# Question regarding light from distant objects

• B
• Robokop
In summary: Gly from us. This is because the space between us and the emitter has expanded during the journey.Answer to Question #2:The light was emitted 10 Gyr ago at a distance of 10 Gly from us at that time. It takes the light 10 Gyr to reach us. Now the emitter is at a distance of 16.1 Gly from us due to the expansion of space. The distance between us and the emitter at the time of emission was 6.1 Gly. This is because the space between us and the emitter has expanded since then.In summary, when we receive light from a distant object, the time and distance at which the light was emitted are affected by the expansion of space.
Robokop
TL;DR 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 can we receive light from a flaslight 10 feet away? The same argument applies, no?

russ_watters
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.)

MikeeMiracle, sysprog, PeroK and 2 others
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.

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.​

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.​

Imager, VEReade, berkeman and 1 other person
That is very interesting. Thank you for that. It is complex to think about.

berkeman
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.

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.

## 1. What is the speed of light?

The speed of light is approximately 299,792,458 meters per second in a vacuum. This is considered to be the fastest speed at which any physical object can travel.

## 2. How does light travel from distant objects?

Light travels in the form of electromagnetic waves from distant objects. These waves are able to travel through the vacuum of space, allowing us to see distant objects such as stars and galaxies.

## 3. How does light from distant objects reach us?

Light from distant objects reaches us through the process of radiation. This means that the light is emitted from the object and travels through space until it reaches our eyes or instruments.

## 4. How can we measure the distance of distant objects using light?

Scientists use a variety of methods to measure the distance of distant objects using light. These include parallax measurements, redshift measurements, and the use of standard candles such as supernovas.

## 5. Can light from distant objects tell us about the history of the universe?

Yes, light from distant objects can tell us about the history of the universe. This is because the light we see from these objects has been traveling for millions or even billions of years, giving us a glimpse into the past and allowing us to study the evolution of the universe.

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