Expanding Universe: What Does it Mean?

[Xidike]

What does it exactly means by the expanding of universe ? Does everything included in it is also expanding ??

 

[Higgs Boson]

If the Big Bang model is correct, and if dark matter does truly exist, (which it obviously does), then yes, the Universe is expanding insomuch as there is an ever-occurring increase in distance, in the spacetime scale of the universe, between any two most distant known points. (See the FLRW metric.)

With regard to "everything included in it also expanding," the answer is technically no, simply because gravity continues to compress matter within the universe - black holes, for instance, compress matter down to a singularity and we're not exactly sure what happens at the singularity of Black Holes but we do know that matter is not expanding at that point - although massive amounts of energy is being released. But that's another matter altogether, pardon the pun - and the Big Bang metric does not rely on the separation of matter but rather, the separation of space itself, over time.

So, if your question regarding "everything included" pertains to whether, or not, space and time appears to be distancing itself from the original point of the Big Bang ... then yes, everything included in the universe, per se, is expanding outward from what was once the singularity, the point of origin of the Big Bang.

Did you have to translate your question from another language by chance? I hope this explanation is suitable for your needs and I am sure someone is going to come along and challenge my attempt at stating this in the simplest terms I know how.

 

[phinds]

So, if your question regarding "everything included" pertains to whether, or not, space and time appears to be distancing itself from the original point of the Big Bang ... then yes, everything included in the universe, per se, is expanding outward from what was once the singularity, the point of origin of the Big Bang.

.

There was no POINT of origin of the big bang. "Singularity" does not mean "point" it means "the place where our models break down and we don't know what is/was going on".

 

[phinds]

What does it exactly means by the expanding of universe ? Does everything included in it is also expanding ??

try this:

www.phinds.com/balloonanalogy

 

[BruceW]

There was no POINT of origin of the big bang. "Singularity" does not mean "point" it means "the place where our models break down and we don't know what is/was going on".

that depends on whether the universe is flat, open or closed (which no-one knows). hehe, sorry, I know that you were just explaining that that singularity does not necessarily mean a point. I agree.

edit: hmm. also, I guess it depends how we define 'point of origin'... Because if the universe is closed and we go back to very early times, we still don't get to a 'point', we just have a much 'smaller' universe, and in the limit, it tends to something of infinitely small size, but this is not necessarily the same as the definition of a point..

 

[BruceW]

What does it exactly means by the expanding of universe ? Does everything included in it is also expanding ??

Yes. For example, between you and me, there will be our normal everyday motion, but also, there will be a very slight movement apart which is due to the expansion of the universe. On small spatial scale such as this, it is pretty much unnoticeable. But when we talk about larger scales, like different galaxies, then they have a much greater motion due to the expansion of the universe.

And as someone already said (sorry to repeat), even though on small scales (for example nearby galaxies), may be moving toward us at some steady speed, the expansion of the universe is still accelerating us apart, although it may be that this acceleration is not enough to prevent a galactic collision.

 

[Higgs Boson]

There was no POINT of origin of the big bang. "Singularity" does not mean "point" it means "the place where our models break down and we don't know what is/was going on".

Yes, I agree, and in my mind I was expressing "a point in time" rather than a "point of origin," (as referenced earlier "we do know that matter is not expanding at that "point" (in time) assumed), but yes, you are entirely correct and thank you for correcting that and preventing any misunderstanding.

 

[pumila]

If you mean are electrons (say) expanding with the universe, then I don't think the big bang theory allows that. Einsteinian expansion might (look up Einstein's "cosmological constant") - the measurements that indicate that the rate of expansion of the universe is increasing appear to challenge the big-bang expansion theory. But the whole field is woefully short on scientific proof.

 

[tiny-tim]

Hi Xidike!

