# B Could dark matter and dark energy be the same thing?

1. Jul 15, 2017

### friend

Dark energy is still energy so couldn't it also gravitate? The question is how large of a region of space must there be before dark energy could sustain itself at a higher density through self-gravitation? I'm under the impression that dark matter works at a smaller scale than dark energy. Or do they exist at the same size scales?

2. Jul 15, 2017

### phinds

No.

3. Jul 15, 2017

### rootone

'Dark matter' is just a place holder name for something which behaves like ordinary matter in that it has mass and produces gravity.
but we can't yet see it, so we call it dark, but it's something similar to normal matter.
Dark energy is a placeholder name for whatever it is that is causing the Universe to expand at an increasing rate.
The only relationship is the 'dark' bit in the name, meaning that we don't know what it is.
Physicists sense of humor is a bit abstract.

4. Jul 15, 2017

### friend

As I understand it, both DM and DE are invisible and don't (if not rarely) interact with the SM particles. Yet, both have an effect on gravity. My question remains. Can dark energy be denser in some regions of space than others? If so, what is the limit of the relative density that it can maintain? Can this approach the density of dark matter, which I understand is quite a bit less dense than the SM matter can get.

5. Jul 15, 2017

### Staff: Mentor

It does--but the way it gravitates is different from the way ordinary matter and energy gravitates.

Heuristically, the general quantity that determines how something gravitates is $\rho + 3 p$, where $\rho$ is the energy density and $p$ is the pressure. (This is actually only true for something that can be modeled as a perfect fluid, but that's a good enough approximation for this discussion.) For ordinary "cold" matter (including dark matter), $p = 0$, so we just have $\rho$, the energy density; this produces the ordinary kind of "gravitation" that we associate with ordinary matter like planets and stars.

Radiation (for example, the CMBR) has $p = \rho / 3$, so heuristically, it "gravitates" twice as much as you would expect from just its energy density. That affects how the expansion rate changes with time in the early universe, because the early universe was dominated by radiation, not cold matter. However, today, and in the future, radiation is negligible since its energy density is so low (because its energy density decreases faster with expansion than the energy density of cold matter does).

Dark energy has $p = - \rho$. That means that the "gravitating" quantity, $\rho + 3p$, is negative (it is $- 2 \rho$). So dark energy produces "gravity" that is repulsive, not attractive--it makes free-falling objects move apart, not together. That is why dark energy causes the expansion of the universe to accelerate.

6. Jul 15, 2017

### friend

Does any of this prevent DE from being more dense in some regions than in others?

7. Jul 15, 2017

### Staff: Mentor

It depends on what "dark energy" actually is. If it's a cosmological constant, then no--its density is the same everywhere. But if it's due to some field (for example, a scalar field can give a pressure-density relationship similar to that of a cosmological constant), then the field could vary from place to place, yes.

8. Jul 16, 2017

### Chronos

There are any number of studies that could reveal if dark energy is, or is not constant. The most obvious test is by measuring the rate of expansion in disparate parts of the universe. If it is slightly faster or slower in any particular region than any other region, that would spell bad news for the cosmological constant idea and good news for BTSM [beyond the standard model] theories. The cosmological constant appears to be holding its own, but, the case is still under litigation. For further discussion, https://ned.ipac.caltech.edu/level5/March01/Carroll/Carroll1.html, might be of interest.

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