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Cosmology :Lecture 16

Cosmology

Cosmolgy is the study of the Universe as a whole and everything in it! Rather an extensive definition. Commonly this really means the study of the large scale structure of the universe. That is the big picture. The building blocks of the Universe as best though of as galaxies. So we shall being with a discusion of these.

Galaxies

Stars are not randomly of evenly spread through the Universe. Instead they are clustered in Galaxies. With modern telescopes millions of galaxies can be seen. Most are objects with a very small angular size indicating their extreme distance from us. Some however are rather large - the Andromeda galaxy has a huge size - 4 times that of the moon, however it is rather a faint object which only takes on its true appearance using a very large telescope and/or a large time exposure.

All galaxies are made up of a very large number of stars however they come in a variety of sizes and shapes. A classification system splits these into four categories. This, the hubble classification scheme, gives four classes

  1. Spiral Galaxy
  2. barred spiral galaxies
  3. elliptical
  4. irregular
This original scheme does have some problems - sometimes it can be difficult to distinguish between spiral and barred spiral. Some facts regarding the types are
Spiral and Barred Spiral Elliptical irregular
percentage 77% 20% 3%
mass (solar masses) 109 to 4x10 11 105 to 10 13 108 to 3x10 10
dimeter (kilo-parsecs) 5 to 250 1 to 200 1 to 10

Some pictures of Galaxies colliding

Our Galaxy

Clusters of Galaxies

Some images of clusters

Hubble's Law

Edwin Hubble was one of the pioneers of Cosmology. He was the astronomer who proved the galaxies were actually very distant (and hence very large!) objects outside our own galaxy. He then made studies of galaxies and came to two conclusions

1) Firstly, the light from galaxies is often colour-shifted. Some galaxies are blue-shifted but most are red shifted. This colour-shifting, Hubble interpreted as meaning that the galaxies are actualy moving away from us at high speeds.

2) The amount of red-shift or rate or recession is proportional to the distance the galaxy is from us. This is known as Hubble's law. In equation form

v = H0 d
where v is the velocity of recession and d is the distance from us. The H0 is a constant which is know as Hubble's constant.

Hubble's observations remain one of the key fact regarding the Universe however they are not completely clear cut. The data is actually rather poor. This is illustrated by the difficulty in measuring Hubble's constant accurately. Hubble gave a value

H0 = 500 km/s/Mpc
However the modern value is radically different from this - is has changed during my scientific career. There is still much debate but the best current value is

H0 = 75 km/s/Mpc
Despite the poor data we must still regard hubble's law as a FACT about the Universe. As we shall see it can be INTERPRETED as indicating an expanding Universe.
We shall now return to ``chew-over'' Hubble's law before advancing to its interpretation.

Distances and Standard Candles

One of the most difficult problems in Cosmology is in determining the distance of objects which are (very) far away. For nearbye stars the method of parallax can determine the distance, however for objects at Cosmological distance the parallax effect is just too small.

One of the most useful methods of determining distance is to use a standard candle . A Standard candle is an object for which we know, with certainty, its absolute magnitude. Recall that an objects absolute magnitude, apparant magnitude and distance are all related. For stars we used the apparant magnitude and distance to determine the absolute magnitude. In this case we use the apparant magnitude and adsolute magnitude to determine distance.
Absolute Magnitude Distance (parsecs) Apparant Magnitude
0.0 10 0.0
0.0 100 +5.0
0.0 1000 +10.0
0.0 10,000 +15.0
0.0 100,000 +20.0
0.0 1,000,000=1Mpar +25.0
For example, if we KNOW an object has absolute magnitude 0.0 and we OBSERVE it to have an apparant magnitude of +20 then it must be 100,000 parsecs distant.

Examples of standard candles include

© Dave Dunbar 2020