# Is the universe expanding?

1. Aug 1, 2011

### Physics_Kid

so, many shows on tv today that keep suggesting our universe is expanding. is it really?

1. if you can say "it is expanding" doesnt that imply you 1st have to know how big it is now, then make a observation (measurement)? so if the universe is already infinite in size how can anyone say its expanding (we cant measure infinity).....?

2. if the universe was a cube of space with the 2-dimensional sides increasing in size at the known max speed limit of C (speed of light), this seems possible, but as an observer inside the cube observing the edge where any two cube surfaces meet the observer would observe the edge moving away faster than C (the vector)! so to me a cube doesnt make sense.

3. going on #2, the universe is a sphere with radius increasing at C ?

4. and to wrinkle the math some, if the universe itself is not expanding at speed of C doesnt this imply that light itself can indeed reach the edge of the universe? and if it does what happens?

just wondering....

2. Aug 1, 2011

### Trevormbarker

1. Yes its expanding
2. It CAN expand faster then the speed of light
I believe there is a FAQ on this in cosmology somewhere

3. Aug 1, 2011

### milkomeda

Although I'm still new to this, I believe I can help out a bit.
The universe is expanding, but that does NOT imply we know the current size or shape of the entire universe. We know the size of the observable universe (a diameter of 93 billion light years), but depending on your model of the universe, that could very well be a drop of water in an ocean.

The farther a galaxy is from the milky way (our point of reference), the faster it is moving away from us. This was determined via the Doppler Effect, with Hubble discovering a correlation between the redshift of a galaxy and the distance it was from the milky way. This type of event (with objects farther away from a center moving faster than those closer to the center) is seen all the time on Earth, as it occurs during an explosion. I believe through this and other discoveries/theories (which I am not qualified to explain), scientists where able to determine that the universe was at one time in an extremely hot and condensed sate, and then started to expand rapidly (The Big Bang, or perhaps more fittingly, The Big Expansion).

Galaxies cannot by themselves travel faster than the speed of light, but will eventually move away from us faster than the speed of light. This is due to the fact that space-time is expanding at an increasingly faster rate due to the propulsive force that is dark energy. So eventually, with the ever increasing rate of expansion, the galaxies will be moving away from us faster than the speed of light, and will disappear from the night sky, as the light from those galaxies can no longer reach us.

I myself am just starting a B.S. in physics, so I would welcome any critique or corrections to my statements/understandings that some of the other posters may have.

4. Aug 2, 2011

5. Aug 2, 2011

### Chronos

You need not know how big the universe is to realize it is expanding. Einstein deduced the universe could not be static, it had to either be expanding or contracting. Hubble confirmed it was indeed expanding, causing Einstein to commit his biggest blunder - withdrawing his cosmological constant idea.

6. Aug 2, 2011

### bapowell

Einstein's biggest blunder was his introduction of the cosmological constant -- not it's removal from the theory. I don't think Einstein ever "deduced" anything about the expansion of the universe -- he simply felt that aesthetics mandated a static cosmology. Hence, his introduction of the CC to render the universe static.

7. Aug 2, 2011

### Chronos

Agreed. Just a little tongue in cheek humor. The cosmological constant was introduced in an effort to model a static universe, which was the prevailing [and Einstein's] belief of the time. Einstein was certainly aware his theory did not model a static universe without the cosmological constant. After Hubble's discovery, Einstein called the cosmological constant his greatest blunder. As we now know denouncing the cosmological constant was his only blunder. see http://www.jb.man.ac.uk/~jpl/cosmo/blunder.html

8. Aug 2, 2011

### Physics_Kid

in regards to post #3, galaxies and stuff is matter (mass). just because we can observe that mass moving away from us does not mean that the universe itself is expanding. what i'm asking is if the the "box" of space-time is expanding and if so at what speed? can we even measure space-time?

i dunno an answer, but i know if its not expanding at min C then we need to come up with the physics/math that explains what happens when light reaches the edge of the universe...

9. Aug 2, 2011

### granpa

when light reaches one end of the box it reappears at the opposite side of the box.

10. Aug 2, 2011

### Physics_Kid

hmmm, this is indeed one theory, like the old asteroids game where spaceship leaves one side and reappears on the other. a theory that needs much explaining, a theory that suggests a finite universe or one that is expanding slower than C.

11. Aug 2, 2011

### DaveC426913

It is that the farther something is, the faster it is moving away. At the very edge of the observable universe things are moving away quite quickly. i.e. the universe seems to be expanding.

The universe has no edge. As granpa says: Google balloon analogy. A balloon's surface expands yet it has no edge.

12. Aug 3, 2011

### -Job-

Ah this reminds me, i had a question recently when watching some documentary on the topic, this is not my field so bear with me.

If stars that are farther away are also farther back in time, then wouldn't the fact that the fastest moving stars are the farthest away from us mean that stars were moving faster a long time ago, suggesting that the universe is deaccelerating?

13. Aug 3, 2011

### bapowell

A given object that today is moving away from earth with the expansion was once closer to earth, and hence, receding more slowly.

14. Aug 3, 2011

### DaveC426913

No. Again, turn to the balloon analogy.

