This week I’m going to talk about what I am learning in my studies of Astro Physics and Cosmology. It may get a bit technical but I am hoping some of the more experienced astronomers that read this blog will find these occasional sessions interesting and thought provoking.
Firstly I would like to explain that there are two fundamental principles/observations around which all modern day cosmology is based. The first is Uniformity – The Universe is the same everywhere. The second is The Hubble Law – Everything is moving away from us, with a speed that is proportional to distance.
Is the universe really homogeneous, the same everywhere? Looking locally on the scale of planetary systems, it is most certainly not uniform, the difference in density between the middle of the earth and outer space is extreme to say the least.
Even on scales of thousands of light-years, it is also very non-uniform; you have dense Galaxies in some regions and empty space in others. Galaxies themselves are gathered into groups and clusters, which themselves gathered into super clusters, separated by voids. So even here we are still not uniform!
But on larger scales still, things do appear to smooth out. Super clusters do not themselves seem to cluster into anything bigger. Surveys for things like quasars that probe distances of billions of light-years do find a very uniform distribution.
So, what we should say is that, on really big scales, the Universe does appear to be homogeneous.
The Hubble Law:
If you take a spectrum of anything out in space, you will usually see spectral lines, caused by electrons jumping up and down energy levels and either emitting or absorbing light. Each element has characteristic wave lengths at which it produces lines: for example the very famous H-alpha line of hydrogen occurs at 656.3nm wavelength. In the photos of this article you can see a plot of a star spectrum showing some of the strongest and most common spectral lines.
If you look at distant galaxies, however, all these spectral lines appear to have shifted to longer wavelengths. This effect is called redshift and the redshift of a given object is given as:
where z is the redshift and Δλ is the shift in wavelength (i.e. observed wavelength minus the wavelength λ0 you would get for this spectral line in a laboratory on Earth.
This shift is caused by the doppler effect: all these distant galaxies are moving away from us at a speed v, which can be found from the equation v=cz where c is the speed of light.
Edwin Hubble showed that the recession velocity is proportional to distance: i.e. that v→=H0r→, where H0 is Hubble’s constant and has a value of around 70km s−1Mpc−1 (i.e. a galaxy that is one mega-parsec from us is moving away at a speed of around 70 km/s).
Could this mean that we are in a special place in the Universe and that nothing else wants to be near us and is in actual fact moving away from us? NO – using vectors it is possible to show that you would see exactly the same thing wherever you were in the Universe.
From these two principles then we can see that we do indeed live in a uniform and seemingly endless universe and that wherever you are in the universe, everything seems to be moving away from you with a speed proportional to its distance.
“Two things are infinite: the universe and human stupidity; and I’m not sure about the universe.”
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