# Big Bang helium nuclei

1. Nov 5, 2016

### DEANS2

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

3. The attempt at a solution
I have no idea.can someone help?

2. Nov 5, 2016

### Staff: Mentor

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

3. Nov 5, 2016

### 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

4. Nov 5, 2016

### Staff: Mentor

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)?

5. Nov 5, 2016

### DEANS2

I have table show that

6. Nov 5, 2016

### Staff: Mentor

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

7. Nov 5, 2016

### DEANS2

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

8. Nov 5, 2016

### Staff: Mentor

What is unclear about "n parts in x"?

9. Nov 5, 2016

### DEANS2

2-5 parts in 10 billion

10. Nov 5, 2016

### Staff: Mentor

Yes, what is unclear about it?

11. Nov 5, 2016

### DEANS2

So It is 1kg x 2 parts in 10 billion？

12. Nov 5, 2016

### Staff: Mentor

Sure. Well, 2 to 5.

13. Nov 5, 2016

### DEANS2

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

14. Nov 5, 2016

### 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?

15. Nov 5, 2016

### Staff: Mentor

Should be 0.2 to 0.5 µg, not mg.

16. Nov 5, 2016

### DEANS2

my calculation is correct？

17. Nov 5, 2016

### Staff: Mentor

Yes.

18. Nov 5, 2016

thanks！