Missing Galaxies? Visible Universe Mystery

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In summary, it is currently believed that the space between us and distant galaxies is expanding at close to the speed of light, causing these galaxies to eventually disappear from our observable universe. However, the expansion of the universe is accelerating, meaning that new galaxies will also continue to appear. The edge of our observable universe is determined by time, not by objects receding faster than the speed of light. Additionally, there is a distinction between the particle horizon (the border of our observable universe) and the event horizon (the location of objects whose light will never reach us in the future).
  • #1
japam
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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
 
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  • #2
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.
 
  • #3
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.
 
  • #4
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.
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
well, somehow I don't think he was interested in the event horizon, since he won't be around at [tex] t= \infty [/tex]!

We should also keep in mind that the Universe was optically thick before the surface of last scattering (around 13.4 billion years ago).
 
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  • #9
matt.o said:
well, somehow I don't think he was interested in the event horizon, since he won't be around at [tex] t= \infty [/tex]!
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.
 
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  • #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 [Broken] 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.
 
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  • #12
Indeed. I think that paper should be read by all who are interested in cosmology!
 

1. What are missing galaxies?

Missing galaxies are galaxies that are not visible in the observable universe, despite being predicted to exist based on our current understanding of the universe. These galaxies are estimated to make up around 95% of all galaxies in the universe.

2. Why are there missing galaxies in the visible universe?

The missing galaxies in the visible universe are thought to be hiding in the form of dark matter. Dark matter is a hypothetical form of matter that does not interact with light and therefore cannot be detected through traditional means. It is believed that dark matter makes up the majority of the mass in the universe and is responsible for the gravitational effects that we observe.

3. How do scientists know that there are missing galaxies?

Scientists have used various methods to study the distribution of matter in the universe, such as studying the rotation of galaxies and the gravitational lensing effect. These studies have shown that there is not enough visible matter in the universe to account for the observed effects, leading to the conclusion that there must be missing galaxies made up of dark matter.

4. Can missing galaxies ever be observed or detected?

At this time, there is no way to directly observe or detect missing galaxies. However, scientists are actively researching and developing new methods and technologies that may one day allow us to detect and study dark matter and its associated galaxies.

5. What impact do missing galaxies have on our understanding of the universe?

The existence of missing galaxies and dark matter has significant implications for our understanding of the universe. It helps to explain the observed structure and behavior of the universe, such as the formation of large-scale structures and the accelerating expansion of the universe. It also presents new challenges for scientists to continue to study and understand this mysterious component of the universe.

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