# The Naked Scientists Forum

### Author Topic: How does light from other galaxies reach us?  (Read 2416 times)

#### sam7

• Jr. Member
• Posts: 48
##### How does light from other galaxies reach us?
« on: 18/02/2016 12:10:03 »
How does light from other galaxies reach us if each galaxy is expanding away from each other faster than the speed of light?

Is it because when people say galaxies are expanding faster than light, they really mean the combined velocities of two galaxies moving in opposite direction? So one galaxy is actually not moving faster than c?

« Last Edit: 19/02/2016 16:06:55 by chris »

#### chiralSPO

• Global Moderator
• Neilep Level Member
• Posts: 1872
• Thanked: 143 times
##### Re: Light from other galaxies
« Reply #1 on: 18/02/2016 17:07:59 »
This is a tricky concept. I'm not sure I grasp it entirely myself, but I'll give it a go.

Nothing can move through space faster than the speed of light. However, there is no mechanism (that we know of) that prevents space between two object from expanding at an arbitrary rate, such that the distance between the objects increases faster than the speed of light.

It appears that space is expanding all around us such that extremely distant objects (galaxies) appear to be moving away from us at speeds approaching the speed of light. If the distance between us and them is increasing faster than light can traverse the space, we can't (and won't ever) observe them.

There are a few other reasons we may not be able to see distant galaxies:
1) They are so far away that not enough light can reach us (too dim).
2) They are so far away that other things get the way and block the light.
3) They are space between us and them is expanding at nearly the speed of light, and the light gets redshifted so far that we cannot observe it
4) They are so far away that the light hasn't had enough time to reach us (the universe has only been transparent for a little over a dozen billion years, so something 100 billion light years away won't be visible for quite some time, even if the other problems don't manifest.)

Hope that helps.

#### Thebox

• Neilep Level Member
• Posts: 3154
• Thanked: 44 times
##### Re: Light from other galaxies
« Reply #2 on: 18/02/2016 22:49:09 »
This is a tricky concept. I'm not sure I grasp it entirely myself, but I'll give it a go.

Nothing can move through space faster than the speed of light. However, there is no mechanism (that we know of) that prevents space between two object from expanding at an arbitrary rate, such that the distance between the objects increases faster than the speed of light.

It appears that space is expanding all around us such that extremely distant objects (galaxies) appear to be moving away from us at speeds approaching the speed of light. If the distance between us and them is increasing faster than light can traverse the space, we can't (and won't ever) observe them.

There are a few other reasons we may not be able to see distant galaxies:
1) They are so far away that not enough light can reach us (too dim).
2) They are so far away that other things get the way and block the light.
3) They are space between us and them is expanding at nearly the speed of light, and the light gets redshifted so far that we cannot observe it
4) They are so far away that the light hasn't had enough time to reach us (the universe has only been transparent for a little over a dozen billion years, so something 100 billion light years away won't be visible for quite some time, even if the other problems don't manifest.)

Hope that helps.

Hello Chiral, you say ''the Universe has only been transparent for a little over a dozen billion years, so something 100 billion light years away won't be visible for quite some time''

Can I ask why it would not be visible for some time  if you state the universe is transparent?

Can we not see through transparent things?

#### chiralSPO

• Global Moderator
• Neilep Level Member
• Posts: 1872
• Thanked: 143 times
##### Re: Light from other galaxies
« Reply #3 on: 18/02/2016 23:09:01 »

Hello Chiral, you say ''the Universe has only been transparent for a little over a dozen billion years, so something 100 billion light years away won't be visible for quite some time''

Can I ask why it would not be visible for some time  if you state the universe is transparent?

Can we not see through transparent things?

A long time ago, the universe was very densely filled with very hot matter, and was therefore not transparent. Then, because it takes light a year (time) to travel a light year (distance), if an object is 100 billion light years away, it would take light 100 billion years for its light to reach us. Since light has only been able to travel through the universe for 13 billion years, there is no hope of seeing the light from such a distant object.

#### Thebox

• Neilep Level Member
• Posts: 3154
• Thanked: 44 times
##### Re: Light from other galaxies
« Reply #4 on: 18/02/2016 23:17:19 »

Hello Chiral, you say ''the Universe has only been transparent for a little over a dozen billion years, so something 100 billion light years away won't be visible for quite some time''

Can I ask why it would not be visible for some time  if you state the universe is transparent?

Can we not see through transparent things?

A long time ago, the universe was very densely filled with very hot matter, and was therefore not transparent. Then, because it takes light a year (time) to travel a light year (distance), if an object is 100 billion light years away, it would take light 100 billion years for its light to reach us. Since light has only been able to travel through the universe for 13 billion years, there is no hope of seeing the light from such a distant object.

