Finite or Infinite Universe

[Tom B]

I posted this in the physics section, but it probably fits here better.

 

I would like to post a few questions and offer a thought experiment or two. I do not have a background in these fields, but I enjoy reading and watching science shows.

 

First, there is widespread agreement on the Big Bang Theory and the age of the universe

 

The age of the universe is largely based upon the residual microwave radiation that is thought to be the oldest observable portion of our universe. We are surrounded by this radiation and it is the same in all directions.

 

First question. In some respects, that seems to put us at the center of the universe. We can see the same distance in every direction. Does that seem mathematically probable?

 

Though experiment.

 

A solar system exists 10 billion light years from earth. Scientists in this solar system have the same understanding, theories, and tools that we have. If they were to look in our direction today and could detect us, they would see us as we were 10 billion years ago. If the universe is 13 billion years old, they would observe the background microwave radiation about 3 billion light years beyond us. Beyond that, galaxies that are clearly detected by us are invisible to them. This assumes the big bang theory and the assumptions of the age of the universe are correct. Now, they look in the opposite direction. What do they see? Is it the background radiation at a distance of 3 billion light years? Would they be unable to see beyond it?

 

Might it have moved further away or evolved into something else? If everything was born from this energy, why not in 10 billion years while so much happened on our side of the universe? Now, if they see more galaxies, can you move another 10 billion light years and repeat this experiment? If the universe is infinite, it seems you could keep doing this, but what does that tell us?

 

If the universe is finite, might an object leaving from their side of the universe enter our side? Could it be that the universe is finite and the region of microwave radiation is at the boundary? At the boundary you simply move in a similar manner as you do when you fly east from New York long enough. You end up back in New York. In this view the universe is a closed system, much as a planet or other geometrical shape with the galaxies on the surface of this shape.

 

If this is the case, couldn't astrophysicists detect an interaction of the galaxies at one side of the universe with those on the other? Gravity would act across the radiation boundary. I know that some unseen interactions have been detected at the far reaches of the universe which have yet to be fully explained.

 

This closed, finite universe could still be expanding. Picture it on the surface of an inflating balloon or a bubble.

 

If this radiation is 13 billion light years from us and has been traveling towards us at the speed of light, how did it get so far away in 13 billion years? Was the early period of expansion after the big bang so extreme to account for these great distances?

[Greg H.]

Based on my understanding (which, admittedly, could be very flawed):We would see in a circle 13 billion light years across - so would they. Both circles are centered on the observer. So they would see things we cannot, and we would see things they cannot.

 

The universe itself is infinite. The observable universe is finite, and what is observed varies based on the location of the observer.

 

Edit to attach a file showing a conceptual drawing.

 

In this drawing the blu dots are the (vastly oversized) planets, while the circles indicate their observable areas. The football shaped area in the middle is the only area where both would be able to see the same things.

 

Now, keep in mind this is a 2d drawing, and the effect would actually be spheres, not circles.

Edited July 13, 2012 by Greg H.

[too-open-minded]

In my opinion, the only thing infinite is nothing. To a human, yes the universe may look so big that its seems infinite but were just really small compared to the universe.

 

The human mind/consciousness can generate things like numbers, lines, circles, patterns. these things do not appear in nature, or atleast they are not precisely perfect. As they are in our minds.

 

Our reality is an illusion ;P

Edited July 14, 2012 by too-open-minded

[robheus]

Our reality is an illusion ;P

 

But a very persistent illusion

[too-open-minded]

What is an illusion other than something you perceive to be a little different than it actually is?

 

I never said our illusions were bad, just saying their illusions and sometimes we forget that. Most people are oblivious too it.

[Binyamin Tsadik]

What is an illusion other than something you perceive to be a little different than it actually is?

 

I never said our illusions were bad, just saying their illusions and sometimes we forget that. Most people are oblivious too it.

 

I think you are straying off topic from what was originally posted.

But perception is a simulation of the brain based on observation.

Every mind has its own simulation of the world, and this is necessary for survival.

Simulations allow us to predict the future and form a direction sense and a sense of our surroundings.

 

A perfect circle may not exist in nature, but it does in mathematics, and mathematics is an extension of the digital information nature of our mind. A perfect circle may also be perception because that is easier for our minds to process and calculate with.

 

Regardless, science is experimental and if a given simulation disagrees with reality it is not a good simulation. The better the simulation of reality that we can come up with, the more accurately we can predict the future (or the past for that matter).

 

The simple fact that you can catch a ball, means that your mind is able to do simple physics intuitively and simulate the path of that ball in order to predict its final destination. Little adjustments of your hand while the ball is in flight is the result of a feedback loop control system in order to correct errors in your simulation.

