# I Question about an equation

1. Jan 2, 2018

### Arman777

Let's suppose we have an equation,
$$Ω_m+Ω_r+Ω_k=1$$
In this equation whats the values that $Ω_k$ can take ?
Only 1,0 or -1 ?

Also this equation is true for only "now" or any time in the universe ?

2. Jan 2, 2018

### PeroK

Assuming the omegas are positive, then $0 \le \Omega_k \le 1$

If you allow negative values, then $\Omega_k$ can be any real number.

3. Jan 2, 2018

### Arman777

For flat universe we can take $Ω_k=0$. In example for positive curvature universe then what could be the value of $Ω_k$ ?
A negative value ?

Cause $Ω_k=-\frac {κ} {a^2H^2}$

Something seems wrong to me. Either this equation only hold for $Ω_k=0$.

Oh okay I understand

4. Jan 2, 2018

### Arman777

For a negative value of $Ω_k$ corresponds to $Ω>1$ which its positively curved universe
For $Ω_k=0$ corresponds to $Ω=1$ which flat universe
For a postive value of $Ω_k$ corresponds to $Ω<1$ which its negatively curved universe

where $Ω=Ω_m+Ω_r$

5. Jan 2, 2018

### kimbyd

$\Omega_k$ can be any real number that makes the equation above true. Because $\Omega_m$ and $\Omega_r$ must be positive, $\Omega_k$ cannot be greater than 1. But it can be as negative as you like.

This is accurate.

6. Jan 3, 2018

### Arman777

Thanks, its more clear now