# What's the Dark Matter Density in Universe?

1. Jul 20, 2015

### RyanH42

In wikipedia says Physical baryon density: $Ω_bh^2=0.02230±0.00014$ and
Physical dark matter density:$Ω_ch^2=0.1188±0.0010$
Matter density:$Ω_m=0.3089±0.0062$
so If we collect baryonic matter density and dark matter density we cannot get matter density

https://en.wikipedia.org/wiki/Lambda-CDM_model

2. Jul 20, 2015

### Orodruin

Staff Emeritus
Notice the appearance of h^2 in the values you quote for DM and baryon abundances.

3. Jul 20, 2015

### RyanH42

Lets suppose I want to calculate dark matter mass/baryonic matter mass ? What should I do

4. Jul 20, 2015

### RyanH42

Whats the meaning of $h^2$ in here

5. Jul 20, 2015

### ruarimac

You ignore the h2 which tells you the result scales with H0. Substituting h~0.7 you get the correct results.

6. Jul 20, 2015

### RyanH42

I got it thanks.

7. Jul 20, 2015

### Orodruin

Staff Emeritus
$h$ is the Hubble constant H today divided by 100, i.e., $h \simeq 0.6780\pm 0.0077$ (PLANCK 2013).

8. Jul 20, 2015

### RyanH42

I add them and I get 0.287 not 0.3089

9. Jul 20, 2015

### Orodruin

Staff Emeritus
What did you use for $h$? Using 0.678 gives me 0.3069.

10. Jul 20, 2015

### RyanH42

I used 0.49

11. Jul 20, 2015

### Orodruin

Staff Emeritus
You mean you used $h^2 = 0.49$? For $h = 0.678$ you will get $h^2 = 0,46$. You should however note that $h$ also comes with an error. The reason that the abundance is given in $\Omega h^2$ is that this quantity is better bounded.

12. Jul 20, 2015

### RyanH42

Yeah I used h2=0.49

13. Jul 20, 2015

### Chalnoth

To get the right answer, you have to use the value of $h$ that was used to measure those parameters. As $h = H_0 / 100 km/s/Mpc$, and $H_0 = 67.74 km/s/Mpc$ in that data set, $h = 0.6774$. Use that number, and it will work. There will be some small differences, due to the fact that these numbers aren't published with full accuracy. But it'll be well within the errors.

14. Jul 20, 2015

### RyanH42

Finally.Thank you