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I don't think the Hubble Space Telescope has anything to do with that term. We use "observable" because the Universe is far larger than what we can ever physica

by rm999 12y ago

I don't think the Hubble Space Telescope has anything to do with that term. We use "observable" because the Universe is far larger than what we can ever physically see or interact with (there's a growing body of evidence that it's infinite and "flat"). The observable part is an upper bound on the part of the Universe that will ever be relevant to us. Kind of makes me feel insignificant in a weird way :) Also, the Hubble can't even see most of the Observable Universe; it can see ~15 billion lights years away, whereas the radius of the observable universe is 47 billion light years.

rquantz 12y ago
How is that the radius of the observable universe? I thought the observable universe was bounded by the age of the universe, which why the furthest distant observations show the universe close to the time of the Big Bang. Shouldn't the radius of the observable universe = the number of years light has been traveling toward us? Edit: Wikipedia says I'm wrong about this. What's new?
- yk 12y ago
  The radius of the observable universe is the distance the furthest stars we could theoretically observe today.[0] So these stars were a lot closer earlier and we can see the light even though the stars are further away than the age of the universe (times the speed of light). [0] Technical details omitted.
- vidarh 12y ago
  The universe has been expanding. So the distances now are vastly larger than they used to be when the light left its sources.
privong 12y ago
> Also, the Hubble can't even see most of the Observable Universe; it can see ~15 billion lights years away, whereas the radius of the observable universe is 47 billion light years. There is a subtlety with the size of the observable Universe versus the distance Hubble can see. The radius of the observable universe is 47 billion lights years (the radial distance to the cosmic microwave background[0]). Hubble has spotted galaxies whose distance corresponds to a light travel time of ~13 billion years. However, the Universe has expanded since that light was emitted 13 billion years ago, so the galaxies which emitted that light are now about 30 billion light years away. So Hubble can see objects which are closer to the edge of the observable Universe than the 13 billion year light travel time implies, compared with the size of the observable universe. There are tentative detections of even more distant galaxies. For reference, I obtained those values using Ned Wright's cosmology calculator[1] with a redshift z=8, and the default values, assuming a flat cosmology. The 13 billion years ago number is the light travel time and the 30 billion light year distance is the comoving radial distance. [0] If you plug the approximate redshift of the cosmic microwave background (z=1100) into [1], you get 45.5 billion light years. [1] http://www.astro.ucla.edu/~wright/CosmoCalc.html http://www.astro.ucla.edu/~wright/CosmoCalc.html Edit: Clarified wording and added CMB distance info.

by rm999 12y ago

rquantz 12y ago

How is that the radius of the observable universe? I thought the observable universe was bounded by the age of the universe, which why the furthest distant observations show the universe close to the time of the Big Bang. Shouldn't the radius of the observable universe = the number of years light has been traveling toward us? Edit: Wikipedia says I'm wrong about this. What's new?

yk 12y ago
The radius of the observable universe is the distance the furthest stars we could theoretically observe today.[0] So these stars were a lot closer earlier and we can see the light even though the stars are further away than the age of the universe (times the speed of light). [0] Technical details omitted.
vidarh 12y ago
The universe has been expanding. So the distances now are vastly larger than they used to be when the light left its sources.

privong 12y ago

> Also, the Hubble can't even see most of the Observable Universe; it can see ~15 billion lights years away, whereas the radius of the observable universe is 47 billion light years. There is a subtlety with the size of the observable Universe versus the distance Hubble can see. The radius of the observable universe is 47 billion lights years (the radial distance to the cosmic microwave background[0]). Hubble has spotted galaxies whose distance corresponds to a light travel time of ~13 billion years. However, the Universe has expanded since that light was emitted 13 billion years ago, so the galaxies which emitted that light are now about 30 billion light years away. So Hubble can see objects which are closer to the edge of the observable Universe than the 13 billion year light travel time implies, compared with the size of the observable universe. There are tentative detections of even more distant galaxies. For reference, I obtained those values using Ned Wright's cosmology calculator[1] with a redshift z=8, and the default values, assuming a flat cosmology. The 13 billion years ago number is the light travel time and the 30 billion light year distance is the comoving radial distance. [0] If you plug the approximate redshift of the cosmic microwave background (z=1100) into [1], you get 45.5 billion light years. [1] http://www.astro.ucla.edu/~wright/CosmoCalc.html http://www.astro.ucla.edu/~wright/CosmoCalc.html Edit: Clarified wording and added CMB distance info.