# How large was the Universe at its moment of creation?

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In summary, scientists don't know how large the universe was at its moment of creation, but they believe it was probably infinite. They believe the Big Bang model is correct, and that the universe expanded from a small, dense point.

How large was the universe at its moment of creation? Do we have a real answer for this? If so, how was this found out?

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How large was the universe at its moment of creation? Do we have a real answer for this? If so, how was this found out?
We don't know. It might well have been infinite, and in fact the current thinking in cosmology (at least as expressed here on PF) is that that is the most likely scenario.

I agree what phinds says.
Oftentimes people then say huh? - how can something infinite be expanding?
There is quite a good analogy called 'Hilberts Hotel' which can explain this.

DrClaude
The size of the universe is conventionally expressed in terms of its scale factor, which is just a way of stating the average distance between galaxies based on the age of the universe. Basically, when the universe was half its present age galaxies were about half as far apart on average as they are today. This leads to an obvious problem - when the universe was age zero, the average distance between galaxies must also have been zero. No one actually takes that seriously, but, we have not yet figured out any solution other than to claim the universe must be infinitely large. Since infinity is always still infinity when divided by any number, problem solved! Division by infinity or zero is cheating since either process yields mathematical nonsense.

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According to Big Bang model (which is the most widely accepted model or not?):

The universe at the moment of creation was a single point (single point means zero dimensions) with infinite density.

Delta² said:
According to Big Bang model (which is the most widely accepted model or not?):

The universe at the moment of creation was a single point (single point means zero dimensions) with infinite density.

Big Bang model does not say ANYTHING about moment of creation. And also it doesn't say that Universe was a single point with infinite density.

weirdoguy said:
Big Bang model does not say ANYTHING about moment of creation. And also it doesn't say that Universe was a single point with infinite density.

Ok I admit I haven't read a serious book about Big Bang. I am here to learn as well.

I thought that Big Bang was basically all about a Big "explosion" that happened some billion years ago and during which the universe started expanding from a single point (or a very tiny region) to a much bigger region. Apparently I am wrong. What does Big Bang says?

Delta² said:
What does Big Bang says?

It says that over 13 bilion years ago (don't if its correct because in Poland bilion is a different number than in USA :P) Universe was in a hot dense state and it was rapidly expandind. Expansion means metric expansion. If Universe is infinite, then it was infinite back then. No explosion, no 'single point' with infinite density, no creation of the Universe. There are plenty of threads here about that, search it up.

weirdoguy said:
It says that over 13 bilion years ago (don't if its correct because in Poland bilion is a different number than in USA :P) Universe was in a hot dense state and it was rapidly expandind. Expansion means metric expansion. If Universe is infinite, then it was infinite back then. No explosion, no 'single point' with infinite density, no creation of the Universe. There are plenty of threads here about that, search it up.

Ok fine I guess I am victim of the most common or non common misunderstandings about Big Bang.

BUT
where does the name Big Bang -comes from/relates to- if not about a big explosion?

Delta² said:
where does the name Big Bang -comes from/relates to- if not about a big explosion?

It was a mock name given by the opponents of the BB in the early days of the theory.

weirdoguy said:
no 'single point' with infinite density,
Well, if you take the model literally then it does predict infinite density. For any finite density there exists some small time such that at all smaller times the density is greater than the target density. That is an infinite density, even if you exclude the singularity (which must be done for mathematical reasons).

However, we have good theoretical reasons to believe that the theory will break down well before that. So many physicists simply say that it starts at a “hot dense state” meaning some large but finite density beyond which the model breaks down.

Feeble Wonk
Delta² said:
Ok fine I guess I am victim of the most common or non common misunderstandings about Big Bang.
Exactly. It's one of the most common, if not THE most common, pop-sci misrepresentations of actual science.

@Delta² I suggest that if it's still open to edit, you add to your post something like "EDIT: incorrect. See below" so as to not further confuse newbies.

Delta2
To address any confusion on the subject, I think that a paper by Charles Lineweaver and Tamara Davis, published as an article in Scientific American several years ago is one of the best, most concise and digestible bits of reading on the subject.

This will address not only the OP's question, but a few of the slightly off base or confused responses.

