RESOLVING SINGULARITIES IN STRING THEORY

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Theory Seminar CERN, Oct. 3, 2007. RESOLVING SINGULARITIES IN STRING THEORY. Finn Larsen U. of Michigan and CERN. INTRODUCTION. Consider a warped geometry such as the KK-compactification Superficially similar metrics describe black holes, RS-throats, FRW-cosmology, .…..

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Theory Seminar

CERN, Oct. 3, 2007.

RESOLVING SINGULARITIES IN STRING THEORY

Finn Larsen

U. of Michigan and CERN

INTRODUCTION
• Consider a warped geometry such as the KK-compactification
• Superficially similar metrics describe black holes, RS-throats, FRW-cosmology, .…..
• In each setting we often consider situations where the conformal factors U1,2 become large somewhere on the base space.
• Then we should ask if the geometry is described accurately by conventional (super)gravity.
EXAMPLE: 5D BLACK HOLE
• Geometry of 5D black hole
• The scale factor U diverges at the horizon.
• For conventional black holes the curvature is finite at the horizon so the geometry is smooth.
• Then corrections to the solutions are small.
• For “small black holes” the horizon size vanishes in the classical approximation, curvature diverges, and “corrections” are important.
TOY MODELS?
• Much exploratory work has focused on toy models of the higher derivative interactions.
• A popular toy model:
• General limitation: there could be other terms at the same order so results cannot be trusted.
• The challenge: compute all terms at four derivative order, then solve the corresponding equations of motion.
A CONCEPTUAL DIFFICULTY
• Suppose we actually determined all the higher derivative corrections up to some order:
• Then: if the leading order solution is regular, the corrections can be computed systematically.
• The problem: if the corrections change the solution qualitatively (like resolving a singularity) then the “corrections” are as important as the “leading order” terms.
• In other words: there is no systematic expansion parameter and so we must generally keep all orders in the Lagrangian, an impossible task.
OUR APPROACH
• This talk: 5D black strings with AdS3xS2 near string geometry.
• Anomalies determine the sizes of the AdS3 and S2 geometries which are then one loop exact.
• Explicit computation of all terms to a given order: using a supersymmetric action.
• Find explicit solution: exploit off-shell SUSY.
• Discussion and applications.

REFS:

A. Castro, J. Davis, P. Kraus, and FL, hep-th/0702072, 0703087, 0705.1847

P. Kraus and FL: hep-th/0506176, hep-th/0508218

P. Kraus, FL, and A. Shah: hep-th/0708.1001

THE EXAMPLE: COSMIC STRINGS
• The ansatz for a string solution is
• The scale factors U1,2 diverge at the horizon.
• Generically, the geometry is nevertheless regular.
• An important case the matter supporting the string solution are the two-form B and the dilaton Ф, as in perturbative string theory.
• This case is singular at the string source.
THE SETTING
• Consider M-theory on CY3 x R4,1. For a small CY3 the theory is effectively D=5.
• In the supergravity approximation M-theory reduces to N=2 SUGRA in D=5.
• Solutions that are magnetically charged with respect to the vector fields AI are black strings.
• Such solutions generically have AdS3 x S2 near horizon geometry.
THE SETTING: MORE DETAILS
• The higher dimensional interpretation of these string solutions: they are M5-branes wrapped on 4-cycles P in CY3 .
• The cIJK are the intersection numbers of the basis cycles PI.
• The magnetic strings have AdS3 x S2 near horizon geometry with scale set by the self-intersection number of the M5-brane cIJKpIpJpK≠0.
• Special case: the CY is K3 x T2 and the M5-brane wraps the 4-cycle P=K3. This solitonic string is the type IIA dual of the heterotic string.
• The dual heterotic strings have singular near horizon geometry in the supergravity approximation since cIJKpIpJpK=0.
• The global symmetry group of AdS3 is
• Diffeormorphism symmetry enhances each of the SL(2)’s acting on the boundary at infinity to a Virasoro algebra.
• Explicit computation from the standard (two-derivative) Einstein action determines the spacetime central charges
• The central charge measures the number of degrees of freedom in the boundary theory but in the bulk it is essentially the size of AdS3.

