# Infinite density to finite density

• mistrbigshot
In summary, the conversation discusses the concept of a singularity in the universe and how it relates to General Relativity. It is mentioned that GR does not exactly predict a singularity, but rather breaks down at that point. The idea of resolving the singularity is also discussed, with the mention of current research in quantum cosmology. The conversation ends with the idea that singularities cannot be taken seriously in GR due to the built-in energy scale of the theory.

#### mistrbigshot

I'm guessing there has been an attempt to address this, and I've looked through some threads but I remain unsatisfied.

I'm not getting it. If relativity predicts that the universe was a singularity (I get that the universe wasn't necessarily a tiny point) with infinite density; then how could that singularity ever not be a singularity? If it was infinitely dense then how could it ever not be infinitely dense?

If you increase the distance between all infinitely close points within the singularity by some set value (say an increase in distance between all points of 10^10000000000%), won't they still be infinitely close together?

Or is this just what is meant by very little being known about the singularity?

Was there some sort of a jump between the infinite density to a finite though very large density where the equations start to work? I'm imagining something of an asymptote and a jump to another part of the graph where it is no longer infinite. I don't know how valid this analogy would be though.

A link that would address these specific questions would satisfy me. Thanks.

Edit- Ok, I now see that it is possible there was no singularity. Though answers to my questions might still be helpful.

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First off, GR doesn't exactly predict a singularity, per se. All GR says is that it doesn't work at that point.

As far as the infinite density goes, any amount of energy crammed into an infinitesimal (zero) volume results in infinite density. Classically speaking, when the universe began to expand, the volume was no longer zero, so the density of the universe was no longer infinite.

mistrbigshot said:
Edit- Ok, I now see that it is possible there was no singularity. Though answers to my questions might still be helpful.

You are getting the idea without anyone helping.

As far as is known, there is no such thing as a singularity in nature.
When a mathematical model predicts a singularity (like infinite density, infinite temperature, infinite curvature) it doesn't mean it's real. It simply means the model has broken, and fails to apply outside a certain range.

Physics models in the past (of the atom, of thermal radiation, etc...) have had singularities, that is breakdowns, and it simply meant you had to come up with a better model that would apply where the old model gave meaningless answers.

GR was invented in 1915. It has a limited range of applicability. It blows up in certain situations and fails to compute meaningful numbers.

All that means is you shouldn't apply it in those situations, and you should try to find a replacement that doesn't blow up. This is called "resolving the singularity". It has been done with other theories about other phenomena in the past. GR is just one case.

Now a lot of people are actually working on resolving the BB singularity.

Here is a database search for research publications since 2005 in the field called "quantum cosmology" which mainly concerns conditions around the t=0 where GR blows up. You can sample it if you want to get a rough impression of the current work. I see this particular search comes up with 282 papers published starting 2006, essentially in the last 3 years. The papers are ranked by how much they are cited for reference in other research, a rough measure of how innovative or useful they have proven to be. The most highly cited papers are listed first:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+and+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]
If you want to glance at a brief summary of any paper, called its abstract, just click on where it says "abstract", a couple of lines below the paper's title and authors. Besides a brief description, the abstract page will normally have a link to a PDF file of the whole paper which is free for download.
I'm not urging detailed study, just offering a way to get a taste of what is going on in this area of research in case anyone is curious.

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A good way of seeing why we can't take the prediction of singularities in General Relativity seriously is the fact that there's a built-in energy scale to the theory: the Planck scale. We typically expect that the theory of gravity should start to break down when energy densities get close to the Planck scale. So obviously a singularity, which goes far beyond the Planck scale in energy density, is not something we can take seriously.

Thanks guys. I'll check out some of those papers Marcus, thanks for posting the link.

## 1. What is "infinite density to finite density"?

"Infinite density to finite density" refers to the concept of a theoretical point in the universe where matter and energy are infinitely compressed and dense, and then expanding outward to a finite density. This is often associated with the Big Bang theory of the creation of the universe.

## 2. How can something go from infinite density to finite density?

According to the Big Bang theory, the universe began as a singularity with infinite density and temperature. However, as the universe rapidly expanded, the density and temperature decreased, eventually reaching a finite density and temperature that we experience today.

## 3. Is it possible to reach infinite density?

In our current understanding of physics, it is not possible to reach infinite density. The laws of physics break down at such extreme conditions, making it impossible to predict what would happen. It is also believed that the expansion of the universe prevents matter from reaching infinite density.

## 4. Can we observe the transition from infinite density to finite density?

Scientists have been able to observe the after-effects of this transition through observations of the cosmic microwave background radiation, which is leftover radiation from the Big Bang. However, we cannot directly observe the transition itself as it occurred in the very early stages of the universe.

## 5. What implications does this transition have for the universe?

The transition from infinite density to finite density is a fundamental aspect of the Big Bang theory and has major implications for our understanding of the universe. It helps explain the expansion of the universe and the formation of galaxies and other structures. It also supports the idea that the universe has a finite age and may have a finite lifespan.