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# The size of the observable universe

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I posted this on philowiki. Please rip it into shreds. :D

A brief comment on the size of the observable universe:

The common belief that the radius of the observable universe is 14 billion light years, because it is 14 billion years old.

We are now receiving light from the most distant observable galaxy , the ones which photons that were just leaked right after the big bang.

This is false.

The observable part of our universe has a radius that exceeds 14 billion years. How can this be? Briefly, while photons travel, the very space it traverses is expanding as well. By the time we receive light, the total distance back to the original source is much larger than the basic calculation that is founded on the travel time, about 3 times as large. Were it static, then the original supposition would be fairly accurate.

In other words, the radius of the observable universe might be 46 billion light years. :)

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The "misconception" that the observable universe is 14 billion years old is wrong on several accounts. First of all, the diameter of the observable universe must be *at least* twice that, since we see equally far in all directions and the observed stars are older the further away they are in *any* direction of the sky. So the diameter is at least 13.7*2 = 27.4 billion lightyears. It is the *age* of the observable universe that is measured to be at least 13.7 billion years. This was measured by the WMAP mission, which concluded that the universe is flat within a margin of error of 2%.

The actual inferred age of the observable universe has hovered somewhere between 9-15 billion years.

However, as has been discussed here several times, due to the expansion of the universe the size of the observable universe is larger than the age implies. When we factor in the expansion the size will be different.

Note that Wikipedia says the distance to the visible edge is 13,7 billion lightyears:

http://en.wikipedia.org/wiki/Universe

which gives the diameter I stated above, not factoring in the apparent increase in the speed of light, nor the cosmic expansion.

BTW, here is an interesting take on the size of the observable universe as related to the actual size of the universe:

http://www.astro.ucla.edu/~wright/CMB-MN-03/inflating_bubble.html

So the "official" estimates are that the distance to the horizon is 13,7 billion lightyears, whereas the size of the observable universe is at least twice that. To my knowledge, and which is actually not the same as I have posted in an earlier thread, this is already with the expansion rate factored in.

I see no point in saying that the observable universe is 46 billion light years. What we observe does not match this so I would like to ask for proof for this assumption.

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thanks for the response, tormod. Here's an excerpt from scientific american on the reasoning behind the 46 billion years conclusion:

Receding Faster Than Light

Another set of misconceptions involves the quantitative description of expansion. The rate at which the distance between galaxies increases follows a distinctive pattern discovered by American astronomer Edwin Hubble in 1929: the recession velocity of a galaxy away from us (v) is directly proportional to its distance from us (d), or v = Hd. The proportionality constant, H, is known as the Hubble constant and quantifies how fast space is stretching--not just around us but around any observer in the universe.

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and the following:

Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years.

Edit: Copyrighted material deleted. Please only post what is required for the discussion.

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In other words, the radius of the observable universe might be 46 billion light years.

Yes. They key word being "might be", of course. Scientific American, while a respectable magazine, often print articles with speculative theories so this must not be taken at face value.

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Yes. They key word being "might be", of course. Scientific American, while a respectable magazine, often print articles with speculative theories so this must not be taken at face value.

I applaud your skepticism, but there's no need to disparage the source and ignoring what it attempts to claim. :)

That it has a reputation for printing speculation has no bearing on whether the claims themselves have any plausibility to them. This is the same charge a sports fan might launch against Peter Vecsey for being trigger-happy with rumors in the past. He may have speculated a bit further than warranted, but that doesn't determine the worth of the current rumor he is pushing these days.

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You misunderstood me. You have only posted a suggestion that the observable universe may be 46 billion light years, followed up with an article which seems to be your only source (if you have more, please share them).

So far you have only posted a suggestion but not backed it up with data! How do you explain that the furthest stars we observe seem to be around 14 billion light years away? How does one go from there to a radius of more than three times that?

I am not discrediting anyone. I read Scientific American but their sensationalist cover stories can sometimes be misleading, although that does not make them less interesting.

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so as the expansion of the universe continues could it go fast enough that light from distant stars could never reach us because more space keeps been created than it can travel through... that would be a boring night sky

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so as the expansion of the universe continues could it go fast enough that light from distant stars could never reach us because more space keeps been created than it can travel through.

Eventually this is what is predicted, yes. This, plus the fact that the stars will eventually die out.

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so the universe is like one big arsed chemical reaction that will eventually reach equilibrium - i hope its a dynamic equilibrium otherwise it would be boring

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• 2 weeks later...

Here is an email I received from physicist Ned Wright (no relation), as we were discussing similar topics via email about a year and a half ago.

Go to http://www.astro.ucla.edu/~wright/CosmoCalc.html and put in a large

z and hit the "Flat" button. Then the comoving radial distance is 46.5

billion light years. So for the current best guess the diameter of the

observable Universe is 93 billion light years. This is bigger than the 78

value on my Web page because we now think the Universe is undergoing an

accelerating expansion. The current best guess for the radius of the

Universe is infinity. Go back to a redshift of

10,000,000,000,000,000,000,000,000,000 = 1E28 at the end of inflation and

the diameter of what is now the observable universe was 93 billion light

years divided by 1E28 = 10 cm. The radius of the whole Universe was

infinity divided by 1E28 = infinity. Of course all we really know is that

the radius of the whole Universe is much bigger than 93 billion light

years, but it could be "only" a trillion light years, for example. It is

now much greater than the spped of light times the age of the Universe and

in the past it was much much much greater than the spped of light times

the age of the Universe. This is true with or without inflation, but

inflation explains the great size of the Universe.[/Quote]

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