Is the Universe growing in size?

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In summary: The professor said that the universe is expanding, but it's not actually expanding into anything. This doesn't make sense to me because if the universe is expanding, then it should be expanding into something.
  • #1
mkarger
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This is a question about the borders of the Universe (if such a thing actually exists).

I got in a discussion with my astronomy professor and inevitably I brought up the question "if the Universe is expanding, and the Universe is contained, what is it expanding into?"

He said that it's not actually expanding into anything. I said that doesn't make any sense. He gave me an odd look and said "I know".

To me, this would suggest that it's not actually growing larger in size.
 
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  • #2
Until about 12 months ago, I had the same difficulty and misunderstanding. As I understand it, there are two theories: The universe is finite, and the universe is infinite. It seems that the latter is the more favored, which I used to have trouble with, but now see its elegance and (within the bounds of my limited knowledge) its validity.

In the infinite model, expansion is discussed in terms of space-time and is difficult, even for the experts, to explain because much of it comes from cosmological/mathematical modeling. But I'll give it a go.

The first thing you have to understand is that the term "big bang" has not helped people's understanding. We think of an explosion as having originated from a coordinate, generally at the center of the explosion. The BB was not like that. With the BB, matter came into existence, everywhere in the same instant. (Suspend philosophical disbelief for the moment, understanding that is a side issue that can be discussed later.)

Next, the typical model of how scientists attempt to explain expansion is by using a 2D analog of our 3D universe. Imagine a balloon as being the universe. Follow a line across that balloon anywhere and you will find no edge. Add some air to that balloon and it expands, creating greater distance between objects on the surface of the balloon.

Objects may be moving on that surface, but the expansion adds distance that has nothing to do with the movement of those objects. Understanding expansion itself is a whole other topic.

If I've explained this wrong, stay tuned for the corrections, but essentially we cannot apply our earthbound understanding of finite things to the universe.
 
  • #3
Well, this is one of those things about mathematics that just can't be visualized properly. The problem is that in General Relativity, there is no background: the geometry of space-time is all there is. So if we're talking about an expanding universe described by General Relativity, then that expanding universe is all there is.* There is no background grid against which the expansion is happening. It's just that objects within the expanding universe are getting further apart. Within General Relativity, it doesn't even make sense to talk about what lies "outside" the universe.

*The caveat here is that there are other models that describe our universe as just one expanding bubble of space-time among many. But in these models, our expanding universe still isn't expanding into anything. Basically, the different expanding bubbles become disconnected from one another and act entirely independently.
 
  • #4
He gave me an odd look and said "I know".

Lol... well, at least you have an honest one... :wink:



OCR
 
  • #5
mkarger said:
To me, this would suggest that it's not actually growing larger in size.

Yes, but it IS. Google Hilbert's Hotel. Infinity can get bigger and still be infinity. Math with infinity doesn't work the same as math with finite numbers.
 
  • #6
Also Google "balloon analogy". Or simply search for it here on PF.
 

FAQ: Is the Universe growing in size?

What evidence supports the idea of the Universe growing in size?

There are several lines of evidence that suggest the Universe is expanding. One of the most significant pieces of evidence is the observation of redshift in light from distant galaxies. This is a phenomenon in which the wavelength of light is stretched as it travels through expanding space. Additionally, the cosmic microwave background radiation, which is leftover radiation from the Big Bang, is also observed to be uniformly distributed in all directions, supporting the idea of a uniformly expanding Universe.

How does the expansion of the Universe work?

The expansion of the Universe is driven by dark energy, a mysterious force that counteracts the pull of gravity between all matter and causes the expansion to accelerate. Think of it like blowing up a balloon - as the balloon expands, all points on its surface move away from each other, just as galaxies move away from each other as the Universe expands.

Will the Universe continue to grow indefinitely?

The current scientific consensus is that the Universe will continue to expand indefinitely. However, the rate of expansion may change over time, and there are theories that suggest the Universe could eventually stop expanding and begin to contract.

Can new matter or energy be created as the Universe expands?

No, the total amount of matter and energy in the Universe remains constant as it expands. However, this does not mean that new galaxies or structures cannot form as the Universe expands - matter and energy can be rearranged and combined in different ways.

Is the expansion of the Universe uniform?

On a large scale, the expansion of the Universe is fairly uniform, meaning that all points in space are moving away from each other at a similar rate. However, on smaller scales, the expansion can be affected by the distribution of matter and energy, causing some regions to expand faster or slower than others.

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