# I Is the increment of the expanding universe part of cosmology

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1. Dec 7, 2018

### Hugh de Launay

{Reference: Wikipedia's Friedmann-Lemaitre-Robertson-Walker (FLRW) Metric article)}

The FLRW (1935) mathematical model of the universe is the one most used by cosmologists. It is differentiable, which means it is based on sound, consistent, mathematical formulations. (The Lambda-CDM model is further developed, but the FLRW model is adequate for this post.) Included in the FLRW mathematics are some of Einstein's field equations from general relativity. Some of the features of the FLRW model are that it is finite, unbounded, expandable, and free of topology defects.

I understand that the model has no borders and can be infinite if needed, and that this is required by its mathematics. What I am curious about is the attitude of cosmologists toward the ongoing increment of the expanding universe. Is it a part of cosmology, or is it not? If it is, then what is the consensus view of its nature?

2. Dec 7, 2018

### Drakkith

Staff Emeritus
Sorry, what does 'increment' mean in this context?

3. Dec 7, 2018

### Hugh de Launay

Assuming a speed of light expansion, the increment would be the speed of light multiplied by your choice of a length of time. I had in mind a small amount of time like a microsecond or less.

4. Dec 8, 2018

### Staff: Mentor

Which is not correct. The expansion of the universe does not have a speed. It has a rate of change of the scale factor in the metric. That's not the same thing.

Because the expansion of the universe does not have a speed, it does not have an "increment" either, at least not as you are using the term.

5. Dec 8, 2018

### Hugh de Launay

I understand the expansion of the universe is a change of the scale factor in the metric, and that because of this the visible universe has a radius greater than the age of the universe in light years. I wanted to avoid bringing that up, but now I see I have to.

The universe expands, so it encroaches upon the three dimensional environment beyond the space it already occupies. The encroachment is an increment of the size of the universe which has occupied space it had not occupied before its expansion. The question is: is this increment in size a part of cosmology or is it not?

6. Dec 8, 2018

### Staff: Mentor

No. There is no "three dimensional environment" beyond the space the universe occupies. The universe is not expanding into a pre-existing "space". That is not what "the universe is expanding" means. It means the universe has a 4-dimensional geometry with a particular shape. There is nothing "outside" this 4-dimensional geometry.

7. Dec 8, 2018

### Hugh de Launay

Okay. I see that the answer to my question is no. Cosmology will not deal with a question that refers to events that are not integrated within the accepted models of the universe, and the increment (encroachment) in size of the universe has the status of not taking place within the cosmological models. I got my answer. Thanks for your help.