Spacetime expansion and conservation of energy

In summary: Mass-energy is, however, always conserved locally in general relativity. More technically, the 4-dvergence of the stress-energy tensor is zero.
  • #1
EasterEggs123
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1
I came across this issue a while ago, when spacetime expands, then energy doesn' seem to be conserved? But does not that violate the law of conservation of energy? I don't get it, how can spacetime expansion happen without energy issues? Thanks in advance
 
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  • #2
Could you be more specific about why you think this is a contradiction? You must take into account that space-time is deformed according to general relativity due to the presence of mass, and therefore it involves energy.
 
  • #3
PabloAMC said:
Could you be more specific about why you think this is a contradiction? You must take into account that space-time is deformed according to general relativity due to the presence of mass, and therefore it involves energy.
Well the wavelengths of photons (CMB) get stretched but the number of them is the same.
 
  • #4
This is actually a complicated issue and depends on what you mean by "energy" and "conserved". There's a good link that explains it that I'll try to find.
 
  • #7
A huge truck that whizzes by me has loads of kinetic energy. I get on my bike and pedal so furiously that the truck has no relative velocity with respect to me, and thus zero kinetic energy. Where did the energy go? Nowhere, I changed frames of reference. Using a non-rigorous analogy, we continuously change frames of reference as the universe expands.

Mass-energy is, however, always conserved locally in general relativity. More technically, the 4-dvergence of the stress-energy tensor is zero.
 
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  • #8
George Jones said:
A huge truck that whizzes by me has loads of kinetic energy. I get on my bike and pedal so furiously that the truck has no relative velocity with respect to me, and thus zero kinetic energy. Where did the energy go? Nowhere, I changed frames of reference. Using a non-rigorous analogy, we continuously change frames of reference as the universe expands.

Mass-energy is, however, always conserved locally in general relativity. More technically, the 4-dvergence of the stress-energy tensor is zero.

Apologize if I sound like a high schooler (because I am) but would not the acceleration of our bike be equal to usage of energy? Would it not take energy to change the frame of reference, which is supposed to explain the energy issue?
 
  • #9
Because of this possible confusion, I carefully chose the two vehicles. Suppose that initially you and I are are stationary with respect to the ground. You remain stationary. After I start pedaling, you see: 1) the truck and me moving with the same speed; 2) the kinetic energy of the truck to be much larger than the kinetic energy of the bike/me system bike, because the mass of the truck is so much larger. The gain that you see for my kinetic energy is much smaller that the loss of kinetic energy that I see for the truck.
 
  • #10
Unfortunately no analogy will be perfect. You simply have to accept that the analogy is only vaguely like expansion and try to increase your knowledge of physics and math until you can learn the real theory.
 
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1. What is spacetime expansion and how does it relate to the conservation of energy?

Spacetime expansion refers to the idea that the universe is constantly expanding, with galaxies and other celestial bodies moving away from each other. This expansion is due to the force of dark energy, which is believed to make up about 70% of the universe. The conservation of energy states that energy cannot be created or destroyed, but can only change forms. In the case of spacetime expansion, the energy is believed to come from the vacuum energy of empty space, which drives the expansion.

2. How does the expansion of spacetime affect the conservation of energy?

The expansion of spacetime does not violate the conservation of energy, as energy is not being created or destroyed. It is simply being transferred from one form (such as vacuum energy) to another (such as the kinetic energy of moving galaxies). This transfer of energy is what drives the expansion of the universe.

3. Can spacetime expansion be explained by the conservation of energy?

While the conservation of energy can explain the transfer of energy that drives spacetime expansion, it is not the only factor at play. The theory of general relativity, which describes the relationship between matter and spacetime, is also necessary to fully understand and explain the expansion of the universe.

4. How does the concept of dark energy fit into the conservation of energy?

Dark energy, which is believed to be the driving force behind the expansion of spacetime, is still a mystery to scientists. It is not fully understood how it fits into the conservation of energy, as it is not a tangible form of energy that can be measured or observed directly. However, some theories suggest that dark energy may be a form of vacuum energy, which would align with the conservation of energy principle.

5. Is the conservation of energy still applicable in the context of spacetime expansion?

Yes, the conservation of energy is still a fundamental principle in the study of spacetime expansion. While the concept of dark energy and the transfer of energy between forms may be complex, they still abide by the basic law that energy cannot be created or destroyed. The conservation of energy is a crucial aspect of understanding the behavior of the universe and its expansion.

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