Sunday, December 8, 2013

Dark Energy Part 14

The Universe Is Not Intuitive

I had a lot of trouble visualizing how the universe is evolving and even where we are within it. There are several online sources that tell us something like "It has taken 13.8 billion years for the radiation from the Big Bang to reach us here on Earth." That statement, though true, led me to think of us as out here away from the Big Bang. Sometimes I imagined that we could trace back to a single point somewhere in space where the Big Bang originated. I also thought of the Big Bang as being long over. We are seeing a mere ghost of it in the CMB (cosmic microwave background). All those imaginings turn out to be totally wrong.

When a pinpoint of unimaginably dense energy burst into existence 13.8 billion years ago, there was nothing outside of it - no space, no kind of theoretical framework, no time and no physics (according to most cosmologists and excepting the multiverse theory). 13.8 billion years later, that Big Bang is still occurring and we are inside it. Thermodynamically, we live inside this nearly perfect blackbody that is our universe. When photons decoupled from matter, they moved at light speed in every possible direction, rather than all shooting away from some point source. The universe was filled with dense gamma radiation at first. As the universe expanded it cooled according to thermodynamic laws, and the radiation stretched into X-rays and then eventually into visible light and beyond. I imagine some period of time when the universe glowed violet, then blue and eventually orange and then red. Over time, photons, going in all directions occasionally struck matter, reflecting off it or being absorbed and readmitted. Photon trajectories bent as they passed through the gravity wells of massive objects. Many of them have been travelling through space never interacting with matter at all.

Now the universe is bathed in microwaves - that is why the CMB fills the entire sky and comes to Earth from all directions not just one. The universe, from 380,000 years old until its very end, if it has one, will always be bathed in CMB radiation, although the radiation is many factors less dense than it once was and that density will continue to decrease. The wavelength of the CMB will eventually stretch into ultra-long radio waves.

As CMB photons traveled, their wavelengths were stretched as space itself stretched, and right now space is stretching faster then the speed of light, and that expansion is ever increasing. As space expands, all matter comes along for the ride. The scaffolding for large-scale structures such as super clusters formed when the universe was very young, within pockets of slightly denser energy that were once as tiny as Planck-scale. Now these super clusters are many billions of light years across. Even with all this clustering, matter, overall, is distributed extremely evenly throughout the universe.

Stars and galaxies are moving away from each other and from us at increasing velocity. If the universe were not expanding, we would eventually be able to observe even the most distant stars as their photons eventually reached us, but instead there exists a spherical boundary beyond which we can never observe any event. This boundary corresponds to the outer horizon of the CMB, 93 billion light years across. Imagine viewing a faint star near this horizon. You may be viewing a star that shone almost 13 billion years ago. It is almost certain that it exploded long ago and many new crops of stars have been born and died since, but you will never be able to see those stars. They are likely in a new location altogether thanks to the curved expansion of space and they are red-shifted out of our view, a cosmic red shift that continues to increase. In the distant future our night skies will be darker as distant stars move away from us at increasing speed and their light will not have time to reach us.

An intelligent creature on some distant planet is looking up at its night sky and realizing the same thing, as the stars in its view move away from it at increasing speed just like ours do. That creature is inside the center of the universe just as we are, because there is no center to the universe. The center is everywhere because we are inside the Big Bang. We are the Big Bang.

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