# How do scientists estimate the total energy of the universe?

1. Mar 23, 2015

### 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?"

2. Mar 23, 2015

### Quarlep

For Mentors: I am very curious apologies me but I cant sleep well If I cant get an answer. I am in high school so I cant understand difficult equatons . I dont want to bother you I just need to know answer.

<complaint about mentor actions removed>

Last edited by a moderator: Mar 24, 2015
3. Mar 23, 2015

### Doug Huffman

4. Mar 23, 2015

### Quarlep

Do you know in which year ? And thank you

5. Mar 23, 2015

### Doug Huffman

6. Mar 23, 2015

### Quarlep

I watched his three lectures about cosmolgy but I dont 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

7. Mar 23, 2015

### Chalnoth

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.

8. Mar 23, 2015

### 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"

9. Mar 23, 2015

### Chalnoth

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.

10. Mar 23, 2015

### Chronos

11. Mar 24, 2015

### Orodruin

Staff Emeritus
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.

12. Mar 24, 2015

### Quarlep

I dont 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 lets suppose its defined somehow (I dont know its impossible or posssible) Does it mean we can calculate Universe energy (Actually this questions answer depends first ones)

13. Mar 24, 2015

### Quarlep

We can never find Energy of Universe I guess

14. Mar 24, 2015

### Bandersnatch

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.

15. Mar 24, 2015

### Quarlep

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

16. Mar 24, 2015

### Staff: Mentor

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.

Last edited: Mar 24, 2015