Brown-Henneaux

ANOMALY INFLOW
• N=2 supergravity has a gravitational Chern-Simons term :
• The interaction violates gauge symmetry and/or diffeomorphism invariance, but only by a total derivative.
• Anomaly inflow: symmetries are preserved in full theory so boundary CFT anomalies must agree precisely with spacetime noninvariance.
• This condition determines the boundary central charges
• These expressions are exact because the underlying symmetries must be exact.

Maldacena, Strominger, Witten

Harvey, Minasian, Moore

Kraus, FL

ASIDE: BLACK HOLE ENTROPY
• A major string theory triumph: the black holes entropy is accounted for by string theory microstates.
• Why does this work?
• Central charges must agree on two sides because of anomaly inflow upholding diffeomorphism invariance and supersymmetry!
• Black holes arise as excitations of the magnetic string considered here so the agreement of entropies follows from Cardy’s formula:
RESOLUTION OF SINGULARITIES
• The dual heterotic string: CY=K3 x T2, P=K3 (so M5 wraps the K3).
• The intersection number CIJKpIpJpK=0 so the central charges are linear in the magnetic charges
• Since c2(K3)=24 we have cL = 12p and cR=24p.
• These are the correct values for p heterotic strings in a physical gauge (Left movers=8B+8F, Right movers=24B).
• The central charge measures the scale of AdS3 and S2. Its only contribution is from the higher derivative terms; so a singularity has been resolved.

Dabholkar

Kraus, FL

EXPLICIT SINGULARITY RESOLUTION
• So far: the resolution of a singularity was inferred from an indirect argument.
• A weakness: we assume AdS3xS2 near-string geometry and then consistency demands nonvanishing geometric sizes.
• Motivation for assumption: fundamental string should have world-sheet CFT and so an AdS3dual, and SU(2) R-symmetry motivates S2.
• Superior to the indirect story: construct asymptotically flat solutions directly.
• This is what we turn to next.
THE NEED FOR OFF-SHELL SUSY
• The essential interaction is the anomalous Chern-Simons term
• SUSY then determines all other four-derivative terms uniquely.
• Complication: on-shell SUSY closes on terms of ever higher order.
• Resolution: use the off-shell (superconformal) formalism.
• Unfamiliar feature: the Weyl multiplet (gravity) has auxiliary two-tensor vab and scalar D.

Hanaki, Ohashi, Tachikawa

SUSY VARIATIONS
• The off-shell action is invariant under the SUSY transformations
• Simplification: these variations are symmetries of each order in the action by itself.
• BPS conditions: these variations must vanish when evaluated on the solution.

(gravitino)

(gaugino)

(auxiliaryWeyl)

THE BPS SOLUTION
• Assume that the metric takes the string form:
• The BPS conditions impose U1=U2 and determine the auxiliary fields:
• Also, the magnetic fields are determined by the scalars (the attractor flow)
• The scalar fields MI and the metric function U are not determined by SUSY alone - they depend on the action!
CHARGE CONSERVATION
• The scalar fields MI are generally determined by the solving the Maxwell equations.
• However, the magnetic field strength is exact because it is topological
• Imposing the Bianchi identity (which is not automatic for the solution to the BPS condition) gives a harmonic equation
• With the standard solution
• Leading order supergravity, in off-shell formalism:
• The equation of motion for the auxiliary D-field gives the familiar special geometry constraint:
• Eliminating also the auxiliary v-field gives the standard on-shell action
• where
OFF-SHELL SUGRA: FOUR DERIVATIVES

Hanaki, Ohashi, Tachikawa

• SUSY completion of the 5D Chern-Simons term
• Definition of Weyl tensor:
• Covariant derivatives include additional curvature terms such as:
DEFORMED SPECIAL GEOMETRY
• Status: the solution has been specified in terms of the metric factor U which is still unknown.
• The equation of motion for the the D-field:
• Evaluated on the solution
• This is an ordinary differential equation for the metric factor U since HI=1 + pI/2r is a given function.
• Interpretation: the special geometry constraint has been deformed.
NEAR STRING ATTRACTOR
• The constraint can be solved analytically near the string where
• Result: the size of the S2 is
• Note: the near horizon geometry remains smooth in the singular case cIJKpIpJpK=0 as long as c2IpI is nonvanishing.
C-EXTREMIZATION
• The central charge is the trace anomali which is the bulk on-shell action, up to known constants of proportionality.
• So: compute on-shell action for our ansatz with (V, D, lA, lS,m) unspecified (m defined by MI=mpI and 6p3=cIJKpIpJpK)
• Consistency: extremizing c relates (V, D, lA, lS,m) as found previously. The value of c at the extremum gives
• This agrees with the anomaly inflow. The agreement relies on most terms in the four-derivative action.

