How do scientists estimate the total energy of the universe?

[Quarlep]

I posted recently on a banned topic, apologies for that. Under advisement of a mentor I've decided to ask a positive question instead, how do scientists estimate the total energy of the universe?"

 

[Quarlep]

For Mentors: I am very curious apologies me but I can't sleep well If I can't get an answer. I am in high school so I can't understand difficult equatons . I don't want to bother you I just need to know answer.

<complaint about mentor actions removed>

 

[Doug Huffman]

Learn the Friedmann–Lemaître–Robertson–Walker equations.

https://en.wikipedia.org/wiki/Friedmann–Lemaître–Robertson–Walker_metric

As I recall, Leonard Susskind derives them in his streamed lectures.

 

[Quarlep]

Do you know in which year ? And thank you

 

[Doug Huffman]

I recommend starting at the beginning of his lecture series for a high school student, as each subject builds on prior knowledge.

https://www.youtube.com/playlist?list=PLQrxduI9Pds1fm91Dmn8x1lo-O_kpZGk8

 

[Quarlep]

I watched his three lectures about cosmolgy but I don't know GR.Should I take GR for cosmolgy lessons or I can handle it I kow special Relatvity a bit and classical mechanics.I watched his all classical mechanics equations

 

[Chalnoth]

I posted recently on a banned topic, apologies for that. Under advisement of a mentor I've decided to ask a positive question instead, how do scientists estimate the total energy of the universe?"

They don't, as it's not possible. The total energy is an ill-defined concept in General Relativity.

Instead, cosmologists measure the energy density. This is estimated using a variety of methods. The most common are measurements of the expansion history and the spatial geometry of our universe.

 

[Quarlep]

Energy density and curvature of the universe are connected . I know In FL equations . If density less than one than k will be negative. Is that means "Universe Total Energy is positive"

 

[Chalnoth]

Energy density and curvature of the universe are connected . I know In FL equations . If density less than one than k will be negative. Is that means "Universe Total Energy is positive"

No. The phrase "Universe Total Energy is positive" is not something that can be evaluated as either true or false within General Relativity: by making different arbitrary choices, you can evaluate it as either true or false with the exact same configuration of the universe.

 

[Chronos]

One possible answer for the total energy of the universe is ... zero: https://www.astrosociety.org/publications/a-universe-from-nothing/

 

[Orodruin]

Energy density and curvature of the universe are connected . I know In FL equations . If density less than one than k will be negative. Is that means "Universe Total Energy is positive"

No, there is simply no way of defining the "total energy of the universe" in a fashion that is unambiguous. You can define a local energy density, but this will also depend on your frame of reference. In addition, even if you study a FRW universe and define the comoving frame as "the" frame, the energy density multiplied by the volume is not going to give you a constant. Energy (however you want to define it) is simply not conserved in a cosmological setting.

 

[Quarlep]

They don't, as it's not possible. The total energy is an ill-defined concept in General Relativity.

I don't know GR but I understand that We have to use GR to calculate universe energy and its noy possible cause its ill-defined.
What do you mean about ill-defined? Is that show us GR is incomplete ? and let's suppose its defined somehow (I don't know its impossible or posssible) Does it mean we can calculate Universe energy (Actually this questions answer depends first ones)

 

[Quarlep]

Instead, cosmologists measure the energy density. This is estimated using a variety of methods. The most common are measurements of the expansion history and the spatial geometry of our universe.

You can define a local energy density, but this will also depend on your frame of reference. In addition, even if you study a FRW universe and define the comoving frame as "the" frame, the energy density multiplied by the volume is not going to give you a constant.

We can never find Energy of Universe I guess

 

[Bandersnatch]

What do you mean about ill-defined? Is that show us GR is incomplete ?

Consider this analogy:
There's a saying that makes sense on Earth: What goes up must come down. If you want to be very precise, you can add 'unless you throw it really, really hard' to account for escape velocity.
It's a good statement about gravity, that holds well locally.

But take it to space, where you're in free-fall (say, in the ISS) and try using that saying. It's not that the saying is now incorrect - it's that up and down are no longer defined. Now you can say that up and down can be defined only in special cases, so any statements using these terms are not generally true.

Does this mean that the theory of gravity is incomplete? No. If anything, it means that your understanding of gravity is good enough to allow you to extend its application to more general regimes, not limited to the special case of the surface of the Earth.

Similarly with energy. Locally, you can say for example that 'the energy is conserved', but take it to a more general regime of the cosmological scales, where the term 'energy' is no longer defined, and the statement about energy conservation stops making sense.

 

[Quarlep]

Thanks for your efforts for more detail I need to learn cosmolgy and GR thank you

 

[Dale]

I can't sleep well If I can't get an answer

You should seek medical attention. Some questions simply do not have answers at present and if you cannot sleep without them then you will suffer from insomnia.

I deleted the part of your post complaining about the mentor actions. Such complaints belong only in the feedback forum, not in the technical forums, and certainly not part of a technical discussion.