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Wednesday, July 5, 2017

The Expanding Universe Conundrum

How fast is the universe expanding? To figure this out, we could take the radius of the known universe and divide it by the universe's age. That should give us the average velocity the universe has expanded since the beginning of time:

Wow! It appears the universe has been expanding at the speed of light since day one! If that's true, if the galaxies, for example, have always been moving apart from each other at light speed, how did their photons reach us? How can we see them? If the space between galaxies were expanding at light speed, the light from the neighboring galaxies would never have made up the increasing distance. As soon as a photon, say, from NGC-2419 covered a light year, another light year of distance would have been added. The result? There would have been no discovery of good old NGC-2419 and more distant galaxies.

But what if the science is wrong? What if we are at rest and space does not expand--and those distant galaxies are moving away from us through space? Their light would eventually reach us no matter how fast they're moving away. Unfortunately this is not the case. If it were, the most distant galaxies would become dimmer and dimmer as they move further and further away. To see stuff beyond our known universe, all we would need is a more powerful telescope. If the science is correct (and it is), there should be a point in the distant cosmos, a cosmological horizon, beyond which we can see nothing (the light can't reach us due to expanding space).

So then how did the light from distant galaxies reach us? Imagine the universe is a sphere. Let's pretend we can draw a small circle anywhere we want on that sphere. That small circle encircles some galaxies (including the Milky Way). The sphere, represented by the 2D larger circle below, expands at light speed (c). The smaller circle within also expands over the same time period (t):

Now, let's double-check the velocity of the circles:

The larger circle expanded at velocity c as expected, but the smaller circle expanded less over the same time period; its velocity is only v or Hr. Therefore, the observable universe has indeed expanded at the speed of light (or more), but galaxies nearer to us have not. Their photons were able to reach us and we can see them.

2 comments:

  1. This is an excellent article. I had to read it twice. I thought that the gravitational force would have been immense at the beginning possibly warping time space that our measurements would be off. I still believe that but your explanation makes more sense than mine

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