Lorentz violations: from quantum gravity to black holes. Thomas P. Sotiriou

Transcription

1 Lorentz violations: from quantum gravity to black holes Thomas P. Sotiriou

2 Motivation Test Lorentz symmetry

3 Motivation Old problem: Can we make gravity renormalizable? Possible solution: Add higher-order curvature invariants Higher order term contain higher order time derivatives This introduces ghosts Simple solution: give up Lorentz invariance. Then Higher order spatial derivatives without higher order time derivatives, i.e. no ghosts Renormalizable theory (at the power counting level) Well, maybe not that simple...

4 Lifshitz scalar Consider the action S = dtdx d 2 a m ( ) m + g n n It is then natural to choose the scaling dimensions [dt] =[ ] z [dx] =[ ] 1 which implies that [ ]=[apple] (d z)/2 [a m ]=[apple] 2(z m) d+z n(d z)/2 [g n ]=[apple] So long as the theory is power counting renormalizable. z d

5 Lifshitz gravity We first need to split spacetime into space and time by introducing a preferred foliation ds 2 = N 2 c 2 dt 2 + g ij (dx i N i dt)(dx j N j dt) Remaining symmetry: foliation preserving diffeomorphisms Time reparametrization: t t(t) Spacetime-dependent 3-diffeos: x i x i (t, x i ) Most general action S = M 2 pl 2 d 3 xdtn g K ij K ij K 2 V (g ij,n) P. Hořava, Phys. Rev. D 79, (2009)

6 General features The potential should be at least 6th order in spatial derivatives V = R a i a i + 1 M 2 L M 4 L 6 a i = i ln N There are 2 types of Lorentz-violating terms: those that come at lower order and those that come at higher order The theory propagates a scalar mode Generically there will be more than 60 couplings So far perfectly consistent and viable theory (with certain assumptions about matter coupling) D. Blas, 0. Pujolas and S. Sibiryakov, Phys. Rev. Let. 104, (2010)

7 Projectable version The lapse is space-independent Drastic simplification - 9 couplings only Same degrees of freedom Severe infrared viability issues Shown to be renormalizable P. Hořava, Phys. Rev. D 79, (2009) T.P.S., M. Visser, S. Weinfurtner, PRL 102, (2009) JHEP 0910, 033 (2009) T.P.S., M. Visser, S. Weinfurtner, JHEP 0910, 033 (2009) D. Blas, O. Pujolas, S. Sibiryakov, JHEP 0910, 029 (2009) K.Koyama, F. Arroja, JHEP 1003, 061 (2010) A. O. Barvinsky et al., PRD 93, (2016) Shown to be asymptotically free in 2+1 dimensions A. O. Barvinsky et al., arxiv:

8 Einstein-aether theory The action of the theory is S æ = where 1 16 G æ d 4 x g( R M µ u µ u ) M µ = c 1 g g µ + c 2 g µ g + c 3 g g µ + c 4 u u g µ and the aether is implicitly assumed to satisfy the constraint u µ u µ =1 Most general theory with a unit timelike vector field which is second order in derivatives T. Jacobson and D. Mattingly, Phys. Rev. D 64, (2001).

9 Hypersurface orthogonality Now assume u = g µ and choose T as the time coordinate T µt T u = T (g TT ) 1/2 = N T Replacing in the action and defining one gets Sæ ho 1 = dt d 3 xn h K ij K ij K 2 + (3) R + a i a i 16 G H with a i = i ln N G H G æ = = and the parameter correspondence 1 1 c 13 = 1+c 2 1 c 13 = c 14 1 c 13 T. Jacobson, Phys. Rev. D 81, (2010).

