Posted by: iLED009 February 12, 2015
Universe & Space
Login in to Rate this Post:     1       ?         Liked by
The size of the observable universe is determined by the number of years since the big bang. The time when the big bang occurred was 13.789 billion years ago. Therefore the size of the observable universe is nominally 13.798 billion light years since there is not enough time for anything further away that that (and further back in time than that) to have emitted light that we can now "observe". That is what is meant by term "the observable universe".
The 13.798 billion years is the current best estimated time to the Big Bang (measured to 0.3% accuracy) and 379,000 years is the number of years after the Big Bang when the universe would have cooled off enough to become transparent. The (mostly visible light) photons from the hot plasma that filled the universe at that time have been traveling since then and have now been red-shifted down into the microwave range. This is the Cosmic Microwave Background (CMB) radiation that has been very accurately measured by the WMAP and Planck satellite.
Btw, for those who were born prior to 90s we've seen this CMB in our television sets as dark and white dots, when we didn't get any signal. CMB data is one of the most important pieces of evidence used to verify and validate our current model of the universe. This model enables us, for example, to say that the Big Bang happened 13.798 billion years ago with an uncertainty of only +/- 0.037 billion years. Two engineers from Bell Labs accidentally detected this CMB while pointing their MW detectors in the sky and got the Nobel.
If we are using only photons, we will not be able to see any further back in time (to before 379,000 years after the Big Bang) since the universe was opaque to photons before that time.

However, if we are ever able to invent and use a neutrino telescopes to measure very low energy cosmic neutrinos (which is probably impossible), then we would be able to see back to just a few minutes or seconds after the Big Bang. Finally, it is also theoretically possible to see back to roughly 10^{-30} seconds after the Big Bang if we could measure the possible gravitational waves that could have been generated at the end of the inflationary period of the Big Bang at that time ( 10^{-30} seconds ).

Will the size of the observable universe increase (or decrease) with time?

Well, if there were no dark energy creating an accelerated expansion of the universe, we would have eventually, as time went on see more and more matter in our observable universe. For example at 50 billion years after the big bang we would have seen up to 50 billion light years away and all the galaxies within that distance could be visible to us.

However, we now know that the rate of the expansion is accelerating (Thanks to Edwin Hubble!). That means that we will see less and less matter in the observable universe as time goes forward. For example, at 50 billion years after the big bang, we would be able to see out to 50 billion light years, however, there will be fewer galaxies visible than we can see right now. This is because galaxies that are currently near the edge of our 13.8 billion light year observable universe will be accelerated away from us at faster than the speed of light due to the accelerated expansion and therefore will be beyond the observable universe horizon by then. In fact, at about 2 trillion years after the big bang, the only galaxies visible in our universe will be the locally gravitationally bound group of galaxies - at most a few dozen!

Some fun facts -> Andromeda galaxy and Milky way galaxy are in collision course. Rough timeline 3-4 billion years from now ! That will be quite a view !!!!
But hey, by then our Sun will have exhausted it's fuel (hydrogen atoms to carry on nuclear fusion and form He atoms and energy ) and stopped providing light. What's up with our Surya Bhagwan !!!!
So, we humans and our imaginary GODS and associated philosophy, religion, language is so insignificant compared to the Universe and its behaviour !!~!!
Read Full Discussion Thread for this article