[Severian]

Obviously one can never prove that the universe is infinite, but there is no evidence that there are any boundaries in space.

[too-open-minded]

I think we can better come to an agreement with

 

Although the universe has infinite boundary it is finite. Thats how I look at it atleast.

[JMJones0424]

Very good questions Tom B. Some of your questions have already been answered by Greg H., but I'd like to add a little bit and point you to a few resources you may find useful. I am going to take some liberties and assume an education level that may be incorrect. It is not my intention to insult your intelligence. I never took any math courses after high school, and cosmology was very difficult for me to wrap my mind around until someone was finally kind enough to break down some of the basic points to me. If you already understand this, then please allow me to explain to some of the other participants of this thread, or to the lurkers who may have questions that they don't wish to ask.

 

I highly recommend you dig around in Ned Wright's website, specifically the Cosmology FAQ. If you are like me and are in need of a quick reference for basic physics questions, you may find GSU's HyperPhysics site useful as well.

 

Is the universe finite or infinite?

This is not an easy question to answer, and the truth is that we don't really know. One may be tempted to conclude that because the Big Bang took place a finite amount of time ago, the universe must be finite. But it's important to remember that Big Bang Theory does not necessitate that the Big Bang happened in a finite volume as is commonly thought. If the Big Bang itself were infinite, then even though it happened a finite amount of time ago, the universe would still be infinite. We only have the observable universe to deal with, which is finite, because the Big Bang occured a finite amount of time ago. Just like Greg H.'s illustration, everyone in the universe appears to be at the center of their observable universe, because the speed of light is finite and the age of the universe is finite. In fact, far in the future, if expansion continues to accelerate, there will come a time where all other galaxies will also be beyond the observable horizon.

 

The answer to this question depends on the geometry of the universe, and we don't yet have accurate enough measurements to determine an answer one way or the other. If Euclidean geometry is an accurate representation of the universe at a large scale, then the universe has no curvature and is unbounded. If the universe is positively or negatively curved, than the universe could very well be finite. Measurements made so far show that the universe is pretty darn close to flat, but we are not able to conclude if there is a slight curvature one way or the other.

 

Curvature? How can 3D space be curved?

If you haven't been exposed to non-Euclidean geometry, this may be a difficult concept to understand. Basically, Euclid is credited with formulating the kind of geometry that is taught in high school. He derived all of what we call geometry from 5 basic postulates (assumptions). The first four are pretty straightforward:

 

1. A straight line segment can be drawn joining any two points.

 

2. Any straight line segment can be extended indefinitely in a straight line.

 

3. Given any straight line segment , a circle can be drawn having the segment as radius and one endpoint as center.

 

4. All right angles are congruent.

 

The fifth one, the

parallel postulate, is the one that was always a thorn in everyone's side. It essentially states that given any straight line and a point not on that line, there exists one and only one line that passes through that point but does not intersect the original line. Stated more simply, parallel lines do not intersect. This seems obvious, because this is the geometry you were taught. However, noone was ever really able to prove this postulate in a way that stood up to intense scrutiny. Much, much later, in the 1800s, people began developing new geometries that do not rely on the parallel postulate. If you give it a moment to consider, you too can see why the fifth postulate isn't necessarily defensible.

 

Take the angles of a triangle, for instance. You know that the angles of a triangle equal 180^o. However, let's say you make a really big triangle. Start at the equator, go straight north to the north pole, turn 90^o and go straight south back to the equator. Including the equator, you now have a triangle made of two lines that intersect the equator at 90⁰ and each other at the north pole at 90⁰ as well, giving a triangle with a total of 270⁰. A die-hard Euclidean would complain that no, you in fact don't have a triangle, you've got a curved section of a sphere that roughly looks like a triangle but isn't itself in a "flat plane" and therefore is not a triangle. However, ask the Euclidean to support his argument with the postulates listed above, and he wouldn't be able to. What you have done is proved that the Earth is not a flat plane, not that you needed proof, but you have also visualized a spherical geometry with positive curvature. (Remember that it's positively curved because there are more than 180^o in a circle.) Another type of curved geometry is hyperbolic geometry, where there are less than 180^o in a circle. Examples of a plane in a hyperbolic geometry would include a horse saddle and the inside of a donut.

Image Source: Wikipedia on Shape of the Universe. From top to bottom, spherical, hyperbolic, and flat.

 

So, what does this mean for the shape of the universe? If the universe is positively curved, like a sphere, then the universe is not infinite. If the universe is flat, then it is infinite, or at least it is unbounded and if expansion continues forever, it would expand infinitely. If the universe is negatively curved, like a horse saddle or a donut, it could be either infinite or finite. The fact of the matter is that we do not yet have a precise enough

measurement of the curvature of the universe to answer definitively whether or not the universe is finite or infinite. We do, however, know that if the universe is curved, the radius of its curvature is so great that light has not had enough time to "wrap around" back to where it started since the Big Bang.

