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Our Motion Through Space Isn’t A Vortex, But Something Far More Interesting

There are a lot of moving parts to the Universe, as nothing exists in isolation. There are literally trillions of large masses in our Solar System, all orbiting around the galactic center on timescales of hundreds of millions of years. But there’s a viral video, parts 1 and 2, that claims that as the Solar System moves through the galaxy, it makes a vortex shape, pulling the planets behind it as it does.

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And that explains part, but not all, of the large-scale motion through the Universe. There’s also one more important effect at play, one that was quantified only recently: the gravitational repulsion of cosmic voids. For every atom or particle of matter in the Universe that clusters together in an overdense region, there’s a region of once-average density that’s lost the equivalent amount of mass.

Just as a region that’s more dense than average will preferentially attract you, a region that’s less dense than average will attract you with a below-average amount of force. If you get a large region of space with less matter than average in it, that lack-of-attraction effectively behaves as a repellent force, just as extra attraction behaves as an attractive one. In our Universe, opposite to the location of our greatest nearby overdensities, is a great underdense void.

The various galaxies of the Virgo Supercluster, grouped and clustered together. On the largest scales, the Universe is uniform, but as you look to galaxy or cluster scales, overdense and underdense regions dominate.

The various galaxies of the Virgo Supercluster, grouped and clustered together. On the largest scales, the Universe is uniform, but as you look to galaxy or cluster scales, overdense and underdense regions dominate. ANDREW Z. COLVIN, VIA WIKIMEDIA COMMONS

Since we’re in between these two regions, the attractive and repulsive forces add up, with each one contributing approximately 300 km/s and the total approaching 600 km/s. When you add all of these motions together: the Earth spinning, the Earth revolving around the Sun, the Sun moving around the galaxy, the Milky Way headed towards Andromeda, and the local group being attracted to the overdense regions and repulsed by the underdense ones, we can get a number for how fast we’re actually moving through the Universe at any given instant.

We find that the total motion comes out to 368 km/s in a particular direction, plus or minus about 30 km/s, depending on what time of year it is and which direction the Earth is moving. This is confirmed by measurements of the cosmic microwave background, which appears preferentially hotter in the direction we’re moving, and preferentially colder in the direction opposite to our motion.

The gravitational attraction (blue) of overdense regions and the relative repulsion (red) of the underdense regions, as they act on the Milky Way.

The gravitational attraction (blue) of overdense regions and the relative repulsion (red) of the underdense regions, as they act on the Milky Way. YEHUDA HOFFMAN, DANIEL POMARÈDE, R. BRENT TULLY, AND HÉLÈNE COURTOIS, NATURE ASTRONOMY 1, 0036 (2017)

If we ignore the Earth’s rotation and revolution around the Sun, we find that our Solar System is moving relative to the CMB at 368 ± 2 km/s. When you throw in the motion of the local group, you get that all of it — the Milky Way, Andromeda, the Triangulum galaxy and all the others — are moving at 627 ± 22 km/s relative to the CMB.

That larger uncertainty, by the way, is mostly due to uncertainty in the Sun’s motion around the galactic center, which is the most difficult component to measure. We know exactly how the Earth moves through the Universe, and it’s both beautiful and simple. Our planet and all the planets orbit the Sun in a plane, and the entire plane moves in an elliptical orbit through the galaxy.

The relative attractive and repulsive effects of overdense and underdense regions on the Milky Way. The combined effect is known as the Dipole Repeller.

The relative attractive and repulsive effects of overdense and underdense regions on the Milky Way. The combined effect is known as the Dipole Repeller. YEHUDA HOFFMAN, DANIEL POMARÈDE, R. BRENT TULLY, AND HÉLÈNE COURTOIS, NATURE ASTRONOMY 1, 0036 (2017)

Since every star in the galaxy also moves in an ellipse, we see ourselves appear to pass in-and-out of the galactic plane periodically, on timescales of tens of millions of years, while it takes around 200-250 million years to complete one orbit around the Milky Way. The other cosmic motions all contribute, too: the Milky Way within the Local Group, the Local Group in our Supercluster, and all of it with respect to the rest-frame of the Universe.

The Solar System isn’t a vortex, but rather the sum of all our great cosmic motions. Thanks to the incredible science of astronomy and astrophysics, we at last understand, to tremendous precision, exactly what that is.