Question about an expanding universe

[hans030390]

I'm not extremely knowledgeable on astronomy and cosmology, but I have taken a couple 100-level college astronomy courses and done a bit of personal reading on it outside of class.

One point that's brought up often is that the universe is expanding. If you observe a distant object in space, the light it emits is redshifted. The farther you look, the greater this redshift becomes (if I am remembering correctly). The idea, then, is that the light from objects farther away will have to travel a longer distance to reach us. If space is expanding, then that light will undergo a larger redshift than another object that is closer to us due to the expansion as the light travels to us.

Now, I have no issues with this. I just feel like I'm missing some details. How do we actually know that the universe is expanding as mentioned above? What if the reason light is more redshifted farther away is because those objects are showing us information (in this case, light) that is older than objects closer to us? To clarify, we aren't actually seeing the sun in it's current state. We're seeing it in a state about 8 minutes before it's current state due to the fact that light is not transmitted instantly. If we're looking billions of light years out into space, we aren't seeing those objects as they are currently...but rather as they were billions of years ago. Given the Big Bang model, if we were to look at objects billions of years ago, it would make sense that they'd be "moving away" from each other faster than objects would be now (I understand that the objects themselves aren't moving away from each other...it's expansion). So, we look at objects billions of light years away and see them "moving" faster than objects closer to us, hence the greater redshifts. Why can't this imply that the expansion of the universe is slowing, as closer objects, which provide more recent information to us than those farther away, show a slower expansion rate than objects in the past? I feel like this fits well with a fast expansion (Big Bang) that has been slowing down over time.

Hopefully what I just said is clear...

Like I said, I'm no expert in astronomy, cosmology, physics, etc. And I feel like there's no way this hasn't been covered already by those that are experts. ATOT seems to have some people that know a good bit about this, or at least have good sources! Google isn't really proving to be very useful (probably because I have no idea how to word all this into a search).

 

[Eli]

Hmm. Interesting question. Maybe just read about redshift itself? You're basically asking whether it is a valid theory - at least, for the purpose we use it for.

I don't know the answer to your question, though I understand what you're asking.

 

[bfdd]

wait... are you asking if the doppler effect is legitimate?

 

[hans030390]

wait... are you asking if the doppler effect is legitimate?

Nope. Like I said, I have no problem with the current model of cosmology and a universe that is expanding at an accelerating rate (I think that's what it is, at least). Doppler effect, redshifts...it's all good stuff, and it fits with the current model! I'm just wondering why it wouldn't also work for the other idea I mentioned. Like I said, I feel like I'm just missing some details!

 

[Kadarin]

One point that's brought up often is that the universe is expanding. If you observe a distant object in space, the light it emits is redshifted. The farther you look, the greater this redshift becomes (if I am remembering correctly). The idea, then, is that the light from objects farther away will have to travel a longer distance to reach us. If space is expanding, then that light will undergo a larger redshift than another object that is closer to us due to the expansion as the light travels to us.

The doppler effect itself has nothing to do with how far away an object happens to be nor how old the light that reaches us happens to be, but rather with how quickly it is moving away from the observer. In an expanding universe, the objects that are farther away from us will be moving with a greater velocity relative to us, and hence light from them will be shifted farther into the red. These characteristics are observable, measurable, and consistent with current scientific theory.

 

[runzwithsizorz]

“The universe is not required to be in perfect harmony with human ambition.” Dr. Carl Sagan
Where is the end to infinity?

 

[hans030390]

The doppler effect itself has nothing to do with how far away an object happens to be nor how old the light that reaches us happens to be, but rather with how quickly it is moving away from the observer.

The Doppler effect "is the change in frequency of a wave for an observer moving relative to the source of the wave" (from Wikipedia). So, in this case, the waves we are observing are light (or EMR in general). If we're observing an object 100 million light years away, then we're seeing that object as it was 100 million years ago. I suppose I just don't understand how the dopper effect can be showing us how fast an object is moving away from us currently if the waves we're observing were emitted 100 million years ago.

