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Originally Posted by sunofsky no one can answer this question of "how this universe is horizontally big, and vertically how much it is long". thousands of years later also the answer will be unanswered. |
Yep, pretty much what Dragon Star said. But, you can define the visible universe which is the universe that we have any hope of ever seeing due to the finite age of the universe and finite speed of light. If just going by these, then our "horizon" is a sphere about 13 billion light-years in radius, which is as far as we can see because light from farther places hasn't had enough time to reach us. But, we can only see back to about 300,000 years, due to the Cosmic Microwave Background radiation, so the sphere is [however old the universe is]-300,000 light-years in radius. A good website explaining the CMB radiation is
http://map.gsfc.nasa.gov/ .
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what is your opinion about this sky.
will its blue color remain forever..
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The sky appears blue because of the gas and dust in the atmosphere. It acts to scatter shorter (bluer) wavelengths of light, while scattering longer (redder) less efficiently. So, as sunlight passes through the atmosphere, shorter (purple and blue) wavelengths are scattered more than longer ones. Thus, the sky appears blue because the scattering makes it appear as though the blue light is coming from the entire sky.
This is why sunsets and sunrises appear red -- the longer path length through the atmosphere (the light's going through 5 or more "atmospheres") scatters the longer wavelengths (think of it as one atmosphere scatters 5% of yellow light, so 10 atmospheres scatters 50%) more, and so all that really filters through from the sun is the red.
This is also why umbral lunar eclipses appear to be red: The only light that can reach the moon has been bent by Earth's atmosphere like a lens, but it's also been filtered by it, so only the red light is left.