Can We See All Galaxies in the Universe?

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Discussion Overview

The discussion centers around the visibility of galaxies in the universe, exploring whether all galaxies can be seen by the light they emit. Participants delve into the implications of distance, redshift, and the limitations of optical observation in understanding the existence of far-off galaxies.

Discussion Character

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

Main Points Raised

  • Some participants assert that not all galaxies are visible due to their distance and the time it takes for light to reach Earth, with light from some galaxies redshifted into the infrared spectrum.
  • There is mention of the Hubble Ultra Deep Field (HUDF) photo, which revealed a vast number of galaxies, surprising many participants.
  • The James Webb Space Telescope is expected to detect galaxies beyond the sensitivity limits of the Hubble Space Telescope, suggesting that many more galaxies exist than currently observed.
  • Some participants discuss the concept of the observable universe (OU) and the universe (U), noting that the OU is limited to what can be detected through radiation, while the U may be much larger or even infinite.
  • There are questions about the implications of the universe's size on the Big Bang theory, with some suggesting that the universe could be much larger than currently understood.
  • Participants express uncertainty about the existence of galaxies beyond the cosmic microwave background (CMB) and the implications of such a statement.
  • Discussion includes the concept of low surface brightness galaxies, which are difficult to detect due to their lack of bright stars and active nuclei.

Areas of Agreement / Disagreement

Participants generally agree that not all galaxies are visible, but there is no consensus on the implications of this visibility or the exact nature of the universe's size. Multiple competing views remain regarding the relationship between the observable universe and the entirety of the universe.

Contextual Notes

Limitations in the discussion include the dependence on definitions of the observable universe and the universe, as well as unresolved questions about the implications of redshift and the nature of galaxies that may exist beyond current observational capabilities.

karl22
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Hi everybody
I just wanted to ask are all the galaxies in the universe visible by the light they send?
In the other words are all the galaxies optically visible ?How do scientists predict the number of galaxies and do they understand the existence of other galaxies(very far galaxies) by optical observation or in some cases no light is visible and they must only rely on the nucleolus radiation ?How do they understand the existence of very far galaxies ?

Thanks you all
Have a nice day
 
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Are ALL galaxies visible? In short, no. But not because they don't emit any light, its because they are beyond our visibility due to their distance and the time it takes for light to reach us.

Some galaxies are so far away that the light has been redshifted from the visible spectrum and into the infrared spectrum, but they are still there.

And I've never heard of "nucleolus radiation" before. What is that?

See here for some more info: http://en.wikipedia.org/wiki/Hubble_ultra_deep_field
 
When the HUDF photo { shown in the wiki link above} was first published, I was dumb-struck. While we all expected many galaxies, no one could of predicted the vast amount that was shown to us.
I agree with Drakkith, the answer is no.
 
hypatia said:
When the HUDF photo { shown in the wiki link above} was first published, I was dumb-struck.

Wow, I'll second that, completely amazed by that photo.
 
jarednjames said:
Wow, I'll second that, completely amazed by that photo.

Same. Bout the only that compared was the view of Jupiter back in November through my telescope. First time I'd ever seen anything other than the moon in a telescope. Amazing...
 
The James Webb telescope is designed for infrared astrophotgraphy. Most scientists anticipate it will detect numerous galaxies beyond the sensitivity limits of the HST. Astrophysicists project the number of galaxies in the universe by extrapolating the number detected in small regions [like the Hubble deep field]. Large field surveys are limited by sensitivity - they only detect the more luminous distant galaxies.
 
karl22 said:
Hi everybody
I just wanted to ask are all the galaxies in the universe visible by the light they send?
In the other words are all the galaxies optically visible ?How do scientists predict the number of galaxies and do they understand the existence of other galaxies(very far galaxies) by optical observation or in some cases no light is visible and they must only rely on the nucleolus radiation ?How do they understand the existence of very far galaxies ?

Thanks you all
Have a nice day

Yes, all galaxies emit light but as Drakkith stated, not all of that light reaches the Earth and most of it that does is so faint as to be invisible to the human eye (but as the Hubble data has shown, it IS there).

