<p>Aside from being, I suspect, totally incompatible with GR, the model fails to match observations.</p>

<p>The universe appears to be homogeneous and isotropic. In the model you suggest, Hubble's law would be different depending on which direction you were looking.</p>

<p>In particular, looking away from the black hole, galaxy redshifts would increase to some asymptotic value, whereas looking towards the black hole they would increase to infinity.</p>

<p>If the argument is that the central black hole is so far away that we cannot measure this asymmetry (i.e. well beyond the observable universe, given the homogeneity of the cosmic microwave background - but then does the model have an explanation for the CMB at all?), then the model also needs to explain why galaxies would move in this particular way, since they would not "feel" the influence of the black hole.</p>

CC BY-SA 3.0
2018-02-13T10:25:52.677Z
Am I right to say that the best point against this scenario is indeed the observation of CMB? In this scenario it won't be a spherical dome. Even supposing it is light emitted by far far objects we still need a rather homogeneous universe and especially there shouldn't be any background looking in transverse directions, ie those perpendicular to our galaxy trajectory. Please confirm that. This kind of question always makes me doubt that I really learned something owing to my little effort to understand cosmology.:)
Alchimista
CC BY-SA 3.0
2018-02-13T10:37:25.380Z
@Alchimista The CMB is the high redshift limit of what we are able to observe in the universe. Indeed it would be anisotropic in the scenario proposed if observed at a given lookback time, though I am not even sure what mechanism would be proposed to produce it in this model. Let alone problems with structure formation; the abundance of chemical elements etc.
ProfRob
CC BY-SA 3.0
2018-02-13T07:48:04.503Z
2018-02-13T07:48:04.503Z
ProfRob
<p>For a contracting universe, space would contract as a ratio, and hence even if we could only see a small distance, we would still see blue-shifted galaxies due to the fact that the galaxies further from the hypothetical centre of the universe (see <a href="https://astronomy.stackexchange.com/questions/874">here</a> for why I say hypothetical) would be moving faster than those closer, and hence any two points in space must be moving towards each other. This holds true even in a finite universe, all points appear to contract towards the point of reference, as can be seen in the diagram below.</p>

<p><a href="https://i.sstatic.net/g7xKM.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/g7xKM.jpg" alt="enter image description here"></a>
The distance between Galaxies 1 and 3 and Galaxy 2 is clearly less after contraction than before.</p>

CC BY-SA 3.0
2018-02-13T10:17:46.773Z
I do not understand why galaxies far away shall move faster than those near the centre. I think the OP means galaxy falling into a center not a rubber analogy. This kind of topics drive me crazy all the time in spite of my cosmology efforts. :)
Alchimista
CC BY-SA 3.0
2018-02-13T03:20:32.557Z
2018-02-13T03:20:32.557Z
damned truths
<p>So in this model, all of the observable universe is expanding. It is simply that it is part of a larger universe which is being locally stretched, as part of a an accelerating convergence to a central point.</p>

<p>I think the answer is that we cannot distinguish this model from an expanding universe (assuming the structure of the convergence is very carefully tuned to look like what we see "locally"). </p>

<p>On the other hand, it makes no difference as far as the observable universe is concerned, so we have to resort to Occam's razor. The expanding universe is simpler, and arises naturally from our theories of gravity, etc.</p>

CC BY-SA 4.0
2018-07-15T20:40:48.340Z
2018-07-15T20:40:48.340Z
Steve Linton
<p>A contracting universe is an attractive proposition as it seems to do away with the need for dark energy.
Equally a primordial black hole is an attractive proposition as its expansion would seem to indicate that time/events can never be  entirely squeezed out of existence.(ie:if time/events ceased to be, there could never be an initial expansion). Both these propositions are currently unproven.
A dirty initial expansion seems feasible (ie: one where super massive black holes split from the primordial black hole and seeded the universe, allowing the creation of galaxies which could not themselves have produced  black holes of this size in the time available).
Obviously we can only observe/photograph what to the observer are present space/ time events (as their light reaches us) which events have taken place previously in the space/time continuum. Presumably a universal gravitational field has always existed within the space time continuum (even inside an event horizon which may have evolved and expanded as the primordial black hole decayed), which will have affected all events and structures in the universe.
Cosmic background radiation  can perhaps be thought of as the remnant of the original event horizon/expansion now massively red shifted.
It is difficult to see how  observed red shifts can definitively prove whether the universe is expanding or contracting.</p>

