Energy Density

Hi -- I'm struggling with the last question on my homework; I've sent
an email to the professor, but I thought I would ask here, too.
Here's a question posed: Calculate the energy density in today's
Universe of matter, the cosmological constant, and CMB radiation.
Assume H_0 - 70 km/s/Mpc, Omega_matter = .3, Omega_Lambda = .7,
Universe = 0 curvature, and CMB temperature is T = 2.7K.

I can't find any equation anywhere in my book for "energy density". I
can find equations for critical density of the universe, mass density
of radiation, and even mass density parament equations, but not this
one. Does anyone have an idea for how I can start on this problem?
I'm not asking for anyone to do my homework, but any help on how to
get started on it would be appreciated.

"Kurt Messick" wrote in message
...
Hi -- I'm struggling with the last question on my homework; I've sent
an email to the professor, but I thought I would ask here, too.
Here's a question posed: Calculate the energy density in today's
Universe of matter, the cosmological constant, and CMB radiation.
Assume H_0 - 70 km/s/Mpc, Omega_matter = .3, Omega_Lambda = .7,
Universe = 0 curvature, and CMB temperature is T = 2.7K.

I can't find any equation anywhere in my book for "energy density". I
can find equations for critical density of the universe, mass density
of radiation, and even mass density parament equations, but not this
one. Does anyone have an idea for how I can start on this problem?
I'm not asking for anyone to do my homework, but any help on how to
get started on it would be appreciated.

You have mass density of radiation and E = mc^2.
Damn, now I've done your homework for you.

In article ,
Kurt Messick writes:
Calculate the energy density in today's
Universe of matter, the cosmological constant, and CMB radiation.
Assume H_0 - 70 km/s/Mpc, Omega_matter = .3, Omega_Lambda = .7,
Universe = 0 curvature, and CMB temperature is T = 2.7K.

I can't find any equation anywhere in my book for "energy density".

That's surprising. Is there a section on "blackbody radiation?" If
not, you'll have to try a different book or a web search. The energy
density of blackbody radiation depends only on temperature, so once
you find the equation, the CMB part should be easy.

I can find equations for critical density of the universe...

The "energy density of matter" is presumably based on E=mc^2, though
I suppose your instructor could have had something else in mind. If
you know the critical density and the definitions of all the
quantities given in the "assume" part of the question, you should be
able to work out the matter density and the equivalent energy
density. Similarly for the cosmological constant.

... mass density of radiation, and even mass density parament
equations

I'm not at all sure what these are. Maybe you should check that part
of the textbook again, though I don't think you need them for this
question.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA