Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)
Posted: Tue Sep 03, 2013 2:07 am
When I was a kid I did an even cruder scale model, really it was just marking the distances of the planets from the Sun across the floor of the house. My sister harshly criticized me saying something like it was stupid. In retrospect, that was more of a self-reflection.Ann wrote:I once built a super-simple model of the inner solar system. I used cotton balls, 0.02 meters in diameter, for the Earth, Venus and Mars. I used yellow peas for Mercury and the Moon, and a round table-cloth, 2 meters in diameter, for the Sun. I then placed the "planets" at the appropriate distances from the Sun. The Earth was about 200 meters from the Sun.
That doesn't seem correct that the whole sky could be as much as 12.7 million times the HUDF area, but it's not like I calculated it. That means the HUDF did an exposure of the area for nearly 29 days straight! That's a lot of valuable Hubble observing time.DavidLeodis wrote:The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.
A million seconds would be only about 11.5 days for the exposure time. One of you (or both) doesn't have this right.BDanielMayfield wrote:Great comment David. So the image this simulation was made from covered only 1/12,700,000th of the sky. There was absolutely nothing special about this location in space. This site was simply just an extremely empty looking patch of sky, with no foreground stars or galaxies in the way. In even large land based telescopes this must look like an untold number of other boring looking dark, empty places with maybe only a few, barely discernable faint fuzzies, if anything at all. But keep Hubble focused on this place for nearly a million seconds and voila, 10,000 galaxies appear! (To make the video Dr. Summer’s team estimated the distances of about 5,000 of these.)
And you're sure that the HUDF didn't image any galaxies closer than 18.4 billion light years?BDanielMayfield wrote:So this random sampling probes a volume of space of between about 18.4 and 30 billion light-years out and found about 10,000 galaxies. Across the entire celestial sphere this would indicate the presence of 10,000 x 12.7 million = 127 billion galaxies, not counting any less than 18.4 B ly nor any more than 30 B ly away. If the density of galaxies inside the 18.4 billion light-year radius is about the same as that inside the volume probed by the HUDF survey we would have another 38 billion galaxies, so this survey suggests the presence of at least around 165 billion galaxies in the observable universe.
This is a good explanation of how it's supposed to work. The only thing I'd add is the age of the universe has been slightly upgraded to 13.8 billion years.BDanielMayfield wrote:We can see objects that are NOW further than 13.7 billion light-years, but no light has been traveling longer of course. But as light travels through space, space itself is expanding, carrying source and observer apart. The estimated distance of 30 billion light-years is the co-moving distance, accounting for the separation that has occurred during the time the light has taken to get to us.DOUGLAS L. MARTIN wrote:HOW CAN SOMETHING TRAVEL THROUGH 30 BILLION YEARS IN A UNIVERSE ONLY 13.7 BILLION YEARS OLD?
That's a good analogy, except for the 13.7 meters corresponds to the current time (universe is 13.7 billion years old) and the 30 meters corresponds to the far distant age of the universe. Therefore, the analogy is centered on an explanation for 16.3 billion years from now.geckzilla wrote:If a guy throws a baseball at you while he is 13.7 meters away from you and then you both immediately run away from the ball after it's released, by the time the ball hits you in the back of the head the guy and yourself are already 30 meters apart from one another. The distance between yourself and the guy at the moment the ball was released and the distance between the both of you the moment it hit you in the head are two different measurements.
Yes, but the observable universe is actually estimated at 93 billion light-years. http://en.wikipedia.org/wiki/Observable_universeAnthony Barreiro wrote:Isn't the observable universe estimated to be 42 billion light years across? So if the universe suddenly stopped expanding, light would take 42 billion years to travel across the universe, right?
I understand why astronomers are calling the observable universe this large. Essentially it's due to the expansion of space, in a nutshell. But I have to say I object to this definition. You're not actually observing anything up to 46–47 billion light-years away (minus the time/distance for the universe to become transparent). This is only observing something that far away as it is at the current time. Looking out in space is looking back in time. So we are not observing anything any further away than 13.8 billion light-years, since we're observing it as it was in the distant past, and at the distance it was then. We're only assuming that what we're looking at now is currently up to 47 billion light-years away, but we don't know that nor do we receive any data from it. For all we know, some mysterious force has halted its recession from us and is really just out of view at 13.9 billion LY, at the current time. For this reason, I don't think astronomers have any business saying the observable universe is any larger than 27.6 billion LY across.
It reminds me of the no-longer-calling-Pluto-a-planet issue. Illogical.
