Space is not infinite.

[jamiestem]

Well nothing is infinite. So why would space be any different?

[Ivan Tuzikov]

According to the recent theory of expanding Universe the space-time itself (with all matter and energy) expands with acceleration.

Metric_expansion_of_space

[Spyman]

Well nothing is infinite. So why would space be any different?

If nothing is infinite then how many digits are there in π?

 

"π is an irrational number, which means that its value cannot be expressed exactly as a fraction m/n, where m and n are integers. Consequently, its decimal representation never ends or repeats."

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

 

 

How do you imagine the Universe, if space has some cind of borders then what would be there on the others side?

 

Currently we don't know if space is curved and finite or infinite and without end, we are not able to observe beyond our visible horizon but there is no known reason why the Universe can't be bigger or even infinite in size.

 

"The universe is immensely large and possibly infinite in volume. The region visible from Earth (the observable universe) is a sphere with a radius of about 46 billion light years, based on where the expansion of space has taken the most distant objects observed. For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years. As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away. There are probably more than 100 billion (10¹¹) galaxies in the observable universe. Typical galaxies range from dwarfs with as few as ten million (10⁷) stars up to giants with one trillion (10¹²) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one sextillion (10²¹) stars in the observable universe; though a 2010 study by astronomers resulted in a figure of 300 sextillion (3×10²³)."

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

[michel123456]

Albert was not sure.

[ajb]

Well nothing is infinite.

 

A little more carefully, infinity cannot be the value of any measurable physical property. Any experiment will never give infinity as the value of some measured property.

 

So why would space be any different?

 

The observable Universe will always be finite in extent at any finite value of cosmic time. What happens outside of our observable horizon is the question here. All of space could be infinite, but we will never observe this and so there is no violation of "only finite values".

[farmboy]

I heard that space was the final frontier.

[John Cuthber]

"Well nothing is infinite"

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

 

If space isn't infinite, in time or extent, how come it goes dark at night?

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

[MigL]

If space isn't infinite, in time or extent, how come it goes dark at night?

http://en.wikipedia..../Olbers_paradox

 

?????? This should read

If space IS infinite in time and extent, how come it goes dark at night?

I don't think you understand Olber's paradox. Given the isotropy of space, the number of stars increases by the same factor that light intensity decreases, so that in any given direction, you would always see a light source, and so the night sky should be bright.

Of course Olber lived long before Einstein and so he didn't take relativity into account.

Even though the universe is infinite or at least unbounded, due to the finite speed of light, the OBSERVABLE universe can never be infinite.

This fact, along with interstellar gas and other non-radiating dark matter, fully explain Olber's (non)paradox.

[John Cuthber]

OK, Oops I got the worms muddled.I meant "If space isn't finite, in time or extent, how come it goes dark at night? "

 

However the finite size of the observable universe only occurs because it hasn't been around for ever, so it only happens if the universe is finite in time.

 

The interstellar gas clouds don't matter to Olber's paradox.

If the universe has always been here, how come those clouds haven't yet heated up until they too are glowing?

 

Olber was right.

[michel123456]

(...)Even though the universe is infinite or at least unbounded, due to the finite speed of light, the OBSERVABLE universe can never be infinite.

(...)

 

I suppose you mean "under the prism of the Big bang Theory".

 

A theoretical infinite Universe in space and in time would produce an infinite observable universe as well.

 

the facts are:

1. the BBT states that the Universe had a beginning (roughly speaking) in our past.

2. the observable universe lies totally in our past.

3. the speed of light is finite.

 

Statements 1 2 & 3 combined produce a finite observable universe in agreement with observation, though with a salt of tautology.

Edited April 21, 2011 by michel123456

[John Cuthber]

"A theoretical infinite Universe in space and in time would produce an infinite observable universe as well."

And, unless there's something odd about our bit of the universe, then all bits of it would have permanent light because of the light from the infinite number of stars (which would have had an infinite time to reach everywhere).

So, we know we don't live in a universe that's infinite in time and space.

That answers the original question.

BBT has nothing to do with it- it's just one possible way in which the universe might be finite, but there are other possibilities. It's just a theory- it may be proven wrong but, even if it is, then the fact that it goes dark at night will still prove that in some way, the universe is finite.

