Our understanding of gravity could be incomplete INTERVIEW WITH PAVEL KROUPA

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Throughout its lifetime, our galaxy has been kidnapping stars from hundreds of smaller galaxies. In the process, the stars that have been thus stripped away form long streams that follow the orbit of their parent satellite galaxy around the Milky Way. These stellar débris have been dubbed tidal streams and their study provides a new means of weighing the dark matter that envelops our galaxy. Understanding the nature of this type of matter is the principal challenge facing
Czech researcher Pavel Kroupa of the Argelander Astronomical Institute (Germany), who is prepared, if necessary to reformulate the present laws of physics.

- In your opinion, how was the Milky Way formed? What explanation could you give us concerning the spatial distribution of satellite galaxies observed around our galaxy?

The Milky Way has a bulge and this suggests that early on, perhaps about 11 Gyr ago, the proto-MW collided with perhaps a similar-sized proto-galaxy. The collision made part of the bulge, and the tidal arms that carried away the angular momentum and the binding energy formed a population of tidal-dwarf galaxies. These remain in the original orbital plane of the two proto-galaxies and this explains why the satellite galaxies of the MW are arranged in a large Disk of Satellites. Another possibility would be that the satellite galaxies were formed as tidal debris from an encounter between the proto-MW and another young galaxy that did not lead to a merger.

- What aspects of fundamental physics could find an explanation through the study of dwarf galaxies?

Dwarf galaxies have a very weak internal gravitational binding force, and by studying the motions of stars in dwarf galaxies we can learn whether Newtonian dynamics is valid there, or whether the gravitational force needs to be modified. Also, star-forming dwarf galaxies probably have much higher star-formation rates than thought until now, and accounting for the origin of the gas that is forming the stars is becoming a very major astrophysical challenge.

- What is the challenge in studying something that does not look like dark matter?

Some young dwarf galaxies that formed out of the debris from interacting galaxies appear to be full of dark matter, because their internal rotation velocities are too high to be explained by Newtonian dynamics. However, according to the dark-matter hypothesis, such objects cannot contain much dark matter. Therefore, Newtonian dynamics is probably wrong, and we need to search for an improved description of gravity. This is the challenge.

- Are you convinced of the existence of dark matter or is it possible that our understanding of gravity is incomplete?

I am not convinced that dark matter exists to the degree required to account for the motions of stars in galaxies, and I take it as a real and also very exciting possibility that our understanding of gravity may be incomplete.

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