# Solving Redshift Confusion Homework Problem

• MoAli
In summary, the conversation discusses the calculation of the distance and redshift at which the Hubble flow dominates over random peculiar velocities in galaxies within a group. The equation used is V = h_0 * D, where D is the distance and h_0 is the Hubble constant. The attempt at a solution involved substituting in numbers and resulted in a distance of 1.36*10^20 km and a velocity-to-speed ratio of 0.001. The difference between the two redshifts mentioned is also mentioned, with the focus being on determining the distance at which the smooth Hubble velocities begin to dominate. The conversation also includes suggestions to use units more commonly used by astronomers and to provide the redshift instead of the
MoAli

## Homework Statement

I encountred a question that says : Assume that gravitational interactions between galaxies in groups lead to random galaxy velocities of 300 km/s At what distance would the redshift predicted by Hubble's law dominate over that produced by individual velocities? and what is the redshift at that distance?
I don't understand what's the difference between the 2 redshifts mentioned here, so I am confused on how to approach this

## Homework Equations

V= h_0 * D
where D is the distance and the Hubble constant is 2.18*10^-18 s^-1

V /c = Δλ/λ

## The Attempt at a Solution

I tried substituting in the numbers and got D= 1.36*10^20 km and v/c=0.001. [/B]

I think you did it right. There is a smoothly increasing velocity moving away from us faster and faster as the distance increases (the "Hubble flow"), and superimposed on this is a random set of what are called "peculiar velocities". You are asked to determine how far away you need to look before the smooth Hubble velocities begin to dominate. A couple of comments: (1) I would put the distance in units more used by astronomers, like parsecs or Megaparsecs in stead of km, and (2) you gave a velocity, but you were asked for the redshift z.

## 1. What is redshift?

Redshift is a phenomenon in which light from distant objects in the universe appears to have a longer wavelength, or be shifted towards the red end of the spectrum. This is caused by the expansion of the universe, which stretches the light as it travels to us.

## 2. How is redshift used to measure the distance of objects in the universe?

The amount of redshift observed can be used to determine how far away an object is in the universe. This is because the amount of redshift is directly proportional to the distance an object is from us. The farther away an object is, the more redshifted its light will be.

## 3. What is the "confusion problem" in redshift measurements?

The confusion problem in redshift measurements refers to the difficulty in accurately determining the redshift of an object due to overlapping spectral lines. This can make it challenging to identify which spectral lines are associated with which objects, leading to potential errors in distance measurements.

## 4. How can the confusion problem be solved?

To solve the confusion problem, scientists use a variety of techniques such as spectroscopy and computer algorithms to identify and separate the spectral lines of different objects. They also rely on large databases of known objects and their redshift values to compare and confirm their measurements.

## 5. What are the implications of accurately solving the redshift confusion problem?

Accurately solving the redshift confusion problem is crucial for understanding the structure and evolution of the universe. It allows scientists to accurately measure the distance and movements of galaxies, which provides insights into the expansion of the universe and the distribution of matter within it. This information is also important in testing and refining theories of cosmology and the origins of the universe.

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