B How much energy does our Universe have?

[vasiladp]

Daniel Vasilaky asks do physicists know "how much energy the universe has"?

An infinite amount seems absurd - no need for conservation laws and so perpetual motion would be possible.

I'd say no to a negative amount also.

So it'd be zero (I'd say probably) or a finite, positive amount (maybe).

In asking this I've made some assumptions that I (inadvertently) may not be allowed to make.

Daniel

 

[russ_watters]

The universe is likely infinitely large, and if so it would have infinite energy too.

 

[vasiladp]

I don't follow that. I thought the universe's age is known from studying light that emerged at the "beginning" of it.
That implies our universe has a finite size. Distance=rate*time, and we know the time elapsed since the Big Bang, and we know the speed of light is a constant. So what you said can't be.

 

[jbriggs444]

I don't follow that. I thought the universe's age is known from studying light that emerged at the "beginning" of it.
That implies our universe has a finite size. Distance=rate*time, and we know the time elapsed since the Big Bang, and we know the speed of light is a constant. So what you said can't be.

The Big Bang is not what you think it is. The standard model of cosmology is way weirder than an explosion in space.

 

[vasiladp]

As I said above, an "infinite supply of energy" strikes me as absurd. Why bother conserving energy then?

 

[berkeman]

As I said above, an "infinite supply of energy" strikes me as absurd. Why bother conserving energy then?

I'm an engineer, not a Physicist, so I'm not sure I can help. But I think the best answer to that specific question is that energy is conserved on local scales (solar systems, galaxies, etc.), but not necessarily on larger scales. So since we live here pretty close to Sol, energy conservation is extremely useful for doing calculations.

I may move have moved your thread from the General Discussion (non-technical) forum to the technical forums for better answers. I'll mark it with the "B" basic prefix to keep the replies on the basic level, which I think would help you the best.

Welcome to PF!

 

[berkeman]

Okay, a digression in the thread caused by the OP's likely pop-science education about Cosmology has been deleted.

@vasiladp -- PF is a great place to ask these questions (even at the B = Basic level), but please be prepared and open to unlearn what you have been reading in the pop-sci literature. Thanks.

 

[Vanadium 50]

Daniel Vasilaky asks

Does Daniel Vasilaky always refer to himself in the third person? Holy Illeism!

strikes me as absurd

The universe is not compelled to arrange itself to your - or mine, or anyone's - liking. If the universe is infinite (and there is no evidence that it is not), any energy density, no matter how small, corresponds to an infinite energy.

 

[jbriggs444]

As I said above, an "infinite supply of energy" strikes me as absurd. Why bother conserving energy then?

The universe is weirder than most of us imagine. Possibly weirder than we can imagine. Just because a thing is absurd, that does not mean that it is untrue.

If you are interested in energy conservation rather than the question of how much energy there is, this article by Michael Weiss and John Baez may be of interest:

"Is Energy Conserved in General Relativity?​

In special cases, yes. In general, it depends on what you mean by "energy", and what you mean by "conserved".

 

[PeterDonis]

Daniel Vasilaky asks do physicists know "how much energy the universe has"?

Where? Please give a specific reference.

 

[Vanadium 50]

Where? Please give a specific reference.

Based on his user name, I think he's referring to himself in the 3rd person. So it's post #1.

 

[Nugatory]

As I said above, an "infinite supply of energy" strikes me as absurd. Why bother conserving energy then?

Consider a smallish box (small on a cosmological scale, so that general relativity curvature effects can be ignored). Conservation of energy says that the amount of energy in the box at time $T$ is equal to the amount of energy in the box at time $T-\Delta{T}$ plus or minus (depending on whether the net flow was in or out) the amount of energy that has flowed across the sides of the box between time $T-\Delta{T}$ and time $T$. This is an enormously useful result that holds no matter how much or little energy there is everywhere else in the universe.

Thinking about it this way will also give you a hint as to why your question "How much energy does our universe have?" is not as meaningful as you're thinking. We can try to calculate the total amount of energy in the universe: imagine the universe divided into a large number (infinite, if the universe is infinite) of these boxes; note the amount of energy in each box at some given time; add up all the energies and we have the total amount of energy in the universe at that time. However, there is a major catch: There is no unique way of defining "at some given time" across all of a curved spacetime, and hence no way of carrying out this calculation across the entire universe.

You may find this link helpful: https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

 

[kimbyd]

Daniel Vasilaky asks do physicists know "how much energy the universe has"?

An infinite amount seems absurd - no need for conservation laws and so perpetual motion would be possible.

I'd say no to a negative amount also.

So it'd be zero (I'd say probably) or a finite, positive amount (maybe).

In asking this I've made some assumptions that I (inadvertently) may not be allowed to make.

Daniel

There's a few different ways to look at this. Two of them are:
1) Given certain conditions and defining energy in a particular way, the universe always has exactly zero energy. If those conditions don't hold, the amount of energy is indefinite and cannot be represented.
2) Energy is not conserved in curved space-time, and total energy is a poorly-defined quantity.

For the second point, I recommend this excellent blog post:
https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/