I previously talked about a thought experiment which would supposedly show that "expansion of the universe" is fundamentally different from just "everything moving away from each other". However, discussion at the Physics Forum has convinced me that light would not be redshifted over time in a perfectly reflecting mirrored box despite the light being stretched along with the expansion of space.
In the comments for another blog post about cosmological distances, I suggested a thought experiment that would highlight how the redshift from distant galaxies is not due to a Doppler shift, that is, light from very distant galaxies is not redshifted because of the movement of the galaxies, but rather because the intervening space itself is expanding:
If someone managed to trap light when the universe first became transparent, about 300,000 years after the Big Bang, inside a perfectly reflecting mirrored box such that the light is contained in perpetuity, what would have happened to the wavelength of the trapped light by now?
Scientists from NASA have found that a large region of our universe is moving in a particular direction using observations of the cosmic microwave background and X-ray clusters. It seems that the local region of our universe, at least a couple of billion light years across, is moving towards a particular patch of the sky relative to the cosmic microwave background.
Gravitational models of the observable universe are not able to account for such a large-scale "dark flow", so they are speculating that the motion might have been due to the pre-inflation gravitational attraction from mass outside the observable universe. From what I can gather, it seems that an uneven distribution of mass and energy throughout the universe set our particular region of the universe moving slightly towards a specific direction. This would have been before the inflationary phase of the Big Bang. After going through inflation, the masses responsible for the gravitational attraction would have moved too far away for any gravity from them to reach us. And yet our local region of the universe would still maintain the motion due to inertia.
I could have misunderstood what the scientists were saying, though, so any corrections would be welcome. In any case, if there really is such a "dark flow", which should be confirmed with further astronomical observations, then it's mind-boggling in that it might give some of the first hints of what the outside of our observable universe looks like.
I'm a sop for articles about fundamental physics. The October issue of Scientific American has an article about how loop quantum gravity might indicate that the universe didn't start with the Big Bang; instead, the Big Bang might actually be a Big Bounce, where the universe started expanding again after a collapse which erased all trace of its previous existence. It appears that at extremely high energy densities, gravity would no longer be an attractive force and instead becomes a repulsive force.
Almost everything around us is made out of matter rather than antimatter. And this is not just true for our solar system or our galaxy, but appears to be true for the entire universe, or at least the observable universe. But except for having opposite charges, matter and antimatter are virtually identical. Then why should everything we see in the universe be made up of matter rather than antimatter? For that matter (pun intended), why is there any matter at all? Why shouldn't there have been an equal amount of matter and antimatter that all annihilated with each other and left nothing?
It is thought that matter and antimatter were formed in nearly equal amounts during the birth of the universe, but there was an extremely tiny excess of matter over antimatter, about one in ten million. Almost all of the matter and antimatter annihilated each other, and only the tiny excess of matter remained to form almost everything we see in the universe today. The process through how this tiny excess of matter was generated is called baryogenesis, which is still a mysterious process where speculation abounds.
Mark of Cosmic Variance is writing and editing a series of excellent posts about baryogenesis. Starting off with an introduction to the problem, so far he has talked about the theories of electroweak baryogenesis and leptogenesis. They are based on speculative theories of particle physics that have not yet been confirmed experimentally, which can be exciting for particle physicists since it means that they still have plenty left to do.
Whatever the mechanism, it must have happened uniformly across the universe after inflation, so mechanisms that are predicted to generate random amounts of excess matter or antimatter cannot be the vehicle for baryogenesis. Mechanisms for baryogenesis will have to take advantage of miniscule asymmetries between matter and antimatter, asymmetries that will have to arise in theories of physics that have yet to be discovered and confirmed.
One question about baryogenesis I had for the longest time was why it did not account for the existence of dark matter. If only the tiny excess of matter forms everything we can see in the universe, wouldn't the energy from the annihilation of the rest of the matter and antimatter be more than enough to account for the gravitational pull of dark matter? It turns out that I did not consider that light, which is the by-product of matter-antimatter annihilation, becomes less energetic with the expansion of the universe because the wavelength is stretched. This means that the energy density of light in the universe falls much faster than the energy density of matter, so gravitation due to light becomes negligible quite early in the life of the universe.
Where did all the energy in the light go? In a sense, the tens of millions of times more energy that used to be in light compared to matter literally all went into thin air; that is, into space-time itself. You could say that it all went into the gravitational energy stored by space-time as the universe expanded. The only caveat is that defining gravitational energy is a quirky thing to do in general relativity, which is the best theory we have so far for explaining gravitation.
Finally, an extra treat with a video from NASA showing the evolution of the universe since the time of the light-dominated period of the universe, when light had a significant gravitational influence, up to today, when light is no longer a major source of gravitation: