# Redshift as a distance indicator

• taylrl3
In summary, the conversation discusses the use of redshift as a distance indicator and the necessity for corroborating evidence to test distance indicators. It also mentions the use of other methods, such as supernovae, to determine Hubble's constant for longer distances.
taylrl3
Surely it is possible to take a spectrum, deduce the redshift, then use this as a definitive distance indicator for all distances.

Why is it we use the distance ladder?

The distance ladder was used to get the conversion factor between redshift and distance - Hubble's constant.

Also for nearer objects hubble's law isn't necessarily true if enough of their velocity is due to relative motion rather than the universe expanding.

We need independent corroborations to test distance indicators. We must be doing something right as all are pretty consistent one to the other.

So for long distances, do we only use other methods (e.g SN) because of the uncertainty in Hubble's constant?

No we use SN to get Hubble's constant.

## 1. What is redshift and how does it relate to distance?

Redshift is a phenomenon where light from an object appears to be shifted towards longer wavelengths, which is caused by the object's motion away from the observer. The amount of redshift is directly proportional to the object's distance from the observer, making it a useful indicator of distance in astronomy.

## 2. How is redshift measured?

Redshift is measured using a spectrograph, which separates light into its component wavelengths. The amount of redshift is calculated by comparing the observed wavelength of a known spectral line to its expected wavelength. This results in a measurement called the redshift z, which is a dimensionless quantity representing the amount of shift in the object's light.

## 3. What is the relationship between redshift and the expansion of the universe?

The relationship between redshift and the expansion of the universe is described by Hubble's Law, which states that the farther an object is from us, the faster it appears to be moving away from us. This is due to the expansion of the universe, which causes objects to appear to be moving away from each other at a rate proportional to their distance.

## 4. Can redshift be used to measure distances to objects beyond our own galaxy?

Yes, redshift can be used as a distance indicator for objects beyond our own galaxy. However, for objects that are extremely far away, the relationship between redshift and distance becomes more complex due to the effects of cosmic expansion and the curvature of space-time.

## 5. What are the limitations of using redshift as a distance indicator?

One limitation of using redshift as a distance indicator is that it assumes a linear relationship between redshift and distance, which may not be accurate for extremely distant objects. Additionally, redshift measurements can be affected by other factors such as the object's peculiar motion and gravitational lensing, which can introduce errors in distance calculations. Furthermore, redshift can only measure relative distances and cannot provide precise measurements of absolute distance.

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