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# 'The denser the universe, the younger it must be'?? Why?

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1. This is written in my CGP revision guide and my intuition is telling me otherwise.

Here is my reasoning:

Increased average density of the universe reduces the rate of expansion.
So if a larger density slows down the rate of expansion then it should take a longer time to expand to its current size with increased density.(This is all assuming the BB theory is correct, not steady-state, and ignoring new theories regarding rapid inflation.) Hence for a larger density, it must have taken a longer time to get ot current size, hence the universe is OLDER.

Also, there is a graph in my book of size against time for flat, open and closed universe and the one for open (with lowest density) reaches any given size in the shortest time, so surely it is 'the denser the universe is, the older it is'?

Or am I talking BS. Thanks
2. Is this statement implying "the denser this universe was, the younger it was when it was in that state" or "a relatively more dense universe is a relatively younger universe"? It seems too ambiguous to me. The first statement is obvious, the second is not necessarily correct.
3. The statement in the title of the thread is simply refering to the fact that there is constant mass, so assuming BB theory, the density will be highter when the universe is younger (smaller volume)
4. (Original post by Phredd)
The statement in the title of the thread is simply refering to the fact that there is constant mass, so assuming BB theory, the density will be highter when the universe is younger (smaller volume)
Yes, but isn't that obvious?

I think OP was implying that his interpretation assumed that "universe" did not necessarily correspond to this universe. I.e. considering 2 different universes of different mass, and considering how they expand with respect to time.
5. (Original post by Llewellyn)
Yes, but isn't that obvious?

I think OP was implying that his interpretation assumed that "universe" did not necessarily correspond to this universe. I.e. considering 2 different universes of different mass, and considering how they expand with respect to time.
No, I meant this universe. My book is referring to the average density of the universe which presumably is assumed to be constant over time at A-Level. It is saying that the larger this average density is, the younger the universe is i.e. the shorter time ago it was that the universe began.

If increased density reduces the rate of expansion then does that not imply it would take longer to reach a given size with higher density?
I meant 'a relatively more dense universe is a relatively younger universe' as what I am questioning.
6. If the mass of the universe is constant, then as it expands, it gets less dense. So it was denser in the past than it is now.

CGP are not known for the depth of their comments.

so it would make sense for it to be denser if its younger
8. But what if...

...the universe began to contract?
9. Assume the universe contains a set amount of mass. All that mass gravitates, pulling together against the expansion of the universe. The more massive the universe, the stronger that effect will be, and the slower the expansion. So the 'heavier' the universe is, the smaller it will be after a given period of time.

However, the universe is still expanding, which means that there's a constant mass occupying an increasingly large volume. The density of the universe is decreasing over time. Higher densities occur at earlier times and lower ones at later times, so the denser it is the younger it is.
10. (Original post by oh_1993)
This is written in my CGP revision guide and my intuition is telling me otherwise.

Here is my reasoning:

Increased average density of the universe reduces the rate of expansion.
So if a larger density slows down the rate of expansion then it should take a longer time to expand to its current size with increased density.(This is all assuming the BB theory is correct, not steady-state, and ignoring new theories regarding rapid inflation.) Hence for a larger density, it must have taken a longer time to get ot current size, hence the universe is OLDER.

Also, there is a graph in my book of size against time for flat, open and closed universe and the one for open (with lowest density) reaches any given size in the shortest time, so surely it is 'the denser the universe is, the older it is'?

Or am I talking BS. Thanks
Our minds are too limited to understand this, let's us all just assume God created the universe to protect us...
11. (Original post by oh_1993)
No, I meant this universe. My book is referring to the average density of the universe which presumably is assumed to be constant over time at A-Level. It is saying that the larger this average density is, the younger the universe is i.e. the shorter time ago it was that the universe began.

If increased density reduces the rate of expansion then does that not imply it would take longer to reach a given size with higher density?
I meant 'a relatively more dense universe is a relatively younger universe' as what I am questioning.
yes, but you are comparing two universes. If you look at one universe, assuming contraction has not begun, and take the average density at two different times, the less dense value would be later in time after the BB/beginning of expansion than the denser.

You are transferring it to apply to multiple unverses, which is extrapolation, and not essentially related. You are taking a theory which operates within certain perameters and extending the relationship beyond them. The theory is not establishing relationship between two different universes, but one at two different times. You would need to update assumptions the theory is based on and establish different endpoints to provide any sound theory.

If you take any universe operating with similar mechanisms to ours, it will get less denser as time passes. You cant relate this to how two universes of different densitys act relative to each other over time.

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