Which is … mind-blowing, if not mind-expanding.
(h/t +Kee Hinckley)
Reshared post from +Brian Koberlein
Traveling Without Moving
With all the news about BICEP2 and the possible detection of early inflation, there have been a lot of misconceptions about what inflation actually is. One of the biggest is the idea that during inflation the universe expanded faster than light. It’s a misconception that even many experts get wrong, and is so common that there’s a technical arxiv paper addressing these misconceptions. It’s easy to see how this misconception arises. After all, during inflation, two atoms a meter apart just before would find themselves about a light year apart within a fraction of a second. How is that not moving faster than light? It all has to do with the subtlety of general relativity.
Before we talk about inflation, let’s talk about a similar effect we see in the universe today, known as cosmic expansion. When we look at distant galaxies, we see that the light of more distant galaxies is redshifted more than closer galaxies. Now we know that light can be redshifted when objects move away from us, known as the Doppler effect. For this reason, this effect is often described as galaxies moving away from us. But redshift can also occur due to the expansion of space itself. That is, space expands, but the galaxies aren’t moving through space. The first is a property of special relativity, and is due to the motion of objects through space. The second is a property of general relativity, and is due to an expansion of space.
Since both effects give a redshift to distant galaxies, how do we know that cosmic expansion is due to an expansion of space and not relative motion? If the redshift were due to relative motion, then the light of distant galaxies would be redshifted when leaving the galaxy, and that means the light would also appear dimmer. If the redshift is due to cosmic expansion, then light leaves a distant galaxy without being redshifted, and therefore also not dimmed. Only later is the light redshifted due to expansion. This means you can compare the brightness of distant supernovae with their redshift, known as the magnitude-redshift relation. What we find is that the magnitude-redshift relation matches expansion extraordinarily well. It doesn’t match the relative motion model at all.
So we know that space is actually expanding, but what does that actually mean?
Cosmic expansion is determined by what is known as the Hubble constant. Currently our best measurement of the Hubble constant is about 20 km/s per million light years. This means that two points in space a million light years apart are moving away from each other at 20 kilometers each second. Since all of space is expanding, the greater the distance between two points in space, the faster they move apart. So two points 10 million light years apart are moving away at 200 km/s, and so on. Because of this, if you consider two points far enough apart, they will be moving away from each other faster than the speed of light. The speed of light is about 300,000 km/s, which, given our current Hubble constant is the separation speed for two points 15 billion light years apart.
Now you might think then that a galaxy 16 billion light years away from us must be moving away from from us faster than light. You could say that the the galaxy appears to be moving faster than light, but in actuality it is space that is expanding between us. The galaxy itself isn’t moving much at all. It’s not as if that distant galaxy is defying relativity. After all, from that distant galaxy’s perspective we are moving away from it faster than the speed of light. The key point to remember is that this is due to cosmic expansion, not galactic motion. And cosmic expansion is not faster than light, even though very distant objects can appear to be moving faster than light.
Which brings us to inflation. Many of the popular articles unfortunately state that during inflation the Universe was expanding faster than light, which isn’t true. What is true is that during inflation the rate of spatial expansion was much larger. This means that the distance at which objects appear to move apart faster than light is much smaller, but it does not mean that the Universe expanded faster than light.
We still don’t understand the mechanism that triggered inflation, but we do know that inflation doesn’t violate the speed of light. During inflation the rate of expansion was tremendous, but even today space continues to expand, just at a much smaller rate.
Image: Cosmic Inflation by Don Dixon.
Paper: Tamara M. Davis, Charles H. Lineweaver. Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe. arXiv:astro-ph/0310808 (2003).
If space is expanding, do objects in space expand? With respect to large objects, is gravity working against expansion? I find the concept of the expansion of space very puzzling.
Well, if it was easy, it wouldn't need the General Theory of Evolution to explain it. 😛
Since the rate of expansion (Higgs?) is dependent somehow on distance, it would appear that there is a difference between the expansion in space for local objects (you, me, the lamppost) and stellar-distance objects, so I don't personally notice (though perhaps that explains my own personal expansion), but it's noticeable in terms of us and distant galaxies.
One would think they do. I'm not a physicist, here, so one can feel free to come in and correct me, but…
The example that gets used in intro physics classes* is that of a balloon. You make a few marks on a balloon, and then blow it up. The balloon being the only point of universal reference, we see the expansion, but when looking at it from the perspective of any one mark on the balloon, everything else is moving further away. And yes, one would expect to see expansion taking effect on the marks themselves, but bearing in mind that the marks are very, very, very, very small relative to the size of the balloon (universe) is such that the effective of spatial expansion on any individual object should be negligible.
I could be completely wrong, here, but that's how it makes sense to me.
It's worth noting that, for every day living purposes, it's not a difference that makes a notable difference, any more than time dilation due to the speed of the Earth. For the most part, it's something that's worth astrophysicists looking at, because of other interesting potential bits, but it's not something I feel particularly dim for not fully understanding, except with balloon metaphors. 🙂
There are three ways I think inflation might not be noticed in our daily life. One is if expansion is so small that it's down within the margin of error of our measuring devices. The second is if everything is growing bigger, then our measuring devices would not show the difference. The third is if space grows, but physical objects don't (perhaps because gravity or other forces pull them back together again as space grows). I'm not puzzled about why we wouldn't notice it, and it doesn't make me feel any more or less dim than the million other things I don't understand. I'm just curious.
+David Newman I think the answer is that we do notice it, only on the macro / many-light-year level, not in our living room. 'Cosmic expansion is determined by what is known as the Hubble constant. Currently our best measurement of the Hubble constant is about 20 km/s per million light years. This means that two points in space a million light years apart are moving away from each other at 20 kilometers each second. Since all of space is expanding, the greater the distance between two points in space, the faster they move apart.'
It's a weird universe, sometimes.
Doing some math, I think that means that if you and I are 1m apart, space is expanding between us at 1/475,000,000,000mm / sec … which is likely beyond detection.
Well, I did say that I was talking about what we don't notice in our daily lives. I certainly can't detect movement on the order of 2x 10^-12 per second with any devices I have in my house. What I really had in mind is this question: do star-sized objects (for example) grow due to expansion, or does gravity counteract their growth so they remain essentially the same size as space expands around them. I think my three things that make expansion harder to notice apply to large objects, though they clearly don't prevent science from detecting expansion using the methods described in the article.