Kraus, FL

THE RESOLVED SINGULARITY
• Now: analyse the differential equation for U in the singular case.
• The attractor has r~p1/3 but the entire region r<<p is described by a p-independent equation

Red: analytical expansion around near string attractor.

Blue: numerical solution.

• Upshot: extends smoothly away from the near string attractor
THE SPURIOUS MODES
• The numerical solution also attaches smoothly to the analytical expansion around flat space.
• The quasiperiodic behavior is due to spurious modes, a characteristic of solutions to higher derivative theories.
• This unphysical artifact is generally present even in flat space but can be removed by a field redefinition.

Sen

Hubeny, Maloney, Rangamani

Blue: numerical solution extended to larger distances.

Green: analytical expansion around flat space.

THE DUAL OF THE HETEROTIC STRING
• Dualizing our solution to the heterotic frame we find that p heterotic strings have near string AdS3 x S2 with
• The space is of string scale but we can still ask: what is the AdS/CFT dual to this space?
• It must be a D=1+1 CFT with (0,8) SUSY and R-symmetry at least SU(2), presumably based on supergroup OSp(4*|4).
• Puzzle: no SCFT with these symmetries exists! They are not consistent with the Jacobi identities.

Lapan, Simons,Strominger

NONLINEAR ALGEBRAS?

Henneaux, Maoz, Schwimmer

Lapan, Simons,Strominger

Kraus, FL

• Suggested resolution: there exists nonlinear superconformal algebras with the correct symmetries!
• Nonlinearity: the OPEs include current bilinears
• Notation:
• The nonlinear superconformal algebras are powerful but unfamiliar relatives to W-algebras.
• We consider multistring states so the suggestion is that NSCAs are important in string field theory.

Knizhnik

• Intriguing fact: nonlinearities determine the central charge. For example, for OSp(4*|4)
• Classical limit (large k) gives the Brown-Henneaux formula. Since k~N~1/g2 the nonlinear algebra determines the quantum corrections to all orders!
• Warning: there are presently a number of loose ends in this story.
• The biggest problem: it seems that non-unitary representations play a central role (the central charge is negative).
MANY MORE EXAMPLES
• This talk: just 5D string solutions with AdS3 x S2 near string geometry. But techniques apply in many other examples.
• Black holes in 5D with AdS2 x S3 near horizon geometry. There are electric charges and so the Maxwell equations are non-trivial.
• Rotating 5D black holes. The solution is much more complicated (all terms in the four-derivative action contribute) but still explicit.
• Black holes on Taub-NUT base space. A smooth interpolation between asymptotically flat 4D and 5D spacetimes.
• Upshot: we check various indirect arguments explicitly, sort out discrepancies in those arguments, and find new results.
EXAMPLE: 5D CALABI-YAU BLACK HOLES
• Based on 4D one loop corrections, the quantum corrections to 5D Calabi-Yau black holes were conjectured as:
• Our explicit solution:
• Understanding of dicrepancy: 4D charges are 5D charges as well as a R2-contribution from the interpolating Taub-NUT geometry
• Topological strings is a powerful technique for computing quantum corrections to holomorphic quantities in 4D. The strong coupling limit is effectively 5D. It appears to confirm the 1/8 shift.

Guica, Huang, Li,

Strominger

Huang, Klemm, Marino, Taranfar

SUMMARY
• Challenge for higher derivative corrections: keep all terms at a given order.
• Additional challenge for singularity resolution: understand why there are no further corrections.
• Our example: controlled by anomalies (so no further corrections) and we employ the complete supersymmetric action (so all important terms are kept).
• Main example: explicit construction of dual fundamental string with asymptotically flat boundary conditions.