Bose-Einstein-condensed scalar field dark matter and the gravitational wave background from inflation: New cosmological constraints and its detectability by LIGO

Abstract

We consider an alternative to WIMP cold dark matter (CDM), ultralight bosonic dark matter (m 10 −22 eV/c 2 ) described by a complex scalar field (SFDM) with a global U (1) symmetry, for which the comoving particle number density, or charge density, is conserved after particle production during standard reheating. We allow for a repulsive self-interaction. In a ΛSFDM universe, SFDM starts relativistic, evolving from stiff (w = 1) to radiation-like (w = 1/3), before becoming nonrelativistic at late times (w = 0). Thus, before the familiar radiation-dominated era, there is an earlier era of stiff-SFDM-domination. During both the stiff-SFDM-dominated and radiation-dominated eras, the expansion rate is higher than in ΛCDM. SFDM particle mass m and quartic self-interaction coupling strength λ, are therefore constrained by cosmological observables, particularly N eff , the effective number of neutrino species during BBN, and zeq, the redshift of matter-radiation equality. Furthermore, since the stochastic gravitational wave background (SGWB) from inflation is amplified during the stiff-SFDM-dominated era, it can contribute a radiationlike component large enough to affect these observables, by further boosting the expansion rate after the stiff era ends. Remarkably, this same amplification makes detection of the SGWB possible at high frequencies by current laser interferometer experiments, e.g., aLIGO/Virgo and LISA. For SFDM particle parameters that satisfy these cosmological constraints, the amplified SGWB is detectable by LIGO for a broad range of reheat temperatures T reheat , for values of the tensor-to-scalar ratio r currently allowed by CMB polarization measurements. For a given r and λ/(mc 2 ) 2 , the marginally-allowed ΛSFDM model for each T reheat has the smallest m that satisfies the cosmological constraints, and maximizes the present SGWB energy density for that T reheat . This SGWB is then maximally detectable for values of T reheat for which modes that reenter the horizon when reheating ended have frequencies today that lie within the LIGO sensitive band. For example, for the family of marginally-allowed models with r = 0.01 and λ/(mc 2 ) 2 = 10 −18 eV −1 cm 3 , the maximally detectable ΛSFDM model has T reheat ≃ 2 × 10 4 GeV and m ≃ 1.6 × 10 −19 eV/c 2 , for which we predict an aLIGO O1 run detection with signal-to-noise ratio of ∼ 10. We show that the null detection of the SGWB recently reported by the aLIGO O1 run excludes the parameter range 8.75 × 10 3 T reheat (GeV) 1.7 × 10 5 for this illustrative family at 95% confidence, thereby demonstrating that GW detection experiments can already place a new kind of cosmological constraint on SFDM. A wider range of SFDM parameters and reheat temperatures should be accessible to aLIGO/Virgo O5, with the potential to detect this unique signature of the ΛSFDM model. For this same illustrative family, for example, a 3σ detection is predicted for 600 T reheat (GeV) 10 7 .

Supplying dark energy from scalar field dark matter

“…which follows from the definition of scalar field energy-momentum tensor (3) in the Schwarzschild background space-time (6), indeed obeys the NFW behavior. After this simple estimations of the scalar BEC galactic DM parameter, which appears to be similar with obtained in the papers [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], let us consider the larger scale, r d. For these distances galaxies can be imagined as the stationary spherical structures of the DM condensate (22) with some scalar 'tails' in outer galactic region, where the density of DM particle is very low,…”

“…All mentioned small-scale structure anomalies can in principle be resolved if the DM is made up of ultra-light bosons that form a Bose-Einstein Condensate (BEC), i.e. a single coherent macroscopic wave function with long range correlation [8][9][10][11][12][13][14][15][16][17][18][19]. Such kinds of bosons are usually modeled by a scalar field either with or without self-interaction.…”

Cosmological implications of ultralight axionlike fields

“…A qualitatively similar stiff dilution phase occurs in complex scalar-field dark-matter models, with the relevant phenomenology discussed in Refs [40,41]…”

Constraints on anharmonic corrections of fuzzy dark matter

“…It is more difficult to find particle-physics models in this case [46], but the model is nonetheless well motivated as the simplest modification leading to a stable potential. This modification has been previously analyzed in some works [47][48][49][50][51][52]. The additional source of pressure from the repulsive self-interactions helps to solve the 'corecusp' problem with larger masses [46].…”