# Dark Matter, any new news about it?

#### Max Born

I know that we don't know what dark matter is but according to observations, dark matter is there. I also knew that the reason why we know dark matter is there is because the universe is supposed to slowly become more compact, but the reality is that its doing the exact opposite, its expanding. I just wanted to know are there any new studies of dark matter. By the way "Hello Physics Forums" I'm a new guy.

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#### mathman

Dark matter is not particluarly related to the expansion of the universe. The reason it is assumed to exist is that galaxies and galactic clusters would fly apart otherwise.

Dark energy is the mechanism that is supposed to be the reason the universe expansion is accelerating.

#### Mark M

Welcome to PF!

As mathman also pointed out, I think you're confusing dark matter with dark energy. Despite the names, they are in no way related. That is a misnomer.

So, I'm guessing your question is about dark energy- so I'll address that. Dark energy is the umbrella term for whatever causes the acceleration of expansion. NOT expansion itself. It was known the universe is expanding since 1929.

Fluids in cosmology have an equation of state - a quantity given by $w = p/ \rho$, where $p$ is the pressure, and $\rho$ is its energy density. For anything that is accelerating the universe, then $w < - \frac {1} {3}$ must be true. Note that this quantity is negative, because dark energy must have a negative pressure. In the FLRW metric, negative pressures can drive the acceleration of the universe if it is already expanding.

So, with this information, there are a few candidates:

Cosmological Constant (CC): Note that, first of all, this is also known as 'vacuum energy'. They are the same thing. In the Einstein Field Equations, there is a term, $\Lambda$, that refers to a sort of vacuum energy - an energy that just comes with space, you can't get rid of it. Einstein introduced the CC after Alexander Friedman showed him that general relativity requires that the universe must be expanding. However, Einstein's CC was negative, it would counteract the effects of expansion to keep the universe static. After it was shown the universe was expanding, interest in the CC was lost. However, after the discovery of dark energy, interest in it has been renewed. The most important property of the CC is the fact it is constant - it will remain the same forever, keeping the universe accelerating until it suffers heat death. For the CC, $w = -1$. That is unchanging. If there is a CC, it's density would be $\Omega_{ \Lambda } \simeq \frac {2} {3}$, keep in mind this remains constant for eternity. In terms of Planck units, its value is astronomically low - about ~10-120. Quantum mechanics also predicts a type of vacuum energy, and it was hoped the value of this would meet the CC. However, it disagrees by a factor of 100 orders of magnitude.

Quintessence (Q) - Q would be a dynamical field that can change over time. W would be $$w = \frac{\frac{1}{2}Q^{2}-V(Q)} {\frac{1}{2}Q^{2}+V(Q)}$$

Theories of Q vary, so I don't really know where to start. But one case includes a Q known as 'phantom energy'. Phantom energy would have a $w < -1$, so acceleration would be non-constant. This would lead to a 'Big Rip' scenario, in which expansion accelerates until everything is pulled to pieces.

Of course, it could be none of these. But, I would lean towards a cosmological constant. Measurements of the equation of state in the future will be the best judge.

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#### turbo

Gold Member
@ OP, Just wait. "Settled" science does not progress except slowly.

#### Max Born

Thank you for clearing that up.

#### turbo

Gold Member
Thank you for clearing that up.
There may be other explanations for the supposed observational effects of Dark Matter. MOND seems to work pretty well for spiral galaxies, but it's ad-hoc and not universally applicable AFAIK. Could the strength of gravity vary with the matter density of its environs? We don't know, though GR would seem to forbid that. The universe has its own rules, and science is here because we are trying to figure the rules out.

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