Expanding universe and age of light

[deuspisi]

OK I might be stupid and english isn't my native language, so I'm sorry in advance.
So:
The red shift is bigger, the further away a galaxy is (they move away faster), and the closer it is, the shift is smaller and it goes to blue for Andromeda (towards). Everyone knows that.
BUT
When you consider that the light from the far galaxy is very very old, and from the close galaxy is new, don't you think that the universe WAS expanding long time ago and now it's collapsing?
It's so simple I can't believe i might be right. I must be wrong. Please tell me where i go wrong...

 

[Orodruin]

It is not what you describe. The expansion is about space itself growing and not really about the galaxies "moving". The redshift is not really a Doppler shift but an effect of space (and therefore the light wavelength) stretching out.

To add to this, the galaxies also do move, but generally in random directions. This causes an additional Doppler effect which when a galaxy moves towards us can give a net blueshift. You have to take all effects into account properly.

 

[deuspisi]

It is not what you describe. The expansion is about space itself growing and not really about the galaxies "moving". The redshift is not really a Doppler shift but an effect of space (and therefore the light wavelength) stretching out.

To add to this, the galaxies also do move, but generally in random directions. This causes an additional Doppler effect which when a galaxy moves towards us can give a net blueshift. You have to take all effects into account properly.

Well I'm a little confused right now. What exactly is "space" then (if not the distance between two things). Am I getting bigger right now, my molecules and atoms, or is the space between them getting bigger? And relative to what?

 

[Orodruin]

Well I'm a little confused right now.

You should be, these are really not concepts which let themselves be well explained with words.

What exactly is "space" then (if not the distance between two things).

Space is generally a part of space-time. It describes a background in which physics occur. A measure of space is distance, but it is possible for this distance to increase without two objects "moving" with respect to each other. I say moving with quotations because in general relativity this requires some additional definitions if the objects are not at the same place.

If you have not seen the balloon analogy, I recommend you look it up (and remember that "moving" means moving relative to the rubber in the balloon, which represents space).

Am I getting bigger right now, my molecules and atoms, or is the space between them getting bigger? And relative to what?

No, you are not getting bigger. The forces involved in holding you together are far stronger than the expansion could overcome.

 

[deuspisi]

First I want to thank you for responding. I was looking for answer for months but somehow didn't get the right article to read.
I know the balloon analogy, but I thought it was simle Doppler, therefore my question about the age of the red shift. Now I know it is cosmological redshift (thank you), and the space universally is stretching the wavelenght, not the movement, therefore the age of light doesn't matter.
One more question: In that case, (if the space is constantly stretching) is this a constant shift (same speed), or moving toward red/blue (accelerating/decelerating) for far and near galaxies. I can't find any information. If you have links, please share. Thank you again.

 

[Orodruin]

The redshift affects all light in the same way regardless of the source. The factor with which the wavelength changes is simply the scale factor today divided by the scale factor at the time the light was emitted. In some sense, it just compares the size of the universe at the different times.

 

[deuspisi]

The redshift affects all light in the same way regardless of the source. The factor with which the wavelength changes is simply the scale factor today divided by the scale factor at the time the light was emitted. In some sense, it just compares the size of the universe at the different times.

I understand that.
But if the wavelenght (red shift) of all galaxies changes over time (from the past 10 years readings for example) toward red/blue, that means space expanding speed is changed, right?

 

[phinds]

I understand that.
But if the wavelenght (red shift) of all galaxies changes over time (from the past 10 years readings for example) toward red/blue, that means space expanding speed is changed, right?

This whole "space is expanding" is really not good terminology (yes, I know you see it EVERYWHERE in pop science). Things just get farther apart, and yes, they are getting farther apart ever faster. That's what it means that the recession is accelerating

 

[deuspisi]

This whole "space is expanding" is really not good terminology (yes, I know you see it EVERYWHERE in pop science). Things just get farther apart, and yes, they are getting farther apart ever faster. That's what it means that the recession is accelerating

So when we observe galaxy 100 mil ly away with some speed (red shift) is this the current speed of the galaxy, or the speed 100 mil years from the past?

