Question about observable universe.

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Discussion Overview

The discussion revolves around the nature of the observable universe, particularly in relation to the expansion of space and the implications for what can be seen from Earth. Participants explore concepts such as the speed of light, redshift, and the conditions under which light from distant galaxies can reach us.

Discussion Character

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

Main Points Raised

  • One participant questions whether the observable universe diminishes due to the expansion of space outpacing the speed of light, seeking clarification on the definition of the observable universe.
  • Another participant asserts that the observable universe is expanding, not diminishing, and explains that the limitation is due to the age of the universe rather than the speed of expansion.
  • A participant raises a question about the visibility of galaxies with redshifts smaller than 1000, given the existence of the cosmic microwave background (CMB).
  • Another participant responds by mentioning the surface of last scattering and the opacity of the universe before the CMB was emitted, suggesting that a neutrino telescope could provide insights back to the Big Bang.
  • One participant speculates that if a star is at the edge of the expanding universe, its light may not reach us due to the expansion speed exceeding that of light, prompting further clarification.
  • A later reply discusses Hubble's law and the concept of the Hubble sphere, explaining that objects outside this sphere can emit photons that may eventually reach us as the radius of the Hubble sphere increases over time.
  • Another participant poses a question about whether all galaxies can be seen with a sufficiently powerful telescope, suggesting that light from all galaxies has reached Earth, except for those formed very recently and at great distances.
  • A follow-up comment notes that new galaxies continue to form, challenging the assumption that all galaxies were formed in the early universe.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the universe's expansion for the observable universe, with some asserting that it is expanding while others question the visibility of distant galaxies. The discussion remains unresolved regarding the extent to which all galaxies can be observed.

Contextual Notes

Participants reference concepts such as the Hubble sphere, redshift, and the cosmic microwave background, but there are unresolved assumptions regarding the definitions and implications of these terms. The discussion also touches on the limitations of current observational capabilities.

f24u7
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If the unobservable universe is defined by the region in which light originated in it is incapable of reaching Earth due to having the expansion of space rate faster than c, the speed of light, thus causing light unable to travel to earth, than under this definition would not our observable universe diminish since light sources located on the edge of our observable universe is constantly outspeed by the expansion of space, and if our observable universe is not diminishing than i must got something wrong here, can someone explain?

thanks in advance
 
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f24u7 said:
If the unobservable universe is defined by the region in which light originated in it is incapable of reaching Earth due to having the expansion of space rate faster than c, the speed of light, thus causing light unable to travel to earth, than under this definition would not our observable universe diminish...

thanks in advance

That's not the definition of the boundary of the observable universe. It doesn't matter that the "expansion speed" is greater than c. The reason why we can't see things from outside of our observable universe is because the light from them has not yet had time to reach us in the age of the universe. As more time passes, light from more and more distant things eventually reaches us. For this reason, our observable universe (or horizon) is actually expanding, not "diminishing."
 
If the farthest thing we can see is the 3 degree microwave background which was cosmolgically redshifted from 3700 degrees Kelvin( a redshift of over 1000), why can't we see eveything (All galaxies, etc) with red shifts smaller than 1000 if we have a large enough telescope?
 
Surface of last scattering is the short answer. The universe was optically opaque before the CMB was emitted. A neutrino telescope could penetrate the haze almost back to the big bang. The aperature required is, however, unimaginably large.
 
Thecla said:
If the farthest thing we can see is the 3 degree microwave background which was cosmolgically redshifted from 3700 degrees Kelvin( a redshift of over 1000), why can't we see eveything (All galaxies, etc) with red shifts smaller than 1000 if we have a large enough telescope?

We have to wait 4 more years.
http://www.jwst.nasa.gov/"
 
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Hmmm... So basically, you have a star on the edge of the expanding universe. And you are saying that because the universe is expanding faster than the speed of light, that we shouldn't be able to see the light from that star? Did I get that correctly?
 
IanTBlack said:
Hmmm... So basically, you have a star on the edge of the expanding universe. And you are saying that because the universe is expanding faster than the speed of light, that we shouldn't be able to see the light from that star? Did I get that correctly?

Not quite. If we take Hubble's law V=HD, we can calculate distance at which recession speed becomes superluminal using V=C, thus D= C/H. This defines so called Hubble sphere. Things outside that sphere are receding faster than light, but it is not horizon of any kind.

If object outside of Hubble sphere emits photon towards us, that photon is going with local velocity of c, but since recession velocity is greater than c , total velocity is away from us.

Now interesting part. Radius of Hubble sphere is increasing with time. So, some of the photons that were on the superluminal side, cross to the subluminal side, and eventually find their way to us. Thus we can observe things that are receding faster than light.
 
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I wanted to make a point(It is really a question)
Can every galaxy in the universe be seen by us if we have a large enough telescope obseving at right wavelengths(visible, infrared and microwave)?
I think the answer is yes.

My thinking: We can see the universe before the galaxies were formed - the microwave background radiation.
Any galaxy was formed shortly after that and would also be visible,i.e. the light has reached the earth. There is no galaxy whose light hasn't reached us.

The only way a distant galaxy's light could not have reached the Earth is if it is very distant , say 1 billion light years away AND formed more recently than a billion years ago.
However, I think, except for collisions, all galaxies were formed in the early universe.
 
Thecla said:
However, I think, except for collisions, all galaxies were formed in the early universe.

Galaxy I Zwicky 18 is less than a billion years old. New galaxies continue to be formed.
 

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