# I Will the acceleration of the expansion of universe ever decrease?

1. Dec 6, 2017

### Souhardya Nandi

This may sound like a noob question but please help me out guys.Will universal acceleration ever retard ? And if it continues to accelerate for all eternity, where is it getting this never ending energy from ?

2. Dec 6, 2017

### Staff: Mentor

Current measurements point to expansion forever, but this could still change - our observations are limited to the present and past, obviously.

The universe doesn't "need energy" to expand, and there is no global conservation of energy anyway in general relativity.

3. Dec 6, 2017

### kimbyd

Removed off-topic discussion - mfb

As for whether the acceleration will slow, nobody knows at present. If the cause of the expansion is some kind of dynamical field that does dilute slowly as the universe expands, then yes, the acceleration will slow. If it's a cosmological constant, then the acceleration will never slow: it will approach a constant value.

Last edited by a moderator: Dec 7, 2017
4. Dec 6, 2017

### Souhardya Nandi

That was helpful. Thanks. However, what about the energy required for continuous expansion ?

5. Dec 6, 2017

6. Dec 6, 2017

### Souhardya Nandi

Sorry. Actually I misplaced my question. Let me reframe my question. He gave 2 possible causes of this energy. I wanted to know which is consistent with the latest observations and models ?

7. Dec 6, 2017

### Souhardya Nandi

Sorry again. Oh yes. I understood mfb answer. Did not spot it at first. Thats why. Never mind. Ignore this subsequent question.

8. Dec 6, 2017

### Mordred

Energy doesn't require a cause, Energy is simply best thought of as the ability to perform work. Any attempt to visualize energy as anything other than a property under that definition will make you go down garden paths. It is a property much like mass is a property or color.
Lets use two basic forms of energy as an example.

Potential energy. Energy due to its position relative to others
kinetic energy. kinetic energy of an object is the energy that it possesses due to its motion.

No matter what stick to those three definitions and never forget them. I can guarantee you, no matter what model under physics you are studying these definitions will always apply

Last edited: Dec 6, 2017
9. Dec 7, 2017

### kimbyd

One way to think of it is that energy is a property of matter (more precisely, it's a property of quantum fields, not just ordinary matter that we're familiar with). It's a property in the exact same way that position or momentum or electric charge are properties. Though these properties have different characteristics, they all follow conservation laws given certain assumptions.

The interesting bit, to me, is how the physical laws of the universe result in these conservation laws. That stems from what is known as Noether's Theorem. The idea there is that if you write the laws of physics (or of the behavior of a system) down in a certain particular way, you'll find that you can change certain variables and the equations are unchanged. Noether's Theorem demonstrates that if the system has this property that changing one variable leaves the system unaffected, then that implies you can write down a parameter which is a constant for the system. If your system is unchanged, in a specific mathematical way, if you move the system from one location to another, then momentum is conserved. If the system is unchanged if you rotate it, then angular momentum is conserved. If the system is quantum-mechanical, and it's unchanged if you change all of the quantum phases of all of the particles that make up the system, then electric charge is conserved. And if the system is unchanged if you examine it at different points in time, then energy is conserved.

That last point is important for General Relativity: it shows that in an expanding universe, energy cannot be conserved (in the simplest sense) because the system is changing over time. You can recover a related conservation law by making some different assumptions, resulting in conservation of the stress-energy tensor (which includes energy). This means that while energy isn't conserved, it changes over time following very specific rules determined by the conservation of this more complicated object.

You can also recover a different sort of energy conservation by using what is known as "Hamiltonian formalism" of General Relativity which ends up applying a sort of energy to the space-time curvature itself (sort of like a gravitational potential energy). But this formulation hasn't been shown to work in all cases. In general it's

10. Dec 7, 2017

### Staff: Mentor

11. Dec 8, 2017

### newjerseyrunner

Both would be. You'd only notice a difference after cosmological timescales. If some energy pool exists that's slowly draining away, it could be trillions of years before it drains away by a noticeable amount. For all practical purposes, we assume the energy amount is constant. It will depend on whatever formulation we come up with for dark energy.

Possible related, the universe is accelerating much much slower than it did at particular times in the past. In the first few moments of the universe, it expanded ridiculously fast in a process called "inflation." Inflation though, stopped very quickly, and nobody is sure what it was or why it stopped.

12. Dec 8, 2017

### phinds

And we're not even 100% sure that it even happened, although if it didn't we're going to have to come up w/ some other explanations for other things.