10 Combined Constraints Einstein-aether theory l c- c StabilityêCherenkov Binary pulsars c c+ rg FIG. 1. (Color online) Constraints on the (c+, c ) plane in Æther theory (left) obtained by combining constraints derived from observations of PSR J D [60] PSR J E. Barausse, T. Jacobson and T.P.S., Phys. Rev. 83,and (2011) [61]. The areas outside the (allowed) shade considerations (light blue), BBN (dark orange) and the combined binary pulsa K. Yagi, D. Blas, N. Yunes and E. Barausse Phys. Rev. Lett. 112, (2014) corresponds to the values of the coupling constants required for the orbital de zero-sensitivity/weak-field limit. Observe that the new constraints are much m ISCO GC is the cosmological gravitational constant µ m

11 Combined Constraints Horava gravity StabilityêCherenkov Binary pulsars l 0 c- m b 0.6 c+ r StabilityêCherenkov Binary pulsars BBN b g ISCO FIG. 2. Binary pulsar constraints on Lorentz-violating theories. The dark, purple shaded surfaces are allowed regions in the 2-dimensional coupling parameter space of Einstein-Æther theory (left) and khronometric gravity (right), given observations E. Barausse, Jacobson and T.P.S., Phys. D 83, (2011) of PSR J [23],T. PSR J [24] and PSR Rev. J [30]. These regions account for possible variability in the K. well Yagi,asD.1 Blas, N. Yunesuncertainties and E. Barausse Lett. 112, (2014) NS EoS, as observational in all Phys. systemrev. parameters, where we have marginalized over ( 1, 2 ) given Solar System constraints. Observe that these regions are significantly smaller than those allowed given stability/cherenkov requirements (light, blue shaded region) and big bang nucleosynthesis constraints (dark, orange shaded region). The red dashed curves show the values of the coupling constants for which the orbital decay rate is exactly the same as in GR in the weak field/low-compactness limit [3, 20]. Thomas P. Sotiriou - 9th Aegean Summer School, Sifnos, September 18th 2017

12 Strong coupling The low energy action exhibits strong coupling at energy M sc = f( 1, )M pl Can be a large scale, but problem with renormalizability A. Papazoglou and T. P. S., Phys. Lett. B 685, 197 (2010) I. Kimpton and A. Padilla, JHEP 1007, 014 (2010) Strong coupling problem can be circumvented if M sc >M D. Blas, O. Pujolas and S. Sibiryakov, Phys.Lett. B 688, 350 (2010) But then potential tension with observations GeV >M >M obs A. Papazoglou and T. P. S., Phys. Lett. B 685, 197 (2010)

13 Hierarchy of scales Consider the dispersion relation E 2 = m 2 + p p 4 M 2 LV + O( p6 M 4 LV ) Assume that there is a universal LV scale, so M LV M? Constraint from synchrotron radiation from the Crab Nebula: M obs > GeV M? cannot be a universal scale S. Liberati, L. Maccione and T. P. S., Phys. Rev. Lett. 109, (2012)

14 Percolations of LV But what about the matter sector and lower order operators? Different speeds for different fields in the IR, with logarithmic running Possible ways out: R. Iengo, J. G. Russo and M. Serone, JHEP 0911, 020 (2009) Some extra symmetry, e.g. supersymmetry S. Groot Nibbelink and M. Pospelov, Phys. Rev. Lett. 94, (2005) Assume Lorentz symmetry in matter and let the weak coupling to gravity (the Lorentz-violating sector) do the rest M. Pospelov and Y. Shang, Phys. Rev. D 85, (2012)

15 Pertinent questions Renormalization group flow: Do couplings get the values we want them to get? Quantization: some real quantum gravity predictions Matter coupling and relevant (possibly worrisome) phenomenology I. Kimpton and A. Padilla, JHEP 1304, 133 (2013) M. Colombo, A. E. Gumrukcuoglu, and T.P.S., Phys. Rev. D 91, (2015); Phys. Rev. D 92, (2015) A. Coates, M. Colombo, A. E. Gumrukcuoglu, and T.P.S., Phys. Rev. D 94, (2016) Vacuum energy Causal structure: do we understand it? Black holes and singularities: are there black holes? Are singularities resolved?