In an expanding universe, the objects that are farther away from us will be moving with a greater velocity relative to us...

Why is this? I was under the impression that everything was expanding uniformly, but that the expansion rate itself was accelerating. Why would objects farther away be moving away with a greater velocity relative to us?

BTW, I'm not trying to argue or anything. I really do just feel like I'm missing details or not understanding everything perfectly, so I'm hoping discussing these things will help me out.

 

[bfdd]

hanns you're assuming we only see the light from 11 billion years ago at a constant. we are moving forward in time and some of those waves are long past us by now and more recent ones are constantly coming in. that's why i asked if you were questioning the doppler effect because that's the only thing i can think of that would make sense into what you're saying. at least for my small brain.

 

[hans030390]

hanns you're assuming we only see the light from 11 billion years ago at a constant. we are moving forward in time and some of those waves are long past us by now and more recent ones are constantly coming in. that's why i asked if you were questioning the doppler effect because that's the only thing i can think of that would make sense into what you're saying. at least for my small brain.

No, that's not what I'm assuming. Even the more recent waves we're receiving from far away objects are still not "recent".

 

[MTDEW]

Interesting, your logic seems sound and after reading it, it does seem more likely the universe is slowing down.
Since the farther back in time the greater the red shift vs less red shift on closer objects.
I don't think there is much dispute that the universe is expanding(since it obviously is), its whether or not its accelerating is still just a theory that is still openly disputed isn't it?

 

[hans030390]

Interesting, your logic seems sound and after reading it, it does seem more likely the universe is slowing down.
Since the farther back in time the greater the red shift vs less red shift on closer objects.
I don't think there is much dispute that the universe is expanding(since it obviously is), its whether or not its accelerating is still just a theory that is still openly disputed isn't it?

At least in my astronomy classes, it seems to be taught more like a fact than a theory. Not that I have a problem with that, as it sounds like a very solid theory/fact. I just feel like I'm missing something that completely solidifies it for me, hence the other things I pointed out. I'm probably just not wrapping my brain around it the right way.

 

[bfdd]

No, that's not what I'm assuming. Even the more recent waves we're receiving from far away objects are still not "recent".

they are recent because we just observed them. i'm not getting your logic here. this light is still coming in from those places, it's not like it came in once and stopped. the light we are receiving now is far more recent than the light we received 10 years ago.

 

[JulesMaximus]

This should answer all your questions.

 

[silverpig]

The redshifted light we see is not doppler shifted. We're talking about two different things.

Doppler redshift: yellow object looks red as it moves quickly away from you

Cosmological redshift: yellow object looks red because the space that the photon travels through is expanding, thus stretching the photon

Photons' wavelengths are a function of a linear space dimension measurement (500 nm for example). As the photon travels, space expands. That 500 nm photon slowly becomes a 501 nm photon as the space it occupies expands. As it travels through more expanding space, it expands more, thus appearing redder. Cosmological redshift is thus a cumulative effect of traveling through space. The more redshift something has, the farther its photons have traveled through space to reach us.

 

[think2]

Why do you think galaxies were moving faster "just after" the big bang compared with how they're moving now?

Anyway, according to the special theory of relativity, the speed of light is independent of the motion of the source so how fast the object is moving relative to us at the time it emitted light isn't going to redshift it.

BTW - the milky way galaxy is moving in the direction of constellation Leo (I think) at 270 km/s.

 

[CoachB]

I can see how Hans would have questions/issues.
The more I learn about the physics of cosmology, the more I feel it's all a house of cards. Each new theory rests on three previous theories which depend on 5 prior theories. Every time a non-conformity with the current theory is found, a supplementary theory is developed to accommodate the non-conformity.
One easy/quick example....
A - the speed of light is absolute; nothing is faster
B - entangled quanta react INSTANTLY regardless of distance
C - so there is a speed greater than C ??
D - there must be dimensions we can't "see" the allow this speed limit breakdown
E - and so on

Try this....next time you have a discussion about physics, substitute the word "magic" in place of relativity and/or quantum.....i.e special "magic" and "magic" mechanics. Works pretty well!!

