  Recent measurements of type Ia supernovae indicate that distant supernovae
are substantially fainter than expected from the standard flat cold dark matter
model. One possible explanation is that the energy density in our universe is
in fact dominated by a cosmological constant. Another possible solution is that
there are large amounts of grey dust in the intergalactic medium. Dust grains
can be grey either because they are non-spherical or very large. We have
numerically investigated whether grey dust can be emitted from high redshift
galaxies without also emitting standard, reddening dust, which would have been
visible in the spectra of high redshift objects. Our finding is that grain
velocities are almost independent of ellipticity so that if greyness are due to
the grains being elongated, it will not be possible to separate grey dust from
ordinary dust. We also find that velocities are fairly independent of grain
size, but we cannot rule out possible sputtering of small grains, so that
large, grey dust grains could be preferentially emitted. Therefore, our
conclusion is that grey dust is an unlikely explanation of the data, but we
cannot rule it out if the grey dust consists of large, spherical grains.

Astrophysics