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Energy of the Universe

by NextFeynman
Tags: energy, universe
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Chalnoth
#19
Nov21-12, 08:36 AM
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Quote Quote by phinds View Post
Yes, so much so that I have heard one theory, I think posted here on this forum, that the universe moves towards a singularity of infinite energy density because of the exponential overall increase in dark energy. I have no idea how solid that theory is.
It's incredibly unlikely. Pure speculation with nothing approaching a realistic model, and some good reasons to expect it's impossible.
phinds
#20
Nov21-12, 08:39 AM
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Quote Quote by Chalnoth View Post
It's incredibly unlikely. Pure speculation with nothing approaching a realistic model, and some good reasons to expect it's impossible.
Thanks. Yeah, it sounded pretty weird to me.
Chalnoth
#21
Nov21-12, 08:47 AM
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Quote Quote by NextFeynman View Post
Seeing as energy is conserved throughout the universe, did all the energy of the universe begin at the same time as our universe?
Energy isn't conserved in General Relativity. Here's a good, detailed look if you're interested:
http://math.ucr.edu/home/baez/physic...energy_gr.html

The short version is that within General Relativity, there simply isn't any unique way of defining overall energy. And if there isn't a unique way of defining it, there isn't a way of conserving it (because somebody else could just come up with a different definition that isn't conserved).

If we take what we humans generally consider the most natural definition of overall energy, then the overall energy of various components of the universe do change in time, and the total is not conserved.

1. The total energy in light is decreasing inversely with the expansion. That is, if the average distance between galaxies doubles, the total energy in light in a given expanding volume is cut in half. In the early universe, this was the dominant form of energy, and now is only a tiny fraction.
2. The total energy in normal and dark matter stays nearly constant. The kinetic and local gravitational potential energy of normal and dark matter reduces slowly, but this is a small fraction of the mass energy anyway. Until relatively recently, this was the dominant form of energy in the universe.
3. The total energy in dark energy is increasing rapidly with the expansion: the density remains constant (or very nearly constant), so that if you double the average distance between galaxies, the energy in an expanding volume multiplies by a factor of eight (2*2*2, one for each direction of space). This energy density is very low, so until recently it didn't mean much. But today it makes up the majority of the density in the universe. In the far future, the growth of energy in dark energy will dominate everything else: effectively the universe will be empty except for the dark energy (the amount of time this takes is tremendous, however. See here: http://en.wikipedia.org/wiki/Future_...nding_universe).

Edit: Incidentally, it's quite possible (given certain other conditions) to define a global gravitational potential energy which exactly cancels the total energy in the rest of the universe, leading to a total energy of precisely zero for all time.
NextFeynman
#22
Nov21-12, 12:08 PM
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At the beginning of our universe, could the total energy of the universe be greater than or equal to the dark energy at that time?
Chalnoth
#23
Nov21-12, 12:22 PM
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Quote Quote by NextFeynman View Post
At the beginning of our universe, could the total energy of the universe be greater than or equal to the dark energy at that time?
Early on, shortly after inflation ended, dark energy was a minuscule fraction of the total energy (inflation itself acted much like a different sort of dark energy, with much higher density).
NextFeynman
#24
Nov21-12, 02:38 PM
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So could dark energy be the same amount of energy since the start of the universe, and its just spreading more space out ever since then.
Chalnoth
#25
Nov21-12, 02:50 PM
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Quote Quote by NextFeynman View Post
So could dark energy be the same amount of energy since the start of the universe, and its just spreading more space out ever since then.
The same amount? No. Dark energy doesn't spread out: it always remains at approximately the same energy density. So as dark energy fills more space, the total energy increases (at least considering the way it's usually looked at).
NextFeynman
#26
Nov21-12, 05:37 PM
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Could dark energy be expanding the universe at the same rate over the entire existence of the universe
phinds
#27
Nov21-12, 05:46 PM
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Quote Quote by NextFeynman View Post
Could dark energy be expanding the universe at the same rate over the entire existence of the universe
No, the expansion is ACCELERATING, not staying the same --- it is changing all the time and always has been and always will be.
NextFeynman
#28
Nov21-12, 06:13 PM
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How do we know that dark energy hasn't been expanding the universe at the same rate? If dark energy has been expanding space since the begining it is going to have more and more space to create. So couldn't dark energy be the creation of the three dimensional space we observe?
phinds
#29
Nov21-12, 06:23 PM
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Rather than continuing to ask serial speculative questions about dark energy, based only on how you think it MIGHT work, how about you go to the internet and fiind a good discourse on dark energy, read it, and then ask questions if there are still things you don't understand.
Drakkith
#30
Nov21-12, 06:59 PM
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Quote Quote by NextFeynman View Post
How do we know that dark energy hasn't been expanding the universe at the same rate? If dark energy has been expanding space since the begining it is going to have more and more space to create. So couldn't dark energy be the creation of the three dimensional space we observe?
Because that isn't how we define dark energy. And you are asking about the "creation of space". Such a thing may not even make sense. Is space even anything? Such a question is not answerable at this time. Our definition of space and dimensions is for another thread, but I will say that dark energy is not related to them in the way you are imagining. There have been several threads asking about what space is and I'm sure using the Search button at the top of the page will allow you to find them.
ImaLooser
#31
Nov21-12, 10:57 PM
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Quote Quote by Drakkith View Post
I believe light loses energy over time due to redshift from expansion.
I thought that total energy was conserved. Energy density of radiation decreases because the volume of the universe increases. Energy density times volume is a constant, as far as I know in a finite Universe. In an infinite Universe then energy density divided by matter density would be a constant.
Chalnoth
#32
Nov21-12, 11:43 PM
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Quote Quote by ImaLooser View Post
I thought that total energy was conserved. Energy density of radiation decreases because the volume of the universe increases. Energy density times volume is a constant, as far as I know in a finite Universe. In an infinite Universe then energy density divided by matter density would be a constant.
Nope. See my first post in this thread.
Chronos
#33
Nov21-12, 11:54 PM
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Keep in mind some forms of energy are positive, and others [e.g., gravity] are considered negative. At the moment, negative energy appears to dominate the universe.


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