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IANAC [1], but here's my Wikipedia-level 2 cents: (Warning: leaky metaphors ahead!) We stayed ahead of the primordial gravitational waves [2] for so long beca

by ggdG 13y ago

IANAC [1], but here's my Wikipedia-level 2 cents: (Warning: leaky metaphors ahead!) We stayed ahead of the primordial gravitational waves [2] for so long because the expanding universe went through an "inflation" period: in a very short time - immediately after the Big Bang - spacetime expanded at a rate that was many orders of magnitude too big for phenomena like light or gravitational waves to catch up to. It leveled off quickly to a more gentle rate of expansion [3], but in the meantime the universe has gotten so big that even its current age of 13.7 billion years isn't enough for us to be able to observe every object. Many objects are so far from us that it will take much more than 13.7 billion years for their photons to reach us. And no matter how distant the objects that will 'come into view' in the future, the light that appears the most distant to us will always be the cosmic microwave background radiation. So here we find ourselves, Earthly observers, sitting in the center of a sphere with an apparent radius of 13.7 billion lightyears - called the observable universe - at the edge of which we can see the immediate aftermath of the Big Bang. No one knows how much bigger the total universe is w.r.t. to the observable universe. It's hard to find out because we have no causal relation to anything outside the observable universe. A safe bet IMHO (given past experiences in the history of cosmology) is this: the entire universe is way, way, way bigger than the observable universe. [1] I Am Not A Cosmologist [2] and also of the cosmic microwave background radiation [3] sub light speed, at least for distances within the observable universe

xenophonf 13y ago
Just a slight correction: The distance from Earth to the edge of the observable universe is 46 Gly, not 13.7 Gly, because of the metric expansion of space.
- ggdG 13y ago
  True. The light has been traveling for "only" 13.7 billion years, but meanwhile the space between origin & destination kept expanding. That's why I wrote about an apparent radius, but on further thought I no longer believe that the travel time will reflect in an apparent distance. It's the amount of redshift that will result in a calculated proper distance.
vl 13y ago
In my layman understanding current prevailing theory is that acceleration of expansion will reduce observable universe, and as consequence there is basically no question of "size beyond observable universe" - there is no possible interaction with it and as such it doesn't exist. http://en.wikipedia.org/wiki/Accelerating_universe http://en.wikipedia.org/wiki/Accelerating_universe
- lutusp 13y ago
  > In my layman understanding current prevailing theory is that acceleration of expansion will reduce observable universe, and as consequence there is basically no question of "size beyond observable universe" ... Yes, but that was true before the Dark Energy discovery. It's predicted to become much worse in the far distant future because of the acceleration caused by Dark Energy, and eventually there will be comparatively small island universes -- galactic clusters -- that will remain gravitationally bound far into the future, after the remainder of the universe has receded from view. This is because galactic clusters are below the threshold for being affected (i.e. torn apart) by Dark Energy. > there is no possible interaction with it and as such it doesn't exist. Yes to the first, no to the second. At present, we cannot see beyond a certain time horizon, but cosmological curvature measurements take this invisible mass-energy into account even though it's not directly observable. Our present conjecture that the universe is infinite in size (based on curvature measurements) obviously implies a universe most of which isn't visible, but all of which affects the measurements.
- evanb 13y ago
  Most physicists[1] would dispute the conclusion that "as such it doesn't exist." They would simply say that there can be no interaction with anything beyond the horizon---that stuff out there is certainly... out there. [1] myself included
- dTal 13y ago
  Similarly: http://www.smbc-comics.com/?id=3290#comic http://www.smbc-comics.com/?id=3290#comic
ggdG 13y ago
Some corrections: * When I wrote "we stayed ahead of the [waves] for so long" I didn't mean that gravitational waves from the Big Bang & its immediate aftermath haven't been hitting us in the past. On the contrary: gravitational waves and background radiation from this event have always kept reaching us as our observable universe expanded. I was only referring to these particular waves that are hitting us now. * I insinuated we will always see the CMB radiation but that might be too strong a statement. When (if ever!) our observable universe has fully expanded to encompass the entire universe, we will no longer see radiation originating from the Big Bang. * I think we should stay skeptical about the existence of gravitational waves as they remained undetected for so long and also this time there's the possibility that someone might have jumped the gun.

by ggdG 13y ago

xenophonf 13y ago

Just a slight correction: The distance from Earth to the edge of the observable universe is 46 Gly, not 13.7 Gly, because of the metric expansion of space.

ggdG 13y ago
True. The light has been traveling for "only" 13.7 billion years, but meanwhile the space between origin & destination kept expanding. That's why I wrote about an apparent radius, but on further thought I no longer believe that the travel time will reflect in an apparent distance. It's the amount of redshift that will result in a calculated proper distance.

vl 13y ago

In my layman understanding current prevailing theory is that acceleration of expansion will reduce observable universe, and as consequence there is basically no question of "size beyond observable universe" - there is no possible interaction with it and as such it doesn't exist. http://en.wikipedia.org/wiki/Accelerating_universe http://en.wikipedia.org/wiki/Accelerating_universe

lutusp 13y ago
> In my layman understanding current prevailing theory is that acceleration of expansion will reduce observable universe, and as consequence there is basically no question of "size beyond observable universe" ... Yes, but that was true before the Dark Energy discovery. It's predicted to become much worse in the far distant future because of the acceleration caused by Dark Energy, and eventually there will be comparatively small island universes -- galactic clusters -- that will remain gravitationally bound far into the future, after the remainder of the universe has receded from view. This is because galactic clusters are below the threshold for being affected (i.e. torn apart) by Dark Energy. > there is no possible interaction with it and as such it doesn't exist. Yes to the first, no to the second. At present, we cannot see beyond a certain time horizon, but cosmological curvature measurements take this invisible mass-energy into account even though it's not directly observable. Our present conjecture that the universe is infinite in size (based on curvature measurements) obviously implies a universe most of which isn't visible, but all of which affects the measurements.
evanb 13y ago
Most physicists[1] would dispute the conclusion that "as such it doesn't exist." They would simply say that there can be no interaction with anything beyond the horizon---that stuff out there is certainly... out there. [1] myself included
dTal 13y ago
Similarly: http://www.smbc-comics.com/?id=3290#comic http://www.smbc-comics.com/?id=3290#comic

ggdG 13y ago

Some corrections: * When I wrote "we stayed ahead of the [waves] for so long" I didn't mean that gravitational waves from the Big Bang & its immediate aftermath haven't been hitting us in the past. On the contrary: gravitational waves and background radiation from this event have always kept reaching us as our observable universe expanded. I was only referring to these particular waves that are hitting us now. * I insinuated we will always see the CMB radiation but that might be too strong a statement. When (if ever!) our observable universe has fully expanded to encompass the entire universe, we will no longer see radiation originating from the Big Bang. * I think we should stay skeptical about the existence of gravitational waves as they remained undetected for so long and also this time there's the possibility that someone might have jumped the gun.