Size of the Observable universe at the Big Bang Singularity

[Drakkith]

Just a quick question. How big was the current observable universe at the point in time where we reach 'singularity conditions' in the early universe? I'm assuming it can't be a single point, as there is no way that I know of to make a zero-dimensional point into a 3-dimensional object or space.

 

[bapowell]

If by "observable universe" you mean the region of space through which light has traveled since the big bang (since the 'singularity conditions') then by necessity it has zero volume. But what do you mean by 'singularity conditions'?

 

[Chalnoth]

Just a quick question. How big was the current observable universe at the point in time where we reach 'singularity conditions' in the early universe? I'm assuming it can't be a single point, as there is no way that I know of to make a zero-dimensional point into a 3-dimensional object or space.

If you take the big bang theory seriously, its size would have been identically zero. That's what the singularity means. And that's why it's nonsense.

In order to get an answer different from zero, you have to make use of another model, such as inflation or the LQC bounce.

 

[Drakkith]

If you take the big bang theory seriously, its size would have been identically zero. That's what the singularity means. And that's why it's nonsense.

In order to get an answer different from zero, you have to make use of another model, such as inflation or the LQC bounce.

Take the radius of the current observable universe. As we look backwards in time, this radius shrinks. I was under the impression that at t=0 this radius is not zero and that the singularity doesn't come from computing the radius/volume of the universe, but from something else in the math. Is that incorrect?

 

[Chalnoth]

Take the radius of the current observable universe. As we look backwards in time, this radius shrinks. I was under the impression that at t=0 this radius is not zero and that the singularity doesn't come from computing the radius/volume of the universe, but from something else in the math. Is that incorrect?

The scale factor at that point goes to zero. You can't say that the entire universe was a single point, because the classic big bang universe is infinite, and infinity multiplied by zero is indefinite. But because the observable universe is finite, its size would have been zero when the scale factor reaches zero.

Of course, in an inflationary or other more sophisticated model, the scale factor would not have been zero at that point in time. Exactly how big it would have been is highly dependent upon the specific model, though it had to be quite tiny. With inflation, for example, it depends upon the energy scale of inflation.

 

[Drakkith]

I see. Thanks for clearing that up!

 

[Chronos]

The 'size' of the observable universe is only an issue if the universe is finite. To extrapolate backwards to t=0 yields a nonsensical result. It would be like extrapolating a person's size backwards in time and concluding it was zero at conception.

 

[Drakkith]

The 'size' of the observable universe is only an issue if the universe is finite. To extrapolate backwards to t=0 yields a nonsensical result. It would be like extrapolating a person's size backwards in time and concluding it was zero at conception.

I'm not sure I understand. The observable portion of the universe is always finite. What does the issue of whether or not the entire universe is infinite have to do with this?

 

[Chronos]

The observable region is always finite in a universe of finite age, as noted by bapowell. Due to the finite speed of light, the observable universe is, by definition, a singularity at t = 0, but, this is unphysical [an illusion] in an infinite universe, which must always be infinite.