CC BY-SA 4.0
2020-10-11T01:26:38.917Z
This isn't written as a proper Stack Exchange answer. It puts forward the author's ideas but does not provide supporting sources or references or even argue for itself using standard tools of science e.g. mathematics. Stack Exchange is a little different than other websites See [How do I write a good answer?](https://astronomy.stackexchange.com/help/how-to-answer)
uhoh
CC BY-SA 4.0
2020-10-09T23:35:03.047Z
2020-10-09T23:35:03.047Z
Guest
<p>What if we just can’t see far enough and the redshift actually shows the galaxies speeding up towards a gigantic black hole to where everything is converging, repeating a cycle that took place countless times “before” the big bang?</p>

<p>Complete text in this link: <a href="http://www.therealebook.com/Astronomy/Redshift_ContractingUniverse.html" rel="nofollow noreferrer">http://www.therealebook.com/Astronomy/Redshift_ContractingUniverse.html</a></p>

<p>Even in a contracting Universe, there is a region around the observer where all the galaxies are moving away; the galaxies that are moving towards the observer could be too far to be seen. If we can demonstrate this, we could challenge the conclusion that the Universe is expanding, based on the redshift effect observed in the “nearby” galaxies.</p>

<p><a href="https://i.sstatic.net/FqWZ2.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/FqWZ2.png" alt="enter image description here"></a></p>

<p>Let’s start with a simpler representation: 1 dimension only (1D). Assume that all the galaxies move along a single line. We could represent an expanding and a contracting Universe as per the images below:</p>

<p><a href="https://i.sstatic.net/lTZgL.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/lTZgL.png" alt="enter image description here"></a></p>

<p>In Situation 1, we have the currently accepted model, where galaxies move away from each other, at increasing speed, which creates the redshift observed by Hubble.</p>

<p>In Situation 2, we assume that in the contracting Universe the galaxies have a speed that is inversely proportional to the distance from the center (as in a sink flow where the particles accelerate towards the drain), galaxies 1 and 3 will display a red shift effect, because they are moving away from each other, while galaxies 4, 5 and 6 will appear with blueshift effect. However, in order to see the blue shifted galaxies it is necessary to see beyond the center of the Universe, which would be not possible either because of the distance or because of the gigantic black hole in the center traps light from galaxies 4, 5 and 6 preventing it from reaching us at galaxy 2.</p>

<p>In fact, assuming that from galaxy 2 we cannot see beyond galaxy 1, it is impossible to distinguish between situation 1 and 2.
With this we have demonstrated that for a 1D Universe it is impossible to determine if it is expanding or contracting, simple because we cannot see far enough. What about a 2D space?</p>

<p>Consider two galaxies (1 and 2); consider that they form a angle beta, relative to the center of the universe (assuming that there is one); Consider that galaxy 1 is at the distance r from the center; consider that galaxy 2 is at a fraction of r (say xr) from the center.</p>

<p>We can calculate the distance between galaxy 1 and 2 as follows:</p>

<p><a href="https://i.sstatic.net/4VjO2.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/4VjO2.png" alt="enter image description here"></a></p>

<p>From the equation above we can calculate alfa and from <a href="https://i.sstatic.net/F241q.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/F241q.png" alt="enter image description here"></a> we can calculate d.</p>

<p>We also suppose that the speed along the radius can be calculated with the equation v = c / r, where c is constant.</p>

<p>Therefore we can calculate the position of the two galaxies after a period of time t, and measure the distance d between them to verify if d increases (redshift) of decreases (blueshift).</p>

<p>We can see that no matter what the fraction x is, there is always a minimum value for the angle beta, such that any galaxy within this angle will be moving away from the reference galaxy. Now assume that the universe is so big that we cannot see galaxies beyond beta. In this way, all that we can see is redshifted galaxies!</p>

<p>This means that even for a contracting universe, depending on how far we can see, it is possible to detect only redshifted galaxies. Does not this disprove the theory of expanding universe based on redshift?</p>

CC BY-SA 4.0
2020-10-10T15:13:17.093Z
Please update dead links or put the relevant content in the question.
ProfRob
<redshift><fate-of-universe>
Could a contracting Universe create the redshift effect observed by Hubble?
Daniel Fabio Zardo 
# Could a contracting Universe create the redshift effect observed by Hubble?

