The structure of the universe

Posted on February 4th, 2012 by george.
Categories: Uncategorized.

My friend Sean asked me a question yesterday, and the answer is not only something I’m learning about in school this semester, but something I’d like to share here.

Q: So, what’s the deal with planets and galaxies? I have always presumed that planets orbited on roughly the same plane around the sun. Is this true, or do they travel about diverse axes from each other? Additionally, I have always presumed that galaxies are spinning away from a central point, on roughly the same plane, in the cosmos. Is that accurate or are they shooting in a more linear fashion away from each other?

A: Your presumption about the planets is correct. The eight planets in our solar system orbit in roughly the same plane (within a few degrees). It’s called the ecliptic.

One of the strikes against Pluto as a planet is that its orbit is tilted 17 degrees from the plane the major planets orbit in.

A galaxy, on the other hand, is totally different than the solar system. The former is characterized by distributed mass, while the latter has point masses (mass concentrations in the form of planets). Yes, gravity is still the force at work in both systems, but there are so many stars per unit volume of a galaxy that their mass is effectively distributed along a density curve, like the atoms in a frisbee. And let’s not even get started on dark matter, which makes galaxies rotate faster than they would if stars were the only matter present.

There is no center of the universe (that’s a difficult one to explain, but I can try to cover it later if you’re curious). Galaxies interact with each other gravitationally, they cluster together, and they collide with each other. All the while, spacetime itself is expanding (but that too is another story). Their interactions are not collisions in the traditional sense, though, because individual stars almost never impact each other. It’s like two clouds of smoke full of embers passing through each other. Yes, their dust and gas is heated and collides and forms stars, but it’s not like two cars crashing. It’s also a crash that takes millions of years to happen, so you can put that slo-mo camera away.

It takes our Sun 250 million years to make one orbit around the galactic center. Considering the Sun is only 4.57 billion years old, that means for its entire life, and the entire life of the solar system, it has made only about 20-25 galactic orbits. So in galactic years, the Sun is only a twenty-something. And from what we know about stellar evolution, it’s only going to live into its forties before becoming a red giant, then devolving into a white dwarf that will live to a ripe old age of who knows how many billion years. All that doesn’t matter for the Earth, though, because the Sun is getting about 10% brighter every billion years (which is probably why life didn’t arise here until about a billion years ago). That means that in about four more galactic years, it’ll be too hot on Earth’s surface for liquid water to exist. So we’ll have to be gone to other planets long before our oceans boil, and four billion years before the Sun swallows the Earth as a red giant. Who knows, maybe we’ll terraform Mars by then and it will be warm enough to walk around outside.

Furthermore, the Sun doesn’t orbit the galaxy like a planet orbits a star. Yes, planetary orbits evolve, but over millions of years, not from one orbit to the next, and they may be elliptic, but they stay in one plane. Because all the mass in the galaxy is widely distributed and the Sun is constantly interacting with nearby stars, the Sun does not travel in a plane around the galactic center. It oscillates up and down in the galactic disk about three times per orbit. So imagine it tracing the surface of a kruller donut with only a few flutes.

So now to your actual question: the galactic structure of the universe is achingly beautiful. It looks like round and spiral pearls made of hundreds of billions of stars, and all those pearls are strung out like cobwebs around vast stretches of empty space. If you consider how long it takes for the Sun to orbit the galaxy, just imagine how long it would take for galaxies to orbit some fictional center of the universe; one orbit would be many times longer than the age of the universe. Astronomers and cosmologists term these large scale structures of the universe walls, filaments, and voids. So you can imagine it as a frothy soup of soap bubbles of different sizes where inside the bubbles are large voids of nothing, and between the bubbles are superclusters of galaxies.

Our Milky Way is part of a local galactic group of 54 galaxies, which is itself part of a mind-boggling huge group of galaxies called the Virgo Supercluster. But the Virgo Supercluster is just one of several “nearby” superclusters, which are themselves only one TINY region of the universe that we can ever see due to the finite speed of light and the amount of time since the big bang.

If you have the patience for the large image to load, this graphic should give you a good idea of the different structures at different scales within our glorious, illimitable universe.



Comment on February 5th, 2012.

I absolutely loved this post. I know I am not the only person who has said this to you in your lifetime, but the way you explain complex things is captivating and simple without a hint of oversimplification or condescension.

I would, however, like to point out the one error I found: you said there is no center of the Universe? Wrong, sir! That’s clearly me. ;) JK, JK.


Comment on February 5th, 2012.

I completely agree with Lorenia you are great at simplifying complex things in your explanation.


Comment on February 5th, 2012.

Lorenia: hilarious, as usual. And thank you. I’m so excited about the universe that I’m just glad when someone wants to learn about it.

Maysoon, thank you. That’s very encouraging. =)


Comment on February 6th, 2012.

I love it! I would be calmed and excited, though, to hear of a comparison of some of these phenomena to those that occur on a microscopic level within our planet or within our bodies would be useful. Do any come to mind?

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