Is dark matter made of mirror matter? Evidence from cosmological data. Paolo Ciarcelluti∗ Web Institute of Physics, www.wiph.org Quentin Wallemacq† IFPA, D´ep. AGO, Universit´ede Li`ege, 4000 Li`ege, Belgium (Dated: August 2, 2018) We present new fast numerical simulations of cosmic microwave background and large scale struc- ture in the case in which the cosmological dark matter is made entirely or partly made of mirror matter. We consider scalar adiabatic primordial perturbations at linear scales in a flat Universe. The speed of the simulations allows us for the first time to use Markov Chain Monte Carlo analyses to constrain the mirror parameters. A Universe with pure mirror matter can fit very well the observa- tions, equivalently to the case of an admixture with cold dark matter. In both cases, the cosmological 2 models estimate the presence of a consistent amount of mirror dark matter, 0.06 . Ωmirrorh . 0.12. PACS numbers: 98.80.-k, 95.35.+d, 98.70.Vc, 98.65.Dx Keywords: cosmology, cosmic microwave background, large scale structure, dark matter The existence of dark matter in the Universe seems to particles have left-handed interactions, mirror particles be an unavoidable evidence, as confirmed by all the cur- have right-handed interactions [11]. Thus, they are sta- rently available astrophysical observations at scales rang- ble exactly as their ordinary counterparts. ing from cosmological to galactic. At the same time, its The ordinary and mirror particles have the same nature is still completely unknown, and is limited to its masses and obey to the same physical laws, but the three qualitative behaviour for the process of structure forma- non-gravitational interactions act on ordinary and mir- tion. Together with Big Bang nucleosynthesis (BBN), the ror sectors completely separately, the only link between most powerful cosmological tests for dark matter candi- all of them being the gravity. Since mirror baryons do dates are the cosmic microwave background (CMB) and not interact with photons, or interact only very weakly, large scale structure (LSS) power spectra, with their in- the presence of mirror matter is felt mainly by its grav- creasing precisions due to the huge observational efforts itational effects, which is exactly the definition of “dark of several groups. matter”. Previous analytical and numerical studies on CMB and Hence mirror matter is a stable self-interacting1 dark LSS power spectra [1–6] have only limited the parameter matter candidate that emerges if one, instead of (or in space of mirror dark matter, without providing a defini- addition to) assuming a symmetry between bosons and tive cosmological answer to its existence. Here we finally fermions (supersymmetry), assumes that nature is parity address this question. symmetric. As suggested many years ago by Lee and Yang [7], Besides being a viable and powerful candidate for dark to suppose the existence of mirror matter is the sim- matter, the increasing interest on mirror matter is due to plest way to restore the parity symmetry of the laws the fact that it provides one of the few potential explana- of nature. Their idea was later developed by other au- tions for the recent DAMA annual modulation signal [15], thors [8–11], and a lot of studies were devoted to it, together with the results of other direct detection exper- showing its compatibility with all the available exper- iments (CDMS, CoGeNT, CRESST, XENON) [16]. The imental and observational constraints (for reviews, see compatibility of this scenario with BBN constraints has Refs. [3, 12–14]). In some cases, as the results of direct already been studied [19, 20]. arXiv:1211.5354v3 [astro-ph.CO] 20 Jan 2014 detection experiments [15, 16] or the observations of neu- Given its consistency with experiments and observa- tron stars [17, 18], there are interesting suggestions of the tions, and the unfruitful attempts to prove the existence existence of mirror matter. of the other dark matter candidates, scientific commu- The original idea was to have a parallel hidden (mirror) nity is facing an emergent question: “is mirror matter sector of particles which is an exact duplicate of the ob- the dark matter of the Universe (or at least a significant servable sector. In the modern context of gauge theories part of it)?” One possibility to answer this question is to this implies the existence of an exact parity (mirror) sym- look at the cosmological signatures of mirror particles. metry between two particle sectors, that are described by It is worthwhile to note that the presence of the mirror the same Lagrangian and coupling constants, and conse- quently have the same microphysics, but where ordinary 1 Astrophysical constraints on self interactions of dark matter present in literature are valid only for homogeneous distributions ∗ [email