Understanding the Expansion of Space: Galaxies Moving Away and Proving Expansion

[PeterDonis]

This is an old thread but my understanding is that there is an idea from quantum physics that says that empty space may be teeming with activity at a quantum level.

This appears in our current cosmological model as dark energy. We don't fully understand why its density is so small (about 120 orders of magnitude smaller than the value that the basic quantum physics calculation you refer to gives), but it is there in the model.

 

[Orodruin]

This appears in our current cosmological model as dark energy. We don't fully understand why its density is so small (about 120 orders of magnitude smaller than the value that the basic quantum physics calculation you refer to gives), but it is there in the model.

I think that is taking things for granted a bit too far. In the cosmological model the cosmological constant is just a parameter without any particular origin. In the quantum zero point energy could theoretically contribute to this but is, as you say, 120 orders of magnitude off which if anything is an indication that the connection to the cosmological constant is not well understood. Ideally a theory of quantum gravity should of course address this.

 

[PeterDonis]

In the cosmological model the cosmological constant is just a parameter without any particular origin.

True, but any "activity of empty space" of the sort the poster I responded to was describing would have to appear in our cosmological model in this parameter. So our cosmological model already takes into account the possibility that such a thing exists. The value we use in our model is based on empirical observation and we don't have a good theoretical understanding of why that is the value we observe, but that doesn't mean our model doesn't take the possibility into account.

 

[vanhees71]

I think that is taking things for granted a bit too far. In the cosmological model the cosmological constant is just a parameter without any particular origin. In the quantum zero point energy could theoretically contribute to this but is, as you say, 120 orders of magnitude off which if anything is an indication that the connection to the cosmological constant is not well understood. Ideally a theory of quantum gravity should of course address this.

Well, in principle our Standard Model of HEP doesn't tell us at all what the absolute value of the vacuum energy (density) is. In the usual perturbative treatment it's set to 0 by imposing "normal ordering" or equivalently by renormalization of the vacuum diagrams. This introduces a renormalization scale, and when using the renormalization group to go from the low-energy scale to very high scales (GUT scale or even the Planck scale) you get these huge 120 orders of magnitude discrepancies.

I think in this sense the vacuum-energy/cosmological-constant problem is the least understood "today observable part" on the crossroad between GR and QT. Maybe it's solved one day when a consistent formulation of "quantum gravity" is found.

 

[timmdeeg]

I think in this sense the vacuum-energy/cosmological-constant problem is the least understood "today observable part" on the crossroad between GR and QT.

Do we have at least a hint why it acts as "repelling" gravity?

 

[PeterDonis]

Do we have at least a hint why it acts as "repelling" gravity?

We have more than a hint. What you are calling "repelling gravity" is what the Einstein Field Equation says you get when you have the stress-energy of a perfect fluid with an equation of state $p = - \rho$. And that's exactly what you have with a cosmological constant.

 

[timmdeeg]

We have more than a hint. What you are calling "repelling gravity" is what the Einstein Field Equation says you get when you have the stress-energy of a perfect fluid with an equation of state $p = - \rho$. And that's exactly what you have with a cosmological constant.

Thanks. I have another question in this context.

The value of the Cosmological Constant according to the Quantum Field Theory is much to high. But apart from this it's sign due to QFT seems correct. In other words QFT having no link to General Relativity doesn't just predict a vacuum energy density, it predicts the vacuum energy density with the correct sign to act like the CC. Is that correct?

 

[PeterDonis]

QFT having no link to General Relativity doesn't just predict a vacuum energy density, it predicts the vacuum energy density with the correct sign to act like the CC. Is that correct?

AFAIK yes, QFT predicts a positive sign for the vacuum energy density.

 

[StandardsGuy]

Space without matter still has fields: electromagnetic field, gravitational field, Higgs field, etc. It also has energy, so "empty" space isn't nothing.