Expert: Philip A. Stahl - 1/31/2008

Question
QUESTION: How do scientists come up with a number of atoms in the universe of 10 to the 80th power?

ANSWER: Hello,

More technically, it is the number of *particles* in the *observable* universe (since one distinguished atoms from ions, and most of the universe - nearly 99% is in the form of ions, or ionized atoms. That is, those atoms that have lost one or more electrons)

Anyway, to make the estimate we use the relation:

# particles ~   8 pi (rho) c^3/ 3 m_p (H)^3

where:

rho = the current mass density of the universe

c = velocity of light (3 x 10^8 m/s)

m_p = proton mass= 1.67 x 10^-27 kg

H = Hubble's constant, or rather the currently estimated value of Hubble's constant.

Hope this helps.

---------- FOLLOW-UP ----------

QUESTION: Where do they get rho?  Thanks, Ford

Answer
Hello,

The estimate of the currently observed cosmological density is a highly complex one and beyond the scope of a detailed answer in this forum.

Basically, what cosmologists have to do is make the best assay they can of all the observable mass-energy that's visible. This will include: galaxies, bright and dark nebulae as well as intergalactic matter, interstellar matter ...plus estimates for the contributions of dark matter, and dark energy.

Currently we know, for example, that the proportions of ordinary matter, dark matter and dark energy are as follows:


7% - ordinary visible matter

93% - dark component, of which:

- 70% is DARK (vacuum) energy and
- 23% is dark matter


Once this is obtained, or more accurately estimated, then the volume is estimated  - also based on observations and assuming a closed spherical space. Then the density can be estimated.

For more details, try to get hold of Michael Rowan -Robinson's book, 'Ripples in the Cosmos'. I seem to recall he has a chapter toward the end about the 'cosmic density parameter' and I believe this also contains understandable details of how the observable density is  arrived at.