# How do you calculate luminosity distance?

## How do you calculate luminosity distance?

Using brightness and luminosity to get distance

- The luminosity of the lightbulb is L = 100 W.
- The brightness is b = 0.1 W/m2.
- So the distance is given by d2 = (100 W)/(4 Pi x 0.1 W/m2).
- Since 4 Pi is approximately 10, this is d2 = (100 / 1) m2.
- Thus d2 = 100 m2.
- We now know what d2 is.
- So d = 10 m.

### How does luminosity change with redshift?

So surface brightness drops as redshift increases….The energy flux you get from an object at that position is:

- reduced because of normal inverse square law of light: f ~ L/4pi(Rr)2.
- reduced again becase the photons are being redshift and therefore reduced in energy: f ~ 1/(1+z)

#### What is luminosity distance dL?

Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object. which gives: where DL is measured in parsecs.

**How do you calculate luminosity from flux and distance?**

Knowing the flux (f) and distance to the object (r), we can calculate its luminosity: L=4πr2f. Therefore, flux and luminosity are intrinsic properties of the object, while brightness depends on our detecting tools (hardware and software).

**What is luminosity distance DL?**

## What is red shift in Doppler effect?

‘Red shift’ is a key concept for astronomers. The term can be understood literally – the wavelength of the light is stretched, so the light is seen as ‘shifted’ towards the red part of the spectrum. Something similar happens to sound waves when a source of sound moves relative to an observer.

### How does luminosity change with distance?

Notice that as the distance increases, the light must spread out over a larger surface and the surface brightness decreases in accordance with a “one over r squared” relationship. The decrease goes as r squared because the area over which the light is spread is proportional to the distance squared.

#### Why is luminosity distance not proper distance?

The proper distance has to reach a maximum because the size of the observable universe is finite. The luminosity distance, though, is a measure of flux, and flux is also affected by redshift.

**What is the redshift formula?**

Furthermore, the magnitude of this blueshift or redshift z is proportional to the object’s velocity v over the velocity at which the waves travel (the constant c for light waves) giving the equation z=v/c.

**What does a redshift of 0 mean?**

Remember: We always observe from a redshift of ZERO! Higher redshift means we are looking farther away and longer ago. Scale Factor: We observe now, when the scale factor of the universe is Rnow. An object we observe at redshift z emitted its light long ago when the universe had scale factor Rz.

## What is luminosity distance cosmology?

The Luminosity Distance depends on cosmology and it is defined as the distance at which the observed flux f is from an object. If the intrinsic luminosity dL of a distant object is known, we can calculate its luminosity by measuring the flux f which is determined by − dL(z)=√L4πf. The Photon Energy gets red shifted.