Missing Galaxies? Visible Universe Mystery

  • Context: Undergrad 
  • Thread starter Thread starter japam
  • Start date Start date
  • Tags Tags
    Galaxies
Click For Summary

Discussion Overview

The discussion revolves around the concept of the "visible universe," particularly focusing on the behavior of galaxies at the limits of our observable universe and the implications of cosmic expansion. Participants explore whether galaxies are moving faster than light, the nature of the observable universe, and the potential for galaxies to disappear from our view over time.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that galaxies at the edge of the visible universe are receding faster than light due to the expansion of space, leading to the idea that they may eventually disappear from our observational reach.
  • Others argue that while galaxies may redshift beyond detectable limits, no currently observable object will "blink" out of view, as they will still maintain a causal connection with our observable universe.
  • One participant clarifies the distinction between the particle horizon and the event horizon, suggesting that the particle horizon is always growing, while the event horizon represents objects whose light will never reach us in the future.
  • Another participant mentions that the universe was optically thick before the surface of last scattering, which could influence observations of distant galaxies.
  • Some contributions highlight that while light from distant galaxies may be received from the past, the current emissions from those galaxies will not be observable in the future due to the expanding universe.
  • A later reply references a paper that discusses common misconceptions about cosmological horizons and the superluminal expansion of the universe, suggesting that the boundary of the observable universe is set by the particle horizon rather than the speed of light limit.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of cosmic expansion on the visibility of galaxies. There is no consensus on whether galaxies will disappear from view or how this relates to the concepts of particle and event horizons.

Contextual Notes

Limitations include varying interpretations of what it means for galaxies to "disappear from our reach," as well as the dependence on definitions of particle and event horizons. The discussion also reflects uncertainty about the implications of redshift and the expansion of the universe on observational astronomy.

japam
Messages
37
Reaction score
0
please correct me if this is a misconception that i have about "visible Universe" . Is true or not that in the limits of the universe visible to us , galaxies move faster than light and because of that we cannot detect it, and that this occurs continually, ( i mean more and more galaxies every day disappear of our reach scope?),
i have other questions related but first need to clarify this
 
Astronomy news on Phys.org
japam said:
please correct me if this is a misconception that i have about "visible Universe" . Is true or not that in the limits of the universe visible to us , galaxies move faster than light and because of that we cannot detect it, and that this occurs continually, ( i mean more and more galaxies every day disappear of our reach scope?),
i have other questions related but first need to clarify this
It is currently considered that the space between us and those distant galaxies is expanding at close to the speed of light. Thus over time these galaxies will disappear over time, along with every other galaxy over a much much greater period of time. However considering how often the wind changes in this particular field, it wouldn't surprise me if the opposite were true. I.E New galaxies showing up all the time.
 
japam said:
please correct me if this is a misconception that i have about "visible Universe" . Is true or not that in the limits of the universe visible to us , galaxies move faster than light and because of that we cannot detect it, and that this occurs continually, ( i mean more and more galaxies every day disappear of our reach scope?),
i have other questions related but first need to clarify this


Based on our current understanding yes.

As we now understand our observations, the expansion of the universe is accelerating, i.e. the universe is blowing itself apart faster and faster. The predicted (not observed) result of this is that, as space expands faster, objects currently at the edge of our vision will move beyond our ability to see them, essentially they'll be moving so fast, their light will never reach us.

Maybe pervect or Spacetiger can help here, I'm not entirely clear on the relativistic issues of this.
 
No object currently observable will ever 'blink' out of view [i.e., sever the causal connection with our observable universe]. They may, however, eventually redshift beyond detectable limits.
 
japam said:
please correct me if this is a misconception that i have about "visible Universe" . Is true or not that in the limits of the universe visible to us , galaxies move faster than light and because of that we cannot detect it, and that this occurs continually, ( i mean more and more galaxies every day disappear of our reach scope?),
i have other questions related but first need to clarify this


no, the edge of the visible universe is the point that space is expanding at the same rate as the speed of light.

because the space between galaxies is expanding at an accelerated rate, the say will come when people will look into the sky and only see the stars in the milkyway. the universe will seem a lot smaller at that point, but it will in fact be much much larger.
 
ComputerGeek said:
no, the edge of the visible universe is the point that space is expanding at the same rate as the speed of light.

the edge of our observable universe is not limited by objects appearing to recede faster than the speed of light due to the Hubble flow. It is limited by time. ie. has the photon emitted by a galaxy at the edge of the observable Universe had enough time to reach us? It only has roughly 13.5 billion years.
 
japam said:
please correct me if this is a misconception that i have about "visible Universe" . Is true or not that in the limits of the universe visible to us , galaxies move faster than light and because of that we cannot detect it, and that this occurs continually, ( i mean more and more galaxies every day disappear of our reach scope?),
i have other questions related but first need to clarify this
You should distinguish between two concepts: the particle horizon and the event horizon.

