ROTATION CURVES OF GALAXIES AS A TEST OF MOND

Galaxies are rotating with such speed that the gravity generated by their observable matter could not possibly hold them together. According to Newtonian gravity, the rotational velocity falls with distance from the center of a galaxy, while the observed data usually show an asymptotically flat rotation curve out to the furthest observationally accessible data points.

One solution to solve this problem is assuming a dark matter halo distributed around each galaxy.

An alternative approach is to replace dark matter by a modification of the Newtonian dynamics known as Milgromian dynamics (MOND). In the MOND the gravitational dynamics of a system is influenced by the external gravitational field in which it is embedded. This so called External Field Effect (EFE) is one of the important implications of MOND and provides a special context to test Milgromian dynamics.

In a recent paper (Haghi et al. 2016) the authors study the rotation curves of 18 spiral galaxies and they show that the EFE can successfully remedy the overestimation of rotation velocities in 80% of the sample galaxies in Milgromian dynamics fits by decreasing the velocity in the outer part of the rotation curves.

Haghi et al. 2016 (Accepted for publication in MNRAS) - Declining rotation curves of galaxies as a test of gravitational theory - (arXiv)

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A UNIVERSE WITHOUT A CENTER?

Image Credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb. According to the standard theories of cosmology, there is no center of the universe. In a conventional explosion, material expand out from a central point and the instinct suggests that with the Big Bang happened something similar. But the Big Bang was not an explosion like that at all: it was an explosion of space, not an explosion in space . The Big Bang happened everywhere in the Universe.

UNIVERSE IS FINITE OR INFINITE?

Art by Moonrunner Design At present there is no answer to this question. However I will try to list the hypothesys currently on the table with related issues.

New research looks at how ‘cosmic web’ of filaments alters star formation in galaxies

Cosmic Web. Credit: NASA Astronomer Gregory Rudnick sees the universe crisscrossed by something like an interstellar superhighway system. Filaments — the strands of aggregated matter that stretch millions of light years across the universe to connect galaxy clusters — are the freeways. “Galaxies will flow along filaments from less dense parts of the universe to more dense parts of the universe, kind of like cars flowing down a highway to the big city. In this case, they are going toward big clusters, being pulled by the gravity of those large concentrations of matter,” he said. “I’m interested in how galaxies are affected by the regions in which they live,” Rudnick said. “Filaments are the first place where galaxies come into contact with higher density regions of the universe. If a galaxy in a ‘rural’ part of the universe enters a dense part, I want to know how its properties change — for example, does it change the number of stars it forms, or does its shape get altered? Us...

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