What does it exactly means by the expanding of universe ? Does everything included in it is also expanding ??

our one-metre "rulers" are not expanding, or at least are not expanding as fast as larger bodies such as galaxies or the universe itself

using the familiar balloon analogy:

the individual galaxies stars or atoms are not fixed to a particular point on the fabric of the expanding balloon … although they are forced to move with the surface, they are free to move along the surface, and their relative positions will depend on their mutual interactions, and the way those interactions decrease with distance

imagine a circle ABCDEF with six springs on it, three very weak ones initially of length 119° (AB CD and EF), and three very strong ones (in between) of length 1° (BC DE and FA) (total 360°)

expand the circle ten time: obviously, the lengths AC CE and EA will always be 120°

but the very strong springs will now be a lot less than 1° …

if we use them as our "rulers", then we measure the long springs as being (almost) ten times as long as before!

so yes, our one-metre rulers are expanding as the universe expands, but much more slowly (because gravity is a "spring" whose strength decreases with distance), and so we do measure an expansion of distances between galaxies

 

[Chronos]

The rate of expansion looks pretty impressive at vast distances, but, vanishingly small at short distances. For example, the size of Earth orbit around the sun would have increased by about 1 Earth diameter over the last 4.5 billion years if it were expanding at the Hubble rate [which it is not].

 

[SC400slide]

The rate of expansion looks pretty impressive at vast distances, but, vanishingly small at short distances. For example, the size of Earth orbit around the sun would have increased by about 1 Earth diameter over the last 4.5 billion years if it were expanding at the Hubble rate [which it is not].

The ending mention is correct.Ω

 

[Xidike]

Is there any solid reason that why the universe is expanding ??

 

[BruceW]

the experimental evidence tells is to us. And theoretically, it is just one of the possible solutions to Einstein's general relativity, so we luckily have the framework to model it too. Although there are many unanswered questions.

Edit: the evidence is in 'standard candles' and redshift and CMB... Also to be honest it is more 'difficult' to conceive of a universe that is perfectly static, so straight away, we should have assumed that the universe would not be static.

 

[bapowell]

Is there any solid reason that why the universe is expanding ??

It could just as easily have started out contracting: whether the universe expands or contracts is just a boundary condition on the equations of motion. There's no dynamical preference for one over the other. However, if the universe had contracted instead of expanded, you wouldn't be here to ask your question...

 

[Mordred]

The expansion is often attributed to dark energy, which is an energy we don't understand. More accurately however its the cosmological constant or vacuum energy. As bapowell pointed out the Eienstein field equations predict either an expanding or contracting universe. A static universe is by nature unstable as the smallest deviation in what made it stable, such as mass density, vacuum energy density would cause and expansion or collapse.

 

[bapowell]

The expansion is often attributed to dark energy, which is an energy we don't understand. More accurately however its the cosmological constant or vacuum energy.

Except that we don't need any of this to have expansion. We should probably keep the idea of expansion as general as possible, and not imply that any sort of dark energy is necessary for it.

 

[phinds]

The expansion is often attributed to dark energy ...

That is not correct. Expansion is NOT attributed to dark energy, it is an initial condition of the universe. The ACCELERATION of the expansion is attributed to dark energy.

 

[BruceW]

Even the acceleration of the expansion is not necessarily due to just dark energy.

 

[Mordred]

I agree I stated "often" I should have expanded on that as most online literature, will state that its attributed to dark energy. Which isn't necessarily correct however if the OP tries to research it online. He will often come across the statement "expansion is attributed to dark energy." Unfortunately pop media articles seldom paint an accurate picture.

 

[marcus]

Even the acceleration of the expansion is not necessarily due to just dark energy.

I also agree.

The cosmological constant may simply be that: a naturally occurring curvature constant which appears on the lefthand side in the equation of GR because it is allowed by the symmetry of the theory.

You may have seen this paper, it's an entertaining and perceptive contribution to the discussion: a favorite of mine. Any time you want to recall the link, just google "why all these prejudices"

http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Submitted on 21 Feb 2010)
The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.
9 pages, 4 figures

 

[BruceW]

hehe, I just read it. pretty good. I agree with what they said, that vacuum energy (in the context of quantum field theory) and cosmological constant are two separate things. For example, the inflaton field (which is inflation), is also a quantum field, but it is not associated with the cosmological constant. In a similar way, maybe the vacuum energy is the same thing as the cosmological constant, but it seems foolish to identify the two concepts a priori.

Also, the point I was making was a different thing. What I meant was that the acceleration of the expansion of the universe is not just due to the cosmological constant (it is also due to the matter and energy in the universe). I was using 'dark energy' as just another word for cosmological constant, but I see that not everyone uses that definition.

 

[Mordred]

Grr I was digging in my archives for the same paper, just found it. Was about to post it then saw that Marcus beat me to it lol