[neutralino]

In this thread I intend to present some of the observational evidence for, firstly, the expansion of the universe, and secondly why we think the universe is accelerating in its expansion today. I will make use of many of the references references that I posted in References and Sources of Info on Standard Cosmology and will try and reference things correctly.

I also invite questions and discussion, but please let's try and avoid religious and/or philosophical comments.

So, let's begin.

In the mid 1920's, Edwin Hubble [1] was observing a group that he called "spiral nebulae," but which we now know were distant sprial galaxies, at Mount Wilson Observatory. These galaxies contained a very special type of stars called Cepheid Variables. Cepheid variables are stars slightly larger than our sun which are growing old to become Red Giants; however they do this in a special way. Heat is trying to escape from the core but it builds up inside the star. This pushes the outer layers up, so the radius of the star increases by a small amount. With a greater surface area, this star now looks brighter. However, due to the fact that the surface area is now greater, it means that the star can radiate this extra heat energy off into space, which in turn allows the radius to return to normal, and so the star's brightness returns to normal. This is a cyclic process, and has period of a few days. In the early 1900's, Henrietta Swan Leavitt [2] had shown that the intrinsic brightness of a Cepheid Variable is related to the period of it's light output variation. So, if you find a Cepheid Variable with a period of, say, 10 days, then it will have the same intrinsic brightness as any other Cepheid Variable with a period of 10 days. If some appear to be dimmer than others, it means that they are further away. (See [3] for a more scientific explanation of Cepehid Variables)

Another thing we need to know is how to measure velocities of distant galaxies. This is done using redshift. See reference [4] for more information.

So, using Cepheid Variables as standard candles to measure distance, and using redshift to measure velocity of distant galaxies, Hubble published a claim [5] that the radial velocities of galaxies are proportional to their distances; i.e. , where is the velocity of the galaxy, is the displacement of the galaxy from the earth, and H is the so-called "Hubble constant" (which is a constant in space but not in time).

Here is the graph that he plotted (from [5]).


So, we can see that this figure confirms Hubble's claim of the proportional relationship between displacement and velocity. Now, one may think by looking at that graph that Hubble has just decided to draw a straight line through some points that look like there is some sort of relationship, but one that's not precisely linear: this was probably because Hubble knew what he was looking for! However, later work has shown that Hubble confused two different types of Cepheid Variables, and hence had a few errors in there. More recent supernovae data has confirmed Hubble's law in this range of redshifts more dramatically.



So, what does this all mean? Well, since Hubble has shown that, for a range of different galaxies, the velocity of recession is proportional to the distance, then we can say that every galaxy is moving away from us. Since we believe in the cosmological principle [6], we know that we cannot be in a special place in the universe, and so we can say that every galaxy is moving away from every other galaxy. This implies that the universe must be expanding.

Is this the end of the story? Is Hubble's law entirely correct? In my next post, we'll take a look at why we now think the universe is accelerating in its expansion.


[1] http://www.time.com/time/time100/sci...le/hubble.html
[2] http://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt
[3] http://map.gsfc.nasa.gov/m_uni/uni_101expand.html
[4] http://www.astro.ucla.edu/~wright/doppler.htm
[5] http://antwrp.gsfc.nasa.gov/diamond_.../hub_1929.html
[6] http://csep10.phys.utk.edu/astr162/l...mology/cp.html