# Dark Energy and Dark Matter

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1. Sep 28, 2014

### gianeshwar

Dear Friends!
I have read that in early universe in the inflationary phase the average density of matter was equal to critical density and now it is 10 percent of that due to which it is probably inferred that there must be hidden dark matter.
Please correct the information if wrong or incomplete or updated now.Moreover is it only matter referred here or (matter plus energy).
Also I want to know how the critical density is decided?

2. Sep 28, 2014

### Orodruin

Staff Emeritus
The critical density (or more accurately critical energy density) is the energy density for which the (homogeneous and isotropic) Universe would have no curvature (or equivalently, infinite radius of curvature). The critical density is derived from the Friedmann equation, which can be expressed as
$$H^2 = \frac{8\pi G}{3} \rho - \frac{\kappa}{R_0^2 a^2},$$
where $a = a(t)$ is the scale factor of the Universe, $R_0$ is the radius of curvature today, $\rho$ the energy density of the Universe, and $H$ is the Hubble parameter $\dot a/a$. The second term on the right hand side is the curvature term and in order to have a flat universe, it must be equal to zero. This defines the critical energy density as
$$H^2 = \frac{8\pi G}{3} \rho_c \quad \Rightarrow \quad \rho_c = \frac{3 H^2}{8\pi G}.$$
Energy density will therefore often be expressed as a ratio compared to the critical energy density: $\Omega = \rho/\rho_c$. With this, the Friedmann equation can be recast to
$$1-\Omega = -\frac{\kappa}{R_0^2 \dot a(t)^2}.$$

In general, $\Omega$ includes all types of energy content in the Universe, currently about 70% dark energy and 30% matter (out of which the majority is dark).

3. Sep 28, 2014

### Chronos

The 10% figure you mention [which seems a little high] was probably only referring to baryonic matter density.

4. Sep 28, 2014

### Matterwave

At last count that I remember, it's 6% baryonic matter, 24% dark matter, and 70% dark energy.

5. Oct 1, 2014

### michelange

Naïvely, just asking : why do we need dark energy and expansion to explain the redshift ? Why can't we just imagine that light is decelerating by itself, through time?

6. Oct 1, 2014

### bapowell

Redshift is not a deceleration of light, but an increase in its wavelength.

7. Oct 1, 2014

### phinds

"Tired light" was proposed and debunked thoroughly years ago.