# Energy in Universe

1. Dec 18, 2008

### chis

Do you guys how much mass energy is known to be in the Universe?

2. Dec 18, 2008

### mgb_phys

The size of the universe depends on your cosmology model.

The simplest and most accurate figure is for the 'observable universe'.
There are around 10^11 galaxies with 10^11 stars in each, a star has a mass of around 10^30kg. Stars make up about 5% of the mass of the universe according to galaxy dynamics measurements.

so 10^11 * 10^11 * 10^30kg * c^2 * 20 = 10^70 J

3. Dec 18, 2008

### chis

Wey hey how many zeros is that.... no in fact what is that! Yes I know nowt, thanks for your patience.

4. Dec 18, 2008

### chis

Jeeze you guy are on it. So the universe has only filled up 5% with known matter?

5. Dec 18, 2008

### marcus

there is another way to approach this

mass-energy could be either baryonic matter, which is estimated to be 4% of total energy density

or you could be asking about all matter including dark matter, that comes to 26% of total.

(Maybe you don't want to include dark energy, which makes up the remaining 74%.)

The total energy density is known rather precisely to be around 0.85 nanojoules per cubic meter.

So if you want the mass-energy due to all forms of matter, you take 26% of that
which comes to about 0.22 nJ per cubic meter. (If you only want ordinary baryonic, make that 4%.)

Now you just need to know the radius of the observable universe---the most recent estimate of that, of which I'm aware, is 46 billion LY. And you need the volume of a sphere with that radius, but in cubic meters. So you can multiply by the 0.22 nanojoule figure.

Google calculator did the whole thing in one step. I put in
(4 pi/3)(46*10^9 lightyears)^3 *0.22 nJ/m^3
and it gave me back something like 8 x 10^70 joules. this is comparable to what MGB said!

Actually when I put that in the window and pressed return it came back with
((4 * pi) / 3) * ((46 * ((10^9) lightyears))^3) * (0.22 (nJ / (m^3))) = 7.59505577 × 10^70 joules

Nice thing is it takes care of converting units by itself so you don't have to worry about mixing lightyears and cubic meters.

Since that 7.6 jillion includes dark matter (but not dark energy) if you want to compare with MGB's estimate, which I think was for ordinary baryonic matter, you need to scale down by a factor of 7 or so. It comes to around 10^70 joules.

Last edited: Dec 18, 2008