Trans-cosmic flow broadens our horizon

Contributed by
Sep 25, 2008
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In one of the weirder astronomy press releases I've seen in a while, it appears that material literally outside the visible Universe is tugging on material that we can see.

A cluster showing traces of an unseen tug.
Hubble image of the Bullet Cluster, which appears to be getting yanked by material from The Other Side. Click to embiggen.

What does this mean? First off, let's take a sec and talk about the visible Universe. If you go outside and look around, you don't see the whole Earth. You only see a small fraction of its surface, because the Earth is a curved ball. The solid planet itself blocks your view. The farthest you can see is out to the horizon, where the curving Earth dips everything else below your view (well, except for tall objects like buildings and ships at sea, but we can ignore them for this analogy).

The Universe is the same way. The fabric of space is expanding, with the cosmos getting bigger every day. This has an odd effect: objects farther away appear to be moving more rapidly away from us. Eventually, an object can be so far away that the space between us is effectively expanding faster than the speed of light! This does't violate any physical laws, because nothing material is actually moving at transluminal speeds; it's just that there is more space itself between us and that object all the time.

This effect naturally provides us with a cosmic horizon. Any object "moving away" from us faster than light can't be seen by us; the photons it emits can't keep up with the expansion of space. They lose energy and fall away from view (like a slow walker on a fast treadmill... or better yet, an ant walking along a rubber band that is being stretched). So, to us, an object far enough away is invisible, beyond the Universal horizon.

Weird, huh? Yeah, as usual, things get even weirder.

Now imagine a third object, say a cluster of galaxies, that lies between us and the one that is beyond our horizon. To the cluster, the object may still be visible, because it's closer, and therefore not receding as rapidly. It's like an island just over the horizon to you as you look seaward from the beach; to you the island is invisible, but to someone a few kilometers out to sea in a yacht the island is still visible.

That cluster can still be affected by the more distant object, pulled by its gravity, for example. To us, farther away, we don't see that distant object, but to the cluster it's sitting right there and still, literally, a force with which to be reckoned.

If you want to go away for a moment and take some Tylenol, I understand. I feel a bit headachy myself just writing this.

The thing is, astronomers now think they've detected this force! Clusters of galaxies are filled with extremely hot gas, or plasma, heated by things like the galaxies' motion in the cluster. As light from objects farther away passes through this gas, it gets affected by it, and we can measure that change. This is called the Sunyaev-Zel'dovich effect, and it's too weak to measure well in individual clusters, but by looking at literally hundreds of clusters, the effect adds up and can be seen. [Edited to add: the photons that are being affected are not from the matter beyond the horizon, but from the cosmic microwave background, the relic radiation from a very early time in the Universe, but still in our visible Universe. Sorry I wasn't clear about that in the original post.]

The total force is fairly big, in fact. Clusters seen in the direction of the constellations Centaurus and Vela appear to have an additional 3 million kilometers per hour added to their usual velocity! That means that some very large clump of matter -- probably a cluster of galaxy clusters, called a supercluster -- lies in that direction, over the horizon to us but very much visible to clusters we can see.

Imagine! It's a sobering reminder that the Universe itself is literally bigger than we can see, with the majority of it forever beyond our ken.

And if your quota of weird isn't yet sated, then ponder this: the expansion is accelerating. That means that objects we can see today, so distant they linger on our current horizon, will eventually fall away from view as the accelerating expansion beats out the velocity of the light they emit. They will literally move beyond the horizon and become invisible. In a sense, it's as if the visible Universe is shrinking, the horizon getting closer to us every day. The physical Universe is getting bigger, but almost paradoxically what we see of it gets smaller. Someday, billions of years from now, only the closest of objects will remain visible.

Everything else will have sailed below the horizon. So we better take a look around while we still can.

[Incidentally, I cover this topic in more detail in my book Death from the Skies!, coming out in October, but already available for pre-order.]