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
- Center. All Universe at center, Big Bang happened everywhere.
- Edge of Universe
- Wht Universe looking bigger as we look back, when the Universe was smaller in past
- Galaxies are not actually moving through space but being carried along with it
- Further away a galaxy is the ffaster it is moving
- All galaxies will see other galaxies moving away from them.
- We see a magnified view of the past.
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.
Rate of Expansion
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.
- Isotropy This is the weakest requirement in so far as it can be more easily
tested by observation. Isotropy requires that there is no preferred direction in space, i.e.
that which ever way we look in the sky, it looks the same. In other words it requires spherical
symmetry around us.
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.
- Homogeneity This requires that the Universe is the same everywhere. In other words, other worlds will view the Universe to be isotopic also.
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.
As mentioned already, the Big Bang Theory is not the only theory to predict an expanding Universe.
The Big Bang is distinctive in predicting a hot beginning to the Universe. The actual beginning will be followed by a period of maybe 100,000 years when the Universe was so hot that matter and radiation freely interacted, and the Universe was consequently opaque.
As the Universe cooled, a certain temperature was reached when matter and radiation became uncoupled. The Universe started to become transparent because light could travel thru matter without interacting. All other things being equal, this radiation, no longer interacting with matter, would stay at the same temperature - but in fact it was red-shifted as the Universe expanded, reducing its temperature.
Cosmic Microwave Background Radiation
Since this background radiation is assumed to be Black Body Radiation, and since Black Body Radiation is characteristic of a particular temperature, such radiation can be determined to be characteristic of a particular temperature, and then extrapolations back in time can estimate the temperature at the time of decoupling.
The microwave background radiation was found (by accident) by Arno Penzias and Robert Wilson. The existence of this radiation is a big blow for the Steady State Theory.
15 billion years