Glue a bunch of pennies to a balloon. Inflate the balloon. The speed at which any two pennies diverge is directly proportional to the distance between those two pennies. Farthest pennies diverge fastest ,despite the fact that the inflation rate has not increased.

15. Aug 3, 2011

### Physics_Kid

i am still having a hard time understanding how the galaxies (mass) are linked to space itself. if we can observe mass moving by doppler then we can say "that mass is moving at speed xyz", but how does that infer that the universe is expanding?

the balloon theory seems flawed........... this balloon theory is more like a balloon inside a infinitely sized box, this seems more analogous to the observable mass moving away from all other observable mass.

16. Aug 3, 2011

### DaveC426913

"linked to space itself"? Don't know what you mean.

If the sum total of the mass that expanded from the BB is growing in volume, then ipso facto, the universe is getting larger.

It is not a theory; it is an analogy. It is designed to help one grasp an elusive concept. "flawed" gives it more credit than its due; it's just a simple analogy.

17. Aug 4, 2011

### zonde

We describe observations using some coordinate system. In that coordinate system average distance between galaxies grows over time. That's our current choice to describe observations this way.

Basically statement that universe itself is expanding is just a way of saying that it is not a big explosion type of expanding. So it says what it is not but do not rally says what it is.

18. Aug 4, 2011

### bapowell

Galaxies are not linked to space itself. There are two important motions to understand here: there is the expansion of the space itself -- this is the rubber in the balloon analogy. Then you have the motions of the galaxies relative to this space (in the balloon analogy, the galaxies are "painted" onto the surface and don't move relative to it, but this is just a simplification.) The motion of a galaxy relative to the expansion is called its peculiar velocity. Now, one can show that in an expanding universe, objects moving relative to the expansion experience a drag force (called Hubble drag) that causes them to asymptotically come to rest relative to the expansion. In any case, peculiar velocities are really quite small in comparison to the recession velocity due to the expansion.

Now, your point about whether the galactic redshifts we observe are due to the Doppler effect or have a cosmological origin is one that has been debated here on these forums at length. I recall the solution is that these two choices are empirically identical, although a universe in which all galaxies happen to have peculiar velocities that agree with Hubble's Law sounds like a very contrived and special universe indeed.

19. Aug 4, 2011

### eah2119

So, is our observable universe shrinking?

I've always thought that our observable universe is expanding because every day more and more light is reaching us. As a result, more and more distant galaxies are being revealed. But when I think about it, considering the constantly acceleration expansion of space, is our observable universe shrinking? If so, at the time before the great expansion, was the observable universe equal to the entire universe? Will we, one day, be limited to view only our own galaxy or less?

20. Aug 4, 2011

### Octonion

Re: So, is our observable universe shrinking?

When we look through telescopes, the average velocities of all the objects distant from our galaxy are pointed radially away from us. Indicating that either (a) the milky way is the center of the universe (which it is not), or (b) the universe is itself expanding, where every single point in space-time is getting further from away from one another. On top of this as we look further and further away from the milky way we see things moving faster and faster away from us. Thus, the velocity you can attribute to the expansion of the universe of a galaxy with respect to our observatories here on earth is proportional to the distance it is away. So when we look at it we can subtract those effects of expansion and study how it is moving with respect to space-time itself. And eventually things are so far away that the space-time they are occupying is expanding away from us faster than the speed of light and thus, the photons from those galaxies can never reach us! This is called the cosmological horizon and because space is expanding everyday more and more things move out of our view. Less and less light is in our viewable portion of the universe.

21. Aug 4, 2011

### bcrowell

Staff Emeritus
We have a FAQ on this topic: https://www.physicsforums.com/showthread.php?t=506993

We also have a FAQ entry that addresses this question: https://www.physicsforums.com/showthread.php?t=508610 [Broken]

Last edited by a moderator: May 5, 2017
22. Aug 4, 2011

### Chronos

The observable universe ends abruptly about 380,000 years after the big bang. We will never see anything older than the universe [13.7 billion years at present]. There are no presently unobservable galaxies that will some day suddenly pop into view, nor any that will suddenly vanish in the future. It appears distant galaxies will eventually redshift beyond detectability over time.

23. Aug 5, 2011

Re: So, is our observable universe shrinking?

I'm not objecting or anything, but seeking to learn. What's the evidence/proof that the milky way is not the center of the universe?

24. Aug 5, 2011

### zonde

Re: So, is our observable universe shrinking?

I doubt that there is any serious evidence/proof of that. Because nobody seriously considers this possibility following http://en.wikipedia.org/wiki/Copernican_principle" [Broken].

Last edited by a moderator: May 5, 2017
25. Aug 5, 2011

### bapowell

Re: So, is our observable universe shrinking?

The Milky Way being the center of the universe is not ruled out by current observations. In fact, a universe in which the Cosmological Principle holds and one in which the Milky Way lies at the center are not empirically distinct. Modern cosmology embraces the Cosmological Principle because it is simpler and requires no assumptions that are not empirically testable (homogeneity and isotropy). If the Milky Way were at the center, scientists would need to understand why we happen to occupy this restricted and privileged location.

EDIT: Also, compare Copernican and Cosmological Principles. The current working assumption is the latter.