**Symmetries of the Universe #1**
Why do most Planetary Systems and Galaxies obey flat symmetry?
 |
"Did you know that our galaxy is flatter than a pancake...!?"
This came out as quite a surprise to me when Dr Tyson said it casually on one of his StarTalk podcasts. Sure enough, the Milky Way is flatter than a pancake, with a ratio of thousand to one thickness to diameter!
And it's not just spiral galaxies or planetary systems that show this strange planar symmetry - from Saturn's majestic rings to the accretion disks around powerful quasars, these celestial systems all appear to be just flat and spinning!
But wait. Why are giant cosmic systems like our Solar System and the Milky Way appear to be 2-D flat disks when the entire fabric of spacetime is said to be spatially 3-dimensional?
For all we know, gravity is supposed to be clumping up stuff into spheres...right!?
Speaking of universal forces, gravity is certainly a game-changer when it comes to the formation of stars to gigantic galaxy clusters floating in spacetime. If there's one thing that we're sure about this force, it is that gravity works in mysterious ways to maintain the balance and symmetry of our universe, and that includes forming giant, symmetric, floating flat disks.
But there's a rather important detail that we haven't brought up yet - the formation of such planetary and galactic flat disks is associated with orbital motion and spin. And whenever rotational dynamics are involved in a physical phenomenon, angular momentum comes into the picture. And that is exactly what is happening here - conservation of angular momentum.
Conservation of Angular Momentum in Rotating Bodies
Before we jump onto how the conservation of angular momentum gives rise to rotating disks in the universe, we need to understand what exactly angular momentum represents in a rotating system.
Simply speaking, angular momentum is a parameter that is associated with a rotating or orbiting body that gives the 'thrust' of the rotating/orbiting body. We know that in linear motion, momentum is the measure of strength or impact of a moving body. Similarly, angular momentum is the measure of strength or impact of a body that is moving in a circular path or a rotating body. For this reason, every object in this universe that is orbiting a central object possesses angular momentum and spin, like the earth, the sun and the Milky Way do.
Even so, what exactly do we mean when we say, 'angular momentum is always conserved in a rotating system'?
The concept of conservation of angular momentum is usually explained with the example of a spinning ice-skater, who spins much more rapidly when their arms are pulled inwards rather than outwards. Of course, we know that if the skater's arms were spread out while spinning, the spin would seem much slower than in the first case (with the arms pulled inwards).
This is the same mechanism that takes place when a massive star undergoes a supernova, and its dense core spins faster than the original star did before it underwent supernova. Since the star in its red giant stage is much broader (outer layers), when it gets ripped off its bulky outer layers in a supernova explosion, all that remains is its dense core, which will now rotate much faster so that its angular momentum is conserved. Conservation of angular momentum is also the reason why planets that are closer to the sun, like Mercury, orbit it at a much faster rate than those that are farther away from the sun.
But again, how does the notion of conservation of angular momentum shape matter into flat rotating disks? We know that the origin and evolution of any planetary system is just a beautifully messy process. The planets that orbit around any star are actually made of the residual gas clouds that were initially involved in forming the new (host) star.
These gas clouds, in their initial stages, are brought together under gravitational forces, so as to clump up and trigger the nuclear fusion inside the core of the protostar that is formed. One slight detail to point out here is that the gas clouds, as a whole, will have an angular momentum or spin that is associated with them, although the individual particles within these gas clouds are moving randomly in every possible direction.
 |
| Simulation by Rob Crain (Leiden Observatory) & the Virgo Consortium |
As gravity pulls in the gas clouds, which are now gradually rotating in a single direction swirling around the star, the individual particles that have up and down motion within the gas clouds cancel out (or dissolve) to blend in with the net rotation of the entire gas cloud. And as gravity pulls in the gas cloud more and more, the overall rotation of the gas cloud speeds up, becomes more stable, and flattens out over time, thereby creating a disk-like structure around the massive center (the star). The disk flattens out because the up and down motion of the particles within the gas cloud gets cancelled in the swirling matter of gas. This is the reason why planets that are further formed out of this swirling disk all orbit the Sun in one direction.
And that's how planetary systems, ring systems and galaxies morph into their flat disk shape, although it takes a couple hundred thousand years (and maybe more) to attain this planar symmetry.
That being said, one might wonder if our Solar System is aligned (or planar) with the plane of our galaxy...and the answer is...not exactly. The plane of the Solar System is almost perpendicular to the plane of the Milky Way, and that is why we can never observe our galaxy as a whole spiral galaxy! Although this is the case, over the next couple of million years, the solar system might align with the plane of our host galaxy.
Again, this is nothing more than a mere coincidence of a rather long chaotic process of maintaining the symmetries of our universe itself!
For more info on the topic, check out:
Comments
Post a Comment