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Abstract:

ABSTRACT In chameleon f(R)-gravity, the fifth force will lead to ‘upturns’ in galaxy rotation curves near the screening radius. The location of the upturn depends on the cosmic background value of the scalar field $\bar{f}_\mathrm{R0}$, as well as the mass, size, and environment of the galaxy. We search for this signature of modified gravity in the SPARC sample of measured rotation curves, using an MCMC technique to derive constraints on $\bar{f}_\mathrm{R0}$. Assuming NFW dark matter haloes and with $\bar{f}_\mathrm{R0}$ freely varying for each galaxy, most galaxies prefer f(R) gravity to ΛCDM, but there is a large spread of inferred $\bar{f}_\mathrm{R0}$ values, inconsistent with a single global value. Requiring instead a consistent $\bar{f}_\mathrm{R0}$ value for the whole sample, models with $\log _{10}|\bar{f}_\mathrm{R0}|\gt -6.1$ are excluded. On the other hand, models in the range $-7.5\lt \log _{10}|\bar{f}_\mathrm{R0}|\lt -6.5$ seem to be favoured with respect to ΛCDM, with a significant peak at −7. However, this signal is largely a result of galaxies for which the f(R) signal is degenerate with the core/cusp problem, and when the NFW profile is replaced with a cored halo profile, ΛCDM gives better fits than any given f(R) model. Thus, we find no convincing evidence of f(R) gravity down to the level of $|\bar{f}_\mathrm{R0}|\sim 6 \times 10^{-8}$, with the caveat that if cored halo density profiles cannot ultimately be explained within ΛCDM, a screened modified gravity theory could possibly provide an alternative solution for the core/cusp problem. However, the f(R) models studied here fall short of achieving this.

Abstract:

The α-attractor inflationary models are nowadays favored by CMB Planck observations. Their similarity with canonical quintessence models motivates the exploration of a common framework that explains both inflation and dark energy. We study the expected constraints that next-generation cosmological experiments will be able to impose for the dark energy α-attractor model. We systematically account for the constraining power of SNIa from WFIRST, BAO from DESI and WFIRST, galaxy clustering and shear from LSST and Stage-4 CMB experiments. We assume a tensor-to-scalar ratio, 10−3 < r < 10−2, which permits to explore the wide regime sufficiently close, but distinct, to a cosmological constant, without need of fine tunning the initial value of the field. We find that the combination S4CMB + LSST + SNIa will achieve the best results, improving the FoM by almost an order of magnitude; respect to the S4CMB + BAO + SNIa case. We find this is also true for the FoM of the w0 − wa parameters. Therefore, future surveys will be uniquely able to probe models connecting early and late cosmic acceleration.

Abstract:

Distinguishing the photon output of an accreting supermassive black hole binary system from that of a single supermassive black hole accreting at the same rate is intrinsically difficult because the majority of the light emerges from near the innermost stable orbits of the black holes. However, there are two possible signals that can distinctively mark binaries, both arising from the gap formed in circumbinary accretion flows inside approximately twice the binary separation. One of these is a "notch" cut into the thermal spectra of these systems in the IR/optical/UV, the other a periodically varying excess hard X-ray luminosity whose period is of order the binary orbital period. Using data from detailed galaxy evolution simulations, we estimate the distribution function in mass, mass ratio, and accretion rate for accreting supermassive binary black holes (SMBBHs) as a function of redshift and then transform this distribution function into predicted source counts for these two potential signals. At flux levels >~10−13 erg cm−2 s−1, there may be ~O(102) such systems in the sky, mostly in the redshift range 0.5 <~ z <~ 1. Roughly 10% should have periods short enough (<~5 yr) to detect the X-ray modulation; this is also the period range accessible to Pulsar Timing Array observations.