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Energy Budget of the Universe November 15, 2008

Posted by keithkchan in Cosmology.

Mr David is lazy and I don’t have new things to say, but I have to say something, so I decide to talk about my own research interest, cosmology. So I will start a series of posts introducing the basic cosmology.
Now we know that the universe consists of 5% of ordinary matter, 25% of dark matter and 70% of dark energy. These contents can be inferred from the Cosmic Microwave Background. The ordinary matter is just the matter you and I made of. The amount of ordinary matter also agree with the Big Bang Nucleosynthesis. The dark matter is more mysterious because we only “see” it through gravity. In many different occasions, we require the existence of dark matter. The most famous evidence is the rotation curve in the galaxies. According to Kepler’s law, the orbital velocity should drop as r^{-1/2}, but observation shows that the rotation curve flattens. To accommodate this fact, one need to introduce dark matter with density distributes as 1/r in a galaxy. There are some dark matter candidates motivated by high energy physics, e.g. axion, WIMP. Some experiments are dedicated to detecting these elusive particles, although they have not been found anything convincingly so far.
The remaining component is even more mysterious. The simplest candidate is the plain cosmological constant. So far the simple cosmological constant is still in agreement with observation. The cosmological constant may be just interpreted as a constant in the Einstein-Hilbert action. From general relativity, we have no clue how large or small it is. If we interpret it as the vacuum fluctuations, and integrate all the ground state energy up to some cut-off scale, a natural scale is the Planck scale, one will get a huge number, which is 120 orders of magnitude bigger than the observed value. The cosmological constant problem is one of the most embarrassing problem in theoretical physics. There is still no satisfactory solution to this problem. Some people argue that the cosmological constant from vacuum fluctuation vanishes for some reason, and the observed cosmological constant is due to some other reasons. Although cosmological constant is still in pretty good agreement with observations, researches are looking at other more interesting alternatives. I will talk about other more interesting possibilities later.



1. lukabrazi - November 17, 2008

hmmm, very good article, looking forward for the next one 🙂
by the way, I added your blog to mine at http://samecniero.net 😉

2. the econophysicist - November 18, 2008

the universe has been investing too much in mortage backed securities and its dark matter index will soon fall from 70% to 30% or so. This is all a big bubble anyway.

3. keithkchan - November 23, 2008

Thanks for adding our blog to your blog, and we just set up the blogroll and added yours to ours. By the way, what exactly is your blog’s name in English? Do you work on LHC?

4. lukabrazi - November 27, 2008

Well, nice to hear that our blogs are friends now 🙂 “samecniero” in English means “scientific”, as we (fizikosi and I) post there articles about science.
No, I don’t, I’m studying medicine, but I love physics other natural sciences. so I love CERN and things like that; That’s why I’ve got CERN avatar 🙂 and wish to get there, but thats very unlikely unfortunately.

5. David - November 27, 2008


Thanks. You guys are doing a good job on your blog- unfortunately theres almost no more science in georgia and it’s always pleasant to meet people still interested in physics especially from other areas.

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