If the dark energy equals the vacuum energy, it explains the flatness and expansion of the universe. But the Cosmological Constant problem is now the biggest issue in theoretical physics because the observed and expected vacuum energy values disagree by 120 orders of magnitude. We used to think it a small problem because the vacuum energy was equal to zero. But the newly discovered dark energy made all the difference.

Two scenarios emerge; see if the vacuum energy is zero, and find some other mechanism for dark energy, or see if dark energy is dynamic, perhaps decreasing to zero. The latter is detectable and will be the focus of this lecture.

The theoretical field of dark energy is Quintessence, ringing the bell of Aristotle in my mind. It would be like the Higgs field of a smooth non-directional scalar distribution of bosons, but unlike the Higgs in that it slowly evolves like a pendulum almost stuck at some non-zero value. It can interact, but would be problematic if some of the interactions were already ruled out.

Could Quintessence then explain the Coincidence Problem where back at a scale factor of 1/1000 the matter to dark energy ratio was one billion, compared to today's surprising ratio of two? It doesn't seem so, because although Quintessence would decrease like matter in the early universe, when galaxies formed Quintessence had to change. If it actually grew over time, this phantom energy could cause a Big Rip. The Hubble constant would increase to the point of creating a singularity, particles would have negative energy allowing the lighter to decay into heavier!

Quintessence would have a value for the field, and contain an amount of energy. It would interact by gravity, but also have hidden affects on the constants of the standard model like the mass and charge of the electron. But we have evidence of constants remaining the same from nucleogenesis and two billion year old fission products from natural uranium reactions. This evidence does put some pressure on speculations about Quintessence.

A new field implies a new "fifth force of nature" that would use a long range boson which implies a very small mass. An gravity experiment with balls on earth affected by the sun does not show any sign of this new force, but there could be something preventing detection.

So the plain vanilla model of a constant dark energy and dark matter where both can only react with anything else by gravity, shows us as just starting to understand. Yes, I am very glad to hear Sean finally make this statement. These concepts and speculations are so new, that one can really question why we are talking so much like we do know what the answers will be. A century from now, or even much sooner, there could be a whole new vocabulary of dark energy and dark matter interactions in addition to the plain vanilla of gravity. Until then we just have to look around as much as possible.

Speculations for other dynamic dark energy are tangled strings, cosmic strings, or variable mass particles. Sean helped develop the latter theory where the mass of particles actually increases. No signs of that yet.

But we can test if the acceleration of the universe is changing. This is the Equation of State Parameter (W). It relates the energy density to the pressure, the same equation from two lectures ago on negative energy. If the energy density is constant, the pressure is -1 in order to balance. Think of W as the changing value of that pressure. The best guess is W > -1, implying a slowly decreasing energy density. If W < -1 the energy density slowly increases by the phantom energy previously mentioned. Either way we need to look at our supernovae data and make some more speculations.

We used to add the two parameters of constant dark energy and matter to get an arbitrary sum, then see what fit the data. The total energy equaled the critical density, so the universe was declared flat. But if we replace the constant dark energy with W and set them equal to the critical density, that places limits on W to be -1 +/- 0.3. This range of -1.3 to -0.7 is good, but we need better. Plans are underway for observations to decrease the uncertainty to +/- 0.05. If W = -1.0, then the dark energy equals the vacuum energy of space itself. But we may not be able to tell if W = -0.999999 or -1.000001. Arg!

Two scenarios emerge; see if the vacuum energy is zero, and find some other mechanism for dark energy, or see if dark energy is dynamic, perhaps decreasing to zero. The latter is detectable and will be the focus of this lecture.

The theoretical field of dark energy is Quintessence, ringing the bell of Aristotle in my mind. It would be like the Higgs field of a smooth non-directional scalar distribution of bosons, but unlike the Higgs in that it slowly evolves like a pendulum almost stuck at some non-zero value. It can interact, but would be problematic if some of the interactions were already ruled out.

Could Quintessence then explain the Coincidence Problem where back at a scale factor of 1/1000 the matter to dark energy ratio was one billion, compared to today's surprising ratio of two? It doesn't seem so, because although Quintessence would decrease like matter in the early universe, when galaxies formed Quintessence had to change. If it actually grew over time, this phantom energy could cause a Big Rip. The Hubble constant would increase to the point of creating a singularity, particles would have negative energy allowing the lighter to decay into heavier!

Quintessence would have a value for the field, and contain an amount of energy. It would interact by gravity, but also have hidden affects on the constants of the standard model like the mass and charge of the electron. But we have evidence of constants remaining the same from nucleogenesis and two billion year old fission products from natural uranium reactions. This evidence does put some pressure on speculations about Quintessence.

A new field implies a new "fifth force of nature" that would use a long range boson which implies a very small mass. An gravity experiment with balls on earth affected by the sun does not show any sign of this new force, but there could be something preventing detection.

So the plain vanilla model of a constant dark energy and dark matter where both can only react with anything else by gravity, shows us as just starting to understand. Yes, I am very glad to hear Sean finally make this statement. These concepts and speculations are so new, that one can really question why we are talking so much like we do know what the answers will be. A century from now, or even much sooner, there could be a whole new vocabulary of dark energy and dark matter interactions in addition to the plain vanilla of gravity. Until then we just have to look around as much as possible.

Speculations for other dynamic dark energy are tangled strings, cosmic strings, or variable mass particles. Sean helped develop the latter theory where the mass of particles actually increases. No signs of that yet.

But we can test if the acceleration of the universe is changing. This is the Equation of State Parameter (W). It relates the energy density to the pressure, the same equation from two lectures ago on negative energy. If the energy density is constant, the pressure is -1 in order to balance. Think of W as the changing value of that pressure. The best guess is W > -1, implying a slowly decreasing energy density. If W < -1 the energy density slowly increases by the phantom energy previously mentioned. Either way we need to look at our supernovae data and make some more speculations.

We used to add the two parameters of constant dark energy and matter to get an arbitrary sum, then see what fit the data. The total energy equaled the critical density, so the universe was declared flat. But if we replace the constant dark energy with W and set them equal to the critical density, that places limits on W to be -1 +/- 0.3. This range of -1.3 to -0.7 is good, but we need better. Plans are underway for observations to decrease the uncertainty to +/- 0.05. If W = -1.0, then the dark energy equals the vacuum energy of space itself. But we may not be able to tell if W = -0.999999 or -1.000001. Arg!