The Most Distant Objects
The most distant objects presently being observed using optical
telescopes have redshifts greater than 5.
From the
table shown in the previous section, this
corresponds to a recessional velocity in excess
of 95 percent of light speed, a
present distance
of about 18 billion light years, and a look-back time of around 10 billion years.
Thus, the light that we presently see from these objects
began its journey to
us some 10 billion years ago. Since that is not far below many estimates for the
age of the
Universe, this light is a kind of "fossil record" of the
Universe not long after its birth! For example, our previous table indicates that a redshift of greater than
5 corresponds to a time that was
less than 7 percent of the present age of the Universe when the light that we see today
was emitted from its source.
The observation of very distant
objects is in a very real sense equivalent to looking backwards in time.
Distant Galaxies
Until recently, the most distant objects that had been observed
were quasars (we shall discuss quasars in the next chapter).
Now however, examination of the Hubble Deep Field
has revealed galaxies that may be further away than the most distant quasars.
The image above right
shows what may be one of the most distant objects
yet observed. It is the faint red smudge at the tip of the
arrow.
If the indirect method of estimating
their distance is reliable, at least six galaxies (including the one in the adjacent image) may be
so far away that we are seeing them when the Universe was less than 1 billion years old. If so, this
implies that the formation of galaxies started relatively soon after the big bang.
A Technique for Identifying Distant Galaxies
The distant galaxy identification
technique used here relies on the Hubble
Law that more distant galaxies will be more redshifted.
By looking at distant galaxies using different
filters to emphasize light of different wavelengths, and then comparing the results for unknown galaxies
with a control group of galaxies for which the distance is already known, it is argued that statistically
one can determine distances to the most distant galaxies. By statistical, we mean that there could be
error in the determination for any one galaxy, but for a set of galaxies the results will be reliable, on
average.
The technique is illustrated in the following figure.
Here four filters have been used on the Hubble Space Telescope to emphasize progressively shorter
wavelength light from right to left in the four images. The galaxy indicated by the arrow is only
seen easily in the near IR region of the spectrum, indicating that it has a very large redshift and
therefore is distant.