16 LV and causal structure Causal structure in special relativity LV with linear dispersion relations k Different modes have different speeds and different light cones But there are still light cones

17 LV and black hole structure What happens to black holes? They will have multiple horizons

18 LV and black hole structure LV with non-linear dispersion relations No light cones 2 k 2 + ak Causal structure without relativity time space Future Simultaneous P No black holes at all?? Past

19 Spacetime diagram E. Barausse, T. Jacobson and T.P.S., Phys. Rev. D 83, (2011) t Singularity Constant preferred time Universal Horizon Metric Horizon r

20 Penrose diagram i + Universal Horizon = φ T i 0 Taken from D. Blas and S. Sibiryakov, Phys. Rev. D 84, (2011)

21 Rotating black holes Slowly rotating BHs in Einstein-aether theory do not have a preferred foliation. Slowly rotating BHs in Horava gravity have universal horizons. E. Barausse and T.P.S., Phys. Rev. Lett. 109, (2012) E. Barausse and T.P.S., Phys. Rev. D 87, (2013) E. Barausse and T.P.S., Class. Quant. Grav 30, (2013) E. Barausse, I. Vega and T.P.S., Phys. Rev. D 93, (2016) 3d rotating black holes can have universal horizons even with flat asymptotics. Universal horizons can lie outside de Sitter horizons. T.P.S., I. Vega and D. Vernieri, Phys. Rev. D 90, (2014)

22 Beyond exact solutions A new toolkit is needed M. Colombo, J. Bhattacharyya, and T.P.S., Class. Quant. Grav. 33, (2016). Can we define this horizon in full generality? Can we have a local definition when we have less symmetry? Theorem Is the universal horizon relevant to astrophysics? Yes (u )=0, (a ) 6= 0 form a set of necessary and sufficient conditions for a hypersurface to be a universal horizon No, it lies always behind the usual horizon

23 Spherical collapse J. Bhattacharyya, A. Coates, M. Colombo, and T.P.S., Phys. Rev. D 93, (2016). One can make the ansatz ds 2 = N 2 dt 2 + S 2 (N R dt + dr) 2 + r 2 (d 2 +sin 2 d 2 ) and then the T-equation takes the R h r 2 SN 2 ~ Æ R i =0, (s ~Æ) = f IM(T ) r 2 N 2 Horava gravity has an instantaneous mode The universal horizon corresponds to N 1 This foliation was used for simulations D. Garfinkle, C. Eling and T. Jacobson, Phys. Rev. D 76, (2007) It does not penetrate the horizon

24 LV and GW propagation E 2 = m 2 g + a 1 M? p + c 2 gp 2 + a 3 p 3 GWs constrain significantly only the mass, M? + a 4 p 4 M? +... p and p 3 Constraining the speed of GW requires knowing distance AND time of travel, or to have EM counterpart Weak constraints on speed from time delays between detectors D. Blas et al., JETP Lett 103, 624 (2016) N. Cornish, D. Blas and G. Nardini, arxiv: [gr-qc] Smoking-gun detection: scalar mode with time delay (but hard to detect) T.P.S., arxiv: [gr-qc]

25 Perspectives LV QG: Well-defined candidate, testable predictions Major IR viability issues resolved (in some versions) Some very interesting predictions. I did not mention: Cosmology: scale invariant spectrum, no horizon problem, novel dark energy model, S. Mukohyama, Class. Quant. Grav. 27, (2010) D. Blas and S. Sibiryakov, JCAP 1107, 026 (2011) Possible contact with discrete QG (CDT) P. Hořava, Phys. Rev. Lett. 102, (2009) T. P. Sotiriou, M. Visser and S. Weinfurtner, Phys. Rev. Lett. 107, (2011) Major challenges ahead: renormalization group flow, quantization What about quantum predictions?

26 Perspectives Black holes are of great interest in Lorentz-violating theories. New notion: universal horizon Non-trivial causal structure Is this horizon stable? Does it form from collapse? D. Blas and S. Sibiryakov, Phys. Rev. D 84, (2011) M. Saravani, N. Afshordi and R. B. Mann, Phys. Rev. D 89, (2014) Testing LV with GW observations? J. Bhattacharyya, A. Coates, M. Colombo, and T.P.S., Phys. Rev. D 93, (2016).