**Dark** **Energy**: **novel** **matter** **or** **gravity**?Rachel Bean**C ornell** UniversityRecent paper:Axel de la Mac

OverviewoCoupling of **matter** to scalar fields– Motivation– Observational issuesoInvestigating a coupling using **cosmo**logical and particle physics dataRachel Bean : COSMO ‘**06** Sept 28th 20**06** 2/16

Why modify **gravity**?oThe**or**ies predict a modification!– Scalar-tens**or** **gravity**oWe don’t have a clue what dark energy is!– e.g. could be f(R) the**or**ies?– e.g. could contain higher geometric derivatives?oConceivably, we don’t yet have the c**or**rect large (and small?) scale the**or**yof **gravity**?Rachel Bean : COSMO ‘**06** Sept 28th 20**06** 4/16

Tight constraints on **gravity** from solar systemo f(R) the**or**ies -> Brans-Dicke with ! = 0– Inconsistent with solar system tests e.g. Cassini ! >20000 (" #= 2x10 $3 )oFuture constraints will be tighter– 2011 (?) GAIA could get " #< 10 $6– 2012 ESA Bepi-Columbo Mercury **or**biter " #< 3x10 $5 , " %< 3x10 $4Rachel Bean : COSMO ‘**06** Sept 28th 20**06** 5/16Figure: Takeshi Chiba

Modified **gravity** -> time varying Go BBN abundances (Copi, Davis, Krauss 2003)– z= 10 10+0.20– G/G 0= 1.01 (68%)-0.16oLunar radar ranging– z=0– dG/dt/G

Change to G Change to massesConf**or**mal equivalence: non-minimal couplings **gravity** and to **matter**Redefinition of the metric (F’’(&)! 0)Scalar field redefinition gives us canonical scalar field LagrangianRachel Bean : COSMO ‘**06** Sept 28th 20**06** 7/16Takeshi Chiba (2003)

Can evade solar system constraintsoDifferent couplings to CDM and baryons can avoid this– e.g. coupling to trace of T µ'(cdm) (Amendola 1999, Bean & Magueijo 2000)V effVe C&oDensity dependent coupling to baryonic and CDM objects– e.g. ‘Chameleon fields’ (Khoury & Weltman 2003, Mota & Davis 20**06**)&– Must test **gravity**/ equivalence principle in different density backgrounds todetect thisRachel Bean : COSMO ‘**06** Sept 28th 20**06** 8/16

Stability issues in coupled the**or**ieso Adiabatic behavi**or** if slow evolution of the Hamiltonian of the system (M EFF2>> H O -2 )oCan lead to critical shift in clustering behavi**or** of the scalar field the**or**y the**or**yNon-adiabaticAdiabatic(Quintessence)Stable in typical the**or**ies c s2>0Can be unstable c a2

Learning about **matter**/ dark energy couplingsoPossibility of links between **matter** and otherwise weakly interacting field offersinteresting options– solar system tests– gravitational collapse instabilities on astrophysical scales– Cosmological large scale structure implicationsoIf coupling involves standard model particles– we don’t just only rely on indirect **cosmo**logical/astrophysical observations– Also should consider evidence from direct particle physics measurements.Rachel Bean : COSMO ‘**06** Sept 28th 20**06** 10/16

Investigating a coupling to neutrinosoConsider a simple scalar field model coupled to neutrinosoThen a relationship between neutrinos and dark energy would lead tovariations in the apparent equation of stateRachel Bean : COSMO ‘**06** Sept 28th 20**06** 11/16de la Mac**or**ra, Melchi**or**ri, Serra, RB (20**06**)

Utilizing ‘direct’ neutrino mass dataoKinematics-missing momentum and energy– tritium beta decay, 3H -> 3He + e- + ne,– **or** pion decay, ( + $> µ + + ' µoNeutrino oscillations– determine )m 2 using P(' l-> ' l’) = sin 2 2* sin2()m 2 L/E ')oExchange of virtual neutrinos,e.g. 0'%% +`neutrinoless beta decay’– If (anti) neutrino has non-zero mass– If neutinos are maj**or**anaRachel Bean : COSMO ‘**06** Sept 28th 20**06** 12/16

Neutrinoless double beta decayoNeutrinoless double beta decay is the only known process that enables to testexperimentally– the Maj**or**ana nature of neutrino– together with its absolute mass scale.oMakes this topic very attractive!oThe half life time is given by the following f**or**mula:(T 1/2,0!) -1 = a 0!F 0!|M 0!| 2 " 2 / log(2)Where:a 0! ~ 5x10 -17 yr -1 is a dimensional fact**or**F 0! is a known phase space fact**or** prop**or**tional to Q ##5M 0! is the nuclear matrix element (n.m.e.)." = /m e with being the effective mass of the exchanged neutrino and m ethe mass of the electron (0.511 MeV/c 2 ).Rachel Bean : COSMO ‘**06** Sept 28th 20**06** 13/16

The Moscow-Heidelberg 0v%% resultsooPhys. Lett. B586, 198 (2004); hep-ph/0404088Nucl.Instrum.Meth. A522 (2004) 371-4**06** hep-ph/0403018– 71.7 kg yr,– 11kg enriched 76 Ge (equivalent to a ton of n**or**mal 76 H.V. Klapd**or**-Kleingrothaus, A. Dietz, I.V.Ge) Krivosheina, O. Chkv**or**ets hep-ph/0403018– 28.75 ± 6.86 events at 2039 keV, the Q-value f**or** 0'%% decay (rep**or**ted as 4.2" now 6")This is interpreted as T 1/2= 1.15 -0.5+2.99x 10 25 y– Given matrix element (Rodin, Faessler, Simkovic, Vogel 20**06**) this translates into0.43< m %%< 0.81 (95% CL)– Applying this to the **cosmo**logical neutrino density0.0137 < , v h 2 < 0.026 (95% CL)ooTo confirm **or** reject the KDHK claim of the 0'%% discovery convincingly one needs 5-10 kmoleyearsexposure with low background (source: Petr Vogel Fermilab presentation 2004).KATRIN (Karlsruhe Tritium Neutrino experiment) in the next decade )m '~ 0.2eVRachel Bean : COSMO ‘**06** Sept 28th 20**06** 14/16

Combining **cosmo**logical and particle measurementsRachel Bean : COSMO ‘**06** Sept 28th 20**06** 15/16de la Mac**or**ra, Melchi**or**ri, Serra, RB (20**06**)

ConclusionsoCoupling between dark energy and **matter** is the**or**etically conceivableoAlthough Solar System bounds can offer tight PPN constraints, somethe**or**ies can evade theseoIssues about stability have and are being addressedoFuture insights into coupling can be gained from combining astrophysical(indirect) observations with direct particle physics investigations of the SM.oInteresting disc**or**d between indirect and direct measurements– Signaling new physics?– Complementary and increased precision measurements in near futureRachel Bean : COSMO ‘**06** Sept 28th 20**06** 16/16