Interesting idea, would hot matter not give off light?

#### chiralSPO

• Global Moderator
• Neilep Level Member
• Posts: 1872
• Thanked: 143 times
##### Re: Light from other galaxies
« Reply #5 on: 19/02/2016 00:04:11 »
Interesting idea, would hot matter not give off light?

Yes. It is now what we call the cosmic microwave background. It was visible light when it was emitted, but has been redshifted so much that it now registers as microwaves.

#### evan_au

• Neilep Level Member
• Posts: 4105
• Thanked: 245 times
##### Re: Light from other galaxies
« Reply #6 on: 19/02/2016 09:39:18 »
''the Universe has only been transparent for a little over a dozen billion years, so something 100 billion light years away won't be visible for quite some time'
This is the concept of the "observable Universe". There are a few odd possibilities, like:
- Things that we see now may become invisible in the future
- Things that we can't see now could become visible in the future (although dark energy makes this outcome less likely)

The following users thanked this post: Thebox

#### Thebox

• Neilep Level Member
• Posts: 3154
• Thanked: 44 times
##### Re: Light from other galaxies
« Reply #7 on: 19/02/2016 09:58:00 »
Interesting idea, would hot matter not give off light?

Yes. It is now what we call the cosmic microwave background. It was visible light when it was emitted, but has been redshifted so much that it now registers as microwaves.

Thanks , so does this CBMR you mention have substance, or is it just a Physical presence like Electromagnetic radiation?

#### evan_au

• Neilep Level Member
• Posts: 4105
• Thanked: 245 times
##### Re: Light from other galaxies
« Reply #8 on: 19/02/2016 10:42:02 »
Quote from: TheBox
does this CBMR you mention have substance, or is it just a Physical presence like Electromagnetic radiation?
CMBR is electromagnetic radiation, with a peak in the microwave part of the spectrum (160 GHz).
See: http://en.wikipedia.org/wiki/Cosmic_microwave_background

..although I wouldn't really call electromagnetic radiation a "Physical presence", like (say) atoms or molecules.

#### Thebox

• Neilep Level Member
• Posts: 3154
• Thanked: 44 times
##### Re: Light from other galaxies
« Reply #9 on: 19/02/2016 12:11:40 »
Quote from: TheBox
does this CBMR you mention have substance, or is it just a Physical presence like Electromagnetic radiation?
CMBR is electromagnetic radiation, with a peak in the microwave part of the spectrum (160 GHz).
See: http://en.wikipedia.org/wiki/Cosmic_microwave_background

..although I wouldn't really call electromagnetic radiation a "Physical presence", like (say) atoms or molecules.
By physical presence I perceive that to be things that exist without solidity such as light or gravity.  I would refer to atoms as physical existence, objects. I consider a physical presence can pass through a physical existence and like wise a physical existence can pass through a physical presence.
Am I correct in thinking that cosmic background microwave radiation remains present in space without what we refer to as light, ''dark''?

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1914
• Thanked: 123 times
##### Re: Light from other galaxies
« Reply #10 on: 19/02/2016 13:43:06 »
Am I correct in thinking that cosmic background microwave radiation remains present in space without what we refer to as light
microwaves are electromagnetic radiation just like light, but much closer in frequency to radio waves. As such they do not depend on the presence of light or dark but propagate independantly.

#### Thebox

• Neilep Level Member
• Posts: 3154
• Thanked: 44 times
##### Re: Light from other galaxies
« Reply #11 on: 19/02/2016 13:44:59 »
Am I correct in thinking that cosmic background microwave radiation remains present in space without what we refer to as light
microwaves are electromagnetic radiation just like light, but much closer in frequency to radio waves. As such they do not depend on the presence of light or dark but propagate independantly.

Thank you Colin, and does the cbmr ''fill'' the visual universe space?

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1914
• Thanked: 123 times
##### Re: How does light from other galaxies reach us?
« Reply #12 on: 19/02/2016 16:33:44 »
does the cbmr ''fill'' the visual universe space
As far as I am aware it is detected in every direction, but in varying intensities.

#### Thebox

• Neilep Level Member
• Posts: 3154
• Thanked: 44 times
##### Re: How does light from other galaxies reach us?
« Reply #13 on: 19/02/2016 19:52:15 »
does the cbmr ''fill'' the visual universe space
As far as I am aware it is detected in every direction, but in varying intensities.

Interesting, how do we distinguish the difference between the CBMR and electromagnetic radiation emitted from stars etc?

