# Expanding universe

Hi,

I have a question about the redshift caused by the expanding universe. I am not a physicist and reading books and articles about physics is just a hobby so I apology if this is obvious to most of you.

I have read somewhere that the light from far away galaxies is redder than light from nearer galaxies. Why is this?

I understand that the redshift is caused by the expanding universe stretching the light but wouldn't the amount of redshift only depend on how long ago the light was emitted? I would have thought that two event happening at the same time would have the same redshift because the universe has expanded by the same amount for both events.

## Answers and Replies

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sylas
You are correct, in general. Note that galaxies that are further away emitted their light longer ago. If we see things from the same time, then we see them at the same distance... apart from local motions over the general Hubble flow.

Some folks here don't like describing the cause of redshift as stretching from expanding space, and they have a point. But it works in the sense of giving you the right answers, and a lot of the simple explanations in cosmology can depend on how you choose to look at it. The underlying maths is the same.

Cheers -- sylas

The farther away something is, the older it appears. Looking, for example, 1 billion light years away is like looking 1 billion years in the past, because the light took 1 billion years to reach you.

So yes, if you were looking at two objects that emitted their light at the same time, then the redshift would be the same. The fact that they are different distances away means that we are looking at light emitted at different times.

Hope it helps :).

turin
Homework Helper
I have read somewhere that the light from far away galaxies is redder than light from nearer galaxies. Why is this?
Nobody knows the answer to that. It is simply a relationship that Hubble noticed when measuring the distances to about 50 galaxies. However, it turns out to be consistent with a simple concept in General Relativity, namely expansion.

I understand that the redshift is caused by the expanding universe stretching the light ...
That is indeed the mainstream interpretation.

I would have thought that two event happening at the same time would have the same redshift because the universe has expanded by the same amount for both events.
What is wrong with that?

turin
Homework Helper
The farther away something is, the older it appears. Looking, for example, 1 billion light years away is like looking 1 billion years in the past, because the light took 1 billion years to reach you.
Not exactly. If space is expanding, then the concept of distance itself is not this straightforward. For instance, something that is 1 billion lyrs away now will be even further away by the time light arrives from it (assuming an open universe). Inversely, light that has traveled 1 billion years from its source has traveled from a source that was actually closer than 1 billion lyrs when it was emitted, and is now actually much further away (assuming that expansion can be extrapolated 1 billion years into the past).