It is a fairly easy but pretty authoritative read. Highly recommended.

https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

diogenesNY

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davenn, Imager and Delta2
I think this issue is a common source of confusion to us non-physicists because of the general nature of the concept... “how large was the universe”. How “large” compared to what? We’re talking about the universe itself here. So “size” has no obvious meaning in terms to an “exterior” relationship. It can only be measured in terms of “interior” relationships. So, is the spatial dimension of the universe itself expanding, or is spatial dimension of the observable objects within the universe contracting? It seems as if it’s a meaningless question because the measurable parameters remain the same regardless.

Feeble Wonk said:
I think this issue is a common source of confusion to us non-physicists because of the general nature of the concept... “how large was the universe”. How “large” compared to what? We’re talking about the universe itself here. So “size” has no obvious meaning in terms to an “exterior” relationship. It can only be measured in terms of “interior” relationships. So, is the spatial dimension of the universe itself expanding, or is spatial dimension of the observable objects within the universe contracting? It seems as if it’s a meaningless question because the measurable parameters remain the same regardless.
But it isn't at all meaningless. As has been discussed here on PF numerous times, "contracting objects" immediately runs into contradictions and is not a workable model.

Feeble Wonk said:
is spatial dimension of the observable objects within the universe contracting?

No. This is easily ruled out by everyday observation; for example, your body is obviously not contracting. If it were, you wouldn't be able to function.

Apologies. It was a loosely phrased comment. Actual “contracture” of dimensionless point particles is not at all what I was suggesting. What I was wondering was whether both sides of that question were essentially describing the same phenomenon. I’m afraid that this discussion will quickly diverge from the OP’s initial question, and that it will also quickly exceed my understanding of the subject matter. However, it’s always seemed to my ignorant conceptualization that the “expansion” of the universe’s spatial dimension is tied to a condensation of universal energy distribution. If that implies increased localization, would that not appear to be manifested in an apparent spatial expansion?

Feeble Wonk said:
Apologies. It was a loosely phrased comment. Actual “contracture” of dimensionless point particles is not at all what I was suggesting. What I was wondering was whether both sides of that question were essentially describing the same phenomenon. I’m afraid that this discussion will quickly diverge from the OP’s initial question, and that it will also quickly exceed my understanding of the subject matter. However, it’s always seemed to my ignorant conceptualization that the “expansion” of the universe’s spatial dimension is tied to a condensation of universal energy distribution. If that implies increased localization, would that not appear to be manifested in an apparent spatial expansion?

I suggest -

Post 13 - read the Sci-Am article

Feeble Wonk said:
What I was wondering was whether both sides of that question were essentially describing the same phenomenon.

If that's the case, then the problem is with the question, not the phenomenon. The phenomenon is something we observe; you can't make it not be what it is by how you frame a question about it.

Feeble Wonk said:
it’s always seemed to my ignorant conceptualization that the “expansion” of the universe’s spatial dimension is tied to a condensation of universal energy distribution

I have no idea why you would think this. As the universe expands, its average density decreases; it doesn't increase.

Grinkle said:
I suggest -

Post 13 - read the Sci-Am article

Thank you Grinkle. I will. Hopefully it will clear things up for me.

## 1. How do scientists determine the size of the Universe at its moment of creation?

Scientists use a variety of methods and evidence, such as the cosmic microwave background radiation and observations of the expansion of the Universe, to estimate the size of the Universe at its moment of creation.

## 2. What is the estimated size of the Universe at its moment of creation?

The size of the Universe at its moment of creation is still a topic of debate among scientists, but current estimates suggest that it was incredibly small, possibly even smaller than a single atom.

## 3. How has the size of the Universe changed since its moment of creation?

The Universe has been expanding since its moment of creation, which is supported by various observations and theories. The exact rate and history of this expansion is still being studied and debated by scientists.

## 4. Is it possible for the Universe to have a finite size at its moment of creation?

Some theories suggest that the Universe may have a finite size at its moment of creation, while others propose that it may have been infinite. These ideas are still being explored and researched by scientists.

## 5. Why is it challenging to determine the exact size of the Universe at its moment of creation?

Due to the extreme conditions at the moment of creation, such as high temperatures and energy levels, it is difficult for scientists to accurately measure the size of the Universe at that time. Additionally, the lack of direct evidence and the limitations of our current technology make it a challenging task.