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You may not notice it, but our Milky Way galaxy is cruising along at 630 kilometers (~391 miles) per second. That speed is often attributed to the influence of a single gravitational source. But in a new study, a group of researchers has found that the motions of the Local Group—the cluster of galaxies that includes the Milky Way—are being driven by two primary sources: the previously known and incredibly massive Shapley Supercluster and a newly discovered repeller, which the researchers dub the Dipole Repeller.
Shapley's contribution was already known, but the Dipole Repeller's hadn't been recognized prior to this study.
The researchers plotted the motions of many galaxies in the nearby Universe in a 3D model, using data from the Cosmicflows-2 database. Since the Universe is expanding, most galaxies are moving away from ours, creating a red-shift in the light they emit. But since the researchers were more interested in the other influences on a galaxy's motion, they simply subtracted the expansion's contribution. The resulting plot shows what the motions of galaxies would look like if space wasn't expanding.
The galaxies in that plot all follow different paths—some proceed through the Great Attractor in the middle of the picture, others curve around the periphery, and so on. They all seemed to have a clear destination: the Shapley Supercluster. But they also seem to have a clear origin point: the Dipole Repeller. When the researchers traced the galaxies' paths backwards, they all originate there. It looks a lot like there's something there repelling the galaxies, as if the Repeller and Shapley formed the negative and positive ends of an electrical dipole, and charges were being driven from one to the other.
That's not what's actually happening. Gravity is the dominant force acting on a galaxy, and gravity, unlike electricity, can't repel—it's only an attractive force. So what's going on?
The Dipole Repeller's true identity is probably, well, nothing. It's actually a void with much less mass than the surrounding space. This has the effect of seeming like a repeller because the nearby space has a much denser concentration of matter, creating a gravitational gradient between the two. The low-density void is the only direction from which there's no force pulling on the galaxy, or at least significantly less force than comes from every other direction.
Source:
Journal Reference:
Nature Astronomy, 2017. DOI: 10.1038/s41550-016-0036
(Score: 0) by Anonymous Coward on Sunday February 05 2017, @07:17PM
You can come up with just about any theory and add "dark matter" and "dark energy" to it as needed. It will fit the data.
(Score: 2, Informative) by Anonymous Coward on Monday February 06 2017, @02:13AM
Fritz Zwicky tries to measure the mass of the Coma cluster by using gravitational effects and comes up with a value that’s much larger than the visible matter that is there, and better measurements since his day still have a large discrepancy. Vera Rubin measures the rotation curves of galaxies and find they’re spinning faster than they should. Other astronomers see colliding galaxies and notice that there are gravitational lenses where there is no visible matter. Scientists analysing the cosmic microwave background power spectrum see certain peaks, which cannot be produced by normal matter. Normal matter subjected to the pressures of the early universe will oscillate due to its electromagnetic interaction, but the peaks they see seem to show a type of matter that doesn’t oscillate like that under pressure. That means that there’s a substantial amount of matter out there that interacts only via gravity and possibly the weak interaction.
These aren’t arbitrary fudge factors, and in any case, there is nothing inherently wrong with adding such things into a scientific theory, as long as you can later figure out a way to later characterise the fudge factor more clearly. Around 1900 Antoine Henri Becquerel and Ernest Rutherford discovered that certain radioactive elements emit ‘beta particles’, which they later realised were high energy electrons, and that their emission resulted in the conversion of a neutron into a proton in the atomic nucleus. If this was so, to satisfy conservation of energy, the energy of the emitted electron must have a specific, well-defined value. However, measurements made of the energy of beta decay electrons showed that it could take on an entire spectrum of values, which meant that either beta decay did not obey conservation of energy (terrible) or there was a third particle involved in beta decay that they couldn’t see that was involved (still bad, it was a huge, huge fudge factor). Wolfgang Pauli took up the latter hypothesis in 1930, and suggested that a particle with no or negligible mass, the neutrino, was also produced in beta decay, so it became a neutron emitting an electron and a neutrino, turning into a proton. It took another twenty years for Pauli’s hypothesis to be proven correct by experiments in nuclear reactors to detect neutrinos, and even today, neutrinos still have many mysteries. Incidentally, they are a particle of the sort that satisfies most of the constraints of dark matter. They can only interact via gravity and the weak force. The only problem is there doesn’t seem to be enough neutrinos to account for all of the dark matter.