The meaning of z number in red shift

In summary: See here: http://en.wikipedia.org/wiki/RedshiftThe basic difference is the background temperature of the universe 'then' vs temperature now. For example, the background temperature at z=2 was nearly three times hotter than we now measure [a z+1 thingy].Thank you doth for your reply's.
  • #1
kc2dws
6
0
Hello

I understand that red shift is a measurement of relative speed. What I am looking for is what the number after the z means. Is it a percent of c or, Or is it it just a scale that physics uses to note the difference between different objects.

Thanks for your time.

Kevin
 
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  • #2
As I understand it, it is simply as you said the relative difference between the observed and emitted frequencies.
 
  • #4
The basic difference is the background temperature of the universe 'then' vs temperature now. For example, the background temperature at z=2 was nearly three times hotter than we now measure [a z+1 thingy].
 
  • #5
Thank you doth for your reply's

kevin
 
  • #6
kc2dws said:
Hello

I understand that red shift is a measurement of relative speed...

Kevin

Mostly not. Most of the redshift that is observed and that we talk about here is not Doppler redshift (depending on relative speed at some given instant of time.)

Mostly we talk about cosmological redshift. Technically different. It is a measure not of relative speed at some time but instead measureshow much distances have expanded while the light was in transit.

You will see that in the Wikipedia article that Drakkith gave link to, if you look far enough down the page. You will see a table and one of the entries will be "cosmological redshift".

that is the z we are talking about when we talk about the observed redshifts of distant galaxies. Or Type Ia supernovae with z = 1 or z = 1.5
Or quasars with z = 6. Or cosmic microwave with z = 1100. That is all cosmological redshift.

The formula does not involve speed. It involves the ratio of distance now/distance then.

z = 1 means that on average a largescale distance DOUBLED while the light was in transit.

You always add one to z to get the ratio by which distances have increased.

1+z = anow/athen you can find that in the Wikipedia Drakkith gave you

In other words if z = 2 then distances have TRIPLED while the light was in transit to us.
they are 3 times longer NOW on the day we receive the light than they were THEN when the light was emitted by the glowing gas or star and started on its way to us.

the general pattern of expansion is not like ordinary motion because nobody gets anywhere by it (everybody gets farther from everybody)
and the rate of expansion has been constantly changing all during the time the light has been traveling, so there is no one obvious rate to pick that would determine z.
 
  • #7
Thank you marcus.

I reread the article, And found the equation
 
  • #8
[itex]z[/itex] is related to the scale factor of the universe by
marcus said:
1+z = anow/athen

and also to the emitted and received wavelengths (and frequencies) of light (for galaxies moving with the Hubble flow).
Irishwake said:
As I understand it, it is simply as you said the relative difference between the observed and emitted frequencies.
[tex]z =\frac{a_{now}}{a_{then}} - 1 = \frac{a_{now} - a_{then}}{a_{then}} = \frac{\lambda_{now} - \lambda_{then}}{\lambda_{then}} = \frac{f_{then} - f_{now}}{f_{now}}.[/tex]
 

1. What is red shift and why is it important in scientific research?

Red shift is the phenomenon in which light from a distant object appears to have a longer wavelength, or shift towards the red end of the spectrum, due to the object's motion away from the observer. In scientific research, red shift is important because it provides evidence for the expanding universe and allows scientists to measure the distance and speed of objects in space.

2. How is red shift related to the concept of the universe expanding?

Red shift is related to the concept of the universe expanding because as objects in the universe move further away from us, their light becomes stretched and appears more red. This is due to the expansion of space itself, which causes the wavelengths of light to stretch out as it travels through the expanding universe.

3. What does z number represent in red shift measurements?

Z number, also known as redshift number, represents the amount of red shift observed in a celestial object's spectrum. It is a measure of the object's recessional velocity and is used to calculate the distance of the object from Earth.

4. How is red shift measured and what units are used?

Red shift is measured using a spectrograph, which breaks down light from an object into its component wavelengths. The amount of red shift is determined by comparing the object's spectrum to a known spectrum. The units used for red shift measurements are typically either kilometers per second (km/s) or a dimensionless unit known as the z number.

5. Can red shift be used to determine the age of the universe?

Yes, red shift can be used to determine the age of the universe. By measuring the red shift of distant objects and calculating their recessional velocities, scientists can estimate the age of the universe using the Hubble's law and the known expansion rate of the universe. This has helped scientists to determine that the universe is approximately 13.8 billion years old.

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