# Thread: The Big Bang

1. Here's some stuff from the lecture dealing with the theory of the big bang:

Based on these formulas, we can calculate how many atoms there are in the universe.

This is the formula for density: the density of normal matter in the universe as 5 x 10^-28/a(t)^3 kg m^-3

a typical atom has mass of 2 x 10^-27 kg

And based on that, to calculate roughly how many atoms there are in the universe today per cubic meter, we get 1 atom per 4 cubic meter.

Now my question is how accurate is this considering we don't know how large the universe is or its approximate dimensions? How do we know the estimate coordinates of a galaxy from billions of years ago using the scale factor? I do understand how distances are being calculated by the redshift that we see through the telescope, but I don't see how we can be sure how many atoms there are.

Though assuming it is correct, how do we determine the actual size of the universe that we can see today?  2.

3. is this some bogus formulas? anyone?  4. The density numbers look roughly right. A bit too high possibly. And nearly all atoms are hydrogen, so that "typical" mass might be a bit high.

None of which tells you anything about the size of the universe or the number of atoms. The observable universe has a radius of about 18 billion light years (because that is the furthest distance that light could travel in the life of the universe). So from that you could work out the mass and number of atoms in the observable universe.

All we know about the whole universe is that it is probably much, much bigger. Maybe infinite.

More here: Mass, Size, and Density of the Universe

Also:
WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). This is equivalent to a mass density of 9.9 x 10-30 g/cm3, which is equivalent to only 5.9 protons per cubic meter.
WMAP- Content of the Universe  5. Originally Posted by Strange The density numbers look roughly right. A bit too high possibly. And nearly all atoms are hydrogen, so that "typical" mass might be a bit high.

None of which tells you anything about the size of the universe or the number of atoms. The observable universe has a radius of about 18 billion light years (because that is the furthest distance that light could travel in the life of the universe).
That value seems off by more than a factor of 2. I believe that the currently accepted value for the radius of the observable universe is more like 45 billion light-years, give or take some.  Bookmarks
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