Some questions about energy conservation

In summary, the professor was discussing how space expansions and vacuums create different types of universes. He said that dark energy is the dominant force in a vacuum-dominated universe while gravity is more dominant in an energy-dominated universe. He also said that there is hope for an anti-gravity universe where we can compress space.
  • #1
Upisoft
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Hello all.

I was watching the prof. Susskind lectures on YouTube. I'm very grateful to Stanford and the professor for making that knowledge so easily available to people willing to learn.

While I did understand almost everything said I still have some question of things that I did not understand or things that were not discussed.

While discussing energy dominated universe the professor said that the expansion of space causes red shift to the photons, thus reducing their energy until the universe became matter dominated. My question is where that energy disappeared?

While discussing vacuum(or dark energy) dominated universe he said that the energy density of vacuum does not change with time, thus expansion of the universe creates more energy. When asked where this energy comes from the professor pointed to the other side of the equation (the one not containing the dark energy density constant). I don't understand what he was talking about. The other side of the equation then has to be unlimited source of energy. How that is possible?
 
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  • #2
1. The short [and not entirely adequate] answer is redshifted photons are also time dilated. The total energy emitted by receeding objects in the universe is thus conserved. The deeper answer is - it depends [as lawyers are won't to say]. In GR, concepts such as energy conservancy are much squishier than in the classical, non-relativistic sense. Some scientists just shrug and say GR does not require conservation of energy, but, I think that is somewhat disengenuous. See: http://www.astronomycafe.net/cosm/expan.html

2. The good professor was saying that, unlike photons, the dark energy density of spacetime is not diluted by expansion. Or simply, more space means more dark energy potential between objects. Dark energy is the antithesis of gravity. Gravity increases as distance decreases. Dark energy increases as distance increases. Dark energy, however, is even more pathetically weak than gravity [which is a good thing]. Its effect is only apparent over vast distances - many millions of light years.
 
  • #3
Chronos said:
2. The good professor was saying that, unlike photons, the dark energy density of spacetime is not diluted by expansion. Or simply, more space means more dark energy potential between objects. Dark energy is the antithesis of gravity. Gravity increases as distance decreases. Dark energy increases as distance increases. Dark energy, however, is even more pathetically weak than gravity [which is a good thing]. Its effect is only apparent over vast distances - many millions of light years.
I see, there is hope for anti-gravity then. All wee need to do is to compress space. :smile:
Thanks for the answer.
 

Related to Some questions about energy conservation

1. What is energy conservation?

Energy conservation refers to the practice of reducing the amount of energy consumed by individuals, organizations, or countries. It involves using energy more efficiently and reducing energy waste in order to preserve natural resources and reduce negative environmental impacts.

2. Why is energy conservation important?

Energy conservation is important because it helps to reduce the amount of natural resources that are used to produce energy. This helps to preserve these resources for future generations and reduces the negative environmental impacts associated with energy production, such as air and water pollution and greenhouse gas emissions.

3. How can I conserve energy in my everyday life?

There are many ways to conserve energy in everyday life, such as turning off lights and electronics when not in use, using energy-efficient appliances and light bulbs, and reducing water usage. Other actions include using public transportation, carpooling, and choosing renewable energy sources.

4. What are the benefits of energy conservation?

The benefits of energy conservation include reduced energy costs, preservation of natural resources, improved air and water quality, and reduced greenhouse gas emissions. Additionally, energy conservation can also create new jobs in the renewable energy sector and promote economic stability.

5. What role do individuals play in energy conservation?

Individuals play a crucial role in energy conservation by making conscious choices to reduce their energy consumption. This can include small actions, such as turning off lights, as well as larger actions like investing in energy-efficient technologies. By making these choices, individuals can collectively make a significant impact on energy conservation efforts.

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