In a rephrased form, how do we know that CBMR is not the detection of light propagating through space if it is detected as ''light''?

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1914
• Thanked: 123 times
##### Re: How does light from other galaxies reach us?
« Reply #14 on: 19/02/2016 22:39:13 »
does the cbmr ''fill'' the visual universe space
As far as I am aware it is detected in every direction, but in varying intensities.

Interesting, how do we distinguish the difference between the CBMR and electromagnetic radiation emitted from stars etc?

In a rephrased form, how do we know that CBMR is not the detection of light propagating through space if it is detected as ''light''?
I will no longer take part in discussions with you as you deliberately misquoted me and are therefore likely to do it again.
I'm out

#### evan_au

• Neilep Level Member
• Posts: 4105
• Thanked: 245 times
##### Re: How does light from other galaxies reach us?
« Reply #15 on: 19/02/2016 23:58:41 »
Quote from: TheBox
By physical presence I perceive that to be things that exist without solidity such as light or gravity.
- By "Physical", I interpret something that can be touched and felt*.
- By "presence", I interpret something that is here, and stationary relative to me.

But:
- Light cannot be touched or felt, so I would call it non-physical.
- Light cannot be stationary relative to me, so I would call it non-presence.
- If I were to make up a description of light, I prefer "non-physical non-presence".

The same could be effectively said for gravitational waves and neutrinos - if they could be stationary, they would be definitely non-touchable.

*Dictionary.com has 8 possible meanings for physical; part of one meaning does allow for energy as a subject for non-biological sciences

Quote
how do we distinguish the difference between the CBMR and electromagnetic radiation emitted from stars etc?
By their spectrum.
- CMBR has an effective temperature of 2.7K, so its spectrum has a peak in the microwave band, and not much at Infra-Red.
- Our Sun has a surface temperature of 5500K, so it has a peak in the visible band, with lots of infra-red, a bit of ultraviolet, and a moderate amount of microwaves.
- Other stars, galactic dust and gas clouds have different temperatures and composition, each with their own spectrum.

By measuring the radiation in space at multiple wavelengths, and by modeling the distribution of gas in the galaxy, it is possible to subtract out the effect of stars and dust clouds, leaving the contribution of the CMBR.

A description of the process and sample images of each step may be found here: http://abyss.uoregon.edu/~js/ast123/lectures/lec23.html http://en.wikipedia.org/wiki/Cosmic_microwave_background#Data_reduction_and_analysis

#### Thebox

• Neilep Level Member
• Posts: 3154
• Thanked: 44 times
##### Re: How does light from other galaxies reach us?
« Reply #16 on: 20/02/2016 09:09:07 »
Quote from: TheBox
By physical presence I perceive that to be things that exist without solidity such as light or gravity.
- By "Physical", I interpret something that can be touched and felt*.
- By "presence", I interpret something that is here, and stationary relative to me.

But:
- Light cannot be touched or felt, so I would call it non-physical.
- Light cannot be stationary relative to me, so I would call it non-presence.
- If I were to make up a description of light, I prefer "non-physical non-presence".

The same could be effectively said for gravitational waves and neutrinos - if they could be stationary, they would be definitely non-touchable.

*Dictionary.com has 8 possible meanings for physical; part of one meaning does allow for energy as a subject for non-biological sciences

Quote
how do we distinguish the difference between the CBMR and electromagnetic radiation emitted from stars etc?
By their spectrum.
- CMBR has an effective temperature of 2.7K, so its spectrum has a peak in the microwave band, and not much at Infra-Red.
- Our Sun has a surface temperature of 5500K, so it has a peak in the visible band, with lots of infra-red, a bit of ultraviolet, and a moderate amount of microwaves.
- Other stars, galactic dust and gas clouds have different temperatures and composition, each with their own spectrum.

By measuring the radiation in space at multiple wavelengths, and by modeling the distribution of gas in the galaxy, it is possible to subtract out the effect of stars and dust clouds, leaving the contribution of the CMBR.

A description of the process and sample images of each step may be found here: http://abyss.uoregon.edu/~js/ast123/lectures/lec23.html http://en.wikipedia.org/wiki/Cosmic_microwave_background#Data_reduction_and_analysis

Thank you evan, I am a little confused.

You say cbmr as a temperature of 2.7k.  Then explain the sun to be being 5500k, CBMR is something we visually observe?

What temperature is light propagating through space, you did not mention the light propagating through space, how do we detect that ?

#### The Naked Scientists Forum

##### Re: How does light from other galaxies reach us?
« Reply #16 on: 20/02/2016 09:09:07 »