# Expanding universe - energy/information conservation

member 342489
I have trouble understanding the following.

Lets take 2 galaxies moving away from each other caused by the expanding universe.

A spaceship travelling between these 2 galaxies, more than one time, will experience a greater distance for each travel.

Therefore my conclusion is, that there must have been created more empty space/vacuum between the 2 galaxies each time the spaceship take another trip.

I have read, that when space expands, then the density of the vacuumenergy remains constant.

So i conclude, that there must have been created more energy and information, coming from nowhere.

But that dosen't make sense to me when I consider the rules about energy/information conservation.

Dosen't conservation in these cases mean constant amount of energy/information?

I already know part of the answer you guys/girls will give me, that there for some reason, haven't been any violation of neither energy nor information conservation.

Im no mathguru, so what I hope for, is some kind of abstract that can give me a basic understanding of why not.

Last edited by a moderator:

Simon Bridge
Homework Helper
Consider: although the vacuum energy density may be a constant, and the amount of vacuum increasing, the mass-energy density also is decreasing (the galaxies are moving apart), and only total energy has to be conserved? Isn't the overall energy-density decreasing with the expansion?

Regular small-scale energy conservation need not apply on cosmological scales. The thing that gets conserved in general relatvity is the "energy-momentum".

How you deal with this depends on the cosmological model you like - general relativity allows for a range of them.

In the end, what you are thinking about is what is behind all those discussions about cosmological constants and dark energy you hear about.

refs:
http://www.cfa.harvard.edu/seuforum/faq.htm
http://en.wikipedia.org/wiki/Conservation_of_energy#Relativity

Last edited by a moderator:
member 342489
Hi Simon.

Thank you for a very good reply.

You are right when you claim that my confusion is "about is what is behind all those discussions about cosmological constants and dark energy you hear about."

I think I get that your point is that I should try to consider the 2 galaxies and the space inbetwen as a closed system where the growing dimension of this system leads to the same density in the system overall.

I also think I get your point when you write "Regular small-scale energy conservation need not apply on cosmological scales" My interpetation of this is, that you consider the 2-galaxy system as a small scale system compared to the entire universe.

My understanding is, that the cosmological constant and dark energy are two sides of the same thing with different names where, as Lawrence Krauss shows, the cosmological constant were put in Einsteins equation to fit them to a steady state universe, and moved to the other side of the equation it describes the precise effect caused by dark energy.

Believe you have been very helpful

Last edited by a moderator:
Simon Bridge
Homework Helper
Some notes:

On the cosmological scale - the two galaxies and the space between them is not a closed system.
But you can consider model universes with very little in them and see how that works out.

If the system is large enough for cosmological/metric expansion to be a factor, that is not a small-scale system.

member 342489

"think I get that your point is that I should try to consider the 2 galaxies and the space inbetwen as a closed system where the growing dimension of this system leads to the same density in the system overall."

Should have been:

I think I get that your point is that I should try to consider the 2 galaxies and the space inbetwen as a closed system where the growing dimension of this system leads to a lower density in the system overall.

I have realized that I have missed your point "gets conserved in general relatvity is the "energy-momentum" , but i'm on it :-)

Simon Bridge
Homework Helper
Well done.
I gave you two answers, one was to direct you to look again at your reasoning and the other was to direct you to the conserved quantity.

You cannot get a good idea of this stuff from pop-science sources though.
If you are serious about exploring these ideas, then you want some more serious sources like:
http://arxiv.org/abs/gr-qc/9712019
http://preposterousuniverse.com/grnotes/grtinypdf.pdf [Broken]

But you need special relativity and a bunch of maths as a prerequisite.

You may prefer something more cosmology oriented like:
http://map.gsfc.nasa.gov/universe/
http://arxiv.org/abs/astro-ph/9312022
http://icc.ub.edu/~liciaverde/CERNCosmo1.pdf
... these should be fairly accessible without requiring an undergrad degree :)

Last edited by a moderator:
member 342489
Thank you again Simon you have been most helpful and patient ;-)

I will look into the links you provided

member 342489
Simon I am impressed!!

The links you have recommended directs me to exactly what i really wants to know.

thank you!!

Simon Bridge