# I How will the universe empty out?

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1. May 6, 2017

### durant35

I have a problem understanding where does the matter/radiation go to because of the expansion of our universe. Eventually all normal matter and radiation will leave our causal patch, but the word leave is pretty unconvincing in this case.

They will stay in their own causal horizon until each particle gets turned to radiation and the radiation also leaves the horizon, but how is this synonimous with emptying out.

Speaking globally, if the matter density as a whole decreases, where does the matter go? How will the universe as a whole/globally be empty?

2. May 6, 2017

### davenn

why do you think it will go anywhere ?

really ? do you have a reference for this ?

Do you really think the earth and any other planet etc will just dissolve away into nothing ?

again ..... why do you think it will go anywhere ?

3. May 6, 2017

### durant35

How not? As far as I know, matter density decreases - therefore universe definitely empties out. If not, can you explain what is the alternative?

4. May 6, 2017

### jerromyjon

Matter that is gravitationally bound doesn't expand, the distance between distant objects expands. It doesn't even become obvious until you look at the distances of millions of light years, so galaxies aren't "stretching out", just the distance between galaxies is growing...

5. May 6, 2017

### kimbyd

For a good amount of detail, I rather like this wiki page:
https://en.wikipedia.org/wiki/Future_of_an_expanding_universe

As for the end state, a universe with zero or one particle per horizon is effectively completely empty.

6. May 6, 2017

### durant35

What is the difference between a zero particle per horizon or one particle per horizon? How would one situation be achieved and how would the other one be achieved?

My motivation for this is basically something about excitations leaving the horizon. We may imagine the whole universe expanding which will 'increase' the number of excitations/fluctuations simply because of the rate multiplied by the expanding spacetime volume. But on the other hand, the universe get colder and emptier per horizon, so fluctuations and excitations don't form, matter hardly ever interacts. Which effect wins in your opinion?

7. May 6, 2017

### davenn

you are still not providing references for these claims of yours

8. May 6, 2017

### rootone

I think the phrase 'empties out' is the problem.
Particles and fields still are there, but not enough for any significant interaction.
Nothing of any consequence can happen any more, maybe a new photon happens once every billion years.

9. May 7, 2017

### kimbyd

Each horizon would contain either zero or one particle. Over time, the number of horizons that could be defined containing zero particles would grow, while the number containing a single massive particle would stay the same.

The latter, since excitations will die out in each and every horizon you could possibly define.

10. May 7, 2017

### Staff: Mentor

11. May 7, 2017

### Staff: Mentor

It doesn't "go" anywhere. Matter density is decreasing due to the expansion of the universe--pieces of matter are moving away from each other.

"Spacetime volume" doesn't expand; spacetime is a 4-dimensional geometry, it doesn't "change". As for "excitation rate", I'm not sure what you mean.

You appear to have some basic misunderstandings about our current best fit model of the universe. Since you have been asked for references and have failed to give them, this thread is closed. You need to give specific sources for where you are getting your understanding from; otherwise it's impossible to tell where your understanding is going wrong.