protected] of dark matter particles, and are therefore not directly applicable † [email protected] to the mirror matter case. 2 ′ sector does not introduce any new parameters in particle photon-baryon equipartitions zbγ and zbγ. The MRE physics (if we neglect the possible weak non-gravitational occurs at the redshift interactions between visible and hidden sectors). But the Ω Ω h2 fact that microphysics is the same in ordinary and mirror m ≈ · 4 m 1+ zeq = 2.4 10 4 , (3) sectors does not mean that also macroscopic realizations Ωr 1+ x should be the same. The different macrophysics is usu- which is always smaller than the value obtained for an ally parametrized in terms of only two “cosmological” ordinary Universe. The MRD takes place in every sec- free parameters: the ratio x of temperatures of the two tor only after most electrons and protons recombine into sectors, in terms of temperatures of the ordinary and neutral hydrogen and the free electron number density mirror photons in the cosmic background radiation; the diminishes, so that the interaction rate of the photons relative amount β of mirror baryons compared to the or- drops below the Hubble expansion rate. Since T ′ ≃ dinary ones. dec Tdec up to small corrections, we obtain 1/3 S′ T ′ Ω′ ′ ≃ −1 x ≡ ≃ and β ≡ b , (1) 1+ zdec x (1 + zdec) , (4) S T Ωb so that the MRD in the mirror sector occurs earlier than ′ ′ ′ where T (T ), Ωb (Ωb), and S (S ) are respectively in the ordinary one. It has been shown [2, 12] that, com- the ordinary (mirror) photon temperature, cosmologi- paring Eqs. (3) and (4), for x smaller than a typical value cal baryon density (normalized, as usual, to the critical xeq the mirror photons would decouple yet during the ra- density of the Universe), and entropy per comoving vol- diation dominated period, and the evolution of primor- ume [12]. dial perturbations in the linear regime is practically iden- The present energy density contains relativistic (ra- tical to the standard cold dark matter (CDM) case. Also diation) component Ωr, non-relativistic (matter) com- the photon-baryon equipartition happens in the mirror ponent Ωm and the vacuum energy (cosmological term sector earlier than in the ordinary one, according to the or dark energy) density ΩΛ. According to the infla- relation tionary paradigm the Universe should be almost flat, ′ Ω Ωb β β Ωtot = Ωm +Ωr +ΩΛ ≈ 1, which agrees well with the ′ b ≃ 1+ zbγ = ′ 4 =(1+ zbγ) 4 > 1+ zbγ . (5) results on the CMB anisotropy. Now both radiation and Ωγ Ωγ x x matter components contain the mirror components2, and the matter composition of the Universe is expressed in Previous analytical and numerical studies on CMB and general by LSS power spectra [1–6, 22] have only shown, using a qualitative comparison with observations, that: (i) for ′ low values of mirror temperatures (x . 0.3) all the dark Ωm =Ωb +Ω +ΩDM =Ωb(1 + β)+ΩDM , (2) b matter can be made of mirror baryons; (ii) for high values x & . where the term ΩDM includes the contributions of any ( 0 3) mirror baryons can be present as an admixture other possible dark matter particles but mirror baryons. with CDM. At the time of BBN the mirror photons γ′, electrons Now we are finally able to fit the cosmological param- ±′ ′ eters and obtain their quantitative estimates. e and neutrinos νe,µ,τ would give a contribution to the energetic degrees of freedom equivalent to an effective 4 number of extra neutrino families ∆Nν ≃ 6.14 x . Cur- TABLE I. Adopted flat priors for the parameters. rent estimates of ∆Nν [21] correspond to an upper bound parameter lower limit upper limit x . 0.7, and hence at the nucleosynthesis epoch the tem- 2 Ωbh 0.01 0.1 perature of the mirror sector should be smaller than that 2 of the ordinary one, T ′ <T . Ωcdmh 0.01 0.8 x Due to the temperature difference between the two sec- 0.05 0.7 β 0.5 9.0 tors, the cosmological key epochs take place at different 100 θs 0.1 10 redshifts, and in particular they happen in the mirror τ 0.01 0.8 sector before than in the ordinary one [2, 12]. The rel- ns 0.7 1.3 10 evant epochs for the cosmic structure formation are re- ln(10 As) 2.7 4 lated to the matter-radiation equality (MRE) zeq, the ′ matter-radiation decouplings (MRD) zdec and zdec due to the plasma recombinations in both sectors, and the We have modified the publicly available cosmological simulation tools CAMB [23] and CosmoMC [24] in or- der to include the effects of mirror matter. Since the physics of the mirror particles is the same as our parti- 2 Since mirror parity doubles all the ordinary particles, even if cles, we have doubled the equations separately in each they are “dark” (i.e., we are not able to detect them now), what- sector, and considered all the particles when describing ever the form of dark matter made by some exotic ordinary par- the gravitational interactions.