It is well known that the expansion of the universe is accelerating due to a mysterious dark energy. Within galaxies, stars also experience an acceleration, although this is due to some combination of dark matter and stellar density. In a new study to be published in Astrophysical journal letters Scientists have now obtained the first direct measurement of the average acceleration that takes place in our home galaxy, the Milky Way. Led by Sukanya Chakrabarti at the Institute for Advanced Study with collaborators from the Rochester Institute of Technology, University of Rochester and University of Wisconsin-Milwaukee, the team used pulsar data to herb radial and vertical accelerations of stars inside and outside the galactic plane. Based on these new high-precision measurements and the known amount of visible matter in the galaxy, scientists were able to calculate the dark matter density of the Milky Way without assuming the usual assumption that the galaxy is in a stable state.
“Our analysis not only gives us the first measurement of the small accelerations experienced by stars in the galaxy, but also opens up the possibility of expanding this work to understand the nature of dark matter and ultimately dark energy on larger scales,” he said. Chakrabarti. , the paper’s lead author and current member and IBM Einstein Fellow at the Institute for Advanced Study.
Stars throw through the galaxy at hundreds of kilometers per second, yet this study indicates that the change in their speed occurs at a literal snail speed – a few centimeters per second, which is about the same speed as a crawling baby. To detect this subtle motion, the research team relied on the ultra-precise timing ability of pulsars widely distributed in the galactic plane and halo – a diffuse spherical region surrounding the galaxy.
“By leveraging the unique properties of pulsars, we were able to measure very small accelerations in the Galaxy. Our work opens a new window into galactic dynamics,” said co-author Philip Chang of the University of Wisconsin-Milwaukee.
The extent outwards approx. 300,000 light-years from the galactic center, halogen can provide important tips for understanding dark matter, which accounts for about 90 percent of the galaxies’ mass and is highly concentrated above and below the star-dense galactic plane. Star movement in this particular region – a primary focus of this study – may be affected by dark matter. Using the local density measurements obtained through this study, researchers will now get a better idea of how and where to look for dark matter.
While previous studies assumed a state of galactic equilibrium to calculate the average density, this research is based on the natural, non-equilibrium state of the galaxy. One can analogy this with the difference between a pond’s surface before and after a stone is thrown in. By taking into account “ripples”, the team was able to get a more accurate picture of reality. Although in this case rather than rocks, the Milky Way is affected by a turbulent history of galactic mergers and continues to be disturbed by external dwarf galaxies such as the small and large Magellanic clouds. As a result, stars do not have flat orbits and tend to follow a path similar to a skewed vinyl record that crosses above and below the galactic plane. One of the key factors that enabled this direct observation method was the use of pulsar data compiled from international collaborations, including NANOGrav (North American Nanohertz Observatory for Gravitational Waves), which has received data from Green Bank and Arecibo telescopes.
This milestone paper is expanded with the work of Jan H. Oort (1932); John Bahcall (1984); Kuijken & Gilmore (1989); Holmberg & Flynn (2000); Jo Bovy & Scott Tremaine (2012) to calculate the average density in the galactic plane (Oort boundary) and the local dark matter density. IAS scholars, including Oort, Bahcall, Bovy, Tremaine, and Chakrabarti, have played an important role in advancing this area of research.
“For centuries, astronomers have measured the position and speed of stars, but these provide only a snapshot of the complex dynamic behavior of the Milky Way galaxy,” said Scott Tremaine, professor emeritus at the Institute for Advanced Study. “The accelerations measured by Chakrabarti and her collaborators are directly caused by the gravitational forces from matter in the galaxy, both visible and dark, thereby providing a new and promising window on the distribution and composition of matter in the galaxy and universe.”
This particular article will enable a large number of future studies. Accurate measurements of accelerations will also soon be possible using the complementary radial velocity method developed by Chakrabarti earlier this year, which measures the change in star velocity with high precision. This work will also enable more detailed simulations of the Milky Way, improve the constraints of general relativity, and provide clues in the search for dark matter. Extensions of this method may ultimately also allow us to measure the cosmic acceleration directly.
While a direct image of our home galaxy – similar to those from Earth taken by the Apollo astronauts – is not yet possible, this study has provided important new details to help imagine the dynamic organization of the galaxy from within.
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Sukanya Chakrabarti et al. A measurement of the density at the galactic plane from binary pulsar acceleration. arXiv: 2010.04018 [astro-ph.GA] arxiv.org/abs/2010.04018
Provided by the Institute for Advanced Study
Citation: Measurements of pulsar acceleration reveal the dark side of the Milky Way (2021, 11 January) retrieved 12 January 2021 from https://phys.org/news/2021-01-pulsar-reveal-milky-dark-side.html
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