Observable universe

tonyman1989 · Posted: Thu Mar 08, 2007 5:24 pm Post subject: Observable universe

what is the appceted size of the Observable universe as of 2007?

I heard some where between 40 and 45 billion lightyears in radius.
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hillbillyatheist · Posted: Thu Mar 08, 2007 5:32 pm Post subject:

how could the universe be any bigger than 26 light years wide?

assuming the universe expanded at the speed of light, then that means it should not be more than about 13 billion light years in either direction from the point of the big bang.

and I don't even think it can be that big, since as far as I know the universe is not expanding at the speed of light.

I don't really know what I am talking about though when it comes to this topic, so I will be watching this thread with anticipation.

Mr_C · Posted: Thu Mar 08, 2007 6:10 pm Post subject:

It's the exact width of God's hand.
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Link

tonyman1989 · Posted: Thu Mar 08, 2007 8:31 pm Post subject:

SteveInSF · Posted: Thu Mar 08, 2007 9:15 pm Post subject:

Space.com claims that its 156 billion light years wide and explains why.

http://www.space.com/scienceastronomy/mystery_monday_040524.html
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benyjets23 · Posted: Thu Mar 08, 2007 9:22 pm Post subject:

I doubt many would assert that the expansion rate of the Universe would be faster than light.

With the estimation that Universe's radius is 40 - 45 Billion light years across, you would have a complete width of 80 - 90 Billion light years. Given current estimates as to the Universe's age at around 13.7 Billion, you are talking exceeding the speed of light by about 6 times. It seems pretty mind bending.

I am also not an expert on this stuff, so I apologize for the non-technicality of what I have said, and any fatal flawa it may contain.

Ben

Philosophos · Posted: Thu Mar 08, 2007 9:33 pm Post subject:

hillbillyatheist · Posted: Thu Mar 08, 2007 10:57 pm Post subject:

Philosophos · Posted: Thu Mar 08, 2007 10:59 pm Post subject:

pjanzen · Posted: Thu Mar 08, 2007 11:04 pm Post subject: Re: Observable universe

tonyman1989 · Posted: Fri Mar 09, 2007 12:40 am Post subject:

SteveInSF thank you for the link.
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Edd · Posted: Fri Mar 09, 2007 5:56 am Post subject:

This isn't a simple a question as it first appears, since the universe is expanding. There's several ways of measuring distance in cosmology. There's a comoving distance, which factors out the expansion compared to the scale of the universe today. There's a luminosity distance which says how much fainter something is (it defines distance so that how bright something is drops off as 1/r^2). There's a light travel time, which is how long it takes light to reach us (which isn't a distance, but you can see how with a speed of light you can make it something like one). There's also the angular diameter distance, which is how much smaller something looks. That one isn't even monotonic - after a while things start looking bigger the further away you get.

So, the edge of the observable universe is at a distance with a light travel time of the age of the universe - a bit under 14 billion years. In comoving distance it's about 40 billion or so light years from us, so the universe is 80 billion light years across (last time I worked it out anyway - I might have made some error! [edit - I did, fixed the numbers!]). The luminosity distance and angular diameter distance don't really matter so much but they differ by a factor of the redshift+1 - for the cosmic microwave background this is about a thousand. They're not so important for this question though, as they're more about how things look than how things are, if you see what I mean.

So depending on how you define distance, both 14 billion and 40 billion light years is correct for the radius, and some other numbers are too.

Note also that the edge of the observable universe isn't getting further away as we build more powerful telescopes. It gets further away by a light year a year, but you don't see beyond the cosmic microwave background because for one thing it's essentially coming from an opaque surface, and for another there's only a few hundred thousand years between it and the big bang itself, and you won't be seeing past that.

And yes, the universe can expand 'faster than the speed of light'. Basically, you only really need to worry if something moves past something else faster than light speed, and this doesn't happen with an expansion over the whole universe, so it's not breaking any laws of physics.

benyjets23 · Posted: Fri Mar 09, 2007 9:35 am Post subject:

Edd,

I guess this is an area where simple logic gives a wrong answer. Thanks for the feed back.

Ben

kmisho · Posted: Fri Mar 09, 2007 11:08 am Post subject:

Edd, my understanding of inflation was that the restriction of the speed of light simply would not apply to space itself.

The real problem with speed and light is that it would take an infinite amount of energy to accelerate a bit of matter to the speed of light. If the big bang was like a planet blowing up, universal expansion could not exceed the speed of light because it would take more energy to speed it up to near the speed of light than the universe itself contains.

But this is not what big bang theory says. What it does say is that the universe following the big bang was super-hot, non-differentiated energy that only segregated itself into quarks and particles and atoms as the universe cooled (froze is more descriptive. There were several phase transitions akin to water turning to ice), the rate of cooling itself a by-product of the rate of expansion.

As far as I know, many still accept some version of Guth's early inflationary period, where the universe (space itself) would have expanded at many times the speed of light for a short period following the big bang.
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