitons.

**Randall-Sundrum Models** > s.a. black-hole
formation; brane
cosmology; higgs mechanism;
torsion.

* __Idea__: The extra dimensions
are compactified, the usual spacetime is a 4D hypersurface or brane, and the
bulk spacetime is AdS; Need a large Λ < 0;
Motivated by claims that it solves the hierarchy problem.

@ __And matter__: Mavromatos & Rizos PRD(00)ht [strings];
Huber & Shafi
PLB(01) [masses, couplings]; Ichinose PRD(02)ht [fermions];
Abazov et D0 PRL(05) [experimental search for gravitons].

@ __And supersymmetry__: Duff et al JMP(01)ht/00, NPB(01)ht/00.

**References** > s.a. cosmological
perturbations; quantum field theory in curved backgrounds;
supersymmetry in field theory.

@ __I__: Abel & March-Russell pw(00)nov;
Kaku 04; Webb 04; Burgess & Quevedo SA(07)nov [as interacting multiverse]; Kakushadze a1410.

@ __Intros, reviews__: Rubakov PU(01);
Maartens in(01)gq;
Dick CQG(01)ht;
Arkani-Hamed et al PT(02)feb;
Förste FdP(02)ht/01 [and
strings]; Maartens LRR(04);
Durrer AIP(05)ht;
Johnson 06; West 12; Raychaudhuri & Sridhar 16 [particle physics].

@ __Precursors__: Pavšič PLA(86)gq/01;
Gibbons & Wiltshire NPB(87)ht/01;
Duff ht/04-in
[historical].

@ __Theory__: Polchinski PRL(95)ht [D-branes];
Wesson et al IJMPA(96)
[5D]; Dienes et al PLB(98)hp [unification
scales]; Ponce de León MPLA(01)gq [vs
spacetime-matter]; Rador EPJC(07)ht/05 [stabilization
of extra dimensions]; Bergman & Lifschytz PLB(06)ht;
Berman PRP(08)
[interactions, in M-theory].

@ __Non-compact extra dimensions__: Arkani-Hamed et al PRL(00).

@ __In supergravity__: Stelle ht/97-ln;
Marolf RMF(03)gq/01-ln
[11D supergravity].

@ __Bounds on extra dimensions__: Milton G&C(02)ht/01-in,
et al MPLA(01);
Uehara MPLA(02)
[rev].

@ __Spacetime structure__: Mavromatos gq/00-conf
[foam/optical properties]; Giddings PRD(03)ht [instability
of 4D]; Greene et al PRD(13)-a1212 [3 as the maximum number of spatial dimensions that can grow large cosmologically from an initial thermal fluctuation].

@ __Dynamics__: Diemand et al ht/01,
Carroll et al PRD(02)ht/01 [(in)stability];
Coley PRD(02)ht/01 [initial
singularity].

@ __Thermodynamics__: Townsend & Zamaklar CQG(01)ht [first
law].

@ __Variations__: Berezhiani et al PLB(01)ht [extra
timelike directions]; Grady TQGR-ht/01 [3D
phase boundary in 4D]; Henty ht/01 [5D
bulk BF]; Kar gq/02 [asymmetrically
warped 5D];
Durrer et al PLB(05)ht [D3
branes in 9+1 bulk]; Hinterbichler et al PLB(12)-a1101 [braneless scenario].

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send feedback and suggestions to bombelli at olemiss.edu – modified
17 jun 2016