It is the isotopy of the U that forces the assumption that the galaxies out at our event horizon, and the ones for at least the next 34 or so billion light years, are there, and the current speculation as to how much further out that isotropy goes ranges from huge numbers to numbers so big as to be pretty much meaningless to the human brain.
 
Every galaxy that can possibly exist must reside in front of the CMB. There are no galaxies beyond the CMB.
 
Chronos said:
Every galaxy that can possibly exist must reside in front of the CMB. There are no galaxies beyond the CMB.

Yes, but that's a statement about TIME, right? I mean as opposed to being a statement about distance. OUR view of the CMB goes out ~45B light years in current distance and 14B yrs old but if we were out at a point currently 45B light years away from earth, we would still see, in ALL directions, a CMB that is 45B LY away and 14B yrs old including a point that is 90B LY away from earch in current distance.
 
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  • #10
Yes, but, bear in mind that point in space is also 14 billion years in our future. Photons currently being emitted by our sun will not arrive there for another 14 billion years [and that assumes no expansion of the universe]. Since the sun is less than 14 billion years of age, no photons from our sun [or earth] are available to observe from that location. The distance to Earth is effectively meaningless.
 
  • #11
I agree completely w/ what you are saying, but saying that such galaxies are beyond our reach is NOT equivalent to saying that they don't exist. I don't think that's just semantics.
 
  • #12
It is physics, not semantics
 
  • #13
Drakkith said:
Are ALL galaxies visible? In short, no. But not because they don't emit any light, its because they are beyond our visibility due to their distance and the time it takes for light to reach us.

Some galaxies are so far away that the light has been redshifted from the visible spectrum and into the infrared spectrum, but they are still there.
Does that mean the limit of the visible specturm is ~14billion light years?
Is it possible for universe to be 100+trillion light years large but we just can't see it? and if that's the case then the big bang theory could be incorrect?
 
  • #14
Chaballa said:
Is it possible for universe to be 100+trillion light years large but we just can't see it? and if that's the case then the big bang theory could be incorrect?

The consensus, to the extent that there is one, seems to be that the U is at least MANY times larger than the OU and perhaps infinite. This would be in accordance with the BB theory, not in contradiction to it. The BB was not an explosion that happened as one point and spread outward. Many physicists believe that 100+ trillion LYs would be a trivial diameter for the U.

Problem is that we can't detect, in any way, anything outside our OU, so everything is just theory. To think that the U ends abruptly at the edge of our OU seems just silly (AND would have massive and extremely weird implications for physics), but we may never know for sure.
 
  • #15
phinds said:
The consensus, to the extent that there is one, seems to be that the U is at least MANY times larger than the OU and perhaps infinite. This would be in accordance with the BB theory, not in contradiction to it. The BB was not an explosion that happened as one point and spread outward. Many physicists believe that 100+ trillion LYs would be a trivial diameter for the U.

Problem is that we can't detect, in any way, anything outside our OU, so everything is just theory. To think that the U ends abruptly at the edge of our OU seems just silly (AND would have massive and extremely weird implications for physics), but we may never know for sure.

What is the U and OU ?
 
  • #17
Jadaav, the OU is a sphear about 50 billion light years radius from us. It represents that part of the U that we are able to detect through various forms of radiation. This is the ACTUAL size; the apparent size is only 15B light years radius but when that radiation started traveling towards us 15B years ago, it's source was moving outward and has been moveing outward for 15B years so it's much further away now than it was when the radiation started towards us. It can be a bit confusing at first but there are lots of introductory explanations on the net.
 
  • #18
Perhaps slightly OT, but I remember reading in an Astronomy mag about 'low surface brightness' galaxies. They don't have many bright stars, they seem to lack an active nucleus, but they contain a high proportion of non-luminous mass...
http://en.wikipedia.org/wiki/Low_surface_brightness_galaxy

As I understand it, they are hard enough to spot in the 'Local Group', so would be almost invisible ( lost against background ) if further away...
 

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