Sun's galactic journey linked to mass extinctions (2013) | Hacker News Reader

Sun's galactic journey linked to mass extinctions (2013)

abc.net.au

48 pointsDrScump20 comments

I remember Stephyen Jay Gould wrote an essay caleed the "cosmic dance of Shiva" where he talked about how the fossil record suggests that mass extinction events at regular intervals of 20 something million years. He wrote that since it would be difficult to find a terrestrial explanation for an extinction event at regular intervals, so it made sense to look at factors from outer space. This could be the sort of missling link in Gould's theory.

I can't find a non-paywalled version of Goud's article but here is another one talking about mass extinctions at regular intervals

http://www.theatlantic.com/science/archive/2015/11/the-next-...

Full paper, with some illustrations (free):

http://saj.matf.bg.ac.rs/187/pdf/043-052.pdf

The authors seem to assume that there's a spiral arm pattern that rotates coherently. What's the evidence for that?

It's believed that the spiral arms are quasi-static and that the stars move in and out of these regions of higher density.

See: https://en.wikipedia.org/wiki/Density_wave_theory

Yes, that's what I vaguely recalled. But we don't have enough observations to test that hypothesis. I wonder, through, whether alternate-path gravitational lensing could provide enough snapshots of some other galaxy to be useful.

I really don't understand how the solar system can orbit the galaxy faster than the spiral arms and be in accordance with Kepler's 2nd law?

Spiral arms aren't static objects, they are a denser region of the galaxy as stars pass through them. There's also a small speed variance between different systems, so on average it all evens out. Then there's dark matter which adds an additional factor to the orbital speed.

It is better to view the spiral arms as 'transient' in the same way you would view a momentary queue in traffic - although the jam itself moves slowly or not at all, the individual cars that make it up move through the jam and eventually exit on the other side.

Interesting: "Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure."

https://en.wikipedia.org/wiki/Spiral_galaxy#Origin_of_the_sp...

Wasn't the higher than expected orbital velocity among the first evidence of dark matter?

Rubin-Ford effect

https://en.wikipedia.org/wiki/Vera_Rubin#Galaxy_rotation_pro...

Kepler's laws don't apply to galactic orbits, because galaxies aren't point masses, nor sufficiently spherical and uniform to be approximated as such.

Welp, we're about to enter one of the arms.

It's been nice knowing you guys.

https://news.ycombinator.com/item?id=11905555

> Summed over the entirety of its orbital path over the last 500 Myr, based on the new weighted symmetrical model, we find that the Sun has spent ∼60% of its time in the spiral arms

This is a statistical observation. In any case, do you have a source for your assertion?

http://saj.matf.bg.ac.rs/187/pdf/043-052.pdf

My prediction is based on their newly weighted symmetrical model given at figure 2.

All previous extinction events are when the sun is in the galaxy arms, and we are currently entering one of them.

Pardon me. Could you point me to a source showing we are entering one of them?

http://i.imgur.com/u69Qd6Y.png

Yellow dot is where we are right now, as you can see we are entering one of the black lines.

6 blue dots represent the known mass extinctions, they all lie on black lines.

5 orange dots represent proposed new mass extinction events proposed in the paper.

Here's a great interview by John Batchelor (free podcast):

https://audioboom.com/boos/4073570-mass-extinction-and-the-s...

not sure this is new... if I remember correctly our galaxies arms rotate along their length and cross each other every (approx) 140 million years which disturbs our kuiper belt which dislodges numerous items which rain down destruction on earth approx 70 million years later, so Jupiter mops up most but not all.

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