Tests of cosmological gravity

Transcription

1 Tests of cosmological gravity Jeremy Sakstein University of Pennsylvania Astrophysics Seminar UC Irvine 23 rd January 2018

2 Who am I? Particle-cosmology (baryogenesis, early universe) Modified gravity (dark energy, new interactions) Astrophysics (tests of gravity, dark matter) Strong field (black holes + neutron stars) Screening mechanisms (see later) Degravitation

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4 Today What can we learn about gravity from: Black holes: GW (GWs from merging NSs):

5 Why modified gravity? Dark energy - new d.o.f. can drive acceleration? (still have CC problem, want well motivated models) Next gen. surveys will test GR on cosmic scales (need new IR modifications) Need alternative theories with different predictions CC problem? (degravitation, UV insensitivity, technical naturalness)

6 The problem with MG gravity Newtonian limit of GR: r 2 N =4 G F N = r N Modified gravity new scalar graviton: r 2 =8 G F 5 = r F 5 F N =2 2 solar system: 2 2 < 10 5 (Shapiro time-delay effect, Cassini)

7 Screening mechanisms Two options: (= 0) r 2 + F 2,...)=8 G + V 0 ( ) non-poisson kinetic terms Vainshtein screening galileons add scalar potential kill off the source no scalar gradient chameleon/symmetron/f(r)

8 Vainshtein screening Change kinetic terms e.g. cubic galileon: apple 1 r 2 d dr 2 r r2 c 3 r 02 =8 G Poisson term Galileon term Coupling to matter (crossover scale r c )

9 Vainshtein Mechanism We can integrate this once: - Vainshtein radius

10 Vainshtein screening

11 Vainshtein screening is generic DGP braneworld gravity Galileons Massive gravity Massive bi-gravity VERY generic scalar-tensor theories with three D.O.F Horndeski Beyond Horndeski breaks down inside objects (+ DHOST)

12 Galileons Self-acceleration (DE but does not solve CC) Nice(ish) UV properties (non-renormlaization) Massive (bi)gravity (Low cut-off for EFT?) Braneworld models Hard to test due to Vainshtein screening

13 Hui & Nicolis 12 Equivalence principle violations M ~x = Mr ext Qr ext gravitational charge scalar charge r ext r ext

14 Hui & Nicolis 12 Equivalence principle violations in galileon theories Baryonic + dark matter: Q = M Black holes described by mass and spin only! Black holes: Q =0 Matter falls faster than BHs r ext Violation of the SEP! r ext

15 Hui & Nicolis 12 Eötvös experiments with black holes

16 Galaxy clusters: nature s leaning towers BH

17 NFW, c=5 M = M Virgo cluster (km/s) 2 /kpc r V ndgp Newtonian Galileon (r c = 500 Mpc) Galileon (r c = 6000 Mpc) -- RMS Cosmological self-accelerating

18 Offset Offset kpc ρ = 0.05M pc -3, M 200 = M ρ = 0.1M pc -3, M 200 = M -- ρ = 0.1M pc -3, M 200 = 2x10 14 M Mpc

19 M87 JS, Jain, Heyl, Hui APJL 17 /(1000 km) -1 self-acceleration LLR M 87 /Mpc 3 = 6M p /r 2 c 1/3

20 Future tests This is one galaxy! More galaxies SDSS, DES, Euclid + X-ray/Radio Morphological distortions Missing SMBHs!

21 GW t apple 1.7s GWs and Photons from merging NSs

22 Geometry of GW NGC Mpc ) c T c c < 10 15

23 Gravitational waves Galileons predict photons move slower than GWs: c 2 c T 2 c 2 = 4c 4x 2 c 4 (H 0 r c ) 4 1 3c 4 x 2 x = = (x, c 4, ) HM pl Probed by GWs Probed by SMBHs

24 JS & Jain 17 PRL Galileons are cosmologically irrelevant SMBHs Speed of Gravitons =

25 JS & Jain 17, Baker et al. 17, Ezquiaga & Zumalacárregui 17, Creminelli & Vernizzi 17 What else is ruled out? Healthy scalar-tensor theories (Horndeski): Three (2 tensor + 1 scalar) degrees of freedom Healthy Can extend to beyond Horndeski c T = c L p = K(,X)+G 3 (,X) + G 4 (,X)R g +G 4,X ( ) G 5,X ( ) 3 (r µ r ) 2 + G 5 (,X)G µ r µ r 3r µ r r µ r +2r r µ + r r r µ r X = (@ ) 2 /2 c T 6= c

26 Other theories that are ruled out Theories: beyond Horndeski (more general) similar story DHOST (even more general) similar story Vector-tensor similar story Exception: can always fine-tune (Crisostomi & Koyama 17)

27 Important: what is allowed Cosmological (scalar acts as dark energy): L p = K(,X)+G 3 (,X) + G 4 ( )R g quintessence k-essence cubic galileons Brans-Dicke f(r) Conformal rescalings (tuned DHOST, beyond Horndeski) g µ = 2 (,X)g µ

28 Important: what is allowed Non-cosmological (no DE): Everything!

29 What else can we do? Test dark energy on small scales Varying speed of light theories

30 Bellini & Sawicki `14, Gleyzes et al. 14, Langlois et al. `17 Effective description of dark energy Parameterize cosmological perturbations: Horndeski: M K B T = c 2 T c 2 Ġ/HG (@ ) n (@ ) 2 speed of tensors beyond Horndeski: DHOST: H V }Vainshtein breaking

31 ( PPN = 1) Unscreened Screened

32 Kobayashi et al. 14, Koyama & JS 15, Saito et al. 15 Vainshtein breaking ds 2 = (1 + 2 )dt 2 +(1+2 ) ij dx i dx j d dr = GM r 2 + 1G 4 d 2 M dr 2 stellar structure, rotation curves d dr = GM r G 4r dm dr + 3G d2 M dr 2 Lensing

33 Example: quartic Horndeski 1 = 2 = 4 H 2 c 2 T (1 + B) H 1 4 H ( H B ) 5[c 2 T (1 + B) H 1] 3 =0 c T 1 can measure cosmology on small scales

34 Bounds: low-mass stars Red dwarf (hydrogen burning) Brown dwarf (no hydrogen burning) Minimum mass for hydrogen burning (MMHB)

35 JS 17 PRL, JS 17 PRD MMHB changes in beyond Horndeski Lightest observed 1 < 1.6 red dwarf 1

36 JS et al 16 Galaxy cluster lensing d dr = GM hydrostatic( 1 ) r 2 d dr = GM lensing( 2, 3 ) r 2 X-ray brightness Weak lensing 1 = =

37 Constraints 2 1 Dwarf Stars 0-1 Galaxy Clusters

38 More general theories (cubic, DHOST) Need more bounds (pulsar decay rate etc.) Need to impose tuning relations so that c T 1 See Crisostomi & Koyama 17, Langlois et al. 17, Dima & Vernizzi 17

39 Disformal transformations g photons µ = g gravitons µ M 4 Varying speed of light theories c 2 c 2 =1 2 M 4 Cosmological time-derivative LIGO/Fermi < (cosmologically irrelevant)

40 Outlook

41 Is MG dead? Depends what you care about Cosmological (DE) theories: Either need tuning or restricted class of theories EFT, naturalness, fine-tuning questions f(r)/chameleon/symmetron fine (cubic galileon in trouble for other reasons) Non-cosmological theories: Very few constraints (graviton mass < ev)

42 Summary Galileons interesting for many reasons Predict SEP violations Can test this with BH offsets New (strongest) constraints on galileons GW vs photon speed constrained by LIGO/Fermi Severe implications for cosmological gravity