# How large does a BH has to be in order to swallow our universe in its current status?

1. Oct 4, 2009

### Cosmicon

In terms of physical dimensional size, what are the measurements a black hole must fit in order for it to actually reverse the expansion of the universe and cause a total suction of the space-time fabric that constitutes our universe, in its current position?

I will appriciate a profound explenation in a given answer which includes structural and causative data regarding the effects of a black hole on the space-time fabric.

2. Oct 4, 2009

### map19

Re: How large does a BH has to be in order to swallow our universe in its current sta

If your definition of a Black Hole is "a volume from which no energy escapes" then our Universe IS a Black Hole.

3. Oct 4, 2009

### qraal

Re: How large does a BH has to be in order to swallow our universe in its current sta

If you can tell me the "current position" of the Universe then we'll both know.

Not sure exactly what you mean. A black hole is just a big concentration of mass in which the gravitation of the mass overcomes all other forces causing it to collapse into a singularity. Just how big the event horizon is depends on how much mass is involved. To collapse our Universe, then the black hole's horizon has to be the size of the Universe, with the corresponding mass. But we can't see all the Universe, merely our cosmic horizon. Past that we've many ideas on what's there but the barest traces of evidence or non-evidence. The Universe could be infinite for all we know.

4. Oct 4, 2009

### EseehC

Re: How large does a BH has to be in order to swallow our universe in its current sta

Black holes aren't vacuums, they don't suck in things.

5. Oct 4, 2009

### Blenton

Re: How large does a BH has to be in order to swallow our universe in its current sta

Somehow that makes complete sense.

6. Oct 4, 2009

### DaveC426913

Re: How large does a BH has to be in order to swallow our universe in its current sta

Not true. It only has to be large enough to cause all matter in the universe to (eventually) fall into it.

Note that, in a non-expanding universe, ALL matter will eventually fall into a BH if we wait long enough. The question is then raised how fast is the universe expanding?
We'd need a BH big enough to reverse this expansion.

Unfortunately, "the universe" is a not well-defined area. We can only talk about our observable universe. That greatly confounds the question.

7. Oct 5, 2009

### Cosmicon

Re: How large does a BH has to be in order to swallow our universe in its current sta

If the universe is partly undetected by any kind of instrument the human kind had possessed so far, then the data collected by the Hubble telescope depicted as the distribution of mass in the universe suppose to be obsolete, considering the fact that the time light travels from different distances is unequal. Furthermore, all the simulations of the evolution of the universe are also false.

Is this correct in any aspect?

Last edited: Oct 5, 2009
8. Oct 5, 2009

### Wallace

Re: How large does a BH has to be in order to swallow our universe in its current sta

This is, on average, not true. We observe that the distribution of galaxies, and the distribution of dark matter (seen via graviational lensing) matches very well the predictions from our cosmological models and simulations. While the Universe clearly has structure (a cluster here a void there) it is on average homogenous.

As for your original question, unfortunately it doesn't really have a sensible answer because if there was a single black hole that had anything like a resonable fraction of the mass of our observable universe then our cosmological models simply would not be correct, since the models predict a certain amount of 'lumpyness' which matches what we observe. The existance of a black hole of such a size would not be compatible with our cosmological models, in which case you are hypothesising about a different Universe to ours, in which case all bets are off and you can't then talk about this black hole being big enough to engulf 'our Universe'.

You could work out the Schwarzchild radius of a Black hole which containted the mass of the observable Universe, and that would give you some number that kind of answers your question, but that would just be playing with numbers, it wouldn't really be the answer you are after.

9. Oct 5, 2009

### Cosmicon

Re: How large does a BH has to be in order to swallow our universe in its current sta

I understand your point, the distortion caused by a black hole big enough to engulf our universe would not let current models be applied, that is clear. but how can we conclude that the current data regarding what we observe, represents the REAL distribution of celestial objects in a specific state in time? Take for example a galaxy 400 million light years away from our own. We observe it through a telescope, which detects the light that had spread out of the object 400 million years ago. Thus, at this very moment, it is possible that its location in the vast spaciousness of the universe, is very different on par with where it was. Ergo, the observation & mapping of the skies can be compared to extracting photos of family members taken in different eras, then analogizing that they are still placed where they where photoed in RELATION to one another.

Am I all mistaken here or can this be a valid assumption ?!

10. Oct 5, 2009

### DaveC426913

Re: How large does a BH has to be in order to swallow our universe in its current sta

I assumed the original querstion was purely hypothetical. I didn't think he was asking if such a thing were actually in existence.

11. Oct 5, 2009

### Wallace

Re: How large does a BH has to be in order to swallow our universe in its current sta

What you are saying is true enough, but it is a feature not a bug. Our models don't just predict what the Universe is like now, but also what is was like in the past. We certainly do observed that objects with different redshifts (the higher the redshift the more distant and hence the 'older' the light) have different properties (what they look like, how many there are, how clustered they are etc ), however we can predict from our models and simulations what those differences with redshift will be, and the results match the observations.

In fact the evolution of many qauntities with redshift is the main game of cosmology. The beauty of the finite speed of light is that you can study the conditions today, or anytime within the last ~11 Billion years (if you are looking at galaxies) or even the last ~13 Billion years (if you include the CMB). There is a bit of a gap between the CMB and the first galaxies (the cosmic dark ages) but apart from that we can look at the Universe at a bunch of ages, and compare the results to our theories, which make specific predictions about how various things will change with time.

12. Oct 5, 2009

### DaveC426913

Re: How large does a BH has to be in order to swallow our universe in its current sta

The effect of time delay due to the speed of light and the concept of 'looking back in time' is well-understood and is indeed taken into account when constructing our models of the universe.

13. Oct 5, 2009

### Wallace

Re: How large does a BH has to be in order to swallow our universe in its current sta

Agreed, but you miss the point. You can't hypothetically place a Black hole into 'our' Universe, because fundamentally if would then not behave like our Universe and hence any understanding we have is lost. It is like asking what your toes would look like if you had no legs, it not a question of something being hypothetical, its about whether what you are hypothesising is logically consistent. There is no sensible answer to what your toes look like if you have no legs, which is the point I made about a black hole swallowing 'our Universe'.

Last edited: Oct 5, 2009