Dark matter clusters could reveal nature of dark energy

Dark matter clusters could reveal nature of dark energy
Gravitational lensing in galaxy clusters such as Abell 370 are helping scientists to measure the dark matter distribution. Credit: NASA, ESA, the Hubble SM4 ERO Team and ST-ECF

Scientists are hoping to understand one of the most enduring mysteries in cosmology by simulating its effect on the clustering of galaxies.

That mystery is dark – the phenomenon that scientists hypothesise is causing the universe to expand at an ever-faster rate. No-one knows anything about dark energy, except that it could be, somehow, blowing pretty much everything apart.

Meanwhile, dark energy has an equally shady cousin – dark matter. This invisible substance appears to have been clustering around galaxies, and preventing them from spinning themselves apart, by lending them an extra gravitational pull.

Such a clustering effect is in competition with dark energy's accelerating expansion. Yet studying the precise nature of this competition might shed some light on dark energy.

"Many dark energy models are already ruled out with current data," said Dr. Alexander Mead, a cosmologist at the University of British Columbia in Vancouver, Canada, who is working on a project called Halo modelling. "Hopefully in future we can rule more out."

Gravitational lensing

Currently, the only way dark matter can be observed is by looking for the effects of its gravitational pull on other matter and light. The intense gravitational field it produces can cause light to distort and bend over large distances – an effect known as gravitational lensing.

By mapping the dark matter in distant parts of the cosmos, scientists can work out how much dark matter clustering there is – and in principle how that clustering is being affected by dark energy.

The link between gravitational lensing and dark matter clustering is not straightforward, however. To interpret the data from telescopes, scientists must refer to detailed cosmological models – mathematical representations of complex systems.

Dr. Mead is developing a clustering model that he hopes will have enough accuracy to distinguish between different dark-energy hypotheses.

"An analogy I like a lot is with turbulence. In turbulent fluid flow you can talk about currents and eddies, which are nice words, but the reality of how fluid in a pipe goes from flowing calmly to flowing in a turbulent fashion is extremely complicated."

Fifth force

One of the more exotic theories is that dark energy is the result of a hitherto undetected fifth force, in addition to nature's four known forces—gravity, electromagnetism, and the strong and weak nuclear forces inside atoms.

A more common hypothesis for dark energy, however, is known as the cosmological constant, which was put forward by Albert Einstein as part of his general theory of relativity. It is often believed to describe an all-pervading sea of virtual particles that are continually popping into and out of existence throughout the universe.

One way to rule out the cosmological constant hypothesis, of course, is to prove that dark energy is not constant at all. This is the goal of Dr. Pier Stefano Corasaniti of the Paris Observatory in France, who – in a project called EDECS – is approaching dark-matter clustering from a different direction.

Instead of attempting to model clustering from data, he is beginning specifically with a dynamical – that is, not constant – hypothesis of dark energy, and trying to predict how dark matter would cluster if this was the case.

Pushing the limits

There are, in principle, infinite ways dark energy can vary in space and time, although many theories have already been ruled out by existing observations. Dr. Corasaniti is focussing his simulations on types of dynamical dark energy that push at the edges of these observational limits, paving the way for tests with future experiments.

The simulations, which trace the evolution of numerous, "N-body' dark matter particles, require supercomputers running for long periods of time, processing several petabytes (one thousand million million bytes) of data.

"We have run among the largest cosmological N-body simulations ever realised," Dr. Corasaniti said.

Dr. Corasaniti's simulations predict that the way dark energy evolves over time ought to affect clustering. This, in turn, alters the efficiency with which galaxies form in ways that would not be the case with constant dark energy.

The predictions his models are making could be tested with the help of forthcoming telescopes such as the Large Synoptic Survey Telescope in Chile and the Square Kilometre Array in Australia and South Africa, as well as by satellite missions such as Euclid (EUropean Cooperation for LIghtning Detection) and WFIRST (Wide Field Infrared Survey Telescope).

"If turns out to be a dynamical phenomenon this will have a profound implication not only on cosmology, but on our understanding of fundamental physics," said Dr. Corasaniti.

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Sep 10, 2018
Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.

The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

Displaced supersolid dark matter is curved spacetime.

Sep 11, 2018
Could both so called dark energy and dark matter be the result of antimatter particles that have negative mass, so cause gravity to push rather than pull.
These antimatter particles would fill the space between galaxies and may be the missing antimatter from the big bang (supposedly created in equal amounts to normal matter).

Sep 11, 2018
Could both so called dark energy and dark matter be the result of antimatter particles that have negative mass, so cause gravity to push rather than pull.
These antimatter particles would fill the space between galaxies and may be the missing antimatter from the big bang (supposedly created in equal amounts to normal matter).

No, for a really wide variety of reasons.
Read the Wikipedia page on antimatter for a start.

Sep 11, 2018
Nice, that dark energy likely is simply vacuum energy needs to be checked by independent means.

@Colbourne: Some obvious problems with that suggestion:
- Antimatter obeys gravity in experiments such as when antiprotons are made and trapped in magnetic bottles. (Though to be fair people are still checking, I think.)
- Dark matter is 5 times matter, not equal amounts.
- We know most matter and antimatter annihilated each other. This nicely makes for the cosmic background photons at 10^9 times the remaining matter and dark matter amount.
- Dark energy is not pushing by somehow curving space "the other way". (How? Gravity is monopolar seen as charge in a quantum field approximation, and simply curves space in a more general [sic] general relativity approximation.) It puts a decreasing pressure differential in a GR stress-energy tensor before/after comparison, sometimes roughly thermodynamic modeled as how you pull a plunger in a closed tube decreases pressure inside.

Oct 01, 2018
I posit this hypothesis that does away with both dark matter and also dark energy. I posit that at a distance of approximately 1.5 million light-years gravity becomes slightly repulsive, gradually increasing with distance to achieve a peak repulsion, and then decreasing with distance to zero.

Thus, cosmological expansion is caused by galaxies pushing against each other, and galactic rotation can be explained by the fact that each galaxy is surrounded by a "womb" of dust, gas, and other galaxies, and this "womb" pushes with repulsive gravity upon the outer stars of a galaxy to keep them in orbit at a higher speed than expected.

I give a cosmological / mathematical justification for this behavior in my Reddit article:


At the bottom, in the responses, I explain how General Relativity can be adjusted so as to retain time dilation while rejecting curved space and retaining flat, 3D, Euclidean space.

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