EDIT: minor grammatical changes

Edited July 18, 2012 by JMJones0424

[too-open-minded]

I wish the universe was round, then I would believe that gravitational fields are infinite

[robheus]

The universe can be both finite and infinite. Now that looks like a ridiculous contradiction. But let me explain.

 

Imagine a straight line, having no begin and end.

 

Now place two points anywhere on the line, and you are free to choose it anywhere on the line.

 

Now we can ask, does the distance between these two points, form a finite or an infinite distance.

 

Let me first show that it is finite.

 

Place the points on the line (anywhere you want) and start measuring the distance using some unit. Will it finish? Yes. We find a finite value.

 

Now let us ask a second question. Can we think of any value (any value at all) for which the measurement of the distance is clearly a limit it can not exceed, so that all measurent values of the distance between any two points on the line does not surpass this distance?

 

Well, we can just take that distance, measure it on the line and place our points there, and then we can show that any such finite value can always be exceeded by placing the points further apart.

 

Which then shows us that there is no limit to the distance between the two points, and that must mean that it is infinite (having no limit).

 

Now these conclusions are in contradiction with each other.

 

How to resolve that?

Edited July 21, 2012 by robheus

[dapifo]

When you discuss about the finite and infinite of the universe...you allways think only in Our Universe.

 

 

But why we don´t consider Our Universe (finite?) like that Universe that we know (from 10 exp -27 to 10 exp +27 meters), where we are just in the middle.

 

And we consider the Global Universe as the Total Universe. (infinite or finite)?...were we could supose higher potencials (from 10 exp <-1000 to 10 exp >+1000 meters)?

 

 

 

[JMJones0424]

I think the term "observable universe" creates the distinction you are trying to make, dapifo.

[too-open-minded]

Why cant empty space be what is infinite and the matter/energy of our universe be what is finite?

 

The empty space being infinity and 0 at the same time.

The material that consists of our universe being finite.

 

 

Our universe is in this void of nothingness or this empty space is part of our universe, so the universe is infinite in a way and finite in a way.

 

Why does it have to be one or the other, is the glass half empty or half full? It all depends on how you look at it and where you stick your labels.

[JMJones0424]

Why cant empty space be what is infinite and the matter/energy of our universe be what is finite?

We don't know that space is infinite for the reasons discussed earlier.

 

Also, space, even when "empty", isn't necessarily devoid of everything. Empty space, even an absolute vacuum, still contains fields, virtual particles, and other "stuff" that make it not nothing.

[too-open-minded]

Yeah I know the vacuum is more of a diffusion than anything else.

 

I'm sorry I don't understand any of that up their^^^ :/

 

I'm good with the theoretical part of physics, the math part scares me. Not gonna lie.

 

 

I know i'm kinda talking out my ass. So we have no proof of empty space? I guess your right even the space between galaxies has photons in it because we see other galaxies.....damn. Opened my mind a bit, thankyou

[JMJones0424]

Think of it this way- The surface of the Earth, a two dimensional sheet, is not flat. At a small scale, it is close to flat, though there are minor hills and valleys. But at a larger scale, the surface of the Earth is relatively spherical, so that if you travel in a straight line long enough, you will end up back where you started. While there is no edge to the 2D surface of the Earth, it is finite.

 

In the same way, but in three spatial dimensions, if the universe curves back on itself, space is not infinite. We do not yet have measurements accurate enough to confirm that space doesn't curve back on itself, but we do have measurements accurate enough to say that if space does curve back on itself, then the radius of the curvature of the universe is so large that the universe hasn't existed long enough for light to end up back where it started.

[too-open-minded]

Dude, gravity boundaries being "infinite" makes sense now. Their not infinite they just seem that way. Or am I wrong?

[JMJones0424]

Not exactly clear what you mean by "gravity boundaries". If you are asking if a gravitational field is bounded or unbounded, I think it is correct to say that it is unbounded. However, due to the inverse square law, gravitational effects are negligible at a large enough distance.

[too-open-minded]

The distance a gravitational field extends to, modern physics says is infinite.

[dapifo]

I think the term "observable universe" creates the distinction you are trying to make, dapifo.

 

 

 

Then the answer is clear.. no discussion...the observable Universe is finite, and it has aprox. 100 billions year light of diameter and 10 exp 80 atoms...and started 13.700 millions ago.

 

 

But this Observable Universe is not All and the Whole Universe.