The particle horizon is the location of a light ray sent from our position at t = 0. This is the border of our observable universe and is about 45,000 million light-years, much more than 13.700 million light-years (13,700 million years is the age of the universe) because space has expanded between our position and the light ray during all this time. The particle horizon tells us about objects whose light sent in past may reach us.

The event horizon is the location of objects whose light will never reach us in future, because space is expanding very fast and light will never be able to overcome the growing distance. The event horizon is more or less located at 13,700 million light-years and this corresponds to the distance at which objects are currently receding at the speed of light d = c / H (the Hubble sphere).

The particle horizon is always growing, but the event horizon will remain at c/H in the current cosmological model.
 
well, somehow I don't think he was interested in the event horizon, since he won't be around at t= \infty!

We should also keep in mind that the Universe was optically thick before the surface of last scattering (around 13.4 billion years ago).
 
Last edited:
matt.o said:
well, somehow I don't think he was interested in the event horizon, since he won't be around at t= \infty!
It depends on what you (or he) understand with "dissapear of our reach scope". For example, a galaxy located at z = 2. We may receive light from it as it was in past, but we will never receive the light it is currently emitting. Of course this does not mean that in future we will see less galaxies than now, because our particle horizon is always growing. However, that galaxy, or the light it emits now, is dissapearing from our (future) reach.

matt.o said:
We should also keep in mind that the Universe was optically thick before the surface of last scattering (around 13.4 billion years ago).
Yes, but the difference in the distance between the last scattering surface and the particle horizon is less than 2%. Note that if the age of the universe is 13,665.8 million years, the last-scattering surface was emitted 13,665.4 million years ago.
 
Last edited:
  • #10
hellfire said:
It depends on what you (or he) understand with "dissapear of our reach scope". For example, a galaxy located at z = 2. We may receive light from it as it was in past, but we will never receive the light it is currently emitting. Of course this does not mean that in future we will see less galaxies than now, because our particle horizon is always growing. However, that galaxy, or the light it emits now, is dissapearing from our (future) reach.

This is completely true. However, sometimes adding extra info can confuse the questioner.

hellfire said:
Yes, but the difference in the distance between the last scattering surface and the particle horizon is less than 2%. Note that if the age of the universe is 13,665.8 million years, the last-scattering surface was emitted 13,665.4 million years ago.

oh well, my little computer program gave me 13.4Gyr for z=1087.
 
  • #11
Perhaps this paper will help clarify some issues for the more technically inclined:

"[URL Confusion:
common misconceptions of cosmological horizons
and the superluminal expansion of the universe[/URL]


japam said:
Is true or not that in the limits of the universe visible to us , galaxies move faster than light and because of that we cannot detect it,

In the current model, all objects beyond redshifts of ~1.5 are receding faster than the speed of light. However, we have already observed objects out to redshifts of > 6, so clearly this doesn't set the boundary of the observable universe. The boundary is actually set by the particle horizon, which hellfire mentioned in one of his posts.
 
Last edited by a moderator:
  • #12
Indeed. I think that paper should be read by all who are interested in cosmology!
 

Similar threads

Undergrad Luminous Fast Blue Optical Transients (LFBOTs)
  • · Replies 0 ·
Replies
0
Views
688
Undergrad How is Dark Matter distributed around our galaxy?
  • · Replies 6 ·
Replies
6
Views
3K
Undergrad How Does the Balloon Analogy Explain the Expansion of the Universe?
  • · Replies 5 ·
Replies
5
Views
1K
Undergrad Ultraluminous X-ray source in the galaxy NGC 55
  • · Replies 0 ·
Replies
0
Views
2K
Undergrad Why don't any stars "look" red? red shift
  • · Replies 7 ·
Replies
7
Views
4K
Undergrad Could fast radio bursts be alien radar signals?
  • · Replies 9 ·
Replies
9
Views
1K
High School The news media on 2 trillion galaxies: Confused, or am I?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Galaxy recession and Universe expansion
  • · Replies 90 ·
4
Replies
90
Views
5K
High School How much of the visible universe have we explored?
  • · Replies 2 ·
Replies
2
Views
22K
Undergrad Can We See All Galaxies in the Universe?
  • · Replies 17 ·
Replies
17
Views
5K