Jeremy Sakstein University of Pennsylvaniajeremysakstein.com/talks/tests_cos_grav.pdf · Jeremy...
Transcript of Jeremy Sakstein University of Pennsylvaniajeremysakstein.com/talks/tests_cos_grav.pdf · Jeremy...
Tests of cosmological gravity
Jeremy Sakstein University of Pennsylvania
Astrophysics Seminar UC Irvine
23rd January 2018
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
TodayWhat can we learn about gravity from:
GW170817 (GWs from merging NSs):
Black holes:
Why modified gravity?
• Dark energy - new d.o.f. can drive acceleration?
• Next gen. surveys will test GR on cosmic scales
• Need alternative theories with different predictions
• CC problem?(degravitation, UV insensitivity, technical naturalness)
(need new IR modifications)
(still have CC problem, want well motivated models)
The problem with MG gravityNewtonian limit of GR:
r2�N = 4⇡G⇢ FN = r�N
Modified gravity — new scalar graviton:
F5 = ↵r�r2� = 8⇡↵G⇢
F5
FN= 2↵2 solar system: 2↵2 < 10�5
(Shapiro time-delay effect, Cassini)
Screening mechanisms
Two options:
non-Poisson kinetic terms Vainshtein screening
galileons
add scalar potential kill off the source no scalar gradient
chameleon/symmetron/f(R)
r2�+ F (@�, @2�, . . .) = 8⇡↵G⇢+ V 0(�)
(= 0)
Vainshtein screening
Change kinetic terms — e.g. cubic galileon:
1
r2d
dr2
r2�0 +
2r2c3
r�02�= 8⇡↵G⇢
Poisson termGalileon term
(crossover scale )rc
Coupling to matter
Vainshtein MechanismWe can integrate this once:
- Vainshtein radius
Vainshtein screening
Vainshtein screening is generic• DGP braneworld gravity
• Galileons
• Massive gravity
• Massive bi-gravity
• Horndeski
• Beyond Horndeski — breaks down inside objects
VERY generic scalar-tensor theories
with three D.O.F
(+ DHOST)
Galileons• Self-acceleration (DE but does not solve CC)
• Nice(ish) UV properties (non-renormlaization)
• Massive (bi)gravity
• Braneworld models
• Hard to test due to Vainshtein screening
(Low cut-off for EFT?)
Hui & Nicolis ‘12
‘gravitational charge’ ‘scalar charge’
r�ext
r�ext
M ~x = �Mr�ext �Qr�ext
Equivalence principle violations
Equivalence principle violations in galileon theories
Baryonic + dark matter: Q = M
Black holes described by mass and spin only!
Hui & Nicolis ‘12
Black holes: Q = 0
Matter falls faster than BHs
Violation of the SEP!
r�ext
r�ext
Hui & Nicolis ‘12
Eötvös experiments with black holes
Galaxy clusters: nature’s leaning towers
BH
Virgo cluster
● Newtonian
● Galileon (rc = 500 Mpc)
● Galileon (rc = 6000 Mpc)
-- RMS Cosmological
0.5 1 5 10
50100
5001000
5000104
(km/s)2/kpc
rVnDGP
self-accelerating
NFW, c=5M = 1015M�
Offset
● ρ = 0.05M☉ pc-3, M200 = 1015M☉
● ρ = 0.1M☉ pc-3, M200 = 1015M☉
-- ρ = 0.1M☉ pc-3, M200 = 2x1014M☉
0.5 1 5 10 Mpc
0.05
0.10
0.50
1
Offsetkpc
M87
M 87
LLR
7.9 5. 3.8
/Mpc
/(1000 km)-1self-acceleration
⇤3 =�6Mp/r
2c
�1/3
JS, Jain, Heyl, Hui APJL ‘17
Future tests
• More galaxies — SDSS, DES, Euclid + X-ray/Radio
• Morphological distortions
• Missing SMBHs!
This is one galaxy!