 

[phinds]

So when we observe galaxy 100 mil ly away with some speed (red shift) is this the current speed of the galaxy, or the speed 100 mil years from the past?

We see things as they were in the past. We also have enough information, having observed LOTS of galactic recessions, to have a good idea what it's doing "now". I put now in quotes 'cause that's a bit of a tricky subject.

This is how we know that the observable universe is about 13+ Billion years old and yet the observable universes has a radius "now" of about 50 Billion light years.

 

[deuspisi]

Thank you, people.
Sorry if i annoyed you with my ignorance.
I have more questions about how the expansion is stretching the wavelenght, but I'll read more (now i know where).
bb

 

[phinds]

Thank you, people.
Sorry if i annoyed you with my ignorance.
I have more questions about how the expansion is stretching the wavelenght, but I'll read more (now i know where).
bb

Hey, that's what we're here for. Glad to see you are learning this stuff.

 

[Chalnoth]

Well I'm a little confused right now. What exactly is "space" then (if not the distance between two things). Am I getting bigger right now, my molecules and atoms, or is the space between them getting bigger? And relative to what?

The average distance between galaxies is getting larger over time. There are some galaxies that are close to one another that are locked in their mutual gravitational attraction (such as the Milky Way and Andromeda), but most galaxies are moving away from one another in a pretty uniform fashion (so that a galaxy that is twice as far away is, on average, receding at twice the speed).

 

[mathman]

The mutual gravitation seems to hold for clusters of galaxies and maybe superclusters.

 

[deuspisi]

The mutual gravitation seems to hold for clusters of galaxies and maybe superclusters.

my original question was "the red shift that we see today: is it old (outdated) data from millions years ago?" because light need time to get to us, and what we see now is actually what was long time ago. that's all I'm asking.

 

[Chalnoth]

my original question was "the red shift that we see today: is it old (outdated) data from millions years ago?" because light need time to get to us, and what we see now is actually what was long time ago. that's all I'm asking.

Yes, this is true. So when we look far away, we see the expansion history of the universe. The rate of expansion has been slowing over the last few billion years, and it appears to be approaching a constant expansion rate (given by the cosmological constant).

 

[phinds]

Yes, this is true. So when we look far away, we see the expansion history of the universe. The rate of expansion has been slowing over the last few billion years, and it appears to be approaching a constant expansion rate (given by the cosmological constant).

That's a very confusing way to express it to a noobie. The rate of the ACCELERATION is slowing to a constant but since the recession is still accelerating and will continue to accelerate, the expansion is getting faster and faster and will continue to get faster still.

 

[Jorrie]

That's a very confusing way to express it to a noobie. The rate of the ACCELERATION is slowing to a constant but since the recession is still accelerating and will continue to accelerate, the expansion is getting faster and faster and will continue to get faster still.

IMO, this is even more confusing. I thought that we have agreed some time ago (will have to look it up) that the (fractional or %) expansion rate is represented by H(t) and that it is dropping to a constant value. The other value is the recession rate of specific galaxies and it is increasing today and in the future due the constant H.

 

[Orodruin]

Yes, this is true.

I believe this is confusing based on what the OP wrote earlier. While it is true that you can infer the expansion history, it is not true that seeing further objects redder implies that they were going away faster.

 

[phinds]

IMO, this is even more confusing. I thought that we have agreed some time ago (will have to look it up) that the (fractional or %) expansion rate is represented by H(t) and that it is dropping to a constant value. The other value is the recession rate of specific galaxies and it is increasing today and in the future due the constant H.

Yes, but I'm talking about how it looks to a noobie who doesn't yet understand those terms.

 

[Chalnoth]

I believe this is confusing based on what the OP wrote earlier. While it is true that you can infer the expansion history, it is not true that seeing further objects redder implies that they were going away faster.

My yes in that statement was solely in response to the quoted question about whether the light we see is outdated. It is.

 

[Orodruin]

My yes in that statement was solely in response to the quoted question about whether the light we see is outdated. It is.

I understand that, but based on the inference the OP made from this fact, I just wanted to make it clear that that inference was not correct.