Cosmology
By far the most popular cosmological theory nowadays is the
Big Bang Theory. An alternative, the
Steady State Theory still has a few adherents.
Both theories encompass an expanding Universe
The work of Hubble first uncovered this phenomenon. He found that the further away a galaxy
is,
the faster it is receding from us.
This is expressed from Hubble's Law of 1929.
v = H r
where v is velocity, H is Hubble's Constant, r is distance
The value of Hubble's Constant is the source of great current debate. Its value is of
extreme importance for predicting the rate at which the Universe is expanding.
Its value is usually quoted to be somewhere between 45 and 85 km per sec per Megaparsec.
The Balloon analogy is a common analogy used to explain to try and explain the Expansion of the Universe. We should be wary of taking it too far but it does have the following advantages
The Universe is represented by the surface
of the balloon. As it expands, the surface expands in a similar way to how the Universe
is assumed to expand.
The important thing to put across is that it is
the Universe itself that is expanding, i.e. space
itself is
expanding, it is not expanding into a pre-existing space.
The surface of the balloon represents the entire Universe.
You could imagine an ant on the surface of the
balloon which has no
knowledge of any dimension other than the surface of the balloon, if that is any
use to you.
Calculations of Hubble's Constant give an indication of the rate of expansion
of the Universe, as well as its deceleration). A given rate of deceleration requires a
given density for the Universe in
order to provide the required gravitational force. Currently there is a great
discrepancy between the amount of observable matter and the amount of matter required by
cosmological theories. To try and explain this, enter the theory of dark matter. Although this
is often reported as requiring exotic new particles to resolve the problem, more conventional
material also counts as 'dark matter', e.g. old stars, or neutrinos with mass.
I ought to point out, in view of misconceptions commonly encountered, dark matter
cannot be detected by visual methods
but its effect can be detected gravitationally, not just its effect on the expansion of the Universe
but also its effect on galactic rotation, for example. Dark matter does not relate
to objects like dark clouds or such like objects that can be detected by visual means.
This is the principle on which the common Robinson-Walker-Friedman models are based.
It has the following two requirements.
Two riders need to be mentioned immediately. First, the observation of matter needs to be
carried out over sufficiently large distances - obviously closer to us, matter is not distributed
evenly across the sky. Secondly, we need to take into account our motion through the Universe
which will disturb our observations. For example, the detection of Background Radiation
which is such
an important part of the theory, will be Doppler shifted because of our own motion, but when this is
taken into account, this Background Radiation appears to be coming from all directions equally.
It should be noted that this is not the same as the Perfect Cosmological Principle which underlies
the Steady State Theory. Another condition would need to be added to the above to
produce the Perfect Cosmological Principle.
Introduction
Expanding Universe
Balloon Analogy
Rate of Expansion
Cosmological Principle