GW170817GWs and Photons from merging NSs
�t 1.7s
Geometry of GW170817
40 Mpc
) cT � c
c< 10�15
NGC 4993
Gravitational wavesGalileons predict photons move slower than GWs:
c2T � c2
c2=
4c4x2
1� 3c4x2
x =�
HMpl
c4 ⇠ (H0rc)4
Probed by GWs
⌦� = ⌦�(x, c4,↵)
Probed by SMBHs
Galileons are cosmologically irrelevant
SpeedofGravitons
SMBHs
-10 -8 -6 -4 -2 0-10
-8
-6
-4
-2
0
↵ = 1
JS & Jain ‘17 PRL
What else is ‘ruled’ out?JS & Jain ’17, Baker et al. ’17, Ezquiaga & Zumalacárregui ’17, Creminelli & Vernizzi ‘17
Healthy scalar-tensor theories (Horndeski):
Lp�g
= K(�, X) +G3(�, X)⇤�+G4(�, X)R
+G4, X
⇥(⇤�)2 � (rµr⌫�)
2⇤+G5(�, X)Gµ⌫rµr⌫�
�1
6G5, X
⇥(⇤�)3 � 3rµr⌫�rµr⌫⇤�+ 2r⌫rµ�+r↵r⌫�rµr↵�
⇤
• Three (2 tensor + 1 scalar) degrees of freedom • Healthy • Can extend to ‘beyond Horndeski’ cT = c
X = �(@�)2/2 cT 6= c
Other theories that are ‘ruled’ out
• beyond Horndeski (more general)— similar story
• DHOST (even more general) — similar story
• Vector-tensor — similar story
Theories:
Exception: can always fine-tune (Crisostomi & Koyama ‘17)
Important: what is allowed
Cosmological (scalar acts as dark energy):
Conformal rescalings (tuned DHOST, beyond Horndeski)
gµ⌫ = ⌦2(�, X)gµ⌫
quintessence k-essence
cubic galileons
Brans-Dicke f(R)
Lp�g
= K(�, X) +G3(�, X)⇤�+G4(�)R
Important: what is allowed
Non-cosmological (no DE):
Everything!
What else can we do?
• Test dark energy on small scales
• Varying speed of light theories
Effective description of dark energy
Parameterize cosmological perturbations:
↵Hbeyond Horndeski:
↵V �1 �2 �3DHOST:
Horndeski: ↵M ↵B↵K
speed of tensors
↵T = c2T � c2
(@�)n (@�)2⇤�G/HG
} Vainshtein breaking
Bellini & Sawicki `14, Gleyzes et al. ’14, Langlois et al. `17
Screened
Unscreened
(�PPN = 1)
Vainshtein breaking
ds2 = �(1 + 2�) dt2 + (1 + 2 )�ij dxi dxj
Kobayashi et al. ’14, Koyama & JS ’15, Saito et al. ‘15
d�
dr=
GM
r2+
⌥1G
4
d2M
dr2
d
dr=
GM
r2� 5⌥2G
4r
dM
dr+⌥3G
d2M
dr2
stellar structure, rotation curves
Lensing
Example: quartic Horndeski
⌥1 =4↵2
H
c2T(1 + ↵B)� ↵H � 1
⌥2 =4↵H(↵H � ↵B)
5 [c2T(1 + ↵B)� ↵H � 1]
⌥3 = 0
cT ⇡ 1 can measure cosmology on small scales
Brown dwarf (no hydrogen burning)
Red dwarf (hydrogen burning)
Minimum mass for hydrogen burning
(MMHB)
Bounds: low-mass stars
MMHB changes in beyond Horndeski
JS ’17 PRL, JS 17’ PRD
���� ���� ���� �
�����������������������������������
1
Lightest observed red dwarf
⌥1 < 1.6
Galaxy cluster lensing
d�
dr=
GMhydrostatic(⌥1)
r2d
dr=
GMlensing(⌥2,⌥3)
r2
X-ray brightness Weak lensing
JS et al ‘16
⌥1 = �0.11+0.93�0.67
⌥2 = �0.22+1.22�1.19
Constraints
Dwarf Stars
Galaxy Clusters
-1.0 -0.5 0.0 0.5 1.0-2
-1
0
1
2
More general theories (cubic, DHOST)
• Need more bounds (pulsar decay rate etc.)
• Need to impose tuning relations so that cT ⌘ 1
See Crisostomi & Koyama ’17, Langlois et al. ’17, Dima & Vernizzi ‘17
Disformal transformations
Varying speed of light theories
gphotonsµ⌫ = ggravitonsµ⌫ +@µ�@⌫�
M4
c2�c2
= 1� �2
M4
Cosmological time-derivative
LIGO/Fermi
< 6⇥ 10�15(cosmologically irrelevant)
Outlook
‘Is MG dead?’Depends what you care about
• 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)
Cosmological (DE) theories:
Non-cosmological theories:
• Very few constraints (graviton mass < 10-22 eV)
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