Archive for the ‘Accelerating Universe’ Category

The Spacetime Diet: Redistributing the Fat of the Universe

October 29, 2017

If you’ve had your nose in spacetime physics at all, you’re familiar with the idea that when you move really fast, other objects look thinner. Or, relatively, you look thinner from the viewpoint of someone else’s reference frame.

This is called the Lorentz-FitzGerald contraction. When you observe a moving object, it appears shorter, or thinner, along the axis of its motion relative to you. Likewise, you appear thinner relative to the moving object (who does not feel thin at all). There’s a formula for this, but it’s irrelevant for the discussion below.

You also appear to gain mass (using a similar formula). This is also irrelevant for the following discussion, but I thought I’d toss it out there.

So here’s the rub. You will often hear someone say or write, “When an object nears the speed of light, the universe flattens to a thin sheet from the viewpoint of an observer on that object.”

Just to clarify, this is bullshit. If you are cruising along at near-light speed, then all matter, relative to your frame of reference, is moving in the opposite direction at near-lightspeed. That’s okay so far. Except, the universe is expanding. And the farther out you go, the faster it’s expanding, such that there are regions of space expanding away faster than the speed of light (the expansion of “space” is apparently able to ignore the whole “speed of light” limit thing; go figure).

So when you attain a certain velocity, you become stationary relative to another part of the universe that is moving away from Earth at the same speed. There is no shortening of length or thickness for that object, that part of the universe.

Take the Andromeda Galaxy for example, moving toward us at 110 kilometers per second. When we measure the galaxy in the direction of its travel, along its axis of motion, it’s foreshortened in that direction. Now, fire up your rockets so you’re traveling at 100 kilometers per second in the same direction, and Shazam! The entire galaxy poofs back out to its real shape in its own frame of reference that happens to coincide with your own. Relative to you, the Andromeda Galaxy is no longer moving.

So, back to the expanding universe; as your spaceship speeds up more and more, there’s always a part of the universe that’s moving at the same speed at which you are traveling (a comoving reference frame). It won’t look compressed or thinner or foreshortened at all. In fact, if we take the viewpoint that all parts of the universe are essentially equal, (that is, there is no “center” of the universe) then the universe doesn’t compress into a pancake at all as you near the speed of light; it’s just that the non-foreshortened part of it, the part that matches your current velocity, is farther and farther away from you. But the overall volume will appear unchanged.

 

Advertisements

LOOKING ALL THE WAY BACK IN TIME

July 31, 2017

If you look back in time, (up in the night sky, at the light emitted from galaxies billions of years ago), you are actually looking at an earlier version of the universe when it was smaller.

images-3

Due to the nature of how light moves through space, when you look back 14 billion years to the farthest reaches of the universe, you are actually looking at a very small volume. But the image, warped as it is, is spread out and fills the farthest regions of the sky, like a view through a concave lens. If you were able to look all the way back to the tiny point of the Big Bang, the image would be smeared and spread out across the 14 billion light-year shell, any detail of the event washed out by turning the fine detail of a tiny event into a picture the size of the universe.

So, when you look up at the night sky, everywhere you look in the blackness of deep space, 14 billion light years away, is actually the same small point.

Does this make sense? I’m trying to think of a good analogy for this, but it just isn’t coming to me. Maybe like starting with a tiny drawing on the surface of a tiny sheet of rubber, then stretching it out so that the sheet of rubber stretches all around you in a sphere, like the inside of a balloon, and then trying to figure out what the original picture looked like.

This, of course, begs the question of what physicists are calculating when they measure the accelerating expansion of the universe. If the universe was physically smaller 14 billion years ago, and now the remaining image of it is spread out over a sphere with a radius of 14 billion light years, that’s going to come off as an acceleration; the farther you look, the smaller the original volume and the more the image is spread out over the apparent warped view of the current volume. And, of course, 14 billion years ago, the universe actually was expanding a lot faster than it is now. It’s a double-whammy of accelerations. Most physicists are a hell of a lot smarter than me, so I’m guessing that both these accelerations have been calculated into the “accelerating expansion of the universe” equation. I can only speculate that there is a third element. I wish I could find out without wading through a lot of really obscure math.