This expansion of space time has an important
consequence. It leads to so called cosmological
red shift, to be distinguished from the red
shift caused by the Doppler Effect or gravitational
red shift when the light is trying to escape
from a massive body.
There are three types of red shift. Let me
explain that. Going back to our two-dimensional
universe, the surface of the sphere, we have
an expanding in this case here, two-dimensional
universe. Say, we've hit a wave of certain
wave length, here, when the universe is this
big.
As the space stretches, the wave itself will
be stretched. What has happened, the same
wave, but as a result of the expansion of
space, its wave length, that is the distance
from one crest to another, has increased.
This type of red shift is what we call cosmological
red shift.
Once again, there are three types of red shifts,
three different causes. One could be, because
the space is not expanding, but the source
of light is moving away from us. The second
type of red shift is resulting when light,
the photons, are trying to escape the gravitational
attraction of the body.
To do that, they are losing energy and as
they lose energy, their wave length increases.
That's called the gravitational red shift.
The third type of red shift is cosmological
red shift, which results from the expansion
of space.
The red shift in spectrum of the galaxies
that is observed, is actually of this nature,
it's cosmological red shift. Although Schliffer
initially interpreted the data, assuming that
the red shift results from doppler shift.
What is happening actually is the space itself
is expanding and that is causing the red shift.
So it's a cosmological red shift. Turns out
that the formula for the amount of red shift
and the rate of the expansion of space, is
basically similar or the same in these cases.
