Energy for expanding universe

• B
Gold Member

Main Question or Discussion Point

What is the source of the energy needed by the ever expanding space-time? It could not be baryonic matter, since it accounts for only 4%. It could not be radiation, since it accounts for only 0.008%. Is it dark matter being converted? Is it quantum fluctuations?
Kurt Ludwig

mfb
Mentor
The universe doesn’t need an energy source to expand.

Drakkith
Staff Emeritus
What is the source of the energy needed by the ever expanding space-time?
There doesn't need to be a source or expenditure of energy. Instead it is the result of the particular geometry of spacetime. Don't interpret the expansion of space as a process involving forces and acceleration as you and I know it from our everday lives (these do require energy). A ball launched out into intergalactic space will recede from us with an increasing velocity over time, but it itself will never experience any acceleration (as measured by an accelerometer). Nor would we. The only known way for our recession velocities to increase without expending energy is for the underlying geometry of spacetime to be such that this happens.

Unfortunately 4-D geometry is rather complicated and non-intuitive, so it may not make any sense right now. I wish I understood it better and could do a better job at explaining it.

timmdeeg
Gold Member
Unfortunately 4-D geometry is rather complicated and non-intuitive, so it may not make any sense right now. I wish I understood it better and could do a better job at explaining it.
Can one boil down the geometry argument saying that matter falls away from each other (if not gravitationally bound) since the decay of the inflaton field. The reason for this is the de Sitter like expansion during inflation which follows from the Friedmann equation.But how is the geometry argument connected with such kind of reasoning?
In my opinion The Meaning of Einstein’s Equation provides good insights why things are moving apart from each other.

Drakkith
Staff Emeritus
But how is the geometry argument connected with such kind of reasoning?
My understanding is that it is because matter 'falling away from each other' happens because of geometry. As in, in the absence of forces the geometry of spacetime at the largest scales makes geodesics move away from each other in space. But I admit my knowledge of the details of GR is limited. Hopefully I haven't strung together a bunch of words that end up meaning nothing.

Gold Member
I still do nor fully understand the following: Space is expanding, but its energy density stays constant. It follows that the total amount of dark energy is increasing. Where does this additional dark energy come from? Kurt Ludwig

PeterDonis
Mentor
2019 Award
Gold Member
Very interesting insights by Sean Carroll. When the space through which particles move is changing, the total energy of those particles is not conserved. Energy is not conserved in General Relativity. (I am currently reading Sean Carroll's book "The Big Picture.") Kurt Ludwig

haushofer