Measuring Hubble's constant in our inhomogeneous Universe
Abstract
Recent observations of Cepheids in the Virgo cluster have bolstered the evidence that supports a Hubble constant in the 7090 km s^1 Mpc^1 range. This evidence, by and large, probes the expansion of the Universe within 100 Mpc. We investigate the possibility that the expansion rate within this region is systematically higher than the true expansion rate due to the presence of a local, large underdense region or void. We begin by calculating the expected deviations between the locally measured Hubble constant and the true Hubble constant for a variety of models. The calculations are done using linear perturbation theory and are compared with results from Nbody simulations wherever possible. We also discuss the expected correlations between these deviations and mass fluctuation for the sample volume. We find that the fluctuations are small for the standard cold dark matter as well as mixed dark matter models, but can be substantial in a number of interesting and viable nonstandard scenarios. In particular, we consider models with features in the primordial power spectrum at k~0.05 h Mpc^1. Deviations in the Hubble flow for a region of radius 200 Mpc are small for virtually all reasonable models. Therefore methods based on supernovae or the SunyaevZel'dovich effect, which can probe 200Mpc scales, will be essential in determining the true Hubble constant. We discuss, in detail, the fluctuations induced in the cosmic background radiation by voids at the lastscattering surface. In addition, we discuss the dipole and quadrupole fluctuations that one would expect if the void enclosing us is aspherical or if we lie offcentre.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 July 1996
 DOI:
 10.1093/mnras/281.2.565
 arXiv:
 arXiv:astroph/9506120
 Bibcode:
 1996MNRAS.281..565S
 Keywords:

 COSMIC MICROWAVE BACKGROUND;
 COSMOLOGY: MISCELLANEOUS;
 DISTANCE SCALE;
 LARGESCALE STRUCTURE OF UNIVERSE;
 Astrophysics
 EPrint:
 20 pages (58K), 8 Postscript figures (111K compressed)