Dark Matter and Dark Energy…

a post by Dreamy

One aspect of the Universe’s expansion was quite certain in the early 1990s. Gravity was bound to slow the expansion over time, whether it had enough energy density to halt its expansion and collapse or not enough energy density to ever stop expanding. Even while the slowing had not yet been noticed, technically, it should have. The gravitational attraction of all matter in the Universe brings it all together. Then in 1998, measurements made by the Hubble Space Telescope (HST) of extremely far-off supernovae revealed that, long ago, the cosmos was actually expanding more slowly than it is right now. Hence, the Universe’s expansion has not slowed down, it is accelerating despite gravity and against everyone’s prediction. Nobody anticipated this, and no one had a good explanation. But something was to blame.

Eventually theorists came up with three explanations.

Maybe it was a result of a long-discarded version of Einstein’s theory of gravity, one that contained what was called a “cosmological constant”.
Maybe there was some strange kind of energy-fluid that filled space.
Maybe there is something wrong with Einstein’s theory of gravity and a new theory should include some kind of field that creates this cosmic acceleration.

Theorists still don’t know what the correct explanation is, but they have given the solution a name. It is called dark energy.

Two small points – the Big Bang was not an explosion. It was the expansion of spacetime. Secondly, in science a theory has been proven, a hypothesis is an idea that has yet to be demonstrated. It is sometimes tricky to separate the everyday meaning of theory from the scientific one.

Scientists have calculated the composition of the Universe to be 68% dark energy, 27% dark matter, and 5% normal matter by fitting a theoretical model of the Universe to the combined set of cosmological measurements.

What is dark matter? We do not know but we are clear it is not normal matter.

First , it is black, therefore we cannot perceive it as we do stars and planets. The 27% dark matter figure is needed to match observations of the cosmos with mathematical calculations of the cosmos. Without it there is not enough mass out there to account for how galaxies behave.
Second, it does not exist as dense clouds of ordinary matter, which is composed of tiny particles known as baryons. We would be able to identify this by the gravitational effects it would have. Large concentrations of matter can bend light coming from farther away objects when it passes through them, but we do not observe enough lensing events to infer that these objects could contribute the necessary 27% of dark matter.

What is dark matter made from?

For decades, the most popular candidate for dark matter has been the weakly interacting massive particle (WIMP). The hypothetical particle was first dreamed up in the 1970s as an expansion of the traditional Standard Model of particle physics. The theory is that the cosmos is swarming with invisible, neutrally charged particles that came into being shortly after the Big Bang. The idea of invisible particles is nothing particularly new. Scientists are already aware of the neutrino – the difficult-to-detect subatomic particle that races across galaxies with a mass fractionally above zero. In comparison, WIMPs are believed to be much heavier and more sluggish, trudging across the sky in dense clumps and intricate structures. That is, if they even exist at all. Despite a large array of experiments, none of the attempts to find WIMPs have been successful. It was originally thought that the LHC in Geneva would be able to shed light on their existence. But almost a decade after it opened, no evidence has been found. Similarly, the highly sensitive tanks of liquid xenon buried deep under South Dakota discovered nothing in their search, either. With scientists continually failing to detect these particles directly, hypotheses surrounding WIMPs are now cast in serious doubt. It would be an oversight to rule out WIMPs altogether. But it looks like scientists must return to the drawing board and consider alternative theories of dark matter.

Other possible explanations include self-interacting dark matter that uses forces as yet unknown to physics to account for its behaviour. There is also dark matter heating that proposes that dark matter is propelled from the centre of a galaxy by energy and wind created during star formation.

So, will we ever find out the truth about this strange and absurd substance?

Only time will tell…