 

The bit about an observable universe is a red herring.

If the universe were infinitely old then light would have reached us from all of it and it would all be observable.

In talking about an observable universe that is different from the universe per se, then you are tacitly accepting that the universe hasn't been here forever. That means you are assuming it's finite, then using that to prove that it's finite.

That's more than a little tautological.

[Airbrush]

"...If the universe were infinitely old then light would have reached us from all of it and it would all be observable....

 

Yes all the pinpoints of light would be observable, but that infinite number of galaxies, extending to infinity, would also be very much smaller and dimmer the further they are, effectively invisible to us unless highly magnified. So the night sky of that universe remains dark.

Edited April 22, 2011 by Airbrush

[Spyman]

Yes all the pinpoints of light would be observable, but that infinite number of galaxies, extending to infinity, would also be very much smaller and dimmer the further they are, effectively invisible to us unless highly magnified. So the night sky of that universe remains dark.

You really should read the link presented:

 

The Paradox

The paradox is that a static, infinitely old universe with an infinite number of stars distributed in an infinitely large space would be bright rather than dark.

To show this we divide the universe in to a series of concentric shells, 1 light year thick (say). Thus a certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away, say. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell between 2,000,000,000 to 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell. Thus the total light received from the second shell is the same as the total light received from the first shell.

 

Thus each shell of a given thickness will produce the same net amount of light regardless of how far away it is. That is, the light of each shell adds to the total amount. Thus the more shells, the more light. And with infinitely many shells there would be a bright night sky.

http://en.wikipedia.org/wiki/Olbers'_paradox

[random]

You really should read the link presented:

 

The Paradox

The paradox is that a static, infinitely old universe with an infinite number of stars distributed in an infinitely large space would be bright rather than dark.

To show this we divide the universe in to a series of concentric shells, 1 light year thick (say). Thus a certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away, say. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell between 2,000,000,000 to 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell. Thus the total light received from the second shell is the same as the total light received from the first shell.

 

Thus each shell of a given thickness will produce the same net amount of light regardless of how far away it is. That is, the light of each shell adds to the total amount. Thus the more shells, the more light. And with infinitely many shells there would be a bright night sky.

http://en.wikipedia.org/wiki/Olbers'_paradox

 

Yes and the moon is made of cheese too! If you put 2 of the same flashlight together the light does not travel further! and there is also the light year theory (possibly proven I dunno) that the actual sunlight hitting the earth on a given day is like 1000 years old or somthing it left the sun 1000 years ago! thats our closest star right? So for a real paradox perhaps the stars newly created or far far away haven't been around long enough for their light to travel here yet.

[Spyman]

Yes and the moon is made of cheese too! If you put 2 of the same flashlight together the light does not travel further! and there is also the light year theory (possibly proven I dunno) that the actual sunlight hitting the earth on a given day is like 1000 years old or somthing it left the sun 1000 years ago! thats our closest star right? So for a real paradox perhaps the stars newly created or far far away haven't been around long enough for their light to travel here yet.

What? Are you really serious?

 

Two flashlights will obviously shine with twice as much light and there is nothing stopping this light from traveling further in space.

 

A lightyear is simply how long light manages to travel in one year in vacuum when it moves with lightspeed.

(Yes we have been able to measure how fast light propagates.)

 

It takes sunlight approximately eight minutes to reach Earth and definitely not 1000 years.

 

Olbers paradox is not about newly created stars but about infinite number of stars that has together been shining on Earth for an infinite time.

[michel123456]

I have read this paradox article at least 20 times, but now that i read it in this Forum, it suddenly comes to my mind that it may be more subtle.

 

If I light a candle 5 cm from my eyes, it will blind me. If I light a billion candles 10 kilometers away, I doubt they will. There must be another relation adding to the simple geometric description above.

Edited April 22, 2011 by michel123456

[StringJunky]

Can I put this thought foward...please shoot it down if it's wrong. If the universe is infinite with an infinite number of stars couldn't the cosmic expansion explain why the sky is not bright all the time because outside the boundary of the Observable Universe the expansion exceeds the speed of light and therefore those photons don't reach our vision...we only see the light of the stars within the Observable Universe which are finite in number.