So....Hans may very well be as correct as Stephan Hawking!!

;>)

 

[DrPizza]

Why do you think galaxies were moving faster "just after" the big bang compared with how they're moving now?

Anyway, according to the special theory of relativity, the speed of light is independent of the motion of the source so how fast the object is moving relative to us at the time it emitted light isn't going to redshift it.

BTW - the milky way galaxy is moving in the direction of constellation Leo (I think) at 270 km/s.

And, how do they know that? (redshift/blueshift) The velocity of the source most certainly affects the redshift.

 

[silverpig]

And, how do they know that? (redshift/blueshift) The velocity of the source most certainly affects the redshift.

It certainly does for "local" things. For stuff that's near the edge of what we can see, doppler redshift due to proper motion is relatively tiny.

What's super cool is I've reduced stellar spectra on pulsing stars and used the red/blue shifts to compute how fast the star expands and contracts to within 1 m/s. Neato.

 

[hans030390]

The redshifted light we see is not doppler shifted. We're talking about two different things.

Doppler redshift: yellow object looks red as it moves quickly away from you

Cosmological redshift: yellow object looks red because the space that the photon travels through is expanding, thus stretching the photon

Photons' wavelengths are a function of a linear space dimension measurement (500 nm for example). As the photon travels, space expands. That 500 nm photon slowly becomes a 501 nm photon as the space it occupies expands. As it travels through more expanding space, it expands more, thus appearing redder. Cosmological redshift is thus a cumulative effect of traveling through space. The more redshift something has, the farther its photons have traveled through space to reach us.

Ok, that more or less cleared things up for me. I think I just needed someone to explain it in a way that clearly addresses my question(s).

Why do you think galaxies were moving faster "just after" the big bang compared with how they're moving now?

I just feel like it's more intuitive that the expansion near the time of the Big Bang would be faster than it is today. However, what is intuitive is not always right!

they are recent because we just observed them. i'm not getting your logic here. this light is still coming in from those places, it's not like it came in once and stopped. the light we are receiving now is far more recent than the light we received 10 years ago.

If we're observing something a billion light years away, the light we're receiving is going to be at least a billion years "old". Just like how the light we're receiving from the sun is 8 minutes "old". I don't know about you, but I don't consider a billion years as recent...in the grand scheme of things, yes. But that's still old as hell to me.

 

[bfdd]

I can see how Hans would have questions/issues.
The more I learn about the physics of cosmology, the more I feel it's all a house of cards. Each new theory rests on three previous theories which depend on 5 prior theories. Every time a non-conformity with the current theory is found, a supplementary theory is developed to accommodate the non-conformity.
One easy/quick example....
A - the speed of light is absolute; nothing is faster
B - entangled quanta react INSTANTLY regardless of distance
C - so there is a speed greater than C ??
D - there must be dimensions we can't "see" the allow this speed limit breakdown
E - and so on

Try this....next time you have a discussion about physics, substitute the word "magic" in place of relativity and/or quantum.....i.e special "magic" and "magic" mechanics. Works pretty well!!

So....Hans may very well be as correct as Stephan Hawking!!

;>)

yeah that's all fun and shit... except you're leaving out math and science.

also, instant isn't a measurement of speed so it doesn't break any sort of "can't travel faster than the speed of light" bs.

 

[think2]

And, how do they know that? (redshift/blueshift) The velocity of the source most certainly affects the redshift.

uh, ok, you're right, my mistake.

So supposing that there's no expansion of the universe, galaxies could be moving slower now than the were 15 billion years ago, due to gravity. But wouldn't that mean that the slowdown in the speed of the receiver compensates for the faster movement of the transmitter so you wouldn't see any redshift compared with what we see in galaxies close to us.

Edit : hmmm, nope that can't be it. You would see a redshift compared to close galaxies. Well, I dunno what the answer is.