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Tuesday, 13 November 2018

Merging galaxies vs expanding Universe

On 31 October 2018, Nature magazine published research by Dutch-Portuguese scientist Amina Helmi that our Milky Way galaxy is actually a merger of two galaxies, which collided some 10 billion years ago. That collision resulted in "the formation of the Milky Way’s inner stellar halo and thick disk" (Nature). The smaller galaxy has been named Gaia-Enceladus (Trouw).

In about 4 billion years, scientists expect another collision between our Milky Way and the (now) far away Andromeda Galaxy: the Andromeda-Milky Way collision. The concepts of merging galaxies in an expanding Universe feels like a contradiction. It would make much more sense finding merging galaxies in a shrinking Universe.

A recent Phys article sheds some light on these collisions and mergers between galaxies. It appears that pairs of supermassive black holes are responsible:
"For the first time, a team of astronomers has observed several pairs of galaxies in the final stages of merging together into single, larger galaxies. Peering through thick walls of gas and dust surrounding the merging galaxies' messy cores, the research team captured pairs of supermassive black holes—each of which once occupied the center of one of the two original smaller galaxies—drawing closer together before they coalescence into one giant black hole." (Phys)
Things can even get weirder. Science Alert of 8 November 2018 reports that "astronomers found a black hole rotating so fast, it could be spinning space itself":
"Black holes, while fascinating, are hardly a new discovery - but a black hole spinning at one of the highest speeds ever, according to the Hindustan Times, is a completely different story - especially when there have only ever been four others like it."
The above could explain the contradiction of merging galaxies within an expanding Universe. Black holes are responsible for both: the spinning or centrifugal force causes the expansion of the Universe. Simultaneously, black holes operate like a vacuum cleaner that suck up entire galaxies. To some extent, this process feels similar to a Dyson vacuum cleaner but without the plastic bin that collects the dust. 

After writing the above, I wondered where the energy - as required for the spinning - would come from. Then I realised that a swirl (a "turn in circles or spirals") automatically appears when you empty your bathtub. Hence, the speed of the spinning around of a black hole is - most likely - depending on the volume of the amount that is being sucked into the black hole.

Spinnin' and Spinnin' Around (1974) by Syreeta (1946-2004)

Note: all markings (bolditalicunderlining) by LO unless stated otherwise

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