The mesure of distances using the redshift

[houhou.trad]

Hi all,
in order to determine the recessional velocity of same galaxy we use the redshift law:
[(observed wavelength) -(source wavelength) ]/source wavelength=v/c
The problem I have found here is how we can know the source wavelength.
thinks.

 

[marcus]

Different elements have distinctive lines in their spectra. These lines have known wavelengths you can measure in the lab. Hot hydrogen gas and plasma have distinctive patterns of spectral lines that you can recognize, and see how much it has been shifted towards longer wavelengths.

Other elements also play a role.

 

[houhou.trad]

Different elements have distinctive lines in their spectra. These lines have known wavelengths you can measure in the lab. Hot hydrogen gas and plasma have distinctive patterns of spectral lines that you can recognize, and see how much it has been shifted towards longer wavelengths.

Other elements also play a role.

Thus, want you say that we know the source wavelength of these galaxies because we know already the elements that constituting them?

 

[marcus]

It is hard NOT to know, most of the universe is made of hydrogen and helium and elements have distinctive PATTERNs of lines, like 3 close together with the longer one twice as far from the middle, and relative brightnesses of the lines within a pattern

It is like a face or fingerprint, you just see "oh! those are the hydrogen lines, but just shifted over!" or Oh I see the bright yellow sodium line but it is shifted over into the red. And the shifts are all consistent, by the same ratio.

So it is hard to mistake the light of a particular abundant element, at least with nearby galaxies where we get a good sample of the light.

 

[sardar]

Hello,

Thank you for your question. I can provide some clarification on the use of redshift in measuring distances.

Firstly, the redshift law you mentioned is known as the Doppler effect, which is a result of the expansion of the universe. This means that as a galaxy moves away from us, the light it emits is stretched, causing a shift towards the red end of the spectrum. The amount of this shift is directly related to the recessional velocity of the galaxy.

To determine the source wavelength, scientists use a technique called spectroscopy. This involves breaking down the light from the galaxy into its component wavelengths, which can then be compared to known sources. By identifying specific spectral lines, we can determine the original source wavelength and calculate the redshift.

It is important to note that the redshift method is most accurate for objects that are very far away, such as galaxies in the distant universe. For closer objects, other methods such as parallax or standard candles are used to measure distances.

I hope this helps to clarify any confusion. Let me know if you have any further questions. Thank you.