Question regarding light from distant objects

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Discussion Overview

The discussion revolves around the question of how we can observe light from distant objects, specifically those that are said to be 10 billion light years away, in the context of an expanding universe. Participants explore the implications of cosmic expansion on the travel time of light and the distances involved, addressing both theoretical and conceptual aspects of cosmology.

Discussion Character

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

Main Points Raised

  • Some participants question how light from an object 10 billion light years away can reach us, given that the universe has been expanding for over 13 billion years.
  • One participant draws an analogy to receiving light from a flashlight 10 feet away, suggesting a similar principle might apply.
  • Another participant emphasizes the need for careful language in cosmology, noting that objects emitting light 10 billion years ago were indeed closer than 10 billion light years at that time, but are now further away due to the expansion of space.
  • A participant explains that the time it takes for light to travel is influenced by the initial distance and the expansion rate of the universe, indicating that the distance corresponding to a time of 10 billion years is less than 10 billion light years in an expanding universe.
  • One participant reflects on their understanding of light, comparing it to a slinky that stretches as objects move apart, relating this to the concept of redshift.
  • Another participant provides calculations using Jorrie’s calculator to illustrate the distances and times involved in light emission and travel, detailing specific scenarios for light emitted from objects at various distances.

Areas of Agreement / Disagreement

Participants express various viewpoints on the implications of cosmic expansion for light travel, with no clear consensus reached. Some agree on the complexities introduced by the expanding universe, while others present differing interpretations of the implications for light from distant objects.

Contextual Notes

The discussion highlights the complexities of cosmological distances and the effects of space expansion on light travel, with participants acknowledging the need for careful consideration of definitions and assumptions in their arguments.

Who May Find This Useful

This discussion may be of interest to those studying cosmology, astrophysics, or anyone curious about the nature of light and the implications of an expanding universe on observational astronomy.

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TL;DR
How can we receive light from an object 10 billion light years away? Since the universe is expanding, wouldn't that source have been a lot closer to us 10 billion years ago. Meaning the light would have already reached us?
Given that the universe is expanding, and we currently believe the universe as we know it began a little over 13 billion years ago, Then when we look at a distant object and say we are looking light from 10 billion years ago, How is that possible. With the universe expanding for 13 billion years, then 10 billion years ago we all would have been closer together. And the light would have been coming from much closer and therefore would have already reached us.
 
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How can we receive light from a flaslight 10 feet away? The same argument applies, no?
 
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In cosmology you need to be very careful when you say ”an object 10 billion ly away” as this is not a statement that is constant over cosmological scales. The objects which emitted lights 10 billion years ago that we receive today were indeed closer than 10 billion light years at the time of emission. (The reason it took light 10 billion years to reach us is because the travel distance remaining expanded during the travel time.) However, they would be further away than 10 billion light years away today. (The distance already covered by the light also expanded after the light passing.)
 
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Robokop said:
Summary:: How can we receive light from an object 10 billion light years away? Since the universe is expanding, wouldn't that source have been a lot closer to us 10 billion years ago. Meaning the light would have already reached us?

Given that the universe is expanding, and we currently believe the universe as we know it began a little over 13 billion years ago, Then when we look at a distant object and say we are looking light from 10 billion years ago, How is that possible. With the universe expanding for 13 billion years, then 10 billion years ago we all would have been closer together. And the light would have been coming from much closer and therefore would have already reached us.
How long light takes to travel from a source to a receiver depends on the initial distance and the expansion rate. If light is emitted from a star 1 billion light years away, then it will take more than 1 billion years to reach us, as the space between the source and receiver expands during the light's journey.

There will be some distance that corresponds to a time of 10 billion years. In a non-expanding universe it would be 10 billion light years. In an expanding universe it is less than 10 billion light years.

That said, you seem to assume that objects emitted a flash of light 13 billion years ago and have been dark ever since. Which is why @Vanadium 50 gave the answer he did.
 
Thank you for the responses. I guess I was thinking of light more like a particle then a wave. I think I am getting a better understanding of it. If I think of the light coming from that object like a slinky, it gets stretched out as they move apart, but they are still connected (so to speak) but the slinky getting stretched out would be similar to the red shift.
 
Robokop said:
Summary:: How can we receive light from an object 10 billion light years away? Since the universe is expanding, wouldn't that source have been a lot closer to us 10 billion years ago. Meaning the light would have already reached us?

Given that the universe is expanding, and we currently believe the universe as we know it began a little over 13 billion years ago, Then when we look at a distant object and say we are looking light from 10 billion years ago, How is that possible. With the universe expanding for 13 billion years, then 10 billion years ago we all would have been closer together. And the light would have been coming from much closer and therefore would have already reached us.

The questions you ask are complicated by the space expansion in the Big Bang model. Please allow me to divide them into two situations and I will answer the questions using Jorrie’s calculator based on PLANCK Data (2015):
  1. Suppose we receive light from an object that is 10 Gly away. We want to know when the received light was emitted and the distance of the object from us at that time.
  2. Suppose we receive light that has traveled 10 Gyr to reach us. We want to know when the light was emitted and the distance of the emitter from us at that time. We also want to know the distance of the emitter from us now.
Answer to Question #1:

The light was emitted 7.39 Gyr ago at a distance of 5.34 Gly from us at that time. It takes the light 7.39 Gyr to reach us. Now the emitter is at a distance of 10 Gly from us.​

Answer to Question #2:

The light was emitted 10 Gyr ago at a distance of 5.84 Gly from us at that time. It takes the light 10 Gyr to reach us. Now the emitter is at a distance of 15.84 Gly from us.​
 
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That is very interesting. Thank you for that. It is complex to think about.
 
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JimJCW said:
  1. Suppose we receive light from an object that is 10 Gly away. We want to know when the received light was emitted and the distance of the object from us at that time.
Answer to Question #1:

The light was emitted 7.39 Gyr ago at a distance of 5.34 Gly from us at that time. It takes the light 7.39 Gyr to reach us. Now the emitter is at a distance of 10 Gly from us.​

Using Jorrie’s calculator, we can obtain the following result about the journeys of the object and the light after they separated 7.39 Gyr ago at a distance of 5.34 Gly from us: the light propagating to us and the object moving with space expansion to its current location.

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Thank you. Your information is helpful. I really appreciate your response.
 

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