(Asked by: username_0 on 2018-02-12)

What if we just can’t see far enough and the redshift actually shows the galaxies speeding up towards a gigantic black hole to where everything is converging, repeating a cycle that took place countless times “before” the big bang?

Complete text in this link: <http://www.therealebook.com/Astronomy/Redshift_ContractingUniverse.html>

Even in a contracting Universe, there is a region around the observer where all the galaxies are moving away; the galaxies that are moving towards the observer could be too far to be seen. If we can demonstrate this, we could challenge the conclusion that the Universe is expanding, based on the redshift effect observed in the “nearby” galaxies.

[![enter image description here](https://i.sstatic.net/FqWZ2.png)](https://i.sstatic.net/FqWZ2.png)

Let’s start with a simpler representation: 1 dimension only (1D). Assume that all the galaxies move along a single line. We could represent an expanding and a contracting Universe as per the images below:

[![enter image description here](https://i.sstatic.net/lTZgL.png)](https://i.sstatic.net/lTZgL.png)

In Situation 1, we have the currently accepted model, where galaxies move away from each other, at increasing speed, which creates the redshift observed by Hubble.

In Situation 2, we assume that in the contracting Universe the galaxies have a speed that is inversely proportional to the distance from the center (as in a sink flow where the particles accelerate towards the drain), galaxies 1 and 3 will display a red shift effect, because they are moving away from each other, while galaxies 4, 5 and 6 will appear with blueshift effect. However, in order to see the blue shifted galaxies it is necessary to see beyond the center of the Universe, which would be not possible either because of the distance or because of the gigantic black hole in the center traps light from galaxies 4, 5 and 6 preventing it from reaching us at galaxy 2.

In fact, assuming that from galaxy 2 we cannot see beyond galaxy 1, it is impossible to distinguish between situation 1 and 2.
With this we have demonstrated that for a 1D Universe it is impossible to determine if it is expanding or contracting, simple because we cannot see far enough. What about a 2D space?

Consider two galaxies (1 and 2); consider that they form a angle beta, relative to the center of the universe (assuming that there is one); Consider that galaxy 1 is at the distance r from the center; consider that galaxy 2 is at a fraction of r (say xr) from the center.

We can calculate the distance between galaxy 1 and 2 as follows:

[![enter image description here](https://i.sstatic.net/4VjO2.png)](https://i.sstatic.net/4VjO2.png)

From the equation above we can calculate alfa and from [![enter image description here](https://i.sstatic.net/F241q.png)](https://i.sstatic.net/F241q.png) we can calculate d.

We also suppose that the speed along the radius can be calculated with the equation v = c / r, where c is constant.

Therefore we can calculate the position of the two galaxies after a period of time t, and measure the distance d between them to verify if d increases (redshift) of decreases (blueshift).

We can see that no matter what the fraction x is, there is always a minimum value for the angle beta, such that any galaxy within this angle will be moving away from the reference galaxy. Now assume that the universe is so big that we cannot see galaxies beyond beta. In this way, all that we can see is redshifted galaxies!

This means that even for a contracting universe, depending on how far we can see, it is possible to detect only redshifted galaxies. Does not this disprove the theory of expanding universe based on redshift?

**Tags:** redshift, fate-of-universe
> **Comments:**
> **username_1**: Please update dead links or put the relevant content in the question.  

---

## 4 answers

### Answer by username_1 on 2018-02-13. Score: 8

Aside from being, I suspect, totally incompatible with GR, the model fails to match observations.

The universe appears to be homogeneous and isotropic. In the model you suggest, Hubble's law would be different depending on which direction you were looking.

In particular, looking away from the black hole, galaxy redshifts would increase to some asymptotic value, whereas looking towards the black hole they would increase to infinity.

If the argument is that the central black hole is so far away that we cannot measure this asymmetry (i.e. well beyond the observable universe, given the homogeneity of the cosmic microwave background - but then does the model have an explanation for the CMB at all?), then the model also needs to explain why galaxies would move in this particular way, since they would not "feel" the influence of the black hole.

> **Comments:**
> **username_2**: Am I right to say that the best point against this scenario is indeed the observation of CMB? In this scenario it won't be a spherical dome. Even supposing it is light emitted by far far objects we still need a rather homogeneous universe and especially there shouldn't be any background looking in transverse directions, ie those perpendicular to our galaxy trajectory. Please confirm that. This kind of question always makes me doubt that I really learned something owing to my little effort to understand cosmology.:)  
> **username_1**: @Alchimista The CMB is the high redshift limit of what we are able to observe in the universe. Indeed it would be anisotropic in the scenario proposed if observed at a given lookback time, though I am not even sure what mechanism would be proposed to produce it in this model. Let alone problems with structure formation; the abundance of chemical elements etc.  

---

### Answer by username_3 on 2018-02-13. Score: 1

For a contracting universe, space would contract as a ratio, and hence even if we could only see a small distance, we would still see blue-shifted galaxies due to the fact that the galaxies further from the hypothetical centre of the universe (see [here](https://astronomy.stackexchange.com/questions/874) for why I say hypothetical) would be moving faster than those closer, and hence any two points in space must be moving towards each other. This holds true even in a finite universe, all points appear to contract towards the point of reference, as can be seen in the diagram below.

[![enter image description here](https://i.sstatic.net/g7xKM.jpg)](https://i.sstatic.net/g7xKM.jpg)
The distance between Galaxies 1 and 3 and Galaxy 2 is clearly less after contraction than before.

> **Comments:**
> **username_2**: I do not understand why galaxies far away shall move faster than those near the centre. I think the OP means galaxy falling into a center not a rubber analogy. This kind of topics drive me crazy all the time in spite of my cosmology efforts. :)  

---

### Answer by username_4 on 2018-07-15. Score: 1

So in this model, all of the observable universe is expanding. It is simply that it is part of a larger universe which is being locally stretched, as part of a an accelerating convergence to a central point.

I think the answer is that we cannot distinguish this model from an expanding universe (assuming the structure of the convergence is very carefully tuned to look like what we see "locally").

On the other hand, it makes no difference as far as the observable universe is concerned, so we have to resort to Occam's razor. The expanding universe is simpler, and arises naturally from our theories of gravity, etc.


---

### Answer by username_5 on 2020-10-09. Score: -3

A contracting universe is an attractive proposition as it seems to do away with the need for dark energy.
Equally a primordial black hole is an attractive proposition as its expansion would seem to indicate that time/events can never be entirely squeezed out of existence.(ie:if time/events ceased to be, there could never be an initial expansion). Both these propositions are currently unproven.
A dirty initial expansion seems feasible (ie: one where super massive black holes split from the primordial black hole and seeded the universe, allowing the creation of galaxies which could not themselves have produced black holes of this size in the time available).
Obviously we can only observe/photograph what to the observer are present space/ time events (as their light reaches us) which events have taken place previously in the space/time continuum. Presumably a universal gravitational field has always existed within the space time continuum (even inside an event horizon which may have evolved and expanded as the primordial black hole decayed), which will have affected all events and structures in the universe.
Cosmic background radiation can perhaps be thought of as the remnant of the original event horizon/expansion now massively red shifted.
It is difficult to see how observed red shifts can definitively prove whether the universe is expanding or contracting.

> **Comments:**
> **username_6**: This isn't written as a proper Stack Exchange answer. It puts forward the author's ideas but does not provide supporting sources or references or even argue for itself using standard tools of science e.g. mathematics. Stack Exchange is a little different than other websites See [How do I write a good answer?](https://astronomy.stackexchange.com/help/how-to-answer)  

---

