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Simulation Suggests 68 Percent of the Universe May Not Actually Exist (newatlas.com)
boley1 quotes a report from New Atlas: According to the Lambda Cold Dark Matter (Lambda-CDM) model, which is the current accepted standard for how the universe began and evolved, the ordinary matter we encounter every day only makes up around five percent of the universe's density, with dark matter comprising 27 percent, and the remaining 68 percent made up of dark energy, a so-far theoretical force driving the expansion of the universe. A new study has questioned whether dark energy exists at all, citing computer simulations that found that by accounting for the changing structure of the cosmos, the gap in the theory, which dark energy was proposed to fill, vanishes. According to the new study from Eotvos Lorand University in Hungary and the University of Hawaii, the discrepancy that dark energy was "invented" to fill might have arisen from the parts of the theory that were glossed over for the sake of simplicity. The researchers set up a computer simulation of how the universe formed, based on its large-scale structure. That structure apparently takes the form of "foam," where galaxies are found on the thin walls of each bubble, but large pockets in the middle are mostly devoid of both normal and dark matter. The team simulated how gravity would affect matter in this structure and found that, rather than the universe expanding in a smooth, uniform manner, different parts of it would expand at different rates. Importantly, though, the overall average rate of expansion is still consistent with observations, and points to accelerated expansion. The end result is what the team calls the Avera model. If the research stands up to scrutiny, it could change the direction of the study of physics away from chasing the ghost of dark energy. "The theory of general relativity is fundamental in understanding the way the universe evolves," says Dr Laszlo Dobos, co-author of the new paper. "We do not question its validity; we question the validity of the approximate solutions. Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion. These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy." The study has been published in the Monthly Notices of the Royal Astronomical Society. You can view an animation that compares the different models here.
There didn't seem to be enough matter to explain the evolution of the universe, so scientists guessed at what might be causing it. It looked like there was more matter that we couldn't see, so they invented the idea of 'dark matter', which was something that had mass but was otherwise pretty inert so we didn't see it. BTW: 'dark' here is an old use meaning 'unable to be seen', such as 'the dark side of the moon' being the side that faces away from earth, not the side that is not lit by the sun. The other possibility was that gravity was somehow slightly different when operating over very large distances and times. This was settled because astronomers got better at calculating the distributions of mass in the universe when they thought there was something interesting to find, and found there were cases, such as the 'bullet nebula' where there were very significant amounts of mass in different places to the star-like matter we could see. This gives credence to the idea that 'dark matter' is a real sort of 'stuff', our can be treated as a sort of stuff, rather than just an systematic difference in the equations.
Okay, suppose we assume for now there is lots of invisible stuff that has mass and momentum, but otherwise does not interact with anything else much (think of neutrinos, but more so). If we take our best assumptions as to the right amount of dark matter, then there is a slight error which means something else is pushing the universe apart. If it looks like extra energy, we call it 'dark energy' and astronomers start looking for ways to detect it. In the meanwhile, other people look for a rival model where there is a systematic error in the equations for very large distances and times. That's pretty much where we are now. Indeed, the two explanations are not different - one just describes the error as 'extra energy' and the other one does not - until we get some new experimental evidence that shows which explanation is more useful.
Dark energy is a small correction term to our universe. If you want something we really don't understand, try the inflationary period of the early universe. We know it got really big, really fast, but really evenly; but we don't have any of the details.
Er, no - that's not entanglement. In quantum entanglement changing the state of one particle instantly changes the state of the other.