# B Singularities forming universes?

1. Mar 2, 2017

### Thomas Clark

The whole universe started off with a Big Bang, before this all matter was contained inside a singularity. Since other singularities are formed when a star forms a black hole hypothetically could this cause the creation of other universes?

2. Mar 2, 2017

### Staff: Mentor

While that's a possibility, the singularity that happens at t=0 is probably just an artifact of our incomplete knowledge of physics at the temperature and density scales present in the very early universe. One thing to note is that the singularity actually occurs everywhere, not at a single point. In other words, as we run the clock back, the size of the universe doesn't change, but the density does. When the density reaches the magnitude where we get a singularity, it reaches it everywhere.

It's a hypothetical prediction by some scientists, but it's really nothing more than a "curiosity". There's no way to observe this process and no way to know if we came from a similar event. At least not at this time.

3. Mar 2, 2017

### Chalnoth

There definitely have been some serious consideration of ideas like this among some theorists (such as Lee Smolin's Fecund Universe), but ultimately it's really hard to test ideas like this with observations.

4. Mar 2, 2017

### Staff: Mentor

Indeed. For now they remain on the "back burner" of mainstream science, just simmering for what could be a long, long time.

5. Mar 2, 2017

### phinds

You are treating the word "singularity" as though it referred to a specific THING. It does not. It just is a stand-in word which is shorthand for "the place where the math model breaks down and we don't know WHAT is or was going on".

6. Mar 2, 2017

### Chronos

The similarity between a black hole singularity and the BB singularity is analogous to the similarity between dark matter and dark energy. It would be fallacious to infer any deeper connection. It's not unlike inferring commonality between cold cuts and a cold sore.

7. May 19, 2017

### Tiger Blood

But what about when Stephen Hawking said "There was in fact no singularity at the beginning of the universe." and yet he still uses that term. What is the difference of that non existing singularity and singularity that he says exists?

8. May 19, 2017

### Staff: Mentor

One's correct, the other's a misconception that's hard to not talk about since it's so ingrained in everyone's mind.

9. May 19, 2017

### Tiger Blood

OK but the false one is supposed to be the universe gets smaller and smaller while the density of the universe and the gravitational field increases. As the size of the universe goes to zero, the density and gravitational field, at least according to the mathematics of general relativity, become infinite. At that point, some theologians claim, time must stop and, therefore, no prior time can exist.
Stephen Hawking repudiated this in his book. Yet he still talks about "The density would have been infinite. It would have been what is called, a singularity." http://www.hawking.org.uk/the-beginning-of-time.html
So what exactly is repudiated about the false singularity?

10. May 19, 2017

### Staff: Mentor

Several things. For one, the common view of the singularity is that the density becomes infinite in a single location. In other words, that the universe exploded from a point in space and expanded from there. The truth is that the density reaches infinity everywhere at the same time. Every point in space becomes a singularity.

Another is the idea of time stopping at t=0 when the density becomes infinite. The problem is that a singularity represents a problem in the math that keeps you from making any predictions based on that math. For example, if I try to plug in $x=1$ into $\frac{1}{1-x}$ I get a singularity because I'm trying to divide by zero. You can talk about any other x-value leading up to $x=1$ but you cannot use 1 itself. The formula simply cannot give you an answer. That's why division by zero is called "undefined" and not "infinity". The answer isn't infinity, it's that there isn't an answer at all. So attempting to predict what happens at t=0 and prior using our current understanding of science just isn't possible. We can't even say that time stops.

Also, note that when Hawking talks about the size of the universe he's talking about the size of the observable universe. The universe in its entirety may be infinite in size.

Finally, a singularity in the math has typically meant that our model and/or theory is incorrect or that the way we're using it is incorrect. So many scientists believe that a quantum theory of gravity will solve this problem and let us understand how physics works at the extremely high energy and density scales of the very early universe. For all we know gravity could become repulsive when the energy density of a volume of space approaches infinity, preventing the formation of an actual singularity.

11. May 19, 2017

### phinds

12. May 19, 2017

### Staff: Mentor

I'd like to clarify something real quick. In my last post, I talked about the density "becoming infinite" at $t=0$. But what's that mean exactly?

If we look at a very simple example, $y=\frac{1}{x}$, then, like what I said in my previous post, there is no answer when $x = 0$. It's undefined. When we say that the density of the universe at a point "is infinite", what we mean is that the density goes to infinity in a finite time. What's this mean exactly?

Well, let's look at probably the simplest example ever, $y = x$. This formula tells us that $y$ increases linearly as we increase $x$. As $x$ goes to infinity, so does $y$. In other words, $y$ increases without limit as $x$ does.

This is different than something like $y=\frac{1}{5-x}$. Here we have a problem. As $x$ approaches 5, $y$ increases without limit. The closer $x$ is to 5, the larger $y$ becomes. We can say that $y$ goes to infinity in a finite time. In this example, $x$ does not need to increase without limit for $y$ to do so. But remember that when $x=5$, $y$ is undefined, not infinity. "Going to infinity" is a process. It's a description of the behavior of the function. It's not a number.

This last example is what happens as we look backwards in time. The density of the universe goes to infinity even though $t$ does not. In other words, the density of the universe increases without limit as $t$ approaches a single value.

That's my understanding at least. As always, someone please correct me if I'm wrong.

13. May 21, 2017

### WWW Mechanical PE

The best explanation I've heard: Tale of two big bangs

Most cosmologists would be very surprised if it turned out that our universe really did have an infinitely dense, infinitely hot, infinitely curved beginning. Commonly, the fact that a model predicts infinite values for some physical quantity indicates that the model is too simple and fails to include some crucial aspect of the real world.

14. Jun 5, 2017

### Saltynuts

"One thing to note is that the singularity actually occurs everywhere, not at a single point. In other words, as we run the clock back, the size of the universe doesn't change, but the density does. When the density reaches the magnitude where we get a singularity, it reaches it everywhere."

Interesting Drakkith. Where are you getting that from? All I've ever heard is that the universe started from basically a single point, and has been expanding sense, and actually expanding at an accelerating rate (at least for the last several billion years). Thanks!

15. Jun 5, 2017

### Staff: Mentor

https://physics.stackexchange.com/questions/136860/did-the-big-bang-happen-at-a-point
http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html
https://www.cfa.harvard.edu/seuforum/faq.htm#e1

16. Jun 5, 2017

### phinds

This is pop-sci nonsense that you ALWAYS see on pop-sci programs because the reality is just slightly more complicated and they don't DO complicated.

17. Jun 6, 2017

### Saltynuts

Got it. Thanks Drakkith! Phinds as well.

So if space is itself expanding, what is there before space expands into it? There has got to be something, even if that something is specifically some kind of nothing.

18. Jun 6, 2017

### phinds

That's a very misleading way to think of it since it implies that space is a THING that can expand. It is not. Things are just getting farther apart. Google "Metric Expansion" and check out the link in my signature
There is no "there before space expands into it"
No, there does not and is not.

EDIT: by the way @Saltynuts, I just realized you have hijacked this thread. That's bad form. If you have a question which is not the same as the original question in a thread, you should start a new thread. This thread was not about the expansion it is about singularities and the big bang

19. Jun 6, 2017

### Saltynuts

Thanks phinds. OK, will start a new thread. Thanks!