Why does expansion redshift light if....

  • Context: High School 
  • Thread starter Thread starter unwillingly ignorant
  • Start date Start date
  • Tags Tags
    Expansion Light Redshift
Click For Summary

Discussion Overview

The discussion revolves around the phenomenon of redshift in light due to the expansion of the universe. Participants explore the relationship between the expansion of space and the behavior of light, including concepts of spacetime curvature and energy conservation on cosmological scales. The conversation includes both conceptual and technical aspects, with a focus on understanding the implications of these ideas.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question whether expansion is simply creating more space rather than moving objects within it, leading to inquiries about the nature of redshift.
  • One participant asserts that redshift occurs because photons lose energy as they travel through expanding space, but this energy loss does not violate conservation laws on cosmological scales.
  • Another participant suggests that the stretching of space could be analogous to stretching the wavelength of light, prompting further clarification about the nature of redshift and spacetime curvature.
  • Some argue that the redshift is a result of spacetime curvature, which affects how photons behave as they travel through the universe.
  • There is a discussion about the implications of redshift being independent of coordinate systems, with some participants expressing confusion about how this relates to the uniformity of shifts across different observers.
  • Participants debate the concept of energy conservation in cosmology, with differing views on whether it applies universally or is context-dependent.
  • One participant expresses frustration over the complexities of cosmological concepts, indicating a struggle to reconcile different aspects of the discussion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on several key points, particularly regarding the nature of energy conservation in cosmology and the implications of redshift. There are competing views on how to interpret the relationship between expansion, light behavior, and energy conservation.

Contextual Notes

Some claims made in the discussion rely on specific definitions and assumptions about cosmological principles, which may not be universally accepted or understood. The conversation reflects ongoing debates in the field of cosmology regarding the application of conservation laws on large scales.

unwillingly ignorant
Messages
15
Reaction score
0
isn't expansion essentially making more space rather than moveing us in it? So why does it redshif light?

Or is that just a super facile explanation of expansion that they explain people like me who wouldn't get it?...
 
Space news on Phys.org
unwillingly ignorant said:
isn't expansion essentially making more space rather than moveing us in it?
No.

So why does it redshif light?
It carries photons away from us (slightly) as they are traveling towards us. Since they always travel at c, they are traveling at c when they reach us but because of the expansion's effect, they are red shifted.

Or is that just a super facile explanation of expansion that they explain people like me who wouldn't get it?...
No, it is an exact description of what is happening.

EDIT: One of the things that will presumably occur to you eventually, if it has not already, is the following. The photons start out at c at a certain frequency. They get to us still traveling at c but at a lower frequency. This CLEARLY says that they lost energy somewhere along the way. OK, since energy has to be conserved, where does that energy GO ?

The answer is that is doesn't "go" anywhere, it just disappears, and this is OK because "conservation of energy" is a local effect and does not apply on a cosmological scale. This is one of the many things that made me tear my hair out and say bad words when I first encountered it.
 
Ok.. I'm grabing clumps of hair ready to tear. Sooo is it essentially, literally streatching the photons' wave frequency as space is added/streached(?)?

Was that just a totally nonsense question?
 
unwillingly ignorant said:
Sooo is it essentially, literally streatching the photons' wave frequency as space is added/streached(?)?

No. The redshift of photons with the expansion of the universe is due to spacetime curvature. In a curved spacetime, photons can redshift (or in some cases blueshift) because of the curvature. More precisely, a photon traveling between two observers in a curved spacetime can redshift (or blueshift) depending on the relationship between the worldlines and the geometry of the curved spacetime.
 
  • Like
Likes   Reactions: QuantumQuest
riiight.. So it's a bit like those depictions of gravity where the grid stretches down into a gravity well except.. Everywhere at once, and not 2D, and it's the bending of those lines that shifts the frequency.. Am i anywhere close with that?
 
unwillingly ignorant said:
it's a bit like those depictions of gravity where the grid stretches down into a gravity well except.. Everywhere at once, and not 2D, and it's the bending of those lines that shifts the frequency.. Am i anywhere close with that?

Not really, because the "grid lines" you are thinking of are coordinate dependent, and the redshift/blueshift of photons is not; it is there regardless of the coordinates you choose (which means regardless of how you draw "grid lines" in spacetime).
 
that's what i meant by "everywhere at once".. I think..?
anyway, wouldn't that just result in a uniformed shift that wouldn't tell us anything?
 
unwillingly ignorant said:
that's what i meant by "everywhere at once".. I think..?

I don't see how.

unwillingly ignorant said:
wouldn't that just result in a uniformed shift that wouldn't tell us anything?

I don't understand what you are referring to here. The redshift/blueshift of photons being independent of your choice of coordinates does not mean it's the same for all photons and all observers. It just means that for a given photon and a given observer, the redshift/blueshift is the same regardless of what coordinates you choose to describe it. But your description in terms of "space stretching" only applies in a particular set of coordinates.
 
unwillingly ignorant said:
isn't expansion essentially making more space rather than moveing us in it? So why does it redshif light?

Or is that just a super facile explanation of expansion that they explain people like me who wouldn't get it?...

Is your "more space" more spacetime, distance or something else?

Redshift is a name of something that is observated, just like (cosmological)expansion and (energy)conservation.
For now they don't add up.
 
  • #10
m k said:
just like (cosmological)expansion and (energy)conservation.
For now they don't add up.
What do you mean "for now they don't add up" ? They will NEVER add up since it is known that energy conservation does not apply on cosmological scales.
 
  • #11
As I understand "known" I wouldn't say so.
 
  • #12
m k said:
As I understand "known" I wouldn't say so.
Well, then you should study some cosmology.
 
  • #13
unwillingly ignorant said:
isn't expansion essentially making more space rather than moveing us in it
Yes.
unwillingly ignorant said:
So why does it redshif light?
Because the speed of light is constant, while wavelength depends on gravity and/or dark energy ... *insert math stuff here*
 
Last edited:
  • #14
If the light wave is traveling through space, and you stretch out space, then you stretch out the wavelength of the wave. The distance between two peaks or troughs has increased.
 
  • #15
phinds said:
Well, then you should study some cosmology.

I have now studied some of it and learned that we know quite a bit of few percents of it and a bit less of the rest.

Still wouldn't say so.

Should you now study some definition [can't include maths here] fine tuning?
 
  • #16
m k said:
I have now studied some of it and learned that we know quite a bit of few percents of it and a bit less of the rest.

Still wouldn't say so.
Then you still have not come to terms with the fact that while yes indeed there are tons of things we know little about, energy conservation on cosmological scales is not one of them. There is no energy conservation on cosmological scales and if you continue to insist that there is, you are simply ignoring reality.
 
  • #17
m k said:
I have now studied some of it and learned that we know quite a bit of few percents of it and a bit less of the rest.

This is not a fair statement of our current theories of cosmology.

m k said:
Still wouldn't say so.

Then you have not understood what you have studied.

In any case, the OP's question has been answered. Thread closed.
 

Similar threads

Undergrad Gravity does negative or positive work with cosmos redshift?
  • · Replies 7 ·
Replies
7
Views
3K
Undergrad How do you get to the conclusion that there is accelerating expansion?
  • · Replies 23 ·
Replies
23
Views
4K
High School The accelerated expansion of the universe; redshift and time/distance
  • · Replies 3 ·
Replies
3
Views
2K
High School How did expansion take 380,000 years to let light travel freely?
  • · Replies 13 ·
Replies
13
Views
3K
High School For object 3.26 million light-years away, light redshifted by 70 km/s
  • · Replies 12 ·
Replies
12
Views
2K
High School Can the "creation" of space in the Universe affect the speed of light?
  • · Replies 7 ·
Replies
7
Views
4K
High School Help with when expansion acceleration began
  • · Replies 23 ·
Replies
23
Views
4K
Undergrad What if the Universe expands faster than the speed of light?
  • · Replies 19 ·
Replies
19
Views
5K
Undergrad Problem interpreting a Distance-Redshift Plot
  • · Replies 55 ·
2
Replies
55
Views
11K
High School Expansion of Space Itself, rather than due to an Explosion like the Big Bang?
  • · Replies 9 ·
Replies
9
Views
3K