The Role of EM Radiation in Cosmology Today

[giann_tee]

Can you tell me the role of electromagnetic radiation in cosmology today?

1. how much energy is in the form of light?
2. what is the spectrum of the majority of galactic clusters?
3. does the EM radiation affect the expansion of the universe or the creation of structure?

 

[Theinvoker]

Can you tell me the role of electromagnetic radiation in cosmology today?

1. how much energy is in the form of light?
2. what is the spectrum of the majority of galactic clusters?
3. does the EM radiation affect the expansion of the universe or the creation of structure?

Not sure about 2 off the top of my head but;

1. Approx 10^-5 %
3. No, but it did at much earlier epochs (assuming the standard model)

 

[giann_tee]

Interesting. I just finished watching the Horizon on BBC
http://www.youtube.com/user/turxxx
"Is Everything We Know About the Universe Wrong?".
I rarely watch cosmology these days. The show was pretty much:
inflation + dark matter + dark energy + dark flow. No Penrose hypothesis there yet. I find the show too strange since it did not step into the topic of interactions. The fundamental forces create structures at different scales. The scale of the early universe was small. That could reflect on the design of the inflation theory.

I hate to hypothesize here, but no importance is given to the EM radiation. There is so much invisible content(s) that is supposed to exist in the universe. Yet, the visible light represents the greatest portion of all EM radiation. When I eliminate the dark matter and energy from my thinking, I remain with EM light: that is to say, I imagine that the EM interaction could be essentially different in some way and explain one or another puzzle.

 

[Theinvoker]

The problem is, at the current time EM radiation only makes up a tiny portion of the universe. It was important in the radiation dominated era (in the standard model) when the universe was small, but it scales as 1/a^4 so drops off very quickly in terms of significance, so in terms of the BB theory, yes it has been accounted for when the universe was small.

The energy density of radiation in the universe is simply too low to explain the mysteries such as dark matter and energy, along with a vast array of other reasons :)

 

[marcus]

...It was important in the radiation dominated era (in the standard model) when the universe was small, but it scales as 1/a^4 so drops off very quickly in terms of significance, so in terms of the BB theory, yes it has been accounted for when the universe was small...

Good point about scaling as an inverse power of size of universe, or of the scale factor a.
because the number of photons per volume goes down as 1/a^3, and also the energy of each photon goes down as 1/a because its wavelength expands with the scalefactor a, and longer wavelength means less energy.

For what its worth here is an attempt to make an "inventory" of the mass-energy of the U. Some of the numbers look right to me and others seem ballpark OK but I don't know how precise or reliable. So I would not quote some of them as authoritative, but they give an idea.
http://www.phy.duke.edu/~kolena/matterinventory.html

EDIT: Theinv. I adjusted this post in accordance with your edit in the preceding. No need to perpetuate the typo. :)

 

[Theinvoker]

The energy density of radiation actually scales as 1/a^4 (not as 1/a^3)

Well that was a schoolboy error! I guess that's the beauty of the internet though, I can still go into work tomorrow without having to hide under my desk until the embaressment passes lol.

I will edit my previous post just incase someone reads it but not the reply :)

 

[giann_tee]

Good point about scaling as an inverse power of size of universe, or of the scale factor a.
because the number of photons per volume goes down as 1/a^3, and also the energy of each photon goes down as 1/a because its wavelength expands with the scalefactor a, and longer wavelength means less energy.

Greetings. Are you talking about the change of wavelength towards red AS the universe is expanding or just the number of photons being "diluted" in the vastness of space?

These are quite interesting relationships. My idea was that the flux of radiation drops with the distance squared. The impact of light is too small then, at this point in history.

For what its worth here is an attempt to make an "inventory" of the mass-energy of the U. Some of the numbers look right to me and others seem ballpark OK but I don't know how precise or reliable. So I would not quote some of them as authoritative, but they give an idea.
http://www.phy.duke.edu/~kolena/matterinventory.html

Amazing chart. The dark matter is mostly located in the voids between something and the dark energy can be anywhere. :-)

I believe more in miniature black holes than in non-baryonic dark matter, although the bullet cluster argument was convincing indeed.

The black holes could be visible, but then again, if the light can bend around them they could be invisible precisely because of that.

 

[Theinvoker]

Greetings. Are you talking about the change of wavelength towards red AS the universe is expanding or just the number of photons being "diluted" in the vastness of space?

It is both, that's why its 1/a^4. As the universe expands (or contracts if you are looking that scenario) it is affected by the 3 spatial dimensions and by wavelength change due to the constant speed of light :)