What is the Relative Concentration of Helium at 3 Minutes After the Big Bang?

[DEANS2]

Homework Statement

the relative concentration by mass of helium at 3 minutes after the big bang is 2-5 parts per 100,000. If you had 1 kilogram of the normal matter from the universe at a time 3 minutes after the big bang, how much of it would be normal helium nuclei?

Homework Equations

The Attempt at a Solution

I have no idea.can someone help?

 

[mfb]

If 2% of all apples are bad, and you have 10000 apples, how many apples are bad? The problem is as easy as this.

 

[DEANS2]

your mean is 1kg x 2x10^-5？but the answer is 250g. I only have the answer. I don't know how to get it

 

[mfb]

250 g sounds reasonable in terms of cosmology (the actual mass fraction was ~25%), but that doesn't fit to the given "2-5 parts per 100,000". Could that number refer to something else (like helium-3 or deuterium)?

 

[DEANS2]

I have table show that

 

[mfb]

That's what I expected. 25% ⁴He, so 250 g in 1 kg. The ³He contribution is negligible.

 

[DEANS2]

how about in 7Li. there isn't have%

 

[mfb]

What is unclear about "n parts in x"?

 

[DEANS2]

2-5 parts in 10 billion

 

[mfb]

Yes, what is unclear about it?

 

[DEANS2]

So It is 1kg x 2 parts in 10 billion？

 

[mfb]

Sure. Well, 2 to 5.

 

[DEANS2]

I have tried it before. 1kgx4x10^(-10)=4x10^(-10)kg. However the answer is 0.4mg=4x10^(-7)kg how to get it?

 

[DEANS2]

there is the Q:If you could take a 1 kg handful of the matter in the universe 3 minutes after the big bang, approximately how much would be lithium nuclei?

 

[mfb]

Should be 0.2 to 0.5 µg, not mg.

 

[DEANS2]

my calculation is correct？

 

[mfb]

Yes.

 

[DEANS2]

thanks！