 

i thinK I'm wrong...I'm sure I've missed something but I thought I'd throw it out here for an explanation why.

[pwagen]

If the universe is infinite with an infinite number of stars couldn't the cosmic expansion explain why the sky is not bright all the time because outside the boundary of the Observable Universe the expansion exceeds the speed of light and therefore those photons don't reach our vision...we only see the light of the stars within the Observable Universe which are finite in number.

To my knowledge, an infinite universe doesn't go very well with an infinite one. Because if it's expanding, it's expanding from something. That means that it originates somewhere, and that means it's not infinite.

 

I read some old 60s book on astronomy that suggested an "ever creating" universe, which is basically creating matter out of nothing at some point, and then expanding like we think our universe does at the moment. So I guess that would be a valid universe by your example, but there's not a lot to suggest ours is one of those.

 

[StringJunky]

To my knowledge, an infinite universe doesn't go very well with an infinite one. Because if it's expanding, it's expanding from something. That means that it originates somewhere, and that means it's not infinite.

 

I read some old 60s book on astronomy that suggested an "ever creating" universe, which is basically creating matter out of nothing at some point, and then expanding like we think our universe does at the moment. So I guess that would be a valid universe by your example, but there's not a lot to suggest ours is one of those.

 

The Universe is not expanding from something , the space expanding between the galaxies at the Supercluster level is increasing...space is created and can do so at superluminal speeds. The rate of expansion increases with the distance two points are away from each other. Beyond a certain point, which is the 'edge' of our Observable Universe the rate of expansion becomes superluminal which at this point leads to the subject of my previous post.

Edited April 22, 2011 by StringJunky

[steevey]

The universe has no observable boundary.

[StringJunky]

The universe has no observable boundary.

 

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

[steevey]

http://en.wikipedia....rvable_universe

 

There's a limit to what we have observed, but that does not mean there isn't any more nor does that mean we can confirm an actual boundary which you cannot go past.

[StringJunky]

There's a limit to what we have observed, but that does not mean there isn't any more nor does that mean we can confirm an actual boundary which you cannot go past.

 

By 'boundary' I meant the nearest point from an observer on Earth at which the rate of cosmic expansion starts to exceed the SOL. The distance outside that 'boundary' increases so fast that the incoming photons can't reach us. I'm not talking about a physical edge. If you don't understand cosmic expansion you won't be able to visualise what I mean. Got this out of Cosmo Basics sticky here:

 

http://www.astro.ucla.edu/~wright/Balloon2.html

 

It doesn't show the photons never reaching a galaxy but does show the increase in distance without the galaxies moving.

 

Edit:

 

The Hubble Distance is the boundary I was on about which is about the same as the distance of the Observable Universe.

 

http://www.universeadventure.org/fundamentals/media/model-hubbledistance.swf

 

I don't know why the link here is bounded by 'media' but just copy and paste the address in between to see an animation.

Edited April 23, 2011 by StringJunky

[pwagen]

The Universe is not expanding from something , the space expanding between the galaxies at the Supercluster level is increasing

Wouldn't this require the space to expand from something though. I thought that was the whole foundation of the Big Bang theory?

[Spyman]

Wouldn't this require the space to expand from something though. I thought that was the whole foundation of the Big Bang theory?

According to the Big Bang theory there is no center or other fixed location inside the Universe that space is expanding from, space expands equally everywhere.

 

 

Can I put this thought foward...please shoot it down if it's wrong. If the universe is infinite with an infinite number of stars couldn't the cosmic expansion explain why the sky is not bright all the time because outside the boundary of the Observable Universe the expansion exceeds the speed of light and therefore those photons don't reach our vision...we only see the light of the stars within the Observable Universe which are finite in number.

 

i thinK I'm wrong...I'm sure I've missed something but I thought I'd throw it out here for an explanation why.

I think this is correct, "Kepler saw this as an argument for a finite observable universe, or at least for a finite number of stars".

Edited April 23, 2011 by Spyman