What is Dark Energy made of? Quintessence? cosmological constant?


What is Dark Energy made of? Quintessence? cosmological constant?

What is Dark Energy made of? Quintessence? cosmological constant?

It’s one of the biggest mysteries in science. It makes up the majority of the universe. What is dark energy? What is dark energy made of? What is quintessence? Is Dark energy the cosmological constant? Does dark energy really exist?

Most scientists believed the expansion rate had to be slowing down over time, because of gravity pulling matter together. Some kind of measurement technique was needed to establish a standard luminosity. This is called a standard candle.

Two groups of astronomers (Saul Perlmutter and Adam Riess, and Brian Schmidt) , came up with a way to measure the true distances and redshifts of very distant galaxies by trying to find recent Supernova explosions – called type 1A supernovae. These provide a standard candle because all type 1A Supernovae explode with about the same luminosity. And by measuring the red shift of the light, they could determine how much space had expanded, What they found was the universe was speeding up.

The energy fueling the accelerating expansion of the universe is called “dark energy.” So what could be causing it?

First, this energy could a property of space itself – the cosmological constant. Lambda can be thought of as the energy density of the vacuum of space. It comes from a non-perfect cancellation of quantum fluctuations.

According to Heisenberg’s uncertainty principle, quantum fields from which all particles come from cannot sit still. Particles are coming in and out of existence causing fluctuations or pressure in the vacuum. But when you use the knows equations of quantum mechanics, the theoretical energy is almost 120 orders of magnitude higher than what is actually measured.

Another mechanism is called Quintessence. This is an energy field the pervades the entire universe. And it is repulsive. It would be a dynamical dark energy. It can change from place to place, and in time. Quintessence, if true, can be measured, because the differences in dark energy in different parts of space would be measurable.

There is a fine tuning problem though, which is called the coincidence scandal. The two densities are almost the same but in the past they were different. Why do we happen to live in a moment where the difference is only about 2.3?

If we believe the dark energy model explained by vacuum fluctuations, then there is something very special about now. And it is a huge coincidence. But if you believe quintessence is true, then this is not a coincidence.

The excitations of the quintessence field would manifest itself as a particle, probably a boson, but unlike every other boson, it would have negative energy. This would mean that lighter particles could decay into heavier particles by emitting quintessence particles with negative energy. This has never been seen. The models that fit the current data indicate that dark energy is not dynamical, but stays constant.

Could dark energy be photons or radiation? No because the energy density of radiation and photons changes as space expands. Dark energy however stays the same. Why is it that if the density doesn’t go down, it makes the universe accelerate?

Because what it means is that every cubic centimeter of space is expanding at a constant rate. So over very large distances, the cumulative effect is an acceleration of the universe.

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