# The redshift is how far/long ago?

• shotgun
In summary: There is no clear connection between what we see as gravity and what we see as an expanding universe.
shotgun
How far into space do we need to look before we can see the first signs of redshift?

In principle there is no lower limit, the redshift of an object is greater the further away it is so at some point objects are close enough to us that we can't really measure the redshift, but how close that is depends on the accuracy of the measurement. In addition, galaxies all have there own small random motions on top of the general expansion so for nearby galaxies the doppler shift due to these dominates over cosmic redshifting.

thank you for that answer. so there is no discernible distance a t which the redshift effect "turns on" vs. gravity? it seems like they might be trying to compete. is there a clear connection between the two?

hmm, unfortunately your question is a little too confused to allow a straightforward answer.

Redshift to a good approximation is simply due to the fact that galaxies are moving away from each other, this velocity causes a doppler shift that shifts spectral lines. That is all redshift is.

There are some subtleties and when redshift gets large enough it becomes inaccurate to think of redshift as a doppler effect, but redshift is that a 'cause' that competes with gravity, rather it is an effect that is caused by motion and gravity.

It sounds like it would be worth your while to read an introductory cosmology textbook to explain this more clearly step by step. It's hard to get a good clear overall picture in your head of these ideas just from ad hoc bits of advice on forums. Better to read a textbook or good website and use forums to clarify the bits you're not understanding.

yes, thank you. i had lept forward in my head to a place that i did not communicate well. i meant that the effect of gravity seems to involve a contaction of the space between objects at a rate that is a function of the extent of that space (distance). the redshift implies that space between distant objects is expanding at a rate that is a function of the extent of that space. i wonder what connection might exist there?

Gravity does not "involve a contaction of the space between objects at a rate that is a function of the extent of that space (distance)". If you think of gravity as doing this you will lead yourself to all kinds of misconceptions!

There are a lot of threads here that deal with the (much abused) concept of expanding space. Use search and have a read of them. To be very brief, the expansion of space is a intellectual shorthand, in other words it doesn't 'really' happen but it can be a convenient way of think about some things, in particular an expanding universe.

so you are saying that gravity does not involve the acceleration of one rest mass towards another with the consequence that the distance between the two is reduced? And then isn't the changing rate at which this distance is being reduced the same thing as saying that they are accelerating towards each other?

shotgun said:
so you are saying that gravity does not involve the acceleration of one rest mass towards another with the consequence that the distance between the two is reduced?

Yes. Two things can be accelerating towards each other but moving apart, this is what happens if you throw a ball in the air.

shotgun said:
And then isn't the changing rate at which this distance is being reduced the same thing as saying that they are accelerating towards each other?

Again, you are mixing the acceleration caused by gravity with velocity. They need not have the same sign.

ok, i can see what you are saying. however, that requires kinetic energy, which may or may not have to do with gravity.

I am just saying that gravitational attraction implies a direction and acceleration (even if the velocity of the body is for some other reason not following the acceleration vector, and when people say the universe is expanding at an accelerating rate, that also implies a direction and acceleration. The direction in this case seems to imply that distance between object and observer should be increasing.

Of course one theory for this may be the big bang, but it seems like a lot of people have put some holes in certain parts of that theory. And still no graviton. It just seems like there is a problem there that people like to speculate about. I was just wondering if anyone had shown any connection between what we see as gravity and what we see as an expanding universe.

## What is redshift and how is it measured?

Redshift is the shift of light towards longer wavelengths, which is caused by the expansion of the universe. It is measured by comparing the observed wavelength of light from an object to the expected wavelength of that light based on its known properties.

## Why is redshift important in astronomy?

Redshift is important in astronomy because it provides evidence for the expansion of the universe and helps us understand the distance and age of objects in the universe. It also allows us to study the evolution of galaxies and the large-scale structure of the universe.

## How does redshift relate to the Big Bang theory?

The Big Bang theory predicts that the universe is expanding, and redshift is a direct consequence of this expansion. As the universe expands, the light from distant objects is stretched, causing their wavelengths to shift towards the red end of the spectrum.

## Can redshift be used to measure the age of the universe?

Yes, redshift can be used to estimate the age of the universe. By measuring the redshift of distant objects and using the known rate of expansion of the universe, we can calculate the age of the universe to be approximately 13.8 billion years.

## Are there different types of redshift?

Yes, there are two types of redshift: cosmological redshift and Doppler redshift. Cosmological redshift is caused by the expansion of the universe, while Doppler redshift is caused by the relative motion between an object and an observer. Both types are important in